JP3913165B2 - Casks and metal gaskets - Google Patents

Description

【００５８】
キャスクの密封用として使用する金属ガスケット２０の使用環境温度は最高１２０℃程度になる。また、金属ガスケット２０の近傍に水分が残留するおそれもある。したがって、このような金属ガスケット２０の使用環境を考慮して、上記試験条件を設定してある。表１から分かるように、高純度鉄はステンレス鋼と電磁軟鉄との中間程度の耐腐食性を示す。
【００５９】
表２は、熱処理又は酸液処理に対する高純度鉄の耐腐食性について評価した結果を示す。熱処理は、所定温度に２０分間保持した後、空気中で放冷したものである。そして、保持温度を２００℃、３００℃、４５０℃の三種類に変化させて評価した。また、酸液処理については、６３体積％のＨＮＯ₃溶液に６０分間浸漬したものと、０．３体積％のＨ₂Ｏ₂溶液に同じく６０分間浸漬したものとを評価した。
【００６０】
耐腐食性は、１００℃の沸騰水中に試験片を１８０分間浸漬して評価した。保持温度が２００℃の試験片は、熱処理なしの試験片と同程度の耐腐食性であり、熱処理による改善効果はみられない。保持温度が３００℃及び４５０℃の試験片は上記条件化で試験しても錆の発生は見られず、熱処理による耐腐食性の改善効果が見られた。これは、この熱処理条件によって試験片の表面に安定した酸化膜が形成されたことが、耐腐食性が改善された要因の一つであると推定される。また、一般に、金属組織中における酸素原子の拡散媒体であるひずみ（転位）の存在が、酸化による腐食を促進させるが、上記条件による熱処理によって、このひずみが除去されたことも耐腐食性が改善された要因の一つであると推定される。
【００６１】
【表２】

【００６２】
一方、ＨＮＯ₃溶液による酸液処理のみでは、耐腐食性は十分には改善されない。Ｈ₂Ｏ₂によって酸液処理のみを施した試験片は、酸液処理及び熱処理なしの試験片と比較して耐腐食性が改善される。これは、Ｈ₂Ｏ₂による酸液処理によって試験片表面に安定した酸化膜が形成されたためであると推定される。なお、熱処理を施した試験片と比較して耐腐食性が劣るのは、酸液処理のみでは試験片中のひずみが除去されないので、この分だけ酸化による腐食が発生したためと考えられる。
【００６３】
これらの結果から、高純度鉄をそのまま外被５０として使用するよりも、３００℃〜４５０℃の温度域で所定時間保持し、放冷する熱処理を施してから外被５０として使用する方が、耐腐食性を改善できるので好ましい。同時に、熱処理を施すと硬度を５０Ｈｖ程度まで低くすることができ、これによって密封性能を向上させることができるので好ましい。特に、圧延工程を経た材料は加工硬化によって硬度が高くなっているので、圧延材料は熱処理してから使用することが望ましい。なお、耐腐食性の改善のみを目的とする場合には、Ｈ₂Ｏ₂による酸液処理を施してもよい。また、イオンプレーティングやスパッタリング等の薄膜形成手段によって金や銀の耐腐食性皮膜を外被５０の表面に形成し、耐腐食性を向上させてもよい。この場合、当該皮膜は薄いので、金や銀を皮膜として使用してもこれによって価格が大きく上昇することはない。また、ニッケル（Ｎｉ）や亜鉛（Ｚｎ）等の耐腐食性に優れた材料を、外被５０の表面にめっきして耐腐食性皮膜を形成してもよい。
【００６４】
次に、外被５０の製法について説明する。外被５０に使用する高純度鉄は、例えば電子ビーム溶融精製法や高周波溶解法等の高純度金属精製法によって製造される。電子ビーム溶融精製法は、偏向コイルを介して電子ビームを精製対象金属に照射し、精製対象金属が溶融したところで偏向コイルによって電子ビームの照射範囲を狭くしながら溶融範囲を狭くする。そして、狭くなった溶融部分に不純物を集めてこの部分を除去し、高純度の金属を精製する方法である。
【００６５】
また、高周波溶解法は、誘導加熱によって金属を溶融させるとともに、るつぼと溶融金属に渦電流を発生させ、この渦電流間に生まれる磁気反発力によって溶融金属をるつぼから浮上させながら、金属を溶融する方法である。この方法によれば、るつぼと溶融金属とが非接触のまま金属を溶融できるので、両者間に反応を起こさせないで高純度の金属を精製することができる。これらの高純度金属精製法によれば、本発明の外被５０に要求される純度の高い高純度鉄を製造することができる。
【００６６】
この高純度鉄は、圧延によって所定の厚さに展延された後、所定の硬度に調整するために、必要に応じて熱処理が施される。この熱処理は、上述したように所定温度に処理対象である高純度鉄を所定時間保持した後、空気中で放冷するものである。なお、上記例において所定温度に保持する時間は２０分であったが、処理対象物の大きさに応じて所定温度に保持する時間は適宜調整される。保持温度の範囲は、上述したように３００℃〜４５０℃が好ましい。また、酸素濃度を調節した炉内雰囲気中で放冷させてもよい。熱処理が終了したら、所定の形状に切断してからスプリング３０又は内被４０を被覆して、金属ガスケット２０が完成する。
【００６７】
次に、金属ガスケット２０に使用される、押圧部材である環状のスプリング３０について説明する。図５は、この発明の実施の形態１に係る金属ガスケットのスプリングを示す説明図である。このスプリング３０はコイルスプリングであり、インコネル（登録商標）やステンレスあるいはピアノ線、りん青銅等の線材を、図５に示すように、隙間を設けないで巻くことによって作られている。これは、隣の線との間に隙間が開いていると、密封のため金属ガスケット２０が押しつぶされたときに、均等な力で内被４０と外被５０（図１、２参照）とを押し返すことができなくなり、長期間の使用において所期の密封性能を発揮できなくなるからである。なお、ピアノ線やりん青銅をスプリング３０として使用する場合には、耐腐食性を向上させるために、予め線材の表面にクロムや金の耐腐食性皮膜を形成しておくことが好ましい。耐腐食性皮膜を形成させる手法としては、例えばめっきやイオンプレーティング等の手法があげられる。
【００６８】
また、図５（ｃ）に示すように、スプリング３２には、断面形状が矩形の素線を使用してもよい。このようにすれば、断面円形のコイルスプリングと比較して、スプリング３２と内被４０又は外被５０（図１、２参照）とはより多くの面積で接するので、より均等にスプリング３２の反発力を内被４０又は外被５０に伝えることができる。これによって、外被５０の応力分布をより均等にできるので、スプリング３２の反力を押さえ込む内被４０を省略するか、又は内被４０を薄くすることもできて、クリープ変形に起因する金属ガスケットの応力緩和現象を低減できる。これによって、長期貯蔵においても密封性能を維持できる。
【００６９】
また、断面円形のスプリング３０と断面矩形のスプリング３２とで、両者の断面係数が同じであれば、断面矩形のスプリング３２の方が素線の幅ｂを小さくできるので、ピッチｐも小さくできる。したがって、同じ曲げ剛性であれば、スプリング３２の巻き数を増やすことができるので、より均一にスプリング３２の反発力を外被５０へ伝えることができる。このように、断面矩形のスプリング３２を使用した場合には、断面円形のスプリング３０を使用するよりも、より金属ガスケットの応力緩和現象を抑制する効果がある。また、ピッチｐを小さくできるので、締付け力Ｐが作用していない場合の初期傾きθ_P=0は断面円形のコイルスプリングよりも小さくできる。これによって、スプリング３２の反力をより有効に利用できる。
【００７０】
また、断面円形をしたスプリング３０と断面矩形のスプリング３２との断面係数が同じであれば、断面矩形のスプリング３２の方が、金属ガスケットの締付け力によってスプリング３２内に発生する応力を小さくできる。さらに、断面矩形のスプリング３２は断面円形のスプリング３０とは異なり、隣り合う素線と面３２ａで接触する。これによって、締付け力Ｐによってこの面３２ａに発生する応力は、断面円形のスプリング３０よりも小さくできる。その結果、締付け力Ｐによる傾き角θ_Pを断面円形のスプリング３０よりも小さくできるので、断面矩形のスプリング３２の素線に作用するねじれ応力も低減できる。これらの作用によって、断面を矩形にすればスプリング３２の長期信頼性をより高くできる。
【００７１】
一般に、金属ガスケット２０（図１、２参照）において密封圧力を得ようとすると、スプリング３０には素線径ｄが太いものを使用して、スプリング３０の曲げ剛性を高くする必要がある。スプリング３０の素線径ｄが太くなればそれだけスプリング３０の巻き線径Ｄ₁は小さくなる。その結果、高いシール圧力が求められる金属ガスケット２０の断面直径Ｄは、シール圧力が低いものと比較して小さくなる。なお、スプリング３０の曲げ剛性とは、スプリング３０の径方向に力Ｐを加えた場合における、スプリング３０の径方向に対する変形のし難さをいう（以下同様）。
【００７２】
金属ガスケット２０のスプリングバック量を大きくすれば、それだけ金属ガスケット２０が変位した際の密封に対する裕度を大きくすることができる。図６は、金属ガスケットに作用させる荷重と金属ガスケットの変位との関係を示した説明図である。図中の点線は、従来の金属ガスケット５２０（図１５（ａ）参照）を、図中の実線は本発明に係る金属ガスケット２０を示す。なお、図６に示してあるのは、ＬＭＰを用いた外挿法による解析結果であり、初期温度１２０℃で６０年後に６０℃まで直線近似のもとに温度降下した場合を温度条件としたものである。なお、この温度条件は、実施のキャスクにおける長期貯蔵の温度条件を十分に再現できるものである。また、ＬＭＰを用いた外挿法は、応力緩和を安全側、すなわち応力緩和を大きく見積もる手法であり、このような解析には好ましい手法である。
【００７３】
本発明に係る金属ガスケット２０の曲げ剛性は、従来の金属ガスケット５２０における曲げ剛性のおよそ６０％としてある。このようにするため、スプリング３０の素線径ｄは従来の金属ガスケット５２０よりも細くしてある。また、この発明に係る金属ガスケットの断面直径Ｄは、従来の金属ガスケット５２０と同じである。なお、本発明に係る金属ガスケット２０の曲げ剛性は、従来の金属ガスケット５２０における曲げ剛性の４０％以上８０％以下の範囲が好ましい。この範囲であれば、ある程度の曲げ剛性を確保しつつスプリングバック量を大きくする効果を得ることができるからであり、さらには従来の金属ガスケット５２０における曲げ剛性の５０％以上７０％以下の範囲がより好ましい。
【００７４】
ここで、この発明に係る金属ガスケット２０の初期締め付け量は、従来の金属ガスケット５２０よりも大きくしてある。これによって、従来の金属ガスケット５２０とこの発明に係る金属ガスケット２０とに作用する初期応力（締付応力）σ₀が等しくなるので、同じ時間経過した後の応力緩和量は同一となる。このときの締め付け応力はσｔである。ここで、締め付け応力σを徐々に開放すると、金属ガスケットは徐々に元の形に戻り、変位δが０に近づく。そして、締め付け応力σが気密限界応力σｃに到達したときの変位δが、金属ガスケットの気密限界変位δｃとなる。
【００７５】
この発明に係る金属ガスケット２０においては、従来の金属ガスケット５２０よりも曲げ剛性を小さくしてあるので、金属ガスケットの変位δに対する締め付け応力σの変化、すなわちσ／δが従来の金属ガスケットよりも緩やかになる。その結果、締め付け応力σの緩和に対して、従来よりも大きく金属ガスケット２０が変位する。すなわち、同じ締め付け応力σであれば、そのときのスプリングバック量を多くとることができるので、気密限界変位δｃもその分だけ大きくすることができる。これによって、従来の金属ガスケット５２０よりも大きなずれが生じても、キャスクの気密を維持できるので、長期貯蔵後であっても安定した密封性能を発揮して安全に輸送できる。
【００７６】
具体的には、スプリング３０にインコネルを使用した従来の金属ガスケット５２０におけるスプリングバック量δｃ₅₂₀は０．０１〜０．０２ｍｍ程度である。これに対し、スプリング３０に細いインコネル素線を使用した本発明の金属ガスケット２０におけるスプリングバック量δｃ₂₀は０．２０ｍｍ程度である。このように、本発明に係る金属ガスケット２０におけるスプリングバック量は、従来の金属ガスケット５２０におけるスプリングバック量の１０〜２０倍である。ここで、キャスクの密封に使用する金属ガスケットに要求されるスプリングバック量はおよそ０．０５ｍｍであるが、本発明に係る金属ガスケット２０は、この値を十分に達成できる。これによって、従来の金属ガスケットでは不十分だった、リサイクル燃料集合体のキャスクにおける数十年の長期貯蔵に対しても十分な信頼性を持つ金属ガスケットを得ることができた。
【００７７】
上記説明から、スプリング３０の曲げ剛性を小さくするとスプリングバック量を増やすことができるが、曲げ剛性を小さくし過ぎると、金属ガスケット２０の変形量が大きくなるため好ましくない。また、外被５０に使用する高純度鉄は、硬度が比較的低いといってもアルミニウムや銅等に比較すれば硬度は高い。したがって、シール面に外被５０を密着させるためには、スプリング３０にある程度の反発力を発生させて外被５０をシール面に押し付ける必要がある。これらの理由から、スプリングの曲げ剛性を小さくし過ぎることは好ましくなく、ある程度の大きさの曲げ剛性を持たせる必要がある。
【００７８】
スプリング３０の曲げ剛性は、材料が決まればスプリング３０の素線径ｄと巻き線径Ｄ₁との関係によって一義的に決定される。ここで、表３〜５に長期貯蔵後における金属ガスケットのスプリングバック量の評価結果を示す。この金属ガスケットに使用するスプリング３０はインコネルである。なお、この評価はＬＭＰを用いた外挿法によるものであり、温度条件は、初期温度１２０℃で６０年後に６０℃まで直線近似のもとに温度降下した場合である。また、キャスクの密封に使用される金属ガスケットに要求されるスプリングバック量が満足できたものを○とする。なお、スプリング３０の巻き線径Ｄ₁は、便宜上金属ガスケット２０の断面直径Ｄ（図２（ａ）参照）で代表させてある。
【００７９】
【表３】

【００８０】
【表４】

【００８１】
【表５】

【００８２】
表３から分かるとおり、金属ガスケット２０の断面直径Ｄが５．６ｍｍである場合には、スプリング３０の素線径ｄが０．３５ｍｍ〜０．５０ｍｍの間で良好なスプリングバック量を示すことが分かる。また、表２から、金属ガスケット２０の断面直径Ｄが１２．０ｍｍである場合には、スプリング３０の素線径ｄが０．８０ｍｍ以下で良好なスプリングバック量を示すことが分かる。表３は、スプリング３０の素線径ｄと金属ガスケット２０の断面直径Ｄとの比ｄ／Ｄで評価結果を整理したものであるが、これからｄ／Ｄが０．０２〜０．０８の範囲で良好なスプリングバック量を示すことが分かる。
【００８３】
上記結果から、スプリング３０の素線径ｄ（図５参照）は、０．３５ｍｍ以上０．８０ｍｍ以下の範囲が好ましく、０．３５ｍｍ以上０．５０ｍｍ以下の範囲がより好ましい。また、あまり金属ガスケット２０の断面直径Ｄが小さい場合には、断面直径Ｄに対して締付け量を大きくしないとスプリングバック量を確保できず、金属ガスケット２０による気密が破壊される場合があるので、無闇に金属ガスケット２０の断面直径Ｄ（図２参照）を小さくすることはできない。一方、金属ガスケット２０の断面直径Ｄが大きくなりすぎると、上記素線径ｄでは必要な曲げ剛性が確保できなくなるので、金属ガスケット２０の断面直径Ｄは、５．０ｍｍ以上１２．０ｍｍ以下が好ましい。また、スプリング３０の素線径ｄと金属ガスケットの断面直径Ｄとの比ｄ／Ｄとしては、０．０２以上０．０８以下が好ましい。これらの数値はスプリング３０にインコネルを使用した場合の値であるが、スプリング３０にインコネルよりも弾性率の大きいステンレスやピアノ線を使用した場合には、およそ１割〜３割程度小さい値でよい。
【００８４】
図７及び図８は、この発明に係る金属ガスケットの取り付け構造を示す説明図である。同図（ａ）に示すように、金属ガスケット２０''は、キャスク５００の一次蓋５０７あるいは二次蓋５０８に、ボルト１００等の取り付け手段によって取り付けられる。リサイクル燃料集合体は、放射線を遮蔽するために水中でキャスク５００に挿入され、あるいは取り出されるが、このときには一次蓋５０７及び二次蓋５０８も水中で取り付け等がされる。このため、蓋側に金属ガスケット２０を取り付けて作業性を向上させるようにしてある。ここで、金属ガスケット２０を一次蓋５０７等に取り付けるためには、図７（ｂ）に示すように、二次蓋５０８等の金属ガスケット取り付け位置に磁石１０２を配置する。金属ガスケット２０''の外被５０は高純度鉄なので、磁石１０２に吸着して二次蓋５０８等に取り付けられる。このようにすれば、ボルト１００を使用しないので金属ガスケット２０''の取り付け作業が容易になる。
【００８５】
また、図８（ａ）に示すように、胴本体５０１側に金属ガスケット２０''を配置してから二次蓋５０８等を胴本体５０１に取り付けてもよい。この場合には、二次蓋５０８等を胴本体５０１から取り外したときに金属ガスケット２０''が胴本体５０１側に残されるが、図８（ｂ）に示すように、金属ガスケット２０''と接する面に磁石１０４を備えた環状の取り外し冶具１０６に金属ガスケット２０を吸着させて、胴本体５０１から金属ガスケット２０''を取り除いてもよい。このように、本発明に係る金属ガスケット２０''は外被５０が高純度鉄なので、磁石を利用することにより金属ガスケット２０''を取り扱う際の自由度を向上させることができる。
【００８６】
以上、この発明の実施の形態１に係る金属ガスケット２０等について説明したが、本発明においては金属ガスケットの外被５０に高純度鉄を使用するので、キャスクによる数十年の長期貯蔵時の密封用に使用しても、ほとんどクリープ変形しない。これによって、金属ガスケット２０等の応力緩和現象を極めて小さく抑えることができるので、長期貯蔵時における密封の信頼性を向上させることができる。また、高純度鉄は腐食しにくいので、この点でも長期貯蔵における密封の信頼性を向上させることができる。
【００８７】
（変形例１）
図９は、実施の形態１の第１変形例に係る金属ガスケット及び密封構造を示す説明である。この金属ガスケット２２は、上記金属ガスケット２０等と略同様の構成であるが、板状材を巻いて端部を一部で重ねることにより、断面略円形の環状板巻状部材を押圧部材として使用する点が異なる。その他の構成は、上記実施の形態１と同様なのでその説明を省略する。
【００８８】
このキャスクのシール構造１１において、金属ガスケット２２は、フープ径Ｄｆが異なる二個の板巻状のスプリング３４ａ、３４ｂとを同心状に配置し、両者を外被５１によって被覆しながら結合する、いわゆるダブルリングタイプのものである。即ち、この金属ガスケット２２において、内輪７０と外輪８０とは外被５１の平面部５１ｃで一体化される。なお、外被５１は、上述したように高純度鉄を使用する。以下の例においても同様である。
【００８９】
前記板巻状のスプリング３４ａ、３４ｂは、板材から重ね部分Ｓが重なるように断面が略円形の板巻状に成形され、シール時の圧縮により、重ね部分Ｓが摺動して全体的に板巻状のスプリング３４ａ、３４ｂの径が小さくなる。即ち、板巻状のスプリング３４ａ、３４ｂの径が小さくなるだけであるから、板巻状のスプリング３４ａ、３４ｂの変形による胴本体５０１等との接触面が大きくなることが抑制され、単位面積当たりの接触圧力の低下を防止できる。このため、確実な密封が可能になる。
【００９０】
また、スプリング３４ｂの内側には、周方向に多数の水抜き穴１０１を穿孔するようにしても良い。この水抜き穴１０１は、内部に侵入した水を外部に出すためのものであり、その位置は、径方向内側で且つ厚さ方向（図中高さ方向）の略中央となる。例えば、下方に設けた場合は、外被５１と干渉するので当該外被５１にも穿孔加工を行う必要が生じるためである。また、シール面９０の近くに設けると、外被５１とシール面９０との間に水が浸入し易くなり、水を抜き難い状態になるからである。具体的に好ましい水抜き穴１０１の位置は、径方向内側の線Ｌを中心に±４５°以内である。一方、胴本体５０１の中心Ｃ側における板巻状の板巻状のスプリング３４ａは、金属ガスケットの内側に重ね部分Ｓが位置するため、シール時の圧縮により重ね部分Ｓが摺動して隠れないような位置に水抜き穴１０１を穿孔する必要がある。
【００９１】
また、上記板巻状のスプリング３４ａ、３４ｂの重ね部分Ｓの代わりに開口３４ａ'ｏを有する、図９（ｂ）、（ｃ）に示すような断面が略Ｃ形の板巻状のスプリング３４ａ'を使用してもよい。このような板巻状のスプリング３４ａ'は、シール時に圧縮されて縮む際に重ね部分Ｓが摺動することはない。また、シール時には全体的に板巻状のスプリング３４ａ'の径が小さくなるので、板巻状のスプリング３４ａ'の変形によって接触面が大きくなるのが抑制され、単位面積当たりの接触圧力の低下を防止できる。このため、確実な密封が可能になる。
【００９２】
上記板巻状のスプリング３４ａや３４ａ'等の材質は、実施の形態１に係るスプリング３０ａ、３０ｂと同様にインコネルが使用できる。また、外被５１の材料である高純度鉄よりも熱膨張率の大きい材料（例えばステンレスやＡｌ等）で前記板巻状のスプリング３４ａ等を構成し、これと外被５１とを合わせたクラッド材構造として金属ガスケット２２を構成してもよい。材料に関してはは、以下の例でも同様に適用できる。
【００９３】
上記板巻状のスプリング３４ａ、３４ｂの重ね部分Ｓ、又は板巻状のスプリング３４ａ'の開口３４ａ'ｏは、製作の容易さを考慮すると胴本体５０１の外側、すなわち金属ガスケット２２の外周側の配置することが好ましい。一方、金属ガスケット２２の密封機能をより十分に発揮させるためには、前記重ね部分Ｓ又は前記開口３４ａ'ｏは、胴本体５０１の中心Ｃ側、すなわち金属ガスケット２２の内周側に配置することが好ましい。
【００９４】
以上のシール構造１１及び金属ガスケット２２によれば、シール面９０に対する単位面積当たりの接触圧力の低下を防止できるとともに、二重リングにより密封性を格段に向上できる。特に、キャスクのような放射性物質を格納する容器の密封に好適である。
【００９５】
（変形例２）
図１０は、実施の形態１の第２変形例に係るキャスクにおけるシール構造を示す説明図である。このシール構造１２において、金属ガスケット２２ａは、スプリング機能を有する内被３５と、その内被３５に積層した外被５２とにより曲げ形成された構造である。この金属ガスケット２２ａを製造するには、まず板状の内被３５と外被５２とを積層して拡散接合等により接合し、図１０に示すような形状に曲げ成形を行う。具体的には、内被３５及び外被５２の両側を略円形板巻状に曲げ、一部で重なる重ね部分Ｓを作る。内輪７１は、内被３５ａと外被５２ａとから構成され、外輪８１は、内被３５ｂと外被５２ｂとから構成される。内輪７１と外輪８１とは平面部３５ｃ、５２ｃで接続される。
【００９６】
この金属ガスケット２２ａを、二次蓋５０８および胴本体５０１の間に介在させ、内部の密封を行う。密封した状態で内輪７１と外輪８１とは、その外被５２でシール面９０ａ、９０ｂに所定圧力で接触する。この金属ガスケット２２ａは、密封した状態で上下方向に圧縮されるが、重ね部分Ｓが摺動して全体的に内輪７１、外輪８１の径が小さくなる。即ち、スプリングとして機能する内輪７１、外輪８１、８１の径が小さくなるだけであるから、外被５２とシール面９０との接触面が大きくならない。この結果、当該金属ガスケット２２ａによる接触圧力の低下を防止し、密封を確実に行える。また、二重リング構造になっているので、キャスクなどの高い機密性を要求される製品に好適なものとなる。
【００９７】
（変形例３）
図１１は、実施の形態１の第３変形例に係るキャスクにおけるシール構造を示す説明図である。図１２は、図１１に示したスプリングの製造方法を示す説明図である。図１３は、図１１の金属ガスケットによるシール状態を示す説明図である。このシール構造１３において、金属ガスケット２２ｂは、フープ径Ｄｆの異なる二個のスプリング３６ａ、３６ｂとを同心状に配置し、これらに内被４４ａ、４４ｂを被覆するとともに、両者を外被５３によって被覆しながら結合する、いわゆるダブルリングタイプのものである。即ち、この金属ガスケット２２ｂは、外被５３により内輪７２と外輪８２とをまとめて被覆した構成で、内輪７３と外輪８３とは外被５３の平面部５３ｃで一体化されている。内輪７３は、スプリング３６ａ、内被４４ａ及び外被５３ａから構成され、外輪８３は、スプリング３６ｂ、内被４４ｂ及び外被５３ｂから構成される。
【００９８】
このシール構造１３は、例えばキャスク５００の二次蓋５０８と胴本体５０１のフランジ部との間に設けられる。なお、キャスク５００の一次蓋と胴本体５０１との間にも、もちろんこの発明に係るシール構造及び金属ガスケットは適用できる。前記スプリング３６ａ、３６ｂは、板材から重ね部分Ｓが重なるように略円形板巻状に成形され、シール時の圧縮により重なり部分Ｓが摺動して全体的にスプリング３２の径が小さくなる。即ち、スプリング３６ａ、３６ｂの径が小さくなるだけであるから、スプリング３６ａ、３６ｂによる接触面が大きくなるのを抑制し、単位面積当たりの接触圧力の低下を防止できる。このため、確実な密封が可能になる。
【００９９】
前記スプリング３６ａ、３６ｂ（両者とも構造が同じなので、以下スプリング３６）は、図１２（ａ）に示すように、矩形短冊状の板の長手方向一端縁をベース３０１として、他端縁に多数のスリット３０３を入れ、多数の櫛歯３０２を形成する。この板を図中一点差線で示す方向を軸として略円形板巻状に折り曲げ、同図（ｂ）に示すような断面板巻状で全体がリング状のスプリング３２を形成する。こうして曲げ成形したスプリング３６には、ベース３０１部分が重ね部分Ｓとなり、櫛歯３０２上を摺動可能となる。
【０１００】
このスプリング３６はその表面が櫛歯のようにスリットが入ったものであるから、高純度鉄で構成した外被５３のみを介して密封すると、スリット３０３に外被５３が入り込み、シール面９０の接触圧力に変動が起きる。このため、図１３に示したように、薄い硬質金属である内被４４ａ、４４ｂを外被５３とスプリング３６との間に介在させ、スプリング３６の櫛歯３０２により一旦内被４４ａ等を押し、その内被４４ａ等により外被５３を押すようにすれば、外被５３に対して均一、且つ全面的に圧力を加えることができる。これにより、外被５３は、均一の接触圧力を持ってシール面９０に接触することになるから、確実な密封が可能になる。
【０１０１】
（実施の形態２）
図１４は、この発明の実施の形態２に係る金属ガスケットを示す断面図である。この金属ガスケット２１は、実施の形態１に係る金属ガスケット２０と略同様の構成であるが、密封対象物である一次蓋５０７や二次蓋５０８と外被５０とが接触する外被５０のシール領域５０ｓを予め平面状に形成した点が異なる。その他の構成は実施の形態１と略同様なのでその説明を省略する。
【０１０２】
図１５は、従来の金属ガスケットを示す断面図である。従来の金属ガスケット５２０においては、断面の外被覆形状は円形であり、円弧の部分がキャスクの胴本体と二次蓋５０８と接した後、締め付け力を受けて図１５（ｂ）のように変形する。このため、図１５（ｃ）に示すように、二次蓋５０８及びキャスクの胴本体５０１と接するシール領域５５０ｓには応力が不均一に作用するので、クリープ変形が促進される。
【０１０３】
この発明に係る金属ガスケット２１においては、図１４（ａ）に示すように、外被５１が二次蓋５０８等と接するシール領域５０ｓを予め平面状に形成してある。これによって、図１４（ｂ）に示すように、この金属ガスケット２１が締め付け力を受けて圧縮された場合には、密封対象物である二次蓋５０８等と接するシール領域５０ｓにおける応力分布が分散されて、従来よりも均一になる。これは、密封対象物である二次蓋５０８等と接するシール領域５０ｓにおける応力分布は、このシール領域以外の領域５０ｙに締め付け力が作用した場合におけるシール領域以外の領域５０ｙの応力分布よりも均一となることを意味する。
【０１０４】
ここで、上述したように、外被５０に使用する高純度鉄の硬度が比較的低いといってもアルミニウムや銅等に比較すれば硬度は高い。また、本実施の形態においては、外被５０のシール領域５０ｓを予め平面状に形成しているので、従来のように断面円形の金属ガスケットと比較してシール面の圧力が低くなる。したがって、シール面に外被５０を密着させるためには、スプリング３０にある程度の反発力を発生させて外被５０を二次蓋５０８等に押し付ける必要がある。これらの理由から、外被５０のシール領域５０ｓを平面状に形成した場合には、密封に十分な圧力をシール領域５０ｓに発生させるだけの曲げ剛性をもつスプリング３０を使用することが好ましい。
【０１０５】
また、密封に十分な圧力をシール領域５０ｓに発生させるためには、シール領域５０ｓがある程度変形する必要があるが、純鉄Ａｌ等と比較して硬度が高いので、シール領域５０ｓの面積によっては二次蓋５０８等を固定する固定用ボルト（図示せず）の締め付け力が大きくなる。このため、シール領域５０ｓの面積は、固定用ボルトの締め付け力が過大にならない範囲に抑えることが好ましい。また、太い固定用ボルトを使用して、より大きな締付け力を発生させることにより、密封に十分な圧力が発生できる程度までシール領域５０ｓを変形させてもよい。
【０１０６】
上記構成によって、従来の金属ガスケット５２０と比較して、シール領域５０ｓにおいては応力分布を均等にする作用が大きくなるので、外被５０のクリープ変形を従来よりも緩やかにすることができる。これによって、外被５０のクリープ変形を従来よりも小さくできるので、クリープ変形に起因する応力緩和現象を抑えることができる。また、外被５０のシール領域５０ｓを予め平面に形成しておくことによって、この金属ガスケット２１の断面形状は円形ではなくなる。
【０１０７】
このような形状とすることによって、断面が円形の場合よりも断面係数が大きくなる。さらに、外被５０のシール領域５０ｓを予め平面に形成しておくことで、クリープ変形しやすい外被５０のシール領域５０ｓの厚みは従来の金属ガスケット５２０（図７参照）よりも薄くなる。これによって、クリープによる変形の絶対量が小さくなり、この分だけ金属ガスケット２１の復元量の減少を抑制できる。また、外被５０には高純度鉄を使用するので、キャスク５００における使用環境下においてほとんどクリープ変形を起こさない。さらに高純度鉄は弾性率がアルミニウムや銅よりも大きいので、それだけ大きな締め付け力に耐えることができる。
【０１０８】
これらの作用によって、この金属ガスケット２１は、シール領域５０ｓに垂直な方向に向かう締め付け力に対して、断面が円形である従来の金属ガスケット５２０と比較して、より大きな締め付け力に耐えることができる。したがって、この金属ガスケット２１においては、より細い素線径ｄのスプリング３０を使用しても、外被５０が従来よりも大きな荷重を支えることができる。これによって、金属ガスケット２１のスプリングバック量を大きくできるので、長期貯蔵においてはさらに高い安全性を発揮できる。
【０１０９】
（実施の形態３）
図１６は、この発明の実施の形態３に係るキャスクにおける密封構造を示す説明図である。このキャスクが備える密封構造は、上記実施の形態１と略同一の構成であるが、同心状に配置したフープ径の異なる二つのスプリングを外被によって被覆しつつ、外被の端部同士を接合して両者を一体としたダブルリング式の金属ガスケットを使用する点が異なる。その他の構成は実施の形態１と同様なのでその説明を省略するとともに、同一の構成要素には同一の符号を付する。
【０１１０】
実施の形態３に係る金属ガスケット２３は、いわゆるダブルリング式のものであり、フープ径Ｄｆの異なる二個のスプリング３３ａ及び３３ｂを同心状に配置する。そして、これらを内被４３で被覆して、外被５０によって両者を被覆してから、外被５０の端部５０ｔ同士を接合部５０ｂで接合して構成する。端部５０ｔ同士の接合には、溶接や摩擦接合が用いられる。溶接にはレーザーや電子ビーム溶接を用いると、接合部に当て板が不要になるので好ましい。
【０１１１】
端部５０ｔ同士は、図１６（ａ）及び（ｃ）に示すように、フープ径Ｄｆの異なる二個のスプリング３３ａ及び３３ｂとの間における接合部５０ｂで接合してもよい。ここで、図１６（ｃ）は突合せ接合の例である。また、図１６（ｂ）に示すように、いずれか一方のスプリングの側部３３ｙにおける接合部５０ｂで接合してもよい。さらに、図１６（ｄ）に示すように、スペーサ６０を外被５０内部に配置して、スペーサ６０とともに接合部５０ｂで外被５０を接合してもよい。また、スペーサ６０を単に接合のバックアップとしてのみ使用して、外被５０のみを接合してもよい。このようにスペーサ６０を接合の際にバックアップとして使用すると、外被５０を容易に接合できる。なお、スペーサ６０には、例えば純アルミニウム等を使用することができる。
【０１１２】
また、図１６（ａ）及び（ｂ）に示す二個のスプリング３３ａと３３ｂとの間に、スペーサ６０と同様に外被５０の変形を拘束するものとして、スプリング３３ａ等よりもやや外径の小さいスプリングを用いてもよい。このようなスプリングを使用すれば、純アルミニウム等を使用する場合と比べて金属ガスケット２３を容易に製造できる。また、スプリングの反発力を適当な値に調整することによって外被５０に過度の応力を与えることもないので好ましい。
【０１１３】
図１７は、外被の端部同士を接合していない一部開放型のダブルリング式の金属ガスケット及びこの発明の実施の形態３に係る金属ガスケットを示す説明図である。同図（ａ）に示すように、一部開放型のダブルリング式の金属ガスケット２０では、二次蓋５０８側のシール領域５０ａｓと５０ｂｓとが、外被５０によって結合されている。このため、外被５０が金属ガスケット２０の径方向Ｘに対してクリープ変形しようとしてもこの変形が拘束される。これによって、シール領域５０ａｓ及び５０ｂｓ側においては外被のクリープ変形が抑制されるので、金属ガスケット２０の応力緩和もその分抑制される。
【０１１４】
一方、キャスクの胴本体５０１側においては、この部分におけるシール領域５０ａｓ₂と５０ｂｓ₂側とは、外被５０によって結合されていない。したがって、外被５０が金属ガスケット２０の径方向Ｘに対してクリープ変形した場合には、この変形を拘束するものはない。このため、二次蓋５０８側と比較して、キャスクの胴本体５０１側では外被５０のクリープ変形が大きくなるので、金属ガスケット２０の応力緩和もその分大きくなる。
【０１１５】
しかし、図１７（ｂ）に示すように、この発明の実施の形態３に係る金属ガスケット２３は、内被４３で被覆したフープ径Ｄｆの異なる二個のスプリング３３ａ及び３３ｂを同心状に配置し、これらを外被５０によって被覆してから、外被５０の端部５０ｔ同士を接合してある。このため、二次蓋５０８側及びキャスクの胴本体５０１側いずれのシール部分においても、スプリング３３ａ及び３３ｂの位置におけるシール領域は、外被５０によって結合される。したがって、二次蓋５０８側及びキャスクの胴本体５０１側いずれのシール領域５０ａｓや５０ｂｓ₂等においても、外被５０のクリープ変形が拘束される。その結果、クリープ変形に伴う応力緩和現象が抑制されるので、長期貯蔵においても密封性能を維持して、安全に輸送できる。
【０１１６】
また、外被５０の端部５０ｔ同士を溶接、摩擦接合その他の接合手段によって接合しているため、金属ガスケット２３内部の気密性も確保できる。一般にキャスクは安全上の配慮から、水を満たした貯蔵プール内でリサイクル燃料集合体が格納される。したがって、キャスクをプールから引き上げた後で輸送したり長期貯蔵したりする前には、水抜き後に真空乾燥等の乾燥手段によって乾燥させる。このとき、一部開放型のダブルリング式の金属ガスケット５２０（図７（ａ）参照）では、乾燥が十分でないと腐食を発生させるおそれがあったため、内部に侵入した水分を完全に除去するには真空乾燥に長時間要していた。しかし、この金属ガスケット２３では内部に水が浸入することはないので、真空乾燥時間を大幅に短縮できる上に、内部に残留した水分による腐食のおそれはない。また、この金属ガスケット２３の外被５０は高純度鉄なので、万一水分が外被５０に残留してもほとんど腐食しない。これらの作用によって、長期間貯蔵においてはより高い信頼性を発揮できる。
【０１１７】
また、図１７（ｃ）に示す金属ガスケット２３'のように、同心状に配置した内被４３で被覆したフープ径の異なる二個のスプリング３３ａ及び３３ｂの間に、予めスペーサ６０を備えてから、外被５０の端部５０ｔ同士を接合してもよい。このようにすれば、スペーサ６０によって外被５０が支えられるので、外被５０の変形を抑えることもでき、端部５０ｔ同士を容易に接合できる。
【０１１８】
ここで、図１７（ｃ）に示すように、締め付け荷重を受けた場合における外被５０の変形を抑えるために、スペーサ６０の高さｈを内被４３の径と同程度の高さにしてもよい。このようにすれば、締め付け後における外被５０の変形を抑えることができるので、外被５０のクリープ変形をより抑制して、長期保管における信頼性をさらに高くできる。ここで、スペーサ６０は、金属ガスケット２３の締め付け力に対しては容易に変形し、不必要に高い締め付け力を要さない形状を選択することができる。また、スペーサ６０の代わりに、スプリング３３ａ等の外径よりもやや外径の小さいスプリングを使用し得ることについては上述した通りである。
【０１１９】
（実施の形態４）
図１８は、この発明の実施の形態４に係る金属ガスケットを示す説明図である。この金属ガスケット２４は、上記実施の形態１に係るキャスクと略同一の構成であるが、二個の異なるフープ径を持つ環状のスプリング３１ａ、３１ｂを同心状に配置し、両者のスプリング直径を異ならせた点が異なる。その他の構成は実施の形態１と同様なのでその説明を省略するとともに、同一の構成要素には同一の符号を付する。
【０１２０】
この金属ガスケット２１は、いわゆるダブルリングタイプであるが、内被４１ａ、４１ｂで被覆されたスプリング直径の異なる二つのスプリング３１ａ及び３１ｂを使用する。図１８（ｂ）に示すように、キャスクの胴本体５０１と二次蓋５０８との間にこの金属ガスケット２４を配置すると、直径が大きい方のスプリング３１ｂ側における外被５０がまず接する。この状態で二次蓋５０８を胴本体５０１に仮止めする。
【０１２１】
ここで、一般にキャスクは安全上の配慮から、水を満たした貯蔵プール内でリサイクル燃料集合体が格納される。したがって、輸送や長期貯蔵前には、真空乾燥等の乾燥手段によって水抜きをする。この金属ガスケット２４では、真空乾燥時に、シールされていない直径が小さいほうのスプリング３１ａ側からガスケット溝部９に残留した水分が蒸発する。真空乾燥が終了したら、キャスクの胴本体５０１と二次蓋５０８とを本締めする。このように、この金属ガスケット２１においては、ガスケット溝部９や金属ガスケット２４内の水分を十分に取り除き、腐食のおそれを極小にできる。また、外被５０は高純度鉄なので、腐食しにくい。これらの作用によって、長期間の貯蔵においても密封性能の信頼性を高くできる。
【０１２２】
ここで、図１８（ｂ）においては、スプリング直径の小さい方のスプリング３１ａをキャスクのキャビティ側に配置しているが、この場合にはキャビティ内側からの真空乾燥に有利である。ここで、スプリング３１ａの径は、蓋固定用のボルトを所定のトルクで締め付けたときには、気密保持に必要な接触圧力が得られる径に選定する。また、図１８（ｃ）に示すように、スプリング直径の小さい方のスプリング３１ａを外側に配置してもよい。この場合には、二次蓋５０８等を固定するボルト孔側から真空乾燥すると効果的に水分を除去できる。
【０１２３】
さらに、図１８（ｄ）に示す金属ガスケット２４ａは、スプリング直径の異なる二個のスプリング３１ａ及び３１ｂ全体を外被５０によって被覆し、スペーサ６０を前記スプリング３１ａと３１ｂとの間に配置して、端部５０ｔ同士を溶接等の接合手段で封止したものである。このような金属ガスケット２４ａでも、金属ガスケット２１の内外周でスプリング３０ａ等の径を異ならせたことによる、上記乾燥促進の効果を得ることができる。
【０１２４】
さらに、この金属ガスケット２４ａでは、内部への水の浸入を防止できるので、図１８（ａ）に示した金属ガスケット２１と比較して、より迅速に乾燥できる効果も得ることができる。また、この金属ガスケット２４ａの外被５０は高純度鉄なので、万一水分が外被５０に残留してもほとんど腐食せず、これによって、長期貯蔵における信頼性を向上させることができる。なお、スペーサ６０は外被５０の変形を拘束するものであり、純アルミニウム等のピースを使用したり、スプリングを使用したりすることができる。
【０１２５】
【発明の効果】
以上説明したように、この発明に係るキャスク（請求項１）では、外被に高純度鉄を用いた金属ガスケットを使用して密封するようにした。これによって、数十年の長期保存においても、金属ガスケットの劣化を抑えて高い密封性能を維持できる。また、金や銀とは異なり素材の価格も安いので、キャスク全体の製造コストを抑えることができる。
【０１２６】
また、この発明に係るキャスク（請求項２）では、外被を押圧する押圧部材にスプリングを使用するようにした。このようにすれば、設計を工夫することによって、外被を均一にキャスクの胴本体や蓋に押し付けて十分な密封性能を発揮させることができる。
【０１２７】
また、この発明に係るキャスク（請求項３）では、外被を押圧する押圧部材として板巻状のスプリングを使用するようにした。このため、全体的にスプリングの径が小さくなるので、スプリングの変形による蓋や胴本体等との接触面が大きくなることが抑制される。これにより、単位面積当たりの接触圧力の低下を防止して、確実に密封できる。
【０１２８】
また、この発明に係るキャスク（請求項４）では、高純度鉄からなる金属ガスケットの外被の硬度を、キャスクの胴本体及び蓋よりも低くした。このため、金属ガスケットの外被はキャスク胴本体や蓋よりも軟らかくなるので、これらを損傷することもなく、キャスク胴本体や蓋の再利用が容易となる。
【０１２９】
また、この発明に係るキャスク（請求項５）では、金属ガスケットの外被の硬度は８０Ｈｖ以下とした。これにより、キャスク胴本体や蓋のシール面とよくなじむので、キャスク胴本体や蓋を保護しつつ十分な密封性能を発揮できる。
【０１３０】
また、この発明に係るキャスク（請求項６）では、金属ガスケットの外被に使用する高純度鉄に、所定温度に保持した後放冷する熱処理を施した。このため、このような熱処理によって硬度を下げることができるので、例えば圧延によって外被の形状に延ばして加工硬化した材料であっても軟化によって優れた密封性能を発揮させることができる。
【０１３１】
また、この発明に係るキャスク（請求項７）では、所定温度に保持した後放冷する熱処理により、外被の表面に不動態皮膜を形成した。このように、外被の表面に不動態皮膜が形成されているので外被の耐腐食性が向上し、数十年の長期貯蔵においても金属ガスケットの劣化を最小限にして十分な密封性能を維持できる。
【０１３２】
また、この発明に係るキャスク（請求項８）では、所定温度に保持した後放冷する熱処理の保持温度を３００℃〜４５０℃とした。これによって、外被の硬度を十分に低下させるとともに耐腐食性の高い不動態皮膜を形成し得るので、効率的に作業を進めることができる。
【０１３３】
また、この発明に係るキャスク（請求項９）では、スプリングと外被との間に、スプリングを被覆する内被をさらに備えた金属ガスケットを使用するようにした。このため、この内被によってスプリングの押圧力が外被に均等に伝わるので、外被の局所的な応力集中を分散させることができる。その結果、外被のクリープ変形に起因する応力緩和現象を抑えて、長期貯蔵における信頼性を高めることができる。
【０１３４】
また、この発明に係る金属ガスケット（請求項１０）では、押圧部材を被覆する外被を高純度鉄で構成した。このため、例えば１２０℃程度の温度で使用しても外被のクリープ変形はほとんど無視でき、数十年の長期保存においても、金属ガスケットの劣化を抑えて高い密封性能を維持できる。また、高純度鉄は金や銀とは異なり価格も比較的安いので、金属ガスケットを比較的安価に提供できる。
【０１３５】
また、この発明に係る金属ガスケット（請求項１１）では、外被同士で個々のスプリングを一体化したので、長期間貯蔵において外被がクリープ変形しても、二個のスプリングが互いにこのクリープ変形を規制する。これによって、外被のクリープ変形に起因する金属ガスケットの応力緩和現象をさらに抑えて、より高い密封に対する信頼性を担保できる。
【０１３６】
また、この発明に係る金属ガスケット（請求項１２）では、フープ径の異なる環状のスプリングを同心状に配置し、これらを外被で被覆した後、外被の端部同士を接合した。このため、例えばキャスクの密封にこの金属ガスケットを使用した場合には、蓋側と胴本体側との両方のシール領域において、二個のスプリングが互いに外被のクリープ変形を規制できる。これによって、外被のクリープ変形に起因する金属ガスケットの応力緩和現象をさらに抑えて、長期貯蔵においてさらに高い安全性を担保できる。また、外被の端部同士を接合することによって、完全な密封構造とすることができるので、金属ガスケット内部に水分が浸入しない。その結果、耐腐食性に優れ、長期貯蔵における信頼性を高くできる。
【０１３７】
また、この発明に係る金属ガスケット（請求項１３）では、異なるフープ径を持つ二個の環状のスプリングを同心状に配置し、両者のスプリング直径を異ならせた。この金属ガスケットを例えばキャスクの密封に使用した場合には、径の大きい方のガスケットリングがキャスクの胴本体及び蓋と接して、金属ガスケットに付着した水分を容易に蒸発させ得る。その結果、金属ガスケット内の水分を十分に取り除くことができるので、金属ガスケットが腐食するおそれを極小にでき、長期間の貯蔵においても密封性能の信頼性を高くできる。
【０１３８】
また、この発明に係る金属ガスケット（請求項１４）では、外被を押圧する押圧部材にスプリングを使用するようにした。このようにすれば、設計を工夫することによって、外被を均一に密封対象物に押し付けて十分な密封性能を発揮させることができる。
【０１３９】
また、この発明に係る金属ガスケット（請求項１５）では、外被を押圧する押圧部材として板巻状のスプリングを使用するようにした。このため、全体的にスプリングの径が小さくなるので、スプリングの変形による密封対象物との接触面が大きくなることが抑制される。これにより、単位面積当たりの接触圧力の低下を防止して、確実に密封できる。
【０１４０】
また、この発明に係る金属ガスケット（請求項１６）では、外被の硬度を密封対象物よりも低くした。このため、金属ガスケットの外被は密封対象物よりも軟らかいので、密封対象物を損傷することもなく、密封対象物の再利用が容易となる。
【０１４１】
また、この発明に係る金属ガスケット（請求項１７）では、外被の硬度を８０Ｈｖ以下としたので、密封対象物のシール面とよくなじむので、密封対象物を保護しつつ十分な密封性能を発揮できる。
【０１４２】
また、この発明に係る金属ガスケット（請求項１８）では、外被表面を腐食に対する耐性を向上させる皮膜で覆うようにした。このため、この金属ガスケットの外被は腐食しにくくなるので、長期にわたり腐食による密封性能の低下を抑えて信頼性を向上させることができる。特に、リサイクル燃料集合体を数十年の長期にわたって貯蔵するキャスクの密封用途に使用した場合には、キャスクの信頼性を向上させることができる。
【０１４３】
また、この発明に係る金属ガスケット（請求項１９）では、外被を構成する高純度鉄を酸液処理することにより、腐食に対する耐性を向上させる皮膜を形成するようにした。このように、酸液処理によれば、比較的簡単な設備でこのような耐腐食性皮膜を形成することができるので、金属ガスケットの製造コストを抑えることができる。
【０１４４】
また、この発明に係る金属ガスケット（請求項２０）では、外被を構成する高純度鉄に対し、これを所定温度に所定時間保持した後、放冷する熱処理が施した。このため、このような熱処理によって外被の硬度を下げることができるので、例えば圧延によって外被の形状に延ばして加工硬化した材料であっても軟化によって優れた密封性能を発揮させることができる。
【０１４５】
また、この発明に係る金属ガスケット（請求項２１）では、外被の表面に不動態皮膜を形成したので外被の耐腐食性が向上し、数十年の使用においても金属ガスケットの劣化を最小限にして十分な密封性能を維持できる。
【０１４６】
また、この発明に係る金属ガスケット（請求項２２）では、高純度鉄の熱処理温度を３００℃〜４５０℃としたので、外被の硬度を十分に低下させることができ、また、耐腐食性の高い不動態皮膜を形成することができる。
【０１４７】
また、この発明に係る金属ガスケット（請求項２３）では、密封対象物である蓋及び胴本体と金属ガスケット外被のシール領域とが平面で接するようにしたので、前記外被のシール領域における応力分布をより均一して外被のクリープ変形の進行を抑制できる。その結果、長期貯蔵においても金属ガスケットの応力緩和現象を抑えることができる。また、応力緩和現象を抑えることによって、金属ガスケットのスプリングバックも多くとることができるので、キャスクに対してこの金属ガスケットを使用した場合には、蓋と胴本体とのずれに対する裕度を大きくできる。その結果、数十年の長期貯蔵においてもより高い安全性を担保できる。
【０１４８】
また、この発明に係る金属ガスケット（請求項２４）では、押圧部材と外被との間に、押圧部材を被覆する内被をさらに備えた。このため、この内被によって押圧部材の押圧力が外被に均等に伝達されるので、外被の局所的な応力集中を分散させることができる。その結果、外被のクリープ変形に起因する応力緩和現象を抑えて、長期貯蔵における信頼性を高めることができる。
【図面の簡単な説明】
【図１】この発明の実施の形態１に係るキャスクの密封構造を示す説明図である。
【図２】この発明の実施の形態１に係る金属ガスケットを示す断面図である。
【図３】金属ガスケットに作用する応力と、金属ガスケットの歪との関係を示した説明図である。
【図４】高純度鉄の純度と硬度との関係を示す説明図である。
【図５】この発明の実施の形態１に係る金属ガスケットのスプリングを示す説明図である。
【図６】金属ガスケットに作用させる荷重と金属ガスケットの変位との関係を示した説明図である。
【図７】この発明に係る金属ガスケットの取り付け構造を示す説明図である。
【図８】この発明に係る金属ガスケットの取り付け構造を示す説明図である。
【図９】実施の形態１の第１変形例に係る金属ガスケット及び密封構造を示す説明である。
【図１０】実施の形態１の第２変形例に係るキャスクにおけるシール構造を示す説明図である。
【図１１】実施の形態１の第３変形例に係るキャスクにおけるシール構造を示す説明図である。
【図１２】図１１に示したスプリングの製造方法を示す説明図である。
【図１３】図１１の金属ガスケットによるシール状態を示す説明図である。
【図１４】この発明の実施の形態２に係る金属ガスケットを示す断面図である。
【図１５】従来の金属ガスケットを示す断面図である。
【図１６】この発明の実施の形態３に係るキャスクにおける密封構造を示す説明図である。
【図１７】外被の端部同士を接合していない一部開放型のダブルリング式の金属ガスケット及びこの発明の実施の形態３に係る金属ガスケットを示す説明図である。
【図１８】この発明の実施の形態４に係る金属ガスケットを示す説明図である。
【図１９】キャスクの構造を示す断面図である。
【図２０】図１９に示したキャスクの一部拡大図である。
【図２１】キャスクのシールに使用する金属ガスケット及びキャスクのシール部を示す拡大図である。
【符号の説明】
１０、１１、１２、１３ シール構造
２０、２０'、２０''、２０'''、２１、２２、２２ａ、２２ｂ、２３、２４、２４ａ 金属ガスケット
３０、３０ａ、３０ｂ、３１ａ、３１ｂ、３２、３３ａ、３６、３６ａ、３６ｂ、５２１ スプリング
３４ａ、３４ａ'、３４ｂ、 板巻状のスプリング
３５、３５ａ、３５ｂ、４０、４０ａ、４３、４４ａ、４４ｂ、５２２ 内被
５０、５１、５２、５２ａ、５２ｂ、５３、５３ａ、５３ｂ 外被
７０、７１、７２、７３ 内輪
８０、８１、８２、８３ 外輪
５００ キャスク
５０１ 胴本体
５０２ 外筒
５０３、５０４、５１２ 中性子遮蔽材
５０５ 底板
５０６ フランジ部
５０７ 一次蓋
５０８ 二次蓋
５０９ キャビティ[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a highly reliable cask and metal gasket that is a pressure vessel that accommodates a recycle fuel assembly that has been combusted and that can maintain sufficient sealing performance even during long-term storage.
[0002]
[Prior art]
A nuclear fuel assembly at the end of the nuclear fuel cycle that cannot be used after combustion is called a recycled fuel assembly. Since the recycled fuel assembly contains a highly radioactive substance such as FP, it needs to be cooled thermally, and is thus cooled for about 10 years in the cooling pit of a nuclear power plant. After that, it is stored in a cask, which is a recycled fuel assembly storage container having a radiation shielding function, and is transported to a reprocessing facility by a transportation means such as a truck and then stored. Since a highly radioactive substance is contained in the cask, strict caution is required for sealing the cask during a storage period of 40 to 60 years. As a conventional example of such a cask, various types are disclosed in Patent Document 1, Non-Patent Document 1, and the like. Refer to these if necessary.
[0003]
FIG. 21 is an enlarged view showing a metal gasket used for cask sealing and a seal portion of the cask. The seal portions between the primary lid 507 and the trunk body 501 and between the secondary lid 508 and the trunk body 501 have the same sealing structure. The gasket groove 525 is formed by machining, and a metal gasket 518 (519) to be used has a double ring structure. This metal gasket 518 (519) is coated with a spring 521 made of Inconel (trade name: nickel-based alloy containing 16% chromium and 7% iron) resistant to corrosion and high-temperature oxidation with an inner cover 522 made of Inconel. Further, the inner and outer rings are collectively covered with an aluminum outer sheath 523. FIG. 16A shows a state before the metal gasket 518 (519) is used. When the secondary lid 508 and the body main body 501 and the like are fixed, the metal gasket 518 (519) is deformed as shown in FIG. 21B by its tightening force and exhibits a sealing function.
[0004]
It fixes to the gasket groove | channel 525 using the bolt hole provided in the jacket 523 part. As the metal gasket 518 (519), for example, “Triback” manufactured by Nippon Valqua Industries Co., Ltd., “Helicoflex Seal” manufactured by CEFILAC of France, etc., which has a long history of use with respect to a nuclear cask, can be used.
[0005]
The temperature and time dependence of the plastic deformation rate and sealing performance of a metal gasket can be expressed by the Larson-Miller Parameter (LMP). The details are described in Non-Patent Document 2, so if necessary Please refer to this as appropriate. In general, the long-term sealing performance of a metal gasket is evaluated by obtaining the LMP of the sealing retention limit and predicting the limit time at a predetermined temperature. Further, the structure for ensuring the sealing performance in such a metal gasket is a single ring type metal gasket, but a metal that suppresses leakage by forming a solid lubricant coating between the outer cover 523 and the inner cover 522. A gasket is disclosed in Patent Document 1.
[Patent Document 1]
Real fairness 5-75154 p1, FIG. 1, FIG.
[Non-Patent Document 1]
Nuclear energy eye issued on April 1, 1998, Nikkan Kogyo Publishing Production p32-p39
[Non-Patent Document 2]
Development of evaluation method for long-term sealing performance of recycled fuel storage cask Kato, Ito, Saegusa Journal of the Atomic Energy Society of Japan, Vol.38, No.6, 95-101, 1996
[0006]
[Problems to be solved by the invention]
By the way, the cask 500 in which the recycled fuel assembly is stored is stored in a storage facility for a long period of several decades. At this time, the metal gasket 518 (519) starts to be used in an environment of about 120 ° C. due to the decay heat of the recycled fuel. Then, during the storage period of several decades, the use environment temperature of the metal gasket 518 (519) gradually decreases, and the metal gasket 518 (519) is used in an environment of about 60 ° C. at the end of the storage period.
[0007]
In order to improve the sealing performance, the outer cover 523 of the metal gasket 518 (519) is generally aluminum which is a soft metal, but the above temperature range is between high temperature creep and low temperature creep for aluminum. For this reason, even in the case of the metal gasket 518 (519) having sufficient sealing performance at the beginning, a stress relaxation phenomenon due to creep deformation occurs due to long-term use in a high temperature environment, resulting in a decrease in sealing performance. There was a fear. In addition, the conventional metal gasket 518 (519) has the performance of keeping the seal under an operating pressure of several MPa, but the outer casing 523 is pressed with a strong force due to the higher operating pressure. For this reason, a metal gasket that is easy to creep the outer cover 523 and can maintain a predetermined sealing performance over a long period of several decades uses a high-grade material with little creep deformation (eg, gold, silver, etc.) for the outer cover 523. It was the current situation. Further, even in the metal gasket having the structure disclosed in Patent Document 1, this creep deformation cannot be reduced, and in the long-term storage, the solid lubricant coating may be deteriorated to cause a stress relaxation phenomenon of the metal gasket. there were.
[0008]
Therefore, the present invention has been made in view of the above, and even when used in a relatively high temperature environment for a long period of several decades, it is sufficient to minimize the deterioration of the metal gasket. It is an object of the present invention to provide a cask and a metal gasket that can achieve at least one of ensuring sealing performance and reducing the price without using a high-grade material for the outer cover of the metal gasket.
[0009]
[Means for Solving the Problems]
In order to achieve the above-described object, a cask according to claim 1 includes a trunk body that shields radiation, in which a cavity that houses a recycled fuel assembly is formed, and a lid that is attached to the cavity opening side of the trunk body. , Purity is 99.9% or more A jacket made of high-purity iron, and a pressing member that is covered with the outer casing and presses the outer casing in the direction of the sealing surface, and is interposed between the trunk body and the lid, And a metal gasket for maintaining a hermetic seal.
[0010]
The cask is sealed using a metal gasket using high-purity iron for the jacket. Since the melting point of high-purity iron is as high as about 1500 ° C. or higher, the creep deformation of the jacket can be ignored at a temperature of about 120 ° C., which is the environment in which the metal gasket is used in the cask. Thereby, even in long-term storage for several decades, it is possible to suppress deterioration of the metal gasket and maintain high sealing performance. In addition, high-purity iron has high corrosion resistance, so that corrosion can be suppressed without special treatment. Furthermore, unlike gold and silver, it is relatively inexpensive, so that the manufacturing cost of the entire cask can be reduced. Here, it is desirable that the high-purity iron referred to in the present invention has a purity of 99.9% or more, and a purity of 99.95% or more in order to exhibit better sealing performance and corrosion resistance. Is preferable (the same applies hereinafter). Moreover, a sealing surface means the surface where the outer cover of the said metal gasket, the said trunk | drum main body, and the said lid will contact if it is the said cask.
[0011]
As in the cask according to claim 2, it is preferable to use a spring for the pressing member. Thus, if a spring is used for the pressing member, the jacket can be uniformly pressed against the trunk main body and the lid of the cask and the sufficient sealing performance can be exhibited by devising the design. The spring includes a plate-like one formed by winding a coil spring or a plate material. Moreover, the cross-sectional shape of the spring is not particularly limited.
[0012]
Further, like the cask according to claim 3, a plate-like spring may be used as the pressing member. A plate-shaped spring generally has a reduced spring diameter due to compression during sealing. That is, since the diameter of the spring is only small, the contact surface with the lid, the trunk body, and the like due to the deformation of the spring is suppressed. Thereby, the fall of the contact pressure per unit area is prevented and reliable sealing becomes possible.
[0013]
Further, as in the cask according to claim 4, it is preferable that the hardness of the jacket is lower than that of the trunk body and the lid. If the hardness is within this range, the outer casing of the metal gasket is softer than the cask trunk body and the lid, so that the cask trunk body and the lid can be easily reused without damaging them. Furthermore, as in the cask according to claim 5, it is preferable that the hardness of the jacket is 80 Hv or less. If the hardness is within this range, it is compatible with the sealing surface of the cask cylinder body and the lid, so that sufficient sealing performance can be exhibited while protecting the cask cylinder body and the lid.
[0014]
Moreover, the cask which concerns on Claim 6 comprises the said jacket in the said cask. Purity is 99.9% or more High-purity iron is characterized by being subjected to a heat treatment that is allowed to cool after being kept at a predetermined temperature for a predetermined time. Since this cask uses high-purity iron for the jacket of the metal gasket used for sealing, the hardness can be lowered by such heat treatment. Thereby, even if it is the material which was extended to the shape of the jacket by rolling, for example and was work-hardened, the outstanding sealing performance can be exhibited by softening.
[0015]
A cask according to claim 7 is characterized in that a passive film is formed on the surface of the outer shell of the cask by the heat treatment. In this way, since the passive film is formed on the surface of the outer cover, the corrosion resistance of the outer cover is improved, and sufficient sealing performance is ensured with minimal deterioration of the metal gasket even during long-term storage for several decades. Can be maintained. In order to form such a passive film, heat treatment is preferably performed in air or in a furnace atmosphere in which the oxygen concentration is adjusted. Moreover, as for the cask which concerns on Claim 8, the temperature of the said heat processing has preferable 300 to 450 degreeC. If high-purity iron, which is a metal to be heat-treated, is held in this temperature range, the hardness of the jacket can be sufficiently reduced, and a passive film having high corrosion resistance can be formed. The holding time varies depending on the size of the heat treatment object.
[0016]
The cask according to claim 9 is characterized in that the cask further includes an inner cover that covers the pressing member between the pressing member and the outer cover. This cask uses a metal gasket further provided with an inner cover that covers the pressing member between the pressing member and the outer cover. For this reason, in addition to the operation and effect exhibited by the cask or the metal gasket, there is obtained an operation in which the pressing force of the pressing member is uniformly transmitted to the outer cover by the inner cover. As a result, the local stress concentration of the jacket can be dispersed, so that the stress relaxation phenomenon caused by the creep deformation of the jacket can be suppressed and the reliability in long-term storage can be improved.
[0017]
The metal gasket according to claim 10 is: Purity is 99.9% or more An annular outer casing made of high-purity iron and maintaining sealing in contact with an object to be sealed, and an annular pressing member that is covered by the outer casing and presses the outer casing toward the sealing surface. It is characterized by that. Since the melting point of high-purity iron used for the jacket of this metal gasket is as high as about 1500 ° C. or higher, it is difficult to creep. For this reason, for example, at a use environment temperature of about 120 ° C., creep deformation of the jacket can be ignored, and deterioration of the metal gasket can be suppressed over a long period of several decades, and high sealing performance can be maintained. In addition, high-purity iron has high corrosion resistance, so that corrosion can be suppressed without special treatment. Furthermore, unlike gold and silver, the price is relatively low, so that a metal gasket can be provided at a relatively low price. In addition, a sealing surface means the surface which the jacket of a metal gasket contacts and seals.
[0018]
A metal gasket according to claim 11 is Purity is 99.9% or more An annular outer casing made of high-purity iron and in contact with an object to be sealed to maintain sealing, and an annular pressing member that presses the outer casing in the direction of the sealing surface, and two hoop diameters different from each other The pressing members are arranged concentrically to form a double ring, and each annular pressing member is covered with the outer cover and integrated.
[0019]
In this metal gasket, annular springs having different hoop diameters are arranged concentrically, and these are integrated by a jacket. Thus, since the individual springs are integrated with each other, the two springs regulate the creep deformation of each other even if the outer shell undergoes creep deformation during long-term storage. As a result, the stress relaxation phenomenon of the metal gasket caused by creep deformation of the outer cover can be further suppressed, and higher sealing reliability can be ensured.
[0020]
A metal gasket according to a twelfth aspect of the present invention is characterized in that, in the metal gasket, the end portions of the outer cover are joined to each other to form an elliptical cross section.
[0021]
This metal gasket has an elliptical cross section in which annular springs having different hoop diameters are concentrically arranged, covered with a jacket, and then the ends of the jacket are joined together. For this reason, for example, when this metal gasket is used for sealing the cask, the two springs can regulate the creep deformation of the outer cover in both the sealing area on the lid side and the trunk body side. As a result, the stress relaxation phenomenon of the metal gasket caused by creep deformation of the outer cover can be further suppressed, and higher safety can be ensured in long-term storage. Further, by joining the end portions of the jacket, it is possible to obtain a complete sealing structure in which the wet area of the metal gasket is minimized.
[0022]
Here, for safety reasons, the cask stores the recycled fuel assembly in a pool filled with water, and is sealed with a main body trunk and a lid via a metal gasket. For this reason, it is necessary to remove moisture by vacuum drying or other means, but since this metal gasket does not enter moisture, it has excellent corrosion resistance and can be highly reliable in long-term storage. Moreover, since the wet area of the metal gasket is small, it can be dried more easily than before. The metal gasket having an elliptical cross section is composed of two circular arcs and two straight lines as shown in FIG. 16B, in addition to the metal gasket having a normal elliptical cross section. A racetrack shape is also included (hereinafter the same).
[0023]
The metal gasket according to claim 13 is characterized in that, in the metal gasket, one of the two annular pressing members has a smaller diameter than the other.
[0024]
This metal gasket has two annular springs having different hoop diameters arranged concentrically, and includes gasket rings having different diameters with different spring diameters. When this metal gasket is used for sealing a cask, for example, the gasket ring having the larger diameter comes into contact with the trunk body and the lid of the cask. And if it vacuum-drys in this state, since a clearance gap will remain in the gasket ring side with a smaller diameter, the water | moisture content adhering to a metal gasket will evaporate easily. As described above, since the moisture in the metal gasket can be sufficiently removed, the risk of corrosion of the metal gasket can be minimized. As a result, the reliability of the sealing performance can be increased even during long-term storage.
[0025]
As in the metal gasket according to claim 14, it is preferable to use a spring for the pressing member. Thus, if a spring is used for the pressing member, the outer cover can be uniformly pressed against the object to be sealed by devising the design, and sufficient sealing performance can be exhibited. The spring includes a plate-like one formed by winding a coil spring or a plate material. Moreover, the cross-sectional shape of the spring is not particularly limited.
[0026]
Further, like the metal gasket according to the fifteenth aspect, a plate-like spring may be used as the pressing member. A plate-shaped spring generally has a reduced spring diameter due to compression during sealing. That is, since the diameter of the spring is only small, the contact surface with the object to be sealed due to the deformation of the spring is suppressed. Thereby, the fall of the contact pressure per unit area is prevented and reliable sealing becomes possible.
[0027]
The metal gasket according to claim 16 is characterized in that, in the metal gasket, the hardness of the jacket is lower than that of the sealing object. In this way, since the outer cover of the metal gasket is softer than the object to be sealed, the object to be sealed can be easily reused without damaging them. Furthermore, as in the metal gasket according to claim 17, it is preferable that the outer shell has a hardness of 80 Hv or less. If the hardness is within this range, it is compatible with the sealing surface of the object to be sealed, so that sufficient sealing performance can be exhibited while protecting the object to be sealed.
[0028]
The metal gasket according to claim 18 is characterized in that, in the metal gasket, the surface of the jacket is covered with a film that improves resistance to corrosion. For this reason, since the jacket of this metal gasket is hard to corrode, it is possible to improve the reliability by suppressing the deterioration of the sealing performance due to the corrosion over a long period of time. In particular, when the recycled fuel assembly is used for sealing a cask that is stored for a long period of several decades, the reliability of the cask can be improved. Such a corrosion-resistant film can be formed by, for example, depositing a material having excellent corrosion resistance such as gold or chromium on the outer surface by plating or ion plating.
[0029]
Further, as in the metal gasket according to claim 19, the outer cover is configured. Purity is 99.9% or more You may form the membrane | film | coat which improves the tolerance with respect to corrosion by acid-processing high purity iron. According to the acid solution treatment, such a corrosion-resistant film can be formed with relatively simple equipment, so that the manufacturing cost of the metal gasket can be suppressed.
[0030]
Moreover, the metal gasket which concerns on Claim 20 comprises the said jacket in the said metal gasket. Purity is 99.9% or more High-purity iron is characterized by being subjected to a heat treatment that is allowed to cool after being kept at a predetermined temperature for a predetermined time. Since this metal gasket uses high-purity iron for the jacket, the hardness can be lowered by such heat treatment. Thereby, even if it is the material which was extended to the shape of the jacket by rolling, for example and was work-hardened, the outstanding sealing performance can be exhibited by softening.
[0031]
The metal gasket according to claim 21 is characterized in that in the metal gasket, a passive film is formed on the surface of the jacket by the heat treatment. In this way, a passive film is formed on the surface of the outer cover, which improves the corrosion resistance of the outer cover and maintains sufficient sealing performance with minimal deterioration of the metal gasket even after decades of use. it can. In order to form such a passive film, heat treatment is preferably performed in air or in a furnace atmosphere in which the oxygen concentration is adjusted. Moreover, like the metal gasket which concerns on Claim 22, the temperature of the said heat processing has preferable 300 to 450 degreeC. If high-purity iron, which is a metal to be heat-treated, is held in this temperature range, the hardness of the jacket can be sufficiently reduced, and a passive film having high corrosion resistance can be formed. The holding time varies depending on the size of the heat treatment object.
[0032]
Further, the metal gasket according to claim 23 is characterized in that, in the metal gasket, a sealing region of the jacket that comes into contact with the object to be sealed is formed in advance on a flat surface. As described above, the lid and the trunk body, which are objects to be sealed, and the seal region of the metal gasket jacket are in contact with each other in a plane, so that the stress concentration in the seal region of the jacket can be dispersed to make the stress distribution more uniform. As a result, the progress of creep deformation of the jacket can be suppressed, so that the stress relaxation phenomenon of the metal gasket can be suppressed even during long-term storage. In addition, by suppressing the stress relaxation phenomenon, a large amount of spring back (effective elastic recovery) of the metal gasket can be obtained. Therefore, when this metal gasket is used for the cask, it is against the displacement between the lid and the trunk body. You can increase your tolerance. As a result, higher safety can be ensured even in long-term storage for several decades. In addition, since the thickness of the outer cover can be selected by flattening the sealing surface, the outer cover can be made thicker than before, and the escape of the outer cover from the seal portion can be further restrained. Here, the sealing area of the metal gasket is a part of the outer casing of the metal gasket and refers to an area that is in contact with and sealed against an object to be sealed (for example, a trunk body and a lid of the cask) (hereinafter the same).
[0033]
According to a twenty-fourth aspect of the present invention, in the metal gasket, an inner cover that covers the pressing member is further provided between the pressing member and the outer cover. The metal gasket further includes an inner cover that covers the spring between the spring and the outer cover. For this reason, in addition to the operations and effects exhibited by the metal gasket, an operation in which the pressing force of the spring is uniformly transmitted to the outer casing by the inner casing is obtained. As a result, the local stress concentration of the jacket can be dispersed, so that the stress relaxation phenomenon caused by the creep deformation of the jacket can be suppressed and the reliability in long-term storage can be improved.
[0034]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to the embodiments. In addition, constituent elements in the following embodiments include those that can be easily assumed by those skilled in the art or those that are substantially the same. Here, the application range of the metal gasket according to the present invention is not particularly limited, and can be applied to, for example, a cask seal or a seal portion of a reactor containment vessel. The metal gasket according to the present invention is particularly suitable for applications in which sealing performance is maintained in units of several decades in a relatively high temperature environment. Among such applications, a recycled fuel assembly is accommodated. Suitable for cask to be transported and stored for a long time.
[0035]
(Embodiment 1)
FIG. 1 is an explanatory view showing a cask sealing structure according to Embodiment 1 of the present invention. Moreover, FIG. 2 is sectional drawing which shows the metal gasket which concerns on Embodiment 1 of this invention. The cask seal structure 10 is characterized in that the casing 50 is sealed with a metal gasket using high-purity iron. The metal gasket 20 having a sealed structure according to the first embodiment is a so-called configuration in which two springs 30a and 30b having different hoop diameters Df are arranged concentrically, and both are covered by a jacket 50 and coupled. Double ring type. However, the metal gasket applicable to the present invention is not limited to this type, and a so-called single ring type in which one spring 30 is covered with an inner cover 40 and an outer cover 50 as shown in FIG. It is applicable also to metal gasket 20 '. The same applies to the following embodiments. The inner jacket 40 is not always necessary. The springs 30a and 30b are also referred to as springs 30 for convenience.
[0036]
In the first embodiment, like the metal gasket 20 shown in FIG. 2A, the outer cover 50 between the springs 30a and 30b is bent downward in FIG. 2A. A case where the present invention is applied to will be described. Further, not only the metal gasket 20 having such a structure but also a structure in which the jacket 50 between the springs 30a and 30b is not bent, such as the metal gasket 20 '' shown in FIG. The present invention is also applicable. Furthermore, as shown in FIG. 2D, when the springs 30a and 30b are coupled not only by the jacket 50 but also by the jacket 40 ′, the escape of the jacket 50 can be reduced.
[0037]
Before describing the metal gasket 20 and the cask sealing structure according to the present invention, the cask that is the main application target of the metal gasket 20 will be outlined. FIG. 19 is a cross-sectional view showing the structure of the cask. FIG. 20 is a partially enlarged view of the cask shown in FIG. The cask 500 includes a stainless steel or carbon steel trunk main body 501, an outer cylinder 502 constituting an outer surface of the cask, and a neutron shielding material 503 containing hydrogen filled between the trunk main body 501 and the outer cylinder 502. The bottom plate 505 is welded to the lower portion of the trunk body 501 and encloses the neutron shielding material 504, and the primary lid 507 and the secondary lid 508 provided on the flange portion 506 integrally welded to the trunk body 501. . The basket 513 that stores the recycled fuel assembly is disposed in the cavity 509 of the trunk body 501. As shown in FIG. 20, the primary lid 507 and the secondary lid 508 are fixed to the flange portion 506 by bolts 510 and 511, and the secondary lid 508 is filled with a neutron shielding material 512.
[0038]
The basket 513 is composed of a plurality of cells formed of a composite material of boron (B) and aluminum. The cavity 509 of the cask 500 is filled with helium gas in a negative pressure state. On the other hand, the inside of the gap between the primary lid 507 and the secondary lid 508 has a positive pressure, thereby forming a pressure barrier between the inside and outside of the cask 500. The secondary lid 508 is provided with a hole 514 for measuring the pressure in the space between the lids, and a pressure sensor 515 is provided at the outlet of the hole 514.
[0039]
The primary lid 507 is provided with a valve 516 for replacing the gas in the cask 500, and the valve 516 is covered with a valve cover 517. The metal gasket 20 according to the present invention is interposed between the primary lid 507 and the trunk body 501 and between the secondary lid 508 and the trunk body 501 to seal the inside of the cask 500 with the outside air.
[0040]
Next, the metal gasket 20 and the cask sealing structure according to the present invention will be described. As shown in FIG. 1, the seal structure 10 is provided, for example, between the secondary lid 508 of the cask 500 and the flange portion of the trunk body 501. Needless to say, the sealing structure and the metal gasket according to the present invention can also be applied between the primary lid of the cask and the trunk body 501 (the same applies hereinafter). As shown in FIG. 2, the metal gasket 20 has a configuration in which an inner spring 40 is covered with an annular spring 30 that is a pressing member, and the inner and outer rings are collectively covered with an outer cover 50. Although the spring 30 uses a coil spring, the pressing member that can be used in the present invention is not limited to this. Other examples will be described later.
[0041]
In the first embodiment, the hoop diameter Df of the metal gasket 20 (diameter of the metal gasket 20) is about 2 m, the cross-sectional diameters D of the inner ring 70 and the outer ring 80 are about 5.5 mm, and the thickness of the outer jacket 50 is 0.00. The thickness of 4 mm and the inner jacket 40 is 0.2 mm. High purity iron is used for the material of the jacket 50, and sealing performance is ensured. High purity iron will be described later. The inner jacket 40 is made of nickel-based alloy such as Inconel (registered trademark) which is resistant to corrosion and high-temperature oxidation, so that elasticity can be maintained even in a high-temperature environment. When it is desired to improve durability at a higher temperature, Nimonic (registered trademark) having a high Co content can be used.
[0042]
In addition, although the inner jacket 40 is used for this metal gasket 20, the inner jacket 40 is not necessarily required and may be used according to the specifications of the cask 500. However, when the inner jacket 40 is provided, the repulsive force of the spring 30 can be more uniformly transmitted to the outer jacket 50, and the local stress concentration generated in the outer jacket 50 can be suppressed to a low level. As a result, the creep deformation of the outer cover 50 can be further reduced, and therefore it is preferable to provide the inner cover 40. In particular, when a metal material with low hardness is used for the outer cover 50, the inner cover 40 is provided. desirable.
[0043]
For the secondary lid 508, the primary lid, and the trunk body 501, stainless steel or carbon steel is used to shield radiation and maintain mechanical strength. On the other hand, high purity iron is used in the present invention in order to exert sealing performance on the jacket 50 of the metal gasket 20. For this reason, a potential difference is generated between the two due to the contact of different metals, and the metal gasket 20 causes electrolytic corrosion, which causes sealing leakage. In order to prevent this, SUS317 or SUS625 containing molybdenum is used as a material for the secondary lid 508 of the cask and the body 501 to prevent electrolytic corrosion.
[0044]
Moreover, since SUS317 or SUS625 is excellent in weldability, it is suitable for those having a large number of welded parts such as cask. Furthermore, in addition to SUS317 and SUS625, SUS314, SUS316, SUS326, SUS345, and the like can also be used. Further, without using SUS317 or the like for the secondary lid 508 and the trunk main body 501, the sealing surface 90 may be formed by overlaying the same material (not shown).
[0045]
Further, a material (for example, Al) having a higher corrosion potential than SUS317 or SUS625 containing molybdenum, which is used as a material for the secondary cover 508 of the cask or the body 501, is coated on the surface of the outer jacket 50 by vapor deposition or the like. May be. In this way, the outer cover 50 of the metal gasket 20 that is easier to replace than the secondary lid 508 and the trunk body 501 is more easily corroded, so that the secondary lid 508 and the trunk body 501 are protected from electrolytic corrosion. .
[0046]
The metal gasket 20 used for the cask is required to have a sealing performance for several decades. Further, the recycle fuel assembly stored in the cask generates decay heat, and the use environment temperature of the metal gasket 20 in the initial storage is about 120 ° C. Then, while the recycled fuel assembly is stored for several decades, the use environment temperature of the metal gasket 20 gradually decreases from about 120 ° C. to about 60 ° C.
[0047]
In such a use environment, creep deformation occurs in the outer cover 50 depending on the type of material used for the outer cover 50. In particular, when aluminum, tin or the like having a relatively low melting point is used for the jacket 50, this creep deformation becomes a problem. FIG. 3 is an explanatory view showing the relationship between the stress acting on the metal gasket and the distortion of the metal gasket. When the secondary cover 508 of the cask is tightened, the metal gasket 20 has a predetermined tightening stress σ. ₀ Acts, which causes the initial strain ε ₀ Will occur. This initial strain ε ₀ Can be expressed as the sum of plastic strain εp and elastic strain εe (t = 0). If creep strain εc occurs in the jacket 50 over time, the tightening stress σ ₀ Is relaxed, the elastic strain εe is reduced. When a certain time t elapses from the time of tightening, the tightening stress decreases to σt as the creep strain εc increases. This is called the stress relaxation phenomenon of the metal gasket.
[0048]
Σc shown in FIG. 3 is a hermetic limit stress, and when the stress acting on the metal gasket 20 falls below the hermetic limit stress σc, the hermeticity is broken by the differential pressure between the inside and outside of the cask. Tightening stress σ on metal gasket 20 ₀ This tightening stress σ ₀ Since the elastic strain εe is removed, the strain of the metal gasket 20 is only the plastic strain εp. And R ₀ Hermeticity is broken at points. However, the tightening stress σ on the metal gasket 20 ₀ When a long time elapses after acting, the tightening stress decreases to σt as described above. When the tightening stress is released in this state, the strain of the metal gasket 20 becomes the sum of the plastic strain εp and the creep strain εc, and R ₁ Hermeticity is broken at points. This is because the initial elastic strain εe (t = 0) is decreased by the increase in the creep strain εc.
[0049]
Since the hermetic limit stress of the metal gasket 20 is σc, the strain of the metal gasket is ε immediately after the metal gasket 20 is tightened. ₁ The airtightness is broken when it decreases. On the other hand, after a long time has elapsed, the strain of the metal gasket 20 is ε. ₂ The airtightness will be broken when it decreases. This means that the hermeticity is broken with less strain, that is, with less displacement, as compared to immediately after the metal gasket 20 is tightened.
[0050]
In order to maintain the sealability of the cask during long-term storage and improve the reliability, it is necessary to sufficiently analyze the stress relaxation phenomenon of the metal gasket 20. As a result of intensive studies on the stress relaxation phenomenon of metal gaskets, the present inventors have found the following matters. First, the stress relaxation phenomenon of the metal gasket 20 is a phenomenon in which the elastic displacement of the spring 30 is released due to creep deformation of the material constituting the outer cover 50. The spring back (effective elastic restoration) after the metal gasket 20 is deteriorated is governed by the bending rigidity after the spring 30 is plastically deformed. The bending stiffness after the spring 30 is plastically deformed is such that the change in characteristics is negligible compared to when the metal gasket 20 is new even after long-term storage of about 60 years. If there is no change. That is, the material deterioration of the spring 30 made of Inconel does not have to be taken into consideration when considering the stress relaxation phenomenon of the metal gasket 20.
[0051]
In order to maintain the sealing performance of the cask during long-term storage and improve the reliability, it is necessary to reduce the creep deformation of the jacket 50. This is because if the creep deformation can be reduced, the decrease in the tightening stress σ after a long period of time is reduced. In order to reduce the creep deformation of the jacket 50 during long-term storage, high purity iron is used for the jacket 50 of the metal gasket 20 according to the present invention. Since the melting point of high-purity iron is as high as about 1500 ° C. or higher, creep deformation of the jacket 50 using high-purity iron does not cause a problem at a temperature of about 120 ° C., which is the environment in which the cask 500 (see FIG. 19) is used.
[0052]
In addition, high-purity iron has high corrosion resistance, so corrosion can be suppressed without special treatment, and corrosion resistance can be further improved by relatively simple treatment such as heat treatment and acid solution treatment. Can do. Furthermore, since the hardness can be lowered to the same level as copper by heat treatment, sealing performance can be ensured. Moreover, since the price is relatively low unlike gold and silver, the manufacturing cost of the entire cask 500 can be suppressed. As described above, high-purity iron is a suitable material as a covering material of the metal gasket 20 used for sealing when the recycled fuel is stored in the cask 500 for a long period of time.
[0053]
Next, the high purity iron used for the jacket 50 will be described. FIG. 4 is an explanatory diagram showing the relationship between the purity and hardness of high-purity iron. Here, the hardness is Vickers hardness (Hv). The material used as the jacket 50 has a hardness of 80 Hv or less. As shown in FIG. 4, it can be seen that when the purity of the high purity iron is higher than 99.9%, the outer shell 50 has a preferable hardness. Accordingly, the purity of the high purity iron used for the jacket 50 is preferably at least 99.9%.
[0054]
As shown in FIG. 4, when the purity of high-purity iron is in the range of 99.95% to 99.995%, there is almost no change in hardness, and the hardness is about 70 Hv. In addition, when heat treatment is performed, the hardness of high-purity iron decreases. Here, in this heat treatment, the high-purity iron to be treated is kept at a predetermined temperature for a predetermined time, and then allowed to cool in air.
[0055]
In order to improve the sealing performance, a lower hardness is preferable, and a hardness of 60 Hv or less is preferable for sealing the cask. As shown in the figure, since the hardness of high-purity iron decreases when the above heat treatment is performed, it is preferable to perform the heat treatment on the jacket 50 in order to obtain such hardness. In addition, when the hardness of high purity iron falls to about 20-25Hv, there exists a possibility that the jacket 50 may creep-deform. For this reason, the hardness of high-purity iron is preferably 30 Hv or higher, and is preferably 40 Hv or higher in consideration of ease of heat treatment.
[0056]
Here, the corrosion resistance of high purity iron will be described. Table 1 shows the results of evaluating the corrosion resistance of high-purity iron, stainless steel (SUS430), and electromagnetic soft iron. Corrosion resistance was evaluated by either exposing the test piece in flowing water at 90 ° C. for 20 minutes or immersing the test piece in boiling water at 100 ° C. for 20 minutes. In the evaluation of corrosion resistance, the appearance of the test piece was observed. The evaluation criteria are as follows. A: No change, B: Spot rust is generated, but metallic luster remains, X: Rust is generated on the whole. In addition,-of a code | symbol shows a somewhat worse grade than the said evaluation.
[0057]
[Table 1]

[0058]
The operating environment temperature of the metal gasket 20 used for sealing the cask is about 120 ° C. at the maximum. In addition, moisture may remain in the vicinity of the metal gasket 20. Therefore, the test conditions are set in consideration of the use environment of such a metal gasket 20. As can be seen from Table 1, high-purity iron exhibits intermediate corrosion resistance between stainless steel and electromagnetic soft iron.
[0059]
Table 2 shows the results of evaluating the corrosion resistance of high-purity iron against heat treatment or acid solution treatment. In the heat treatment, after being kept at a predetermined temperature for 20 minutes, it is allowed to cool in air. And it evaluated by changing holding temperature into three types, 200 degreeC, 300 degreeC, and 450 degreeC. For acid solution treatment, 63% by volume of HNO _Three Soaked in the solution for 60 minutes and 0.3% by volume of H ₂ O ₂ The solution was similarly soaked for 60 minutes.
[0060]
The corrosion resistance was evaluated by immersing the test piece in 100 ° C. boiling water for 180 minutes. The test piece having a holding temperature of 200 ° C. has the same degree of corrosion resistance as that of the test piece without heat treatment, and the improvement effect by the heat treatment is not observed. The test pieces having the holding temperatures of 300 ° C. and 450 ° C. did not show rust even when tested under the above conditions, and an effect of improving the corrosion resistance by the heat treatment was seen. This is presumed that the formation of a stable oxide film on the surface of the test piece by this heat treatment condition is one of the factors that improved the corrosion resistance. In general, the presence of strain (dislocation), which is a diffusion medium for oxygen atoms in the metal structure, promotes corrosion due to oxidation, but the fact that this strain has been removed by heat treatment under the above conditions also improves corrosion resistance. It is estimated that this is one of the factors.
[0061]
[Table 2]

[0062]
On the other hand, HNO _Three Corrosion resistance is not sufficiently improved only by acid solution treatment with a solution. H ₂ O ₂ Thus, the test piece subjected only to the acid solution treatment is improved in corrosion resistance as compared with the test piece without acid solution treatment and heat treatment. This is H ₂ O ₂ It is estimated that this is because a stable oxide film was formed on the surface of the test piece by the acid solution treatment. The reason why the corrosion resistance is inferior to that of the heat-treated test piece is thought to be that corrosion due to oxidation has occurred by this amount because the strain in the test piece is not removed only by the acid solution treatment.
[0063]
From these results, rather than using high-purity iron as the jacket 50 as it is, it is better to use it as the jacket 50 after being subjected to a heat treatment that is held for a predetermined time in a temperature range of 300 ° C. to 450 ° C. and allowed to cool. It is preferable because the corrosion resistance can be improved. At the same time, heat treatment is preferable because the hardness can be lowered to about 50 Hv, thereby improving the sealing performance. In particular, since the material that has undergone the rolling process has a high hardness due to work hardening, the rolled material is preferably used after being heat-treated. If the purpose is only to improve the corrosion resistance, ₂ O ₂ An acid solution treatment may be performed. Further, a corrosion resistant film of gold or silver may be formed on the surface of the jacket 50 by thin film forming means such as ion plating or sputtering to improve the corrosion resistance. In this case, since the film is thin, even if gold or silver is used as the film, the price does not increase significantly. Further, a material having excellent corrosion resistance such as nickel (Ni) or zinc (Zn) may be plated on the surface of the outer cover 50 to form a corrosion-resistant film.
[0064]
Next, the manufacturing method of the jacket 50 is demonstrated. The high-purity iron used for the jacket 50 is manufactured by a high-purity metal purification method such as an electron beam melting purification method or a high-frequency melting method. In the electron beam melting and refining method, the metal to be purified is irradiated through a deflection coil, and when the metal to be purified is melted, the melting range is narrowed while the irradiation range of the electron beam is narrowed by the deflection coil. And it is the method of collecting impurities in the molten part which became narrow, removing this part, and refining a high purity metal.
[0065]
In addition, the high frequency melting method melts the metal by induction heating, generates eddy currents in the crucible and the molten metal, and melts the metal while floating the molten metal from the crucible by the magnetic repulsive force generated between the eddy currents. Is the method. According to this method, since the metal can be melted while the crucible and the molten metal are not in contact with each other, a highly pure metal can be purified without causing a reaction between them. According to these high-purity metal refining methods, high-purity iron with high purity required for the jacket 50 of the present invention can be produced.
[0066]
This high-purity iron is spread to a predetermined thickness by rolling, and then heat-treated as necessary in order to adjust it to a predetermined hardness. In this heat treatment, as described above, high-purity iron to be treated is kept at a predetermined temperature for a predetermined time, and then allowed to cool in air. In the above example, the time for holding at the predetermined temperature is 20 minutes, but the time for holding at the predetermined temperature is appropriately adjusted according to the size of the object to be processed. As described above, the range of the holding temperature is preferably 300 ° C. to 450 ° C. Moreover, you may let it cool in the furnace atmosphere which adjusted oxygen concentration. When the heat treatment is completed, the metal gasket 20 is completed by cutting into a predetermined shape and then covering the spring 30 or the inner cover 40.
[0067]
Next, the annular spring 30 that is a pressing member used for the metal gasket 20 will be described. FIG. 5 is an explanatory view showing a spring of the metal gasket according to the first embodiment of the present invention. The spring 30 is a coil spring, and is formed by winding Inconel (registered trademark), stainless steel, piano wire, phosphor bronze or the like without providing a gap as shown in FIG. This is because, when there is a gap between the adjacent lines, when the metal gasket 20 is crushed for sealing, the inner cover 40 and the outer cover 50 (see FIGS. 1 and 2) are evenly applied. This is because it cannot be pushed back and the desired sealing performance cannot be exhibited in a long-term use. When piano wire or phosphor bronze is used as the spring 30, it is preferable to previously form a corrosion resistant film of chromium or gold on the surface of the wire in order to improve the corrosion resistance. Examples of the method for forming the corrosion-resistant film include methods such as plating and ion plating.
[0068]
Further, as shown in FIG. 5C, an element wire having a rectangular cross section may be used for the spring 32. In this way, the spring 32 and the inner cover 40 or the outer cover 50 (see FIGS. 1 and 2) are in contact with each other over a larger area than the coil spring having a circular cross section. The force can be transmitted to the inner jacket 40 or the outer jacket 50. As a result, the stress distribution of the outer jacket 50 can be made more uniform, so that the inner jacket 40 that suppresses the reaction force of the spring 32 can be omitted or the inner jacket 40 can be made thinner, and a metal gasket caused by creep deformation. The stress relaxation phenomenon can be reduced. As a result, the sealing performance can be maintained even during long-term storage.
[0069]
Further, if the section modulus of the spring 30 having a circular cross section and the spring 32 having a rectangular cross section are the same, the spring 32 having a rectangular cross section can reduce the width b of the element wire, so that the pitch p can also be decreased. Therefore, since the number of turns of the spring 32 can be increased with the same bending rigidity, the repulsive force of the spring 32 can be transmitted to the jacket 50 more uniformly. Thus, when the spring 32 having a rectangular cross section is used, the stress relaxation phenomenon of the metal gasket is more effectively suppressed than when the spring 30 having a circular cross section is used. Further, since the pitch p can be reduced, the initial inclination θ when the tightening force P is not acting is provided. _{P = 0} Can be made smaller than a coil spring having a circular cross section. Thereby, the reaction force of the spring 32 can be used more effectively.
[0070]
Further, if the section modulus of the spring 30 having a circular cross section and the spring 32 having a rectangular cross section are the same, the spring 32 having a rectangular cross section can reduce the stress generated in the spring 32 by the tightening force of the metal gasket. Further, unlike the spring 30 having a circular cross section, the spring 32 having a rectangular cross section comes into contact with an adjacent element wire at the surface 32a. As a result, the stress generated on the surface 32 a by the tightening force P can be made smaller than that of the spring 30 having a circular cross section. As a result, the inclination angle θ due to the tightening force P _P Can be made smaller than the spring 30 having a circular cross section, and the torsional stress acting on the wire of the spring 32 having a rectangular cross section can also be reduced. By these actions, if the cross section is rectangular, the long-term reliability of the spring 32 can be further increased.
[0071]
Generally, in order to obtain a sealing pressure in the metal gasket 20 (see FIGS. 1 and 2), it is necessary to use a spring 30 having a large wire diameter d and to increase the bending rigidity of the spring 30. When the wire diameter d of the spring 30 is increased, the winding diameter D of the spring 30 is increased accordingly. ₁ Becomes smaller. As a result, the cross-sectional diameter D of the metal gasket 20 that requires a high sealing pressure is smaller than that having a low sealing pressure. The bending rigidity of the spring 30 refers to the difficulty of deformation in the radial direction of the spring 30 when a force P is applied in the radial direction of the spring 30 (the same applies hereinafter).
[0072]
If the amount of springback of the metal gasket 20 is increased, the tolerance for sealing when the metal gasket 20 is displaced can be increased accordingly. FIG. 6 is an explanatory diagram showing the relationship between the load applied to the metal gasket and the displacement of the metal gasket. The dotted line in the figure indicates a conventional metal gasket 520 (see FIG. 15A), and the solid line in the figure indicates the metal gasket 20 according to the present invention. In addition, what is shown in FIG. 6 is an analysis result by an extrapolation method using LMP, and the temperature condition is a case where the temperature drops to 60 ° C. after 60 years at an initial temperature of 120 ° C. under a linear approximation. Is. In addition, this temperature condition can fully reproduce the temperature condition of long-term storage in the implementation cask. Further, the extrapolation method using the LMP is a method for estimating stress relaxation on the safe side, that is, stress relaxation, and is a preferable method for such analysis.
[0073]
The bending rigidity of the metal gasket 20 according to the present invention is approximately 60% of the bending rigidity of the conventional metal gasket 520. For this purpose, the wire diameter d of the spring 30 is smaller than that of the conventional metal gasket 520. The cross-sectional diameter D of the metal gasket according to the present invention is the same as that of the conventional metal gasket 520. The bending rigidity of the metal gasket 20 according to the present invention is preferably in the range of 40% to 80% of the bending rigidity of the conventional metal gasket 520. This is because within this range, it is possible to obtain an effect of increasing the amount of springback while ensuring a certain degree of bending rigidity. Furthermore, the range of 50% or more and 70% or less of the bending rigidity in the conventional metal gasket 520 is obtained. More preferred.
[0074]
Here, the initial tightening amount of the metal gasket 20 according to the present invention is larger than that of the conventional metal gasket 520. As a result, the initial stress (clamping stress) σ acting on the conventional metal gasket 520 and the metal gasket 20 according to the present invention. ₀ Therefore, the amount of stress relaxation after the same time elapses is the same. The tightening stress at this time is σt. Here, when the tightening stress σ is gradually released, the metal gasket gradually returns to the original shape, and the displacement δ approaches zero. The displacement δ when the tightening stress σ reaches the hermetic limit stress σc becomes the hermetic limit displacement δc of the metal gasket.
[0075]
In the metal gasket 20 according to the present invention, since the bending rigidity is made smaller than that of the conventional metal gasket 520, the change of the tightening stress σ with respect to the displacement δ of the metal gasket, that is, σ / δ is gentler than that of the conventional metal gasket. become. As a result, the metal gasket 20 is displaced more largely than in the past with respect to the relaxation of the tightening stress σ. That is, if the tightening stress σ is the same, the amount of springback at that time can be increased, and the hermetic limit displacement δc can be increased accordingly. As a result, even if a larger displacement than the conventional metal gasket 520 occurs, the airtightness of the cask can be maintained. Therefore, even after long-term storage, stable sealing performance can be exhibited and safe transportation can be achieved.
[0076]
Specifically, the spring back amount δc in the conventional metal gasket 520 using Inconel as the spring 30. ₅₂₀ Is about 0.01 to 0.02 mm. On the other hand, the spring back amount δc in the metal gasket 20 of the present invention using a thin Inconel wire for the spring 30. ₂₀ Is about 0.20 mm. Thus, the spring back amount in the metal gasket 20 according to the present invention is 10 to 20 times the spring back amount in the conventional metal gasket 520. Here, the amount of spring back required for the metal gasket used for sealing the cask is approximately 0.05 mm, but the metal gasket 20 according to the present invention can sufficiently achieve this value. As a result, it was possible to obtain a metal gasket having sufficient reliability even for long-term storage for several decades in a cask of a recycled fuel assembly, which was insufficient with a conventional metal gasket.
[0077]
From the above description, if the bending rigidity of the spring 30 is reduced, the amount of spring back can be increased. However, if the bending rigidity is too small, the amount of deformation of the metal gasket 20 increases, which is not preferable. Further, even though the high-purity iron used for the jacket 50 has a relatively low hardness, the hardness is higher than that of aluminum or copper. Therefore, in order to bring the outer cover 50 into close contact with the seal surface, it is necessary to generate a certain repulsive force on the spring 30 and press the outer cover 50 against the seal surface. For these reasons, it is not preferable to make the bending rigidity of the spring too small, and it is necessary to have a certain degree of bending rigidity.
[0078]
The bending stiffness of the spring 30 is determined by determining the material diameter and the wire diameter d and the winding diameter D of the spring 30. ₁ Is uniquely determined by the relationship. Here, the evaluation result of the springback amount of the metal gasket after long-term storage is shown in Tables 3 to 5. The spring 30 used for this metal gasket is Inconel. This evaluation is based on an extrapolation method using LMP, and the temperature condition is the case where the initial temperature is 120 ° C. and the temperature drops under linear approximation to 60 ° C. after 60 years. In addition, the case where the amount of spring back required for the metal gasket used for sealing the cask was satisfied was marked with “◯”. The winding diameter D of the spring 30 ₁ Is represented by the sectional diameter D of the metal gasket 20 (see FIG. 2A) for convenience.
[0079]
[Table 3]

[0080]
[Table 4]

[0081]
[Table 5]

[0082]
As can be seen from Table 3, when the cross-sectional diameter D of the metal gasket 20 is 5.6 mm, a good spring back amount is exhibited when the element wire diameter d of the spring 30 is between 0.35 mm and 0.50 mm. I understand. Further, it can be seen from Table 2 that when the cross-sectional diameter D of the metal gasket 20 is 12.0 mm, a good springback amount is exhibited when the element wire diameter d of the spring 30 is 0.80 mm or less. Table 3 summarizes the evaluation results based on the ratio d / D between the wire diameter d of the spring 30 and the cross-sectional diameter D of the metal gasket 20, and the d / D is in the range of 0.02 to 0.08. It can be seen that a good springback amount is exhibited.
[0083]
From the above results, the wire diameter d (see FIG. 5) of the spring 30 is preferably in the range of 0.35 mm to 0.80 mm, and more preferably in the range of 0.35 mm to 0.50 mm. Further, when the cross-sectional diameter D of the metal gasket 20 is too small, the amount of spring back cannot be secured unless the tightening amount is increased with respect to the cross-sectional diameter D, and the airtightness by the metal gasket 20 may be destroyed. The cross-sectional diameter D (see FIG. 2) of the metal gasket 20 cannot be reduced without darkness. On the other hand, if the cross-sectional diameter D of the metal gasket 20 is too large, the necessary bending rigidity cannot be secured with the above-mentioned wire diameter d. Therefore, the cross-sectional diameter D of the metal gasket 20 is preferably 5.0 mm or more and 12.0 mm or less. . Further, the ratio d / D between the wire diameter d of the spring 30 and the cross-sectional diameter D of the metal gasket is preferably 0.02 or more and 0.08 or less. These numerical values are values when Inconel is used for the spring 30, but when the spring 30 is made of stainless steel or piano wire having a higher elastic modulus than Inconel, the value may be about 10% to 30% smaller. .
[0084]
7 and 8 are explanatory views showing a metal gasket mounting structure according to the present invention. As shown in FIG. 5A, the metal gasket 20 ″ is attached to the primary lid 507 or the secondary lid 508 of the cask 500 by attachment means such as a bolt 100. The recycled fuel assembly is inserted into or removed from the cask 500 underwater in order to shield radiation. At this time, the primary lid 507 and the secondary lid 508 are also attached underwater. For this reason, the metal gasket 20 is attached to the lid side to improve workability. Here, in order to attach the metal gasket 20 to the primary lid 507 or the like, as shown in FIG. 7B, the magnet 102 is disposed at the metal gasket attachment position of the secondary lid 508 or the like. Since the jacket 50 of the metal gasket 20 ″ is high-purity iron, it is attracted to the magnet 102 and attached to the secondary lid 508 or the like. In this way, since the bolt 100 is not used, the mounting work of the metal gasket 20 '' is facilitated.
[0085]
Further, as shown in FIG. 8A, the secondary lid 508 and the like may be attached to the trunk body 501 after the metal gasket 20 ″ is arranged on the trunk body 501 side. In this case, when the secondary lid 508 or the like is removed from the trunk body 501, the metal gasket 20 ″ is left on the trunk body 501 side. However, as shown in FIG. The metal gasket 20 ″ may be removed from the trunk body 501 by adsorbing the metal gasket 20 to an annular removal jig 106 having a magnet 104 on the contact surface. As described above, since the outer cover 50 of the metal gasket 20 '' according to the present invention is high-purity iron, the degree of freedom in handling the metal gasket 20 '' can be improved by using a magnet.
[0086]
As mentioned above, although the metal gasket 20 etc. which concern on Embodiment 1 of this invention were demonstrated, since the high purity iron is used for the jacket 50 of a metal gasket in this invention, the sealing at the time of long-term storage for several decades by a cask Almost no creep deformation even when used for As a result, the stress relaxation phenomenon of the metal gasket 20 and the like can be suppressed to an extremely low level, and the reliability of sealing during long-term storage can be improved. Moreover, since high-purity iron does not corrode easily, the reliability of sealing in long-term storage can also be improved in this respect.
[0087]
(Modification 1)
FIG. 9 is a diagram illustrating a metal gasket and a sealing structure according to a first modification of the first embodiment. The metal gasket 22 has substantially the same configuration as the metal gasket 20 and the like, but an annular plate-shaped member having a substantially circular cross section is used as a pressing member by winding a plate material and partially overlapping an end portion. The point to do is different. Since other configurations are the same as those of the first embodiment, the description thereof is omitted.
[0088]
In the cask seal structure 11, the metal gasket 22 is a so-called two plate-wound springs 34 a and 34 b having different hoop diameters Df arranged concentrically and joined together while being covered with an outer jacket 51. Double ring type. That is, in the metal gasket 22, the inner ring 70 and the outer ring 80 are integrated by the flat portion 51 c of the outer jacket 51. The jacket 51 uses high-purity iron as described above. The same applies to the following examples.
[0089]
The plate-like springs 34a and 34b are formed into a plate-like shape having a substantially circular cross section so that the overlapping portion S overlaps the plate material, and the overlapping portion S slides as a whole by compression during sealing. The diameters of the coiled springs 34a and 34b are reduced. That is, since the diameters of the plate-like springs 34a and 34b are only reduced, the contact surface with the trunk body 501 and the like due to the deformation of the plate-like springs 34a and 34b is suppressed, and per unit area. It is possible to prevent a decrease in contact pressure. For this reason, reliable sealing becomes possible.
[0090]
In addition, a large number of drain holes 101 may be drilled in the circumferential direction inside the spring 34b. This drain hole 101 is for letting out water that has entered the outside, and the position thereof is the inside in the radial direction and the approximate center in the thickness direction (the height direction in the figure). For example, when it is provided below, it interferes with the outer jacket 51, so that it is necessary to drill the outer jacket 51 as well. In addition, if it is provided near the seal surface 90, water easily enters between the outer jacket 51 and the seal surface 90, and it becomes difficult to drain water. A particularly preferable position of the water drain hole 101 is within ± 45 ° with respect to the radially inner line L. On the other hand, the plate-like plate-like spring 34a on the center C side of the trunk body 501 has the overlapping portion S located inside the metal gasket, so that the overlapping portion S is not slid by the compression at the time of sealing. It is necessary to drill the drain hole 101 at such a position.
[0091]
Further, instead of the overlapping portion S of the plate-like springs 34a and 34b, an opening 34a'o is provided, and the plate-like spring 34a having a substantially C-shaped cross section as shown in FIGS. 9B and 9C. 'May be used. Such a plate-like spring 34a ′ is compressed at the time of sealing, and the overlapping portion S does not slide when contracted. Further, since the diameter of the plate-like spring 34a ′ is reduced as a whole at the time of sealing, an increase in the contact surface due to the deformation of the plate-like spring 34a ′ is suppressed, and the contact pressure per unit area is reduced. Can be prevented. For this reason, reliable sealing becomes possible.
[0092]
Inconel can be used as the material of the plate-like springs 34a, 34a ′, etc., like the springs 30a, 30b according to the first embodiment. The clad spring is made of a material having a coefficient of thermal expansion larger than that of the high-purity iron that is the material of the jacket 51 (for example, stainless steel or Al), and the plate-like spring 34 a is combined with the jacket 51. The metal gasket 22 may be configured as a material structure. Regarding materials, the following examples can be similarly applied.
[0093]
The overlapping portion S of the plate-like springs 34a and 34b or the opening 34a'o of the plate-like spring 34a 'is outside the body main body 501, that is, on the outer peripheral side of the metal gasket 22 in view of ease of manufacture. It is preferable to arrange. On the other hand, in order to fully exhibit the sealing function of the metal gasket 22, the overlapping portion S or the opening 34 a ′ o is disposed on the center C side of the trunk body 501, that is, on the inner peripheral side of the metal gasket 22. Is preferred.
[0094]
According to the seal structure 11 and the metal gasket 22 described above, a decrease in contact pressure per unit area with respect to the seal surface 90 can be prevented, and the sealing performance can be significantly improved by the double ring. In particular, it is suitable for sealing a container for storing a radioactive substance such as a cask.
[0095]
(Modification 2)
FIG. 10 is an explanatory diagram showing a seal structure in the cask according to the second modification of the first embodiment. In the seal structure 12, the metal gasket 22 a is a structure formed by bending an inner jacket 35 having a spring function and an outer jacket 52 laminated on the inner jacket 35. In order to manufacture the metal gasket 22a, first, the plate-shaped inner jacket 35 and the outer jacket 52 are laminated and bonded by diffusion bonding or the like, and then bent into a shape as shown in FIG. Specifically, both sides of the inner jacket 35 and the outer jacket 52 are bent into a substantially circular plate winding shape to form an overlapping portion S that partially overlaps. The inner ring 71 includes an inner cover 35a and an outer cover 52a, and the outer ring 81 includes an inner cover 35b and an outer cover 52b. The inner ring 71 and the outer ring 81 are connected by plane portions 35c and 52c.
[0096]
The metal gasket 22a is interposed between the secondary lid 508 and the trunk body 501 to seal the inside. In a sealed state, the inner ring 71 and the outer ring 81 are in contact with the sealing surfaces 90a and 90b with a predetermined pressure by the outer jacket 52. The metal gasket 22a is compressed in the vertical direction in a sealed state, but the overlapping portion S slides to reduce the diameters of the inner ring 71 and the outer ring 81 as a whole. That is, only the diameters of the inner ring 71 and the outer rings 81, 81 functioning as springs are reduced, so that the contact surface between the outer jacket 52 and the seal surface 90 is not increased. As a result, a decrease in contact pressure due to the metal gasket 22a can be prevented and sealing can be performed reliably. Moreover, since it has a double ring structure, it is suitable for products that require high confidentiality such as cask.
[0097]
(Modification 3)
FIG. 11 is an explanatory view showing a seal structure in a cask according to a third modification of the first embodiment. FIG. 12 is an explanatory view showing a manufacturing method of the spring shown in FIG. FIG. 13 is an explanatory view showing a sealed state by the metal gasket of FIG. In this seal structure 13, the metal gasket 22 b has two springs 36 a and 36 b having different hoop diameters Df arranged concentrically, and covers the inner jackets 44 a and 44 b, and both are covered by the outer jacket 53. It is a so-called double ring type that joins together. That is, the metal gasket 22 b is configured such that the inner ring 72 and the outer ring 82 are collectively covered with the outer jacket 53, and the inner ring 73 and the outer ring 83 are integrated with each other by the flat surface portion 53 c of the outer jacket 53. The inner ring 73 includes a spring 36a, an inner cover 44a, and an outer cover 53a, and the outer ring 83 includes a spring 36b, an inner cover 44b, and an outer cover 53b.
[0098]
The seal structure 13 is provided between the secondary lid 508 of the cask 500 and the flange portion of the trunk body 501, for example. Of course, the seal structure and metal gasket according to the present invention can also be applied between the primary lid of the cask 500 and the trunk body 501. The springs 36a and 36b are formed in a substantially circular coil shape so that the overlapping portion S overlaps the plate material, and the overlapping portion S slides due to compression at the time of sealing, and the diameter of the spring 32 is reduced as a whole. That is, since only the diameters of the springs 36a and 36b are reduced, an increase in the contact surface by the springs 36a and 36b can be suppressed, and a decrease in contact pressure per unit area can be prevented. For this reason, reliable sealing becomes possible.
[0099]
As shown in FIG. 12 (a), the springs 36a and 36b (hereinafter both springs 36) have a plurality of rectangular strips with one end in the longitudinal direction as a base 301 and a large number at the other end. A plurality of comb teeth 302 are formed by inserting slits 303. This plate is bent into a substantially circular plate winding shape with the direction indicated by the one-dotted line in the figure as an axis, and a spring 32 having a ring-like cross-sectional shape as shown in FIG. In the spring 36 bent in this way, the base 301 portion becomes the overlapped portion S and can slide on the comb teeth 302.
[0100]
Since the surface of the spring 36 has a slit like a comb tooth, when the seal is sealed only through the jacket 53 made of high-purity iron, the jacket 53 enters the slit 303 and the seal surface 90 Variations in contact pressure occur. For this reason, as shown in FIG. 13, inner jackets 44a and 44b, which are thin hard metals, are interposed between the outer jacket 53 and the spring 36, and the inner jacket 44a and the like are once pushed by the comb teeth 302 of the spring 36, If the outer jacket 53 is pushed by the inner jacket 44a or the like, pressure can be applied to the outer jacket 53 uniformly and entirely. As a result, the outer jacket 53 comes into contact with the sealing surface 90 with a uniform contact pressure, so that reliable sealing is possible.
[0101]
(Embodiment 2)
FIG. 14 is a cross-sectional view showing a metal gasket according to Embodiment 2 of the present invention. The metal gasket 21 has substantially the same configuration as that of the metal gasket 20 according to the first embodiment, but the seal of the outer cover 50 where the primary cover 507 or the secondary cover 508 and the outer cover 50 are sealed. The difference is that the region 50s is formed in a planar shape in advance. Since other configurations are substantially the same as those of the first embodiment, the description thereof is omitted.
[0102]
FIG. 15 is a cross-sectional view showing a conventional metal gasket. In the conventional metal gasket 520, the outer covering shape of the cross section is circular, and after the arc portion comes into contact with the trunk body of the cask and the secondary lid 508, it is deformed as shown in FIG. To do. For this reason, as shown in FIG. 15C, since the stress acts on the seal region 550s in contact with the secondary lid 508 and the trunk main body 501 of the cask, creep deformation is promoted.
[0103]
In the metal gasket 21 according to the present invention, as shown in FIG. 14 (a), a sealing region 50s in which the outer jacket 51 is in contact with the secondary lid 508 and the like is formed in a flat shape in advance. As a result, as shown in FIG. 14B, when the metal gasket 21 is compressed by receiving the tightening force, the stress distribution in the seal region 50s in contact with the secondary lid 508, which is the object to be sealed, is dispersed. It becomes more uniform than before. This is because the stress distribution in the seal region 50s in contact with the secondary lid 508 or the like, which is the object to be sealed, is more uniform than the stress distribution in the region 50y other than the seal region when a tightening force is applied to the region 50y other than the seal region. Means that
[0104]
Here, as described above, even if the hardness of the high-purity iron used for the jacket 50 is relatively low, the hardness is higher than that of aluminum or copper. Further, in the present embodiment, since the sealing region 50s of the outer jacket 50 is formed in a flat shape in advance, the pressure on the sealing surface is lower than that of a metal gasket having a circular cross section as in the prior art. Therefore, in order to bring the outer cover 50 into close contact with the seal surface, it is necessary to generate a repulsive force to the spring 30 and press the outer cover 50 against the secondary lid 508 or the like. For these reasons, when the sealing region 50s of the outer cover 50 is formed in a planar shape, it is preferable to use the spring 30 having a bending rigidity sufficient to generate a sufficient pressure for sealing in the sealing region 50s.
[0105]
Further, in order to generate a pressure sufficient for sealing in the seal region 50s, the seal region 50s needs to be deformed to some extent. However, since the hardness is higher than that of pure iron Al or the like, depending on the area of the seal region 50s. The tightening force of a fixing bolt (not shown) for fixing the secondary lid 508 and the like is increased. For this reason, it is preferable to suppress the area of the seal region 50s within a range in which the fastening force of the fixing bolt does not become excessive. Further, the seal region 50s may be deformed to such an extent that a sufficient pressure can be generated for sealing by generating a larger tightening force using a thick fixing bolt.
[0106]
With the above configuration, compared with the conventional metal gasket 520, the action of equalizing the stress distribution is increased in the seal region 50s, so that the creep deformation of the jacket 50 can be made more gradual than in the conventional case. As a result, the creep deformation of the outer jacket 50 can be made smaller than before, so that the stress relaxation phenomenon caused by the creep deformation can be suppressed. Further, by forming the sealing region 50s of the outer jacket 50 in a plane in advance, the cross-sectional shape of the metal gasket 21 is not circular.
[0107]
By setting it as such a shape, a section modulus becomes larger than the case where a cross section is circular. Further, by forming the sealing region 50s of the outer cover 50 in a flat surface in advance, the thickness of the sealing region 50s of the outer cover 50 that is likely to undergo creep deformation becomes thinner than the conventional metal gasket 520 (see FIG. 7). As a result, the absolute amount of deformation due to creep is reduced, and a reduction in the amount of restoration of the metal gasket 21 can be suppressed by this amount. Moreover, since high purity iron is used for the jacket 50, creep deformation hardly occurs in the use environment of the cask 500. Furthermore, since high-purity iron has a higher elastic modulus than aluminum or copper, it can withstand such a large clamping force.
[0108]
By these actions, the metal gasket 21 can withstand a larger clamping force as compared with the conventional metal gasket 520 having a circular cross section with respect to a clamping force directed in a direction perpendicular to the seal region 50s. . Therefore, in this metal gasket 21, even if the spring 30 having a smaller wire diameter d is used, the outer jacket 50 can support a larger load than the conventional one. As a result, the amount of springback of the metal gasket 21 can be increased, so that higher safety can be exhibited in long-term storage.
[0109]
(Embodiment 3)
FIG. 16 is an explanatory view showing a sealing structure in a cask according to Embodiment 3 of the present invention. The sealing structure provided in the cask has substantially the same configuration as that of the first embodiment, but the two ends of the hoops arranged concentrically are covered with the outer cover, and the ends of the outer cover are joined to each other. The difference is that a double-ring type metal gasket in which both are integrated is used. Since other configurations are the same as those of the first embodiment, the description thereof is omitted and the same components are denoted by the same reference numerals.
[0110]
The metal gasket 23 according to the third embodiment is a so-called double ring type, and two springs 33a and 33b having different hoop diameters Df are arranged concentrically. Then, these are covered with the inner jacket 43, and both are covered with the outer jacket 50, and then the end portions 50t of the outer jacket 50 are joined together with the joint portion 50b. For joining the end portions 50t, welding or friction joining is used. It is preferable to use laser or electron beam welding for the welding because a contact plate is not required at the joint.
[0111]
As shown in FIGS. 16A and 16C, the end portions 50t may be joined at a joining portion 50b between two springs 33a and 33b having different hoop diameters Df. Here, FIG. 16C is an example of the butt joint. Moreover, as shown in FIG.16 (b), you may join by the junction part 50b in the side part 33y of any one spring. Furthermore, as shown in FIG. 16D, the spacer 60 may be disposed inside the outer jacket 50, and the outer jacket 50 may be joined together with the spacer 60 at the joint portion 50 b. Alternatively, only the outer cover 50 may be joined using the spacer 60 only as a backup for joining. Thus, when the spacer 60 is used as a backup at the time of joining, the jacket 50 can be easily joined. For the spacer 60, for example, pure aluminum can be used.
[0112]
In addition, as in the case of the spacer 60, between the two springs 33a and 33b shown in FIGS. 16A and 16B, the outer cover 50 is restrained from being deformed. Small springs may be used. If such a spring is used, the metal gasket 23 can be easily manufactured as compared with the case where pure aluminum or the like is used. Further, it is preferable that excessive stress is not applied to the jacket 50 by adjusting the repulsive force of the spring to an appropriate value.
[0113]
FIG. 17 is an explanatory view showing a partially open double ring type metal gasket in which the end portions of the jacket are not joined to each other and a metal gasket according to Embodiment 3 of the present invention. As shown in FIG. 4A, in the partially open double ring type metal gasket 20, the seal regions 50 as and 50 bs on the secondary lid 508 side are joined by the outer cover 50. For this reason, even if the outer cover 50 is creep-deformed with respect to the radial direction X of the metal gasket 20, this deformation is restrained. As a result, the creep deformation of the jacket is suppressed on the seal regions 50as and 50bs side, so that the stress relaxation of the metal gasket 20 is also suppressed accordingly.
[0114]
On the other hand, on the trunk body 501 side of the cask, the seal region 50as in this portion. ₂ And 50bs ₂ The side is not joined by the jacket 50. Therefore, when the outer cover 50 undergoes creep deformation with respect to the radial direction X of the metal gasket 20, there is nothing that restrains this deformation. For this reason, compared with the secondary lid 508 side, since the creep deformation of the jacket 50 becomes larger on the cask body 501 side, the stress relaxation of the metal gasket 20 is also increased accordingly.
[0115]
However, as shown in FIG. 17 (b), the metal gasket 23 according to the third embodiment of the present invention has two springs 33a and 33b with different hoop diameters Df covered by the inner sheath 43 arranged concentrically. These are covered with the jacket 50, and then the end portions 50t of the jacket 50 are joined together. For this reason, the sealing region at the position of the springs 33 a and 33 b is coupled by the outer cover 50 in both the sealing portions on the secondary lid 508 side and the trunk main body 501 side of the cask. Therefore, the seal region 50as or 50bs on the secondary lid 508 side or the cask trunk body 501 side. ₂ Also in the above, the creep deformation of the outer jacket 50 is restrained. As a result, since the stress relaxation phenomenon accompanying creep deformation is suppressed, the sealing performance can be maintained even during long-term storage, and safe transportation can be achieved.
[0116]
Further, since the end portions 50t of the outer jacket 50 are joined together by welding, friction joining, or other joining means, airtightness inside the metal gasket 23 can be secured. In general, for safety reasons, a cask stores a recycle fuel assembly in a storage pool filled with water. Accordingly, before the cask is pulled out of the pool and transported or stored for a long time, it is dried by a drying means such as vacuum drying after draining. At this time, the partially open double-ring type metal gasket 520 (see FIG. 7A) may cause corrosion if it is not sufficiently dried. Took a long time for vacuum drying. However, in this metal gasket 23, water does not enter the inside, so that the vacuum drying time can be greatly shortened and there is no risk of corrosion due to moisture remaining inside. Further, since the jacket 50 of the metal gasket 23 is high-purity iron, even if moisture remains in the jacket 50, it hardly corrodes. By these actions, higher reliability can be exhibited in long-term storage.
[0117]
Further, after providing a spacer 60 in advance between two springs 33a and 33b having different hoop diameters covered with a concentrically arranged inner sheath 43 as in a metal gasket 23 'shown in FIG. 17 (c). The end portions 50t of the jacket 50 may be joined together. In this way, since the jacket 50 is supported by the spacer 60, deformation of the jacket 50 can be suppressed, and the end portions 50t can be easily joined to each other.
[0118]
Here, as shown in FIG. 17 (c), the height h of the spacer 60 is set to the same height as the diameter of the inner jacket 43 in order to suppress deformation of the outer jacket 50 when receiving a tightening load. Also good. If it does in this way, since the deformation | transformation of the jacket 50 after clamping can be suppressed, the creep deformation | transformation of the jacket 50 can be suppressed more, and the reliability in long-term storage can be improved further. Here, the spacer 60 can be easily deformed with respect to the tightening force of the metal gasket 23, and a shape that does not require an unnecessarily high tightening force can be selected. Further, as described above, instead of the spacer 60, a spring having a slightly smaller outer diameter than the outer diameter such as the spring 33a can be used.
[0119]
(Embodiment 4)
FIG. 18 is an explanatory view showing a metal gasket according to Embodiment 4 of the present invention. The metal gasket 24 has substantially the same configuration as the cask according to the first embodiment, but two annular springs 31a and 31b having different hoop diameters are arranged concentrically, and the spring diameters of the two are different. Different points. Since other configurations are the same as those of the first embodiment, the description thereof is omitted and the same components are denoted by the same reference numerals.
[0120]
This metal gasket 21 is a so-called double ring type, but uses two springs 31a and 31b covered with inner jackets 41a and 41b and having different spring diameters. As shown in FIG. 18B, when the metal gasket 24 is disposed between the cask trunk body 501 and the secondary lid 508, the jacket 50 on the spring 31b side having the larger diameter first comes into contact. In this state, the secondary lid 508 is temporarily fixed to the trunk body 501.
[0121]
Here, in general, a cask has a recycle fuel assembly stored in a storage pool filled with water for safety reasons. Therefore, before transport and long-term storage, water is drained by a drying means such as vacuum drying. In this metal gasket 24, the moisture remaining in the gasket groove 9 evaporates from the side of the spring 31a having the smaller unsealed diameter during vacuum drying. When the vacuum drying is completed, the cask trunk body 501 and the secondary lid 508 are finally tightened. Thus, in this metal gasket 21, the moisture in the gasket groove 9 and the metal gasket 24 can be sufficiently removed, and the risk of corrosion can be minimized. Moreover, since the jacket 50 is high-purity iron, it is difficult to corrode. By these actions, the reliability of the sealing performance can be enhanced even during long-term storage.
[0122]
Here, in FIG. 18B, the spring 31a having the smaller spring diameter is arranged on the cavity side of the cask, but in this case, it is advantageous for vacuum drying from the inside of the cavity. Here, the diameter of the spring 31a is selected such that a contact pressure required for airtightness can be obtained when the lid fixing bolt is tightened with a predetermined torque. Moreover, as shown in FIG.18 (c), you may arrange | position the spring 31a with a smaller spring diameter outside. In this case, moisture can be effectively removed by vacuum drying from the bolt hole side that fixes the secondary lid 508 and the like.
[0123]
Further, in the metal gasket 24a shown in FIG. 18 (d), two springs 31a and 31b having different spring diameters are entirely covered with an outer cover 50, and a spacer 60 is disposed between the springs 31a and 31b. The ends 50t are sealed with a joining means such as welding. Even in such a metal gasket 24a, the effect of promoting the drying can be obtained by changing the diameter of the spring 30a and the like on the inner and outer circumferences of the metal gasket 21.
[0124]
Further, in this metal gasket 24a, water can be prevented from entering the inside, so that it is possible to obtain an effect of drying more quickly than the metal gasket 21 shown in FIG. Further, since the jacket 50 of the metal gasket 24a is high-purity iron, even if moisture remains in the jacket 50, it hardly corrodes, thereby improving the reliability in long-term storage. The spacer 60 restrains the deformation of the outer cover 50, and a piece such as pure aluminum or a spring can be used.
[0125]
【The invention's effect】
As described above, in the cask according to the present invention (Claim 1), a metal gasket using high-purity iron is used for the outer cover to seal the casing. Thereby, even in long-term storage for several decades, it is possible to suppress deterioration of the metal gasket and maintain high sealing performance. In addition, unlike gold and silver, the price of the material is low, so the manufacturing cost of the entire cask can be reduced.
[0126]
In the cask according to the present invention (Claim 2), a spring is used as a pressing member that presses the jacket. In this way, by devising the design, the outer cover can be uniformly pressed against the trunk main body and the lid of the cask and sufficient sealing performance can be exhibited.
[0127]
In the cask according to the present invention (Claim 3), a plate-like spring is used as a pressing member for pressing the outer cover. For this reason, since the diameter of the spring is reduced as a whole, an increase in the contact surface with the lid, the trunk body and the like due to deformation of the spring is suppressed. Thereby, the fall of the contact pressure per unit area can be prevented, and it can seal reliably.
[0128]
In the cask according to the present invention (Claim 4), the hardness of the outer casing of the metal gasket made of high-purity iron is made lower than that of the trunk body and the lid of the cask. For this reason, since the outer cover of the metal gasket is softer than the cask trunk body and the lid, the cask trunk body and the lid can be easily reused without damaging them.
[0129]
Moreover, in the cask according to the present invention (Claim 5), the hardness of the outer jacket of the metal gasket is 80 Hv or less. Thereby, since it fits well with the seal surface of the cask cylinder body and the lid, sufficient sealing performance can be exhibited while protecting the cask cylinder body and the lid.
[0130]
In the cask according to the present invention (Claim 6), the high-purity iron used for the outer jacket of the metal gasket is subjected to a heat treatment that is allowed to cool after being kept at a predetermined temperature. For this reason, since the hardness can be lowered by such heat treatment, for example, even a material that has been work-hardened by rolling to the shape of a jacket by rolling can exhibit excellent sealing performance by softening.
[0131]
Further, in the cask according to the present invention (Claim 7), a passive film is formed on the surface of the jacket by a heat treatment that is allowed to cool after being held at a predetermined temperature. In this way, since the passive film is formed on the surface of the outer cover, the corrosion resistance of the outer cover is improved, and sufficient sealing performance is ensured with minimal deterioration of the metal gasket even during long-term storage for several decades. Can be maintained.
[0132]
Further, in the cask according to the present invention (invention 8), the holding temperature of the heat treatment to be cooled after being held at a predetermined temperature is set to 300 ° C to 450 ° C. As a result, the hardness of the jacket can be sufficiently lowered and a passive film having a high corrosion resistance can be formed, so that the work can be carried out efficiently.
[0133]
In the cask according to the present invention (Claim 9), a metal gasket further provided with an inner cover covering the spring is used between the spring and the outer cover. For this reason, the pressing force of the spring is uniformly transmitted to the jacket by the inner jacket, so that local stress concentration of the outer jacket can be dispersed. As a result, the stress relaxation phenomenon resulting from the creep deformation of the jacket can be suppressed, and the reliability in long-term storage can be improved.
[0134]
In the metal gasket according to the present invention (Claim 10), the jacket covering the pressing member is made of high-purity iron. For this reason, even if it is used at a temperature of about 120 ° C., for example, the creep deformation of the jacket can be almost ignored, and even during long-term storage for several decades, deterioration of the metal gasket can be suppressed and high sealing performance can be maintained. In addition, since high-purity iron is relatively cheaper than gold or silver, a metal gasket can be provided at a relatively low cost.
[0135]
Further, in the metal gasket according to the present invention (invention 11), since the individual springs are integrated with each other, even if the outer cover undergoes creep deformation during long-term storage, the two springs are subjected to this creep deformation. To regulate. As a result, the stress relaxation phenomenon of the metal gasket caused by creep deformation of the outer cover can be further suppressed, and higher sealing reliability can be ensured.
[0136]
Further, in the metal gasket according to the present invention (Claim 12), the annular springs having different hoop diameters are arranged concentrically, and these are covered with the jacket, and then the ends of the jacket are joined together. For this reason, for example, when this metal gasket is used for sealing the cask, the two springs can regulate the creep deformation of the outer cover in both the sealing area on the lid side and the trunk body side. As a result, the stress relaxation phenomenon of the metal gasket caused by creep deformation of the outer cover can be further suppressed, and higher safety can be ensured in long-term storage. Further, by joining the end portions of the jacket, a completely sealed structure can be obtained, so that moisture does not enter the metal gasket. As a result, it is excellent in corrosion resistance and can be highly reliable in long-term storage.
[0137]
Further, in the metal gasket according to the present invention (Claim 13), two annular springs having different hoop diameters are arranged concentrically, and the spring diameters of both are made different. When this metal gasket is used for sealing a cask, for example, the gasket ring having the larger diameter comes into contact with the trunk main body and the lid of the cask, so that the water adhering to the metal gasket can be easily evaporated. As a result, the moisture in the metal gasket can be sufficiently removed, so that the risk of corrosion of the metal gasket can be minimized, and the reliability of the sealing performance can be increased even during long-term storage.
[0138]
In the metal gasket according to the present invention (Claim 14), a spring is used as a pressing member for pressing the outer cover. In this way, by devising the design, the outer cover can be uniformly pressed against the object to be sealed, and sufficient sealing performance can be exhibited.
[0139]
In the metal gasket according to the present invention (Claim 15), a plate-like spring is used as a pressing member for pressing the jacket. For this reason, since the diameter of the spring is reduced as a whole, an increase in the contact surface with the object to be sealed due to deformation of the spring is suppressed. Thereby, the fall of the contact pressure per unit area can be prevented, and it can seal reliably.
[0140]
In the metal gasket according to the present invention (Claim 16), the hardness of the jacket is made lower than that of the object to be sealed. For this reason, since the jacket of the metal gasket is softer than the sealed object, the sealed object can be easily reused without damaging the sealed object.
[0141]
Further, in the metal gasket according to the present invention (Claim 17), since the hardness of the jacket is 80 Hv or less, it is compatible with the sealing surface of the object to be sealed, so that sufficient sealing performance is exhibited while protecting the object to be sealed. it can.
[0142]
In the metal gasket according to the present invention (Claim 18), the outer jacket surface is covered with a film that improves resistance to corrosion. For this reason, since the jacket of this metal gasket becomes difficult to corrode, it is possible to improve the reliability by suppressing the deterioration of the sealing performance due to the corrosion over a long period of time. In particular, when the recycled fuel assembly is used for sealing a cask that is stored for a long period of several decades, the reliability of the cask can be improved.
[0143]
In the metal gasket according to the present invention (claim 19), a high purity iron constituting the outer sheath is treated with an acid solution to form a film that improves resistance to corrosion. As described above, according to the acid solution treatment, such a corrosion-resistant film can be formed with relatively simple equipment, so that the manufacturing cost of the metal gasket can be suppressed.
[0144]
In the metal gasket according to the present invention (invention 20), the high-purity iron constituting the outer cover is subjected to a heat treatment for cooling after being kept at a predetermined temperature for a predetermined time. For this reason, since the hardness of the jacket can be lowered by such a heat treatment, an excellent sealing performance can be exhibited by softening even a material that is work-hardened by extending to the shape of the jacket by rolling, for example.
[0145]
Further, in the metal gasket according to the present invention (Claim 21), since the passive film is formed on the surface of the outer cover, the corrosion resistance of the outer cover is improved, and the deterioration of the metal gasket is minimized even after use for several decades. Limited sealing performance can be maintained.
[0146]
Further, in the metal gasket according to the present invention (Claim 22), the heat treatment temperature of the high purity iron is set to 300 ° C. to 450 ° C., so that the hardness of the jacket can be sufficiently reduced, and the corrosion resistance is high. A high passive film can be formed.
[0147]
Further, in the metal gasket according to the present invention (Claim 23), the lid and the trunk main body, which are objects to be sealed, and the seal region of the metal gasket jacket are in contact with each other in a plane. The distribution can be made more uniform, and the progress of creep deformation of the jacket can be suppressed. As a result, the stress relaxation phenomenon of the metal gasket can be suppressed even during long-term storage. In addition, by suppressing the stress relaxation phenomenon, it is possible to increase the spring back of the metal gasket, so that when this metal gasket is used for the cask, the tolerance for the displacement between the lid and the trunk body can be increased. . As a result, higher safety can be ensured even in long-term storage for several decades.
[0148]
The metal gasket according to the present invention (Claim 24) further includes an inner cover covering the pressing member between the pressing member and the outer cover. For this reason, since the pressing force of the pressing member is evenly transmitted to the outer cover by the inner cover, the local stress concentration of the outer cover can be dispersed. As a result, the stress relaxation phenomenon resulting from the creep deformation of the jacket can be suppressed, and the reliability in long-term storage can be improved.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram showing a cask sealing structure according to Embodiment 1 of the present invention;
FIG. 2 is a cross-sectional view showing a metal gasket according to Embodiment 1 of the present invention.
FIG. 3 is an explanatory diagram showing the relationship between the stress acting on the metal gasket and the distortion of the metal gasket.
FIG. 4 is an explanatory diagram showing the relationship between the purity and hardness of high-purity iron.
FIG. 5 is an explanatory view showing a spring of the metal gasket according to the first embodiment of the present invention.
FIG. 6 is an explanatory diagram showing the relationship between the load applied to the metal gasket and the displacement of the metal gasket.
FIG. 7 is an explanatory view showing a metal gasket mounting structure according to the present invention.
FIG. 8 is an explanatory view showing a metal gasket mounting structure according to the present invention.
FIG. 9 is an explanatory view showing a metal gasket and a sealing structure according to a first modification of the first embodiment.
FIG. 10 is an explanatory diagram showing a seal structure in a cask according to a second modification of the first embodiment.
FIG. 11 is an explanatory diagram showing a seal structure in a cask according to a third modification of the first embodiment.
12 is an explanatory view showing a manufacturing method of the spring shown in FIG. 11. FIG.
13 is an explanatory view showing a sealed state by the metal gasket of FIG. 11. FIG.
FIG. 14 is a sectional view showing a metal gasket according to a second embodiment of the present invention.
FIG. 15 is a cross-sectional view showing a conventional metal gasket.
FIG. 16 is an explanatory view showing a sealing structure in a cask according to Embodiment 3 of the present invention.
FIG. 17 is an explanatory view showing a partially open double-ring type metal gasket in which the ends of the jacket are not joined to each other and a metal gasket according to Embodiment 3 of the present invention.
FIG. 18 is an explanatory view showing a metal gasket according to a fourth embodiment of the present invention.
FIG. 19 is a cross-sectional view showing the structure of a cask.
20 is a partially enlarged view of the cask shown in FIG.
FIG. 21 is an enlarged view showing a metal gasket used for sealing a cask and a seal portion of the cask.
[Explanation of symbols]
10, 11, 12, 13 Seal structure
20, 20 ′, 20 ″, 20 ′ ″, 21, 22, 22a, 22b, 23, 24, 24a Metal gasket
30, 30a, 30b, 31a, 31b, 32, 33a, 36, 36a, 36b, 521 Spring
34a, 34a ', 34b, plate-wound spring
35, 35a, 35b, 40, 40a, 43, 44a, 44b, 522
50, 51, 52, 52a, 52b, 53, 53a, 53b
70, 71, 72, 73 Inner ring
80, 81, 82, 83 Outer ring
500 casks
501 trunk
502 outer cylinder
503, 504, 512 Neutron shielding material
505 Bottom plate
506 Flange
507 Primary lid
508 Secondary lid
509 cavity

Claims (24)

A trunk body that shields radiation, in which a cavity for housing the recycled fuel assembly is formed;
A lid attached to the cavity opening side of the trunk body;
A high purity iron jacket having a purity of 99.9% or more, and a pressing member that covers the jacket and presses the jacket in the direction of the sealing surface, and is between the trunk body and the lid A metal gasket that is interposed in between to maintain a seal in the cavity;
A cask characterized by comprising:   The cask according to claim 1, wherein the pressing member is a spring.   The cask according to claim 1, wherein the pressing member is a plate-like spring.   The cask according to any one of claims 1 to 3, wherein a hardness of the jacket is lower than that of the trunk body and the lid.   The cask according to any one of claims 1 to 4, wherein the outer shell has a hardness of 80 Hv or less. 6. The high purity iron having a purity of 99.9% or more constituting the outer cover is subjected to a heat treatment for allowing to cool after being kept at a predetermined temperature for a predetermined time. The cask according to item 1.   The cask according to claim 6, wherein a passive film is formed on the surface of the jacket by the heat treatment.   The cask according to claim 6 or 7, wherein a temperature of the heat treatment is 300 ° C to 450 ° C.   The cask according to claim 1, further comprising an inner cover that covers the pressing member between the pressing member and the outer cover. An annular jacket made of high-purity iron having a purity of 99.9% or more and in contact with the object to be sealed to maintain the sealing;
An annular pressing member that is covered with the outer cover and presses the outer cover toward the sealing surface;
A metal gasket characterized by comprising: An annular jacket made of high-purity iron having a purity of 99.9% or more and in contact with the object to be sealed to maintain the sealing;
An annular pressing member that presses the outer cover in the direction of the sealing surface;
The two pressing members having different hoop diameters are arranged concentrically to form a double ring, and each annular pressing member is covered with the outer cover and integrated. Metal gasket.   The metal gasket according to claim 11, wherein the end portions of the jacket are joined to form an elliptical cross section.   The metal gasket according to claim 11 or 12, wherein one of the two annular pressing members has a smaller diameter than the other.   The metal gasket according to claim 10, wherein the pressing member is a spring.   The metal gasket according to any one of claims 10 to 13, wherein the pressing member is a plate-like spring.   The metal gasket according to any one of claims 10 to 15, wherein a hardness of the jacket is lower than that of the sealing object.   The metal gasket according to any one of claims 10 to 16, wherein the outer shell has a hardness of 80 Hv or less.   The metal gasket according to any one of claims 10 to 17, wherein the outer jacket surface is covered with a film that improves resistance to corrosion. 19. The metal gasket according to claim 18, wherein high purity iron having a purity of 99.9% or more constituting the jacket is treated with an acid solution. The high-purity iron having a purity of 99.9% or more constituting the outer jacket is subjected to a heat treatment for allowing to cool after being held at a predetermined temperature for a predetermined time. The metal gasket according to item 1.   21. The metal gasket according to claim 20, wherein a passive film is formed on the surface of the jacket by the heat treatment.   The metal gasket according to claim 21, wherein the temperature of the heat treatment is 300 ° C to 450 ° C.   The metal gasket according to any one of claims 10 to 22, wherein a sealing region of the jacket that comes into contact with the object to be sealed is formed in advance on a flat surface.   The metal gasket according to any one of claims 10 to 23, further comprising an inner jacket that covers the pressing member between the pressing member and the outer jacket.

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