JP3763035B2 - Method and apparatus for controlling oxidative combustion of graphite that may be radioactively contaminated by being used in a nuclear reactor, etc. - Google Patents

Description

【００２６】
前記特許第3051859号の発明時には、これら不純物重金属に触媒作用があることは解明されていなかったため、原子炉材料用黒鉛をそのまま実験試料として使用し、その空気中での顕著燃焼温度は６８０℃であった。
【００２７】
しかしながら、本発明は上記不純物重金属の作用を確かめながら行うものであるので、本発明では黒鉛試料には上記不純物が含まれない純炭素片（以下、単に「黒鉛片」という。）を用いることにした。
この黒鉛片の空気中での顕著燃焼温度を事前に測定したところ、８５０℃であった（ＴＧ−ＤＴＡを用いて１０ｍｇサンプルで測定。ＴＧ−ＤＴＡについては以下に詳述。）。
【００２８】
黒鉛片に添加した元素化合物を表２に示す。該化合物の水溶液を予め秤量した黒鉛片に滴下し、付着させてから乾燥させる。表２中の「化合物添加量」は、先に秤量した黒鉛片の乾燥試料重量からの増加量から求められる。
又、同表２中の「黒鉛への元素添加率」は、黒鉛量に対して化学計算によって表した元素値での百分率である。
【００２９】
上記のようにして作成した黒鉛サンプルを熱分析装置ＴＧ−ＤＴＡ（マックスサイエンス社製）にかけ、常温から２０℃／分の昇温速度で温度上昇させた。
本実験(I)は、触媒特性を顕著に示す元素の絞り込みを目的とし、燃焼空間は先ず空気中で行った。
【００３０】
【表２】

【００３１】
又、ＴＧ−ＤＴＡチャートを図１に示す。ＴＧ−ＤＴＡチャートは、時間に対して３つの曲線が得られ、温度を示す曲線はほぼ右上がりの直線である。又、ＤＴＡは示差熱曲線で、サンプルの吸発熱を示す。更に又、ＴＧは熱天秤であり、サンプルの重量の変化を連続的に表示する。
【００３２】
図１から、黒鉛の酸化燃焼は、燃焼開始温度近辺になるとＤＴＡが急激に立ち上がり、発熱のピークを示す。そして、燃焼を終了すると、ＤＴＡは平坦に戻る。
一方、ＴＧは燃焼開始と共に、急激に右下がりを示し、燃焼が終わるまで下がり続け、以降平坦となる。このことは、燃焼によってサンプルの重量が減少していく過程を示している。
以上の様にして得られたＴＧ−ＤＴＡチャートに、日本工業規格（例えばＪＩＳ Ｈ７１０１）に定める熱分析の方法に従って、チャートの曲線上に接線を引き、該両接線の交点Ｐ₁，Ｐ₂から燃焼開始点及び燃焼終了点の温度と時間を求める。
ここで、温度を求めるには、前記Ｐ₁，Ｐ₂から温度軸に平行に該温度曲線に対して線を下ろして、その交点Ｔ₁，Ｔ₂から各温度を読み取る。
【００３３】
実験結果を表２にまとめて示す。試みた３９元素の順番は実験順序に従っており、実際に黒鉛片に添加した化合物を分子式で示した。
又、黒鉛片になんら塩類を添加しないで塩類の代わりに蒸留水を滴下して乾燥させたブランク試料は、その燃焼開始温度が平均８５０℃であり、この８５０℃を基準温度として各３９種のサンプルデータとの差を「燃焼開始温度偏移」として示した。
【００３４】
表２から、黒鉛の燃焼開始温度に影響を及ぼす元素を、次の様に分類した。
ア）燃焼開始温度を下げる元素。効果の大きい順に、
Ｐｂ，Ｂｉ，Ｒｂ，Ｃｓ，Ｐｄ，Ｋ，Ｖ，Ａｇ，Ｎａ，Ｃｒ，Ｃｅ，Ｍｏ，Ｌｉ，Ｃｏ，Ｃｕ，Ｓｒ，Ｒｈ，Ｃｄ，Ｆｅ，Ｗ，Ｃａ，Ｓｂ
イ）燃焼開始温度を上げる元素。効果の大きい順に、
Ｐ，Ａｌ，Ｍｇ，Ｂ，Ｎｂ，Ｍｎ
ウ）燃焼開始温度への影響が顕著でない元素、即ち燃焼開始温度が８５０℃±５０℃より大きな効果を示さなかった元素は、
Ｌａ，Ｚｎ，Ｓｎ，Ｙ，Ｈｇ，Ｂａ，Ｉｎ，Ｇａ，Ｚｒ，Ｔｉ，Ｎｉ
以上の結果から、今回実験していない前記Ｒｕ，Ｏｓ，Ｉｒ，Ｐｔ，Ａｕ等の白金族元素、Ｐｒ，Ｎｄ，Ｐｍ，Ｓｍ，Ｅｕ，Ｇｄ，Ｔｂ，Ｄｙ，Ｈｏ，Ｅｒ，Ｔｍ，Ｌｕ，等のレアアース元素は、黒鉛の燃焼開始温度を低下させる作用を有すると推測される。
【００３５】
(II)化合物の添加量及び複合添加の効果実験
次に、発明者は、化合物の添加量及び複合添加の効果について、実験した。
化合物としては、前記表２中から燃焼開始温度に顕著な効果を示した元素の中からカリウム（アルカリ金属）と鉛（重金属）を選んだ。
そして、添加した化合物はそれぞれ硝酸塩で、前記実験(I)の手順と同様に、黒鉛試料に水溶液として付着・乾燥後燃焼させ、空気中燃焼での触媒効果を調べた。
【００３６】
実験結果を表３に示す。
【００３７】
【表３】

【００３８】
表３において、蒸留水を添加乾燥させた試料の結果が上段であり、燃焼開始温度は（実験(I)と同様に）８５０℃であった。
これに対して、化合物を添加した試料では、Ｋ、Ｐｂ単独でも複合して添加しても、燃焼開始温度が下がった。
又、添加量に関して述べると、添加量が一定値を越えるとかえって燃焼開始温度は上昇しており、いずれも添加量１％以下程度のものが触媒効果が高いようである。
【００３９】
従って、原子炉用黒鉛を酸化燃焼させるためには、Ｋ、Ｐｂなどの元素を多量に添加する必要はなく、しかもこれらの化合物は単独でも複合でも黒鉛重量に対して１％以下程度の添加量で最適な触媒効果が得られることがわかった。
【００４０】
(III)高濃度酸素中での燃焼における添加化合物の効果実験
黒鉛の燃焼開始温度は、高濃度酸素中の方が空気中より低下し、燃焼速度も速く、ブロック形状のまま酸化燃焼が可能であることは、上記特許第3051859号より明らかである。
本発明では、純炭素片を用いて化合物を添加した試料を作成し、高濃度酸素中で燃焼させて空気中での燃焼と比較した。
実験の手法は前記実験(I),(II)と同様であり、結果を表４に示す。
【００４１】
【表４】

【００４２】
表３と表４を比較することにより、化合物を添加した場合においても、特許第3051859号で述べたように、空気中で燃焼させた場合（表３）より高濃度酸素中で燃焼させた場合（表４）の方が、燃焼開始温度が低下することが確かめられた。
【００４３】
(IV)燃焼温度と燃焼速度との関係
表４において、黒鉛が燃焼し切るに要した時間から算出される「燃焼速度」に注目すると、化合物無添加のものに比べて、化合物を添加したものはその燃焼速度が減少していることがわかる。換言すれば、化合物を添加すると、無添加のものよりも黒鉛を燃焼し尽くすのに長時間を要することになる。
この点を解明するため、発明者はＰｂを添加した場合におけるＴＧ−ＤＴＡチャートを実験(I)と同様の手法で入手した（図２）。
【００４４】
図２より、鉛を添加した黒鉛の酸化反応は、ＤＴＡにいくつかの発熱反応ピークが見られることがわかる。
このことは、鉛が黒鉛酸化反応の途中で還元炭化され、触媒作用を失うが、再び酸化されて燃焼触媒となるため、と考えられる。
【００４５】
ここで、発明者は、ＰｂとＫとを複合添加した際の結果に着目し、黒鉛の燃焼途中でＰｂが触媒失効するのを防止するのに役立つことがわかった。
前記表４において、実験Ｎｏ．４５に示す如くＰｂを０．９７％、Ｋを０．８０％添加した際には、温度低下にして燃焼開始温度は２２８℃、燃焼終了温度は１６４℃低下したにも拘わらず、その燃焼速度4.20mg/minは化合物無添加の場合のそれ4.31mg/minとほとんど変わらない程度速いのである。
これは、「化学反応速度は温度の上昇につれて急上昇する」という温度依存性の常識から考えて、全く驚くべき結果である。
【００４６】
即ち、ＰｂとＫとを複合した触媒を用いると、先ずＰｂの反応開始温度低下効果が発揮され、無触媒のものに比べて２２８℃低い５５２℃で黒鉛は酸化しはじめる。そして、反応途中ではＫによって触媒失効を防がれて、酸化反応は順調に進行し、実験Ｎｏ．４０の無触媒の燃焼終了温度（８１６℃）に比べてそれより１６４℃低い６５２℃で酸化反応は終了し、黒鉛は燃焼し尽くされる。更に、その燃焼速度は、同実験Ｎｏ．４０の無触媒の場合のそれ（4.31mg/min）とほとんど変わらない4.20mg/minであった。
【００４７】
又、表４中で実験Ｎｏ．４５とＮｏ．４６とを比較することにより、触媒の添加量には（空気中での燃焼と同様に）触媒適正添加量があることがわかる。触媒の添加率が１％を越えたＮｏ．４６は、Ｎｏ．４５に比べて燃焼開始温度・燃焼終了温度・燃焼速度のすべての点で触媒効果が低減した。
【００４８】
以上の事実から考察すると、温度低下に効果のある重金属は、同時に温度低下に効果のあるアルカリ金属と複合すると、燃焼開始・燃焼終了・燃焼速度に制御を加えることができると考えられる。
また、Ｒｂ，Ｃｓ，Ｎａ，Ｌｉなどのアルカリ金属をＫに替えて添加したり、Ｂｉ，Ｐｄ，Ｖ，Ａｇ，Ｃｒ，Ｃｅ，Ｍｏ，Ｃｕ，Ｓｒ，Ｒｈ，Ｃｄ，Ｆｅ，Ｗ，Ｃａ，Ｓｂ，白金族元素，レアアース元素等をＰｂに替えて添加できる。
更に、上記元素は、単独のみならず各元素の持つ特徴から２種類以上を組み合わせて添加することも有効であると推察される。
【００４９】
【実施例】
本発明の実施例を図３及び図４を用いて説明する。
原子炉で使用された黒鉛１は、本発明で酸化燃焼に付される材料である。詳細な図示は省略するが、断面がほぼ正六角形の柱状体で、中心に燃料体を挿入する円型の穴を有している。原子炉内では各種の長さ寸法の六角柱を組み合わせて大きなブロック減速体を形成するため、該六角柱には夫々長手方向に線状の凹凸が設けられている。又、個々の構成ブロックの寸法は、対角線長さ約２４ｃｍ、長さ約４０〜８５ｃｍ、重量約３５〜７０ｋｇである。
【００５０】
原子炉ではこの他に、燃料のスリーブを構成したり、特別の原子炉機能のための黒鉛体が用いられているが、それらの黒鉛体を燃焼処理を目的として特別に粉砕したもの以外の黒鉛であれば、例え多少破壊されていても、本発明の方法においてはブロック状黒鉛と一括呼称する。
又、破片状の黒鉛で、ブロック単体としては取扱いが不便なものは、燃焼に支障のないプラスチック、紙、又は、木製などの包装材を使って一体化した形状の物であっても、該ブロック状黒鉛の概念に含むものである。
【００５１】
黒鉛ブロック１は、触媒溶液塗布装置２まで移動され、ここで予め触媒を溶解・濃度調整する触媒溶液溶解貯蔵装置３により、触媒溶液２ａを添加される。
該触媒溶液２ａの塗布方法は種々選択可能であるが、その一例として、図示しないが触媒溶液２ａに黒鉛ブロック１を浸したり、刷毛で塗ったり、或いは図４に示す如く噴霧する方法が考えられる。その中でも噴霧する法は、触媒溶液２ａの量的判断が容易である点で、最も簡便な方法である。
又、スプレーは、ポンプ圧力方式以外にも空気圧利用方式でもよいことは言うまでもない。
【００５２】
更に又、添加元素を複合して用いる場合には、予め複合した溶液で添加しても、順次別々に添加しても、本発明を構成する上で特段の差違はない。
【００５３】
そして、界面活性剤は、前記貯蔵装置３の触媒溶液２ａ中に攪拌手段３ａで混合しても、又は予め触媒の添加に先立って添加塗布してもよい。
【００５４】
次に、触媒溶液２ａを塗布された黒鉛ブロック１は、触媒溶液中の溶媒を除くため、触媒乾燥装置４に送られて乾燥される。
この乾燥方法は、自然乾燥させても、別途乾燥させてもよく、種々の選択が可能である。
又、燃焼炉が作動を始めると、系全体としては熱過剰となるので、回収熱を利用することも考えられる。
【００５５】
特段の乾燥手段を用いないで、溶媒中の水分が多量でなければ、炉内の熱で該水分は乾燥・蒸発するため問題はないと考えられるが、万一乾燥の途中で触媒溶液２ａが垂れ集まり、結果的に触媒が偏在する恐れがあるので、注意が必要である。
但し、該触媒乾燥装置４の有無は、本発明を構成する上で必ずしも必要とされる条件ではない。
【００５６】
図３及び図４に示した触媒乾燥装置４には、乾燥用熱源５から高温の気体が送られている。
又、該乾燥装置が電気加熱方式の場合であれば、該熱源５からは電力の供給が行われる。
この熱源５は、燃焼炉が稼動してから後には、化学反応熱を直接又は間接に熱交換して回収利用することもできる。
又、該乾燥装置４で溶媒中の水分を分離させて生じた湿りガス６ａは、乾燥排気装置６により、排気される。この湿りガス６ａは、前記熱源５に放射能を持たない限り、通常放射性物質を含んでおらず、外部に排気できる。
【００５７】
この様にして、触媒を添加された黒鉛ブロック１は、その後機密型黒鉛投入装置７から燃焼炉８へ送られる。
該燃焼炉８には、高濃度酸素発生装置１０から酸素１０ａが送気され、黒鉛ブロック１を燃焼させる。
この高濃度酸素発生装置１０として、モレキュラシーブを利用した圧力スイング吸着法（ＰＳＡ）によるものや、選択透過膜を使ったものや、或いは液体空気からの分離によるものなどが用いられる。
【００５８】
又、酸化燃焼開始に際しては、前述した様に、炉内の黒鉛ブロック１の一部は燃焼開始温度まで昇温させなければならないため、助燃用燃料９を炉内で燃焼させ、昇温する。
１１は、残灰排出蓋である。
【００５９】
燃焼炉８での燃焼で得られたガスは、温度を低下させる目的で、ガス冷却器１２へ送られる。
該ガス冷却器１２は、事実上ボイラーであり、冷却用水１３が燃焼ガスと伝熱チューブを通して伝熱され、該ガスは温度が下げられ、水は水蒸気又は温水１４になる。
温度の下げられた燃焼ガスは、その後の集塵、ガス処理機１５で容易に処理される。
このガス処理機１５は、これらのガス中の飛散灰の清浄化、気体不純物の減少の目的で使われる一式の装置を集合的に表現したものである。
【００６０】
また、燃焼炉を含めて、全体の系を負圧で操作し、各設備及び連結部分の圧力損失を補うために、ファン１６がある。これは、同一の目的のために集塵、ガス処理装置の前などに設置することが可能である。
ファン１６を出た燃焼ガス１７は、それぞれの場合によってプロセス処理される。
該燃焼ガス１７は、一酸化炭素、炭酸ガス以外にも多少の窒素、アルゴンなどを含むので、適宜必要により、分離しなければならない。また、環境的条件が許されるならば、直接大気へ放出させたり、海洋注入される場合の可能性もありうる。
【００６１】
【発明の効果】
本発明は以上の様に構成したので、原子炉用黒鉛などの放射能汚染された黒鉛（以下原子炉用黒鉛という）をブロック形状のまま、酸化開始温度・酸化速度を制御することが可能である。この制御の利点を以下に述べる。
ｉ）酸化開始温度を下げる場合の有効性
原子炉用黒鉛をブロック形状のままで燃焼させるに際に、前述したように、予備加熱が行われる。この段階で、低い温度で酸化が始まると、酸化設備の運転開始が容易となる。
部分的にでも酸化燃焼が開始されれば、その後燃焼が燃焼炉全体に及び、広範囲に酸化燃焼を起こすことができるようになる。
【００６２】
また、酸化反応温度が低いと、酸化時に発生する一酸化炭素の比率が下がり、炭酸ガスの組成濃度が上昇する。そこで、発生ガスの炭酸ガス濃度を高くしたいときには、燃焼温度を下げることが有効となる。そして、触媒を複合して添加することで、燃焼温度を下げすぎた際に生じる酸化反応の部分的停止を防止することができ、安定して酸化を継続させることが可能である。
【００６３】
更に、燃焼温度が下がると、酸化反応炉の耐熱設計が大変楽になり、安価で取り扱いの容易な耐火材の使用が可能となる。水冷金属壁を計画することも可能である。
炉内の温度が低いと、燃焼灰分の溶融の心配がなく、炉体や耐火材の損耗を防止し、熱交換器のパイプに腐食性の溶融灰分が固着することがないので、熱交換のパイプなどの金属材料の高温腐食を防止できる。ひいては、燃焼炉、ガス冷却器の運転、保守が容易となる。燃焼残灰の取り扱いも融解固着しないので、簡便である。
又、燃焼後の工程で得られたガスを圧縮液化する場合には、有害な一酸化炭素をアフターバーナーで燃焼させ、炭酸ガスに転換する必要があり、この工程を省略でき、工程を簡素化できる。
【００６４】
ｉｉ）酸化開始温度を上げる場合の有効性
黒鉛の酸化反応は、高温で行うほど一酸化炭素の発生比率が高くなる。後工程で放射性同位元素である炭素１４の分離濃縮を考える場合には、分離工程に供給する気体として一酸化炭素の方が、二酸化炭素よりも酸素の比率が少ない分、分子重量での炭素含有比が大きくなるため、同位体分離にどの様な方法を用いるにしても、一酸化炭素の方が同位体炭素の分離効率上有利である。
【００６５】
又、高温酸化を行うにしても、黒鉛燃焼炉の運転開始などにあたって、部分的に低温で燃焼開始をさせ、その後燃焼温度を上昇させて操作するために、炉内の黒鉛の初期着火を行う一部には低温燃焼触媒を、そのほかの部分の黒鉛には高温燃焼触媒を付着させるという組み合わせも操業上有効である。
【００６６】
以上より、本発明によれば、原子炉で使用された放射能を有する黒鉛の酸化方法として、触媒技術の発明により、燃焼炉の設計構造・燃焼炉の作動条件・燃焼炉と付帯設備の保守・発生ガスの組成と後工程等を総合評価して燃焼開始温度・燃焼速度を幅広い範囲で選択できる。
【図面の簡単な説明】
【図１】本発明の黒鉛の燃焼実験における燃焼開始温度・燃焼終了温度を測定するＴＧ−ＤＴＡチャートである。
【図２】本発明の実施の形態を示す図で、鉛触媒を使用した黒鉛の酸化反応を示すＴＧ−ＤＴＡチャートである。
【図３】本発明の実施例を示すチャート図である。
【図４】本発明の実施例を示す縦断面図である。
【符号の説明】
１ 黒鉛
２ 触媒溶液塗布装置
２ａ 触媒溶液
３ 触媒溶液溶解貯蔵装置
３ａ 触媒溶液攪拌機
４ 触媒乾燥装置
５ 乾燥用熱源
６ 乾燥排気装置
６ａ 湿りガス
７ 機密型黒鉛投入装置
８ 燃焼炉
９ 助燃用燃料
１０ 高濃度酸素発生装置
１０ａ 酸素
１１ 残灰排出蓋
１２ ガス冷却器
１３ 冷却水
１４ 温水
１５ ガス処理機
１６ ファン
１７ 燃焼ガス[0001]
BACKGROUND OF THE INVENTION
In the present invention, when the nuclear reactor is decommissioned, graphite used as a moderator or fuel body in the core of the reactor, unnecessary when changing the fuel, etc., there is a possibility of activation by neutron irradiation. Control method for oxidative combustion of graphite, nuclear technology, experiment, measurement of radioactive materials, graphite contaminated in storage containers, suspected graphite, and unused graphite suspected of contamination And its equipment(Here, graphite that is suspected of being contaminated is measured at a radiation handling facility or a nuclear facility, so it can not be proved that it is completely free of contamination, or the activated part is separated and divided. (This means graphite that is treated in the same way as graphite that is clearly contaminated because it is difficult to guarantee and specify that it has no radioactivity.)Further, the present invention relates to a method and apparatus for controlling the oxidation combustion start temperature and oxidation rate by adding a catalyst to the graphite when the graphite is subjected to oxidation combustion treatment.
[0002]
[Prior art]
The core of a graphite moderation nuclear reactor that uses graphite as a moderator is assembled by stacking so-called molded blocks formed by molding graphite into a block shape.
This graphite block is activated because it is irradiated with a large amount of neutrons emitted from the nuclear fuel body for a long time during operation of the nuclear reactor. Therefore, it is important to separate the radioactive material contained in the graphite when disassembling and reducing the volume of the graphite slow reactor.
[0003]
As a treatment method, a method of burning spent graphite to convert carbon into a fluid such as carbon monoxide or carbon dioxide gas and separating the radioactive metal oxide as a residue can be considered.
However, the graphite block molded for the core of a graphite moderation nuclear reactor has been subjected to high-purity processing, has very few impurities, and is dense and dense, and therefore is flame retardant in air. Specifically, the graphite block that has been subjected to the high purity treatment is dense and has a small surface area, so that the combustion surface area per volume is insufficient, and the reaction heat can maintain the combustion temperature against the heat dissipation. Because it is difficult, combustion does not continue.
Since the flame retardance of graphite blocks cannot be solved in this way,Of graphite as it isThere are many examples of neglected in the world.
[0004]
Although it has already been proved through experiments that graphite blocks can be oxidized by external heating in an electric furnace or direct current heating, there are still problems with heat retention and secondary contamination of equipment.
[0005]
Further, a method and apparatus for pulverizing graphite blocks in advance to increase the oxidation reaction area and combusting in a fluidized bed or the like can be considered.
However, in this case, the coarse particles are deposited in the combustion zone and do not burn, and the fine powder may be scattered unburned, resulting in a low overall combustion rate. There is currently no grinding technology for graphite that does not produce fine powder, it takes time and effort to classify and remove fine powder, reprocessing fine powder after classification, and secondary contamination of equipment required for the treatment, and problems Pile up.
[0006]
In view of the above, the present inventor, in order to solve the flame retardancy of the block without pulverizing the graphite block and to perform the combustion treatment, the oxygen concentration of the graphite block is reduced. It was invented to overcome the flame retardancy of the graphite block and continue combustion by supplying high air (Japanese Patent No. 305189).
[0007]
[Problems to be solved by the invention]
Usually, graphite burns to carbon dioxide or carbon monoxide as described below.
C + O₂→ CO₂
2C + O₂→ 2CO
Further, in order for the graphite block and oxygen to react and burn, it is necessary to preheat to a predetermined “reaction start temperature” and maintain the combustion temperature even during combustion.
[0008]
In general, the rate of carbon monoxide generation increases as the oxidation reaction of graphite is performed at a higher temperature. On the other hand, if the oxidation reaction temperature is low, the ratio of carbon monoxide generated during oxidation decreases and the generation ratio of carbon dioxide gas increases.
Therefore, if the degree of freedom for controlling the reaction start temperature is obtained, the gas obtained by the oxidation reaction can be selected.
[0009]
However, in the method for treating graphite according to Patent No. 3051859, although the reaction start temperature in high-concentration oxygen is lower than that in air, the reaction start temperature has not been controlled.
[0010]
In view of the above circumstances, an object of the present invention is to control the oxidation reaction temperature in the oxidation treatment of graphite used in a nuclear reactor.
[0011]
[Means for Solving the Problems]
  The graphite oxidation combustion control method and the control apparatus thereof, which may be radioactively contaminated by being used in the nuclear reactor in the present invention, or the like, may be used to convert a used graphite block into a combustion furnace. In the treatment method of graphite blocks of the same shape as used in a nuclear reactor where oxygen is supplied from a high-concentration oxygen generator and burned into the combustion furnace, the graphite block of that shape is burned A catalyst element compound for controlling the oxidation reaction start temperature and / or the oxidation reaction continuation temperature is applied to the surface of the graphite block before being put into the furnace with a catalyst solution coating apparatus, and the catalyst element compound is applied. The graphite block is charged into the combustion furnace with an airtight graphite charging device and oxygen is supplied from the high-concentration oxygen generator to burn it.At that timeOxidation reaction start temperature and / or oxidation reaction continuation temperatureDepending on the identification of the catalyst element compound and its addition amountIs to control.
[0012]
Further, in the oxidation method of graphite used in the nuclear reactor, an alkali metal for lowering the oxidation reaction start temperature and / or the oxidation reaction continuation temperature of the catalyst element compound added to the surface of the graphite block, and / or A compound containing a heavy metal element.
[0013]
Further, a surfactant is added to the catalyst compound in the graphite block oxidation method, or a surfactant is applied to the surface of the graphite block in advance.
[0014]
Furthermore, it is to arrange a combustion furnace connected with a high-concentration oxygen generator at the subsequent stage of a device for adding a catalytic element compound such as a catalyst solution device to the surface of a graphite block used in a nuclear reactor.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
In the invention leading to the above-mentioned Patent No. 3051859, the present inventor stated that “metal oxides such as iron oxide, copper oxide and zinc oxide promote the oxidation of graphite and lower the remarkable combustion temperature in air by 15 ° C. (See the patent specification [0007]). That is, it seems that the impurity element derived from the brass plated on the piano wire used for cutting the sample seems to have an effect of lowering the combustion start temperature of graphite.
[0016]
The inventor selects from the 39 types of elemental compounds that are available, the compound in which the oxide of the target element remains even after pyrolysis at a temperature lower than the combustion start temperature of ordinary graphite for a nuclear reactor. An oxidation combustion experiment was conducted by adding to graphite.
[0017]
As a result, it was found that by adding a small amount of a specific element compound in advance to the surface of the graphite block to be oxidized, the compound acts as a catalyst for the oxidation reaction.
Further, these compounds may be added alone or in combination, and it has also been clarified that the effect is diminished if the amount exceeds an appropriate amount.
[0018]
Therefore, some experiments that are the basis of the embodiment of the present invention will be described.
[0019]
(I) Specific experiment of element having catalytic effect
The elements adopted in the oxidation catalyst effect experiment are in the order of their atomic numbers.
Li, B, Na, Mg, Al, P, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, Nb, Mo, Rh, Pd, Ag, Cd, In, Sn, Sb, Cs, Ba, La, Ce, W, Hg, Pb, Bi
In actual experiments, aqueous solutions of nitrates, sulfates, ammonium salts, oxy salts, halides, boric acid, phosphoric acid and the like that are higher than the primary reagents of these elements were used in actual experiments.
[0020]
Further, elements that are diffused during heating of graphite and cannot adhere to graphite (S, halogen, rare gas), elements that are meaningless for the purpose of the invention (C, N, O), highly toxic elements (Be, As) , Se, Te, Po), radioactive elements, elements that are difficult to obtain aqueous solution compounds as single elements (Si, Ge, Sc, Au, platinum group), elements that are difficult to obtain high-purity reagents (most of rare earths) Although consciously excluded from the experiment, the catalytic effects of these elements can be inferred from the experimented elements.
[0021]
Furthermore, the salt addition is performed in an aqueous solution for the convenience of implementation. However, even if the compound remains pyrolyzed up to the combustion temperature of graphite and the target element remains, any solvent other than water may be used. No problem.
[0022]
In fact, unlike the small sample graphite used in the experiment, when applying the present invention to a large graphite block, the graphite surface is resistant to wetting by repelling water, so a very small amount of surfactant should be added to the water. Is preferred.
[0023]
Here, in many cases, the surfactant is a Na salt or a K salt, but the Na or K element in the surfactant also has an oxidation promoting effect as described below. It is necessary to consider.
[0024]
As described above, graphite for nuclear reactor material is highly pure, but actually contains impurities shown in Table 1.
[0025]
[Table 1]

[0026]
At the time of the invention of Patent No. 3051859, it was not elucidated that these impurity heavy metals have a catalytic action, so graphite for nuclear reactor material was used as an experimental sample as it was, and the remarkable combustion temperature in air was 680 ° C. there were.
[0027]
However, since the present invention is performed while confirming the action of the impurity heavy metal, in the present invention, a pure carbon piece (hereinafter simply referred to as “graphite piece”) containing no impurity is used for the graphite sample. did.
It was 850 degreeC when the remarkable combustion temperature in the air of this graphite piece was measured in advance (it measured with a 10 mg sample using TG-DTA. TG-DTA is explained in full detail below).
[0028]
Table 2 shows elemental compounds added to the graphite pieces. An aqueous solution of the compound is dropped onto a pre-weighed piece of graphite and allowed to adhere before drying. The “compound addition amount” in Table 2 is determined from the amount of increase from the dry sample weight of the graphite piece weighed previously.
Also, the “element addition rate to graphite” in Table 2 is the graphiteamountIs the percentage of the elemental value expressed by chemical calculation.
[0029]
The graphite sample prepared as described above was subjected to a thermal analyzer TG-DTA (manufactured by Max Science Co., Ltd.), and the temperature was increased from room temperature at a rate of temperature increase of 20 ° C./min.
The purpose of this experiment (I) was to narrow down the elements that markedly show the catalyst characteristics, and the combustion space was first conducted in air.
[0030]
[Table 2]

[0031]
A TG-DTA chart is shown in FIG. In the TG-DTA chart, three curves are obtained with respect to time, and the curve indicating temperature is a straight line that rises to the right. Moreover, DTA is a differential heat curve and shows the endothermic heat generation of a sample. Furthermore, TG is a thermobalance that continuously displays the change in the weight of the sample.
[0032]
From FIG. 1, in the oxidation combustion of graphite, DTA rises abruptly when the temperature approaches the combustion start temperature, and shows a peak of heat generation. When the combustion is finished, the DTA returns to a flat state.
On the other hand, TG suddenly decreases to the right with the start of combustion, continues to decrease until the end of combustion, and thereafter becomes flat. This indicates a process in which the weight of the sample is reduced by combustion.
A tangent line is drawn on the curve of the TG-DTA chart obtained as described above on the curve of the chart according to the method of thermal analysis defined in Japanese Industrial Standard (for example, JIS H7101), and the intersection point P of the two tangent lines is drawn.₁, P₂The temperature and time at the combustion start point and combustion end point are obtained from the above.
Here, to obtain the temperature, the P₁, P₂A line is drawn from the temperature curve parallel to the temperature axis to the intersection T₁, T₂Read each temperature from.
[0033]
The experimental results are summarized in Table 2. The order of the 39 elements tried was in accordance with the experimental order, and the compounds actually added to the graphite pieces were shown by molecular formulas.
Also, without adding any salt to the graphite piece,GenerationInstead, the blank sample, which was dried by dropping distilled water, had an average combustion start temperature of 850 ° C., and the difference from the 39 types of sample data with “850 ° C.” as the reference temperature was defined as “combustion start temperature shift”. Indicated.
[0034]
From Table 2, the elements that affect the combustion start temperature of graphite were classified as follows.
A) An element that lowers the combustion start temperature. In descending order of effect
Pb, Bi, Rb, Cs, Pd, K, V, Ag, Na, Cr, Ce, Mo, Li, Co, Cu, Sr, Rh, Cd, Fe, W, Ca, Sb
B) An element that increases the combustion start temperature. In descending order of effect
P, Al, Mg, B, Nb, Mn
C) Elements that do not significantly affect the combustion start temperature, that is, elements that did not show an effect greater than 850 ° C. ± 50 ° C.
La, Zn, Sn, Y, Hg, Ba, In, Ga, Zr, Ti, Ni
From the above results, the platinum group elements such as Ru, Os, Ir, Pt, and Au that have not been tested this time, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Lu, Such rare earth elements are presumed to have an action of lowering the combustion start temperature of graphite.
[0035]
(II) Effect of compound addition and compound addition
Next, the inventor experimented about the addition amount of a compound and the effect of compound addition.
As the compounds, potassium (alkali metal) and lead (heavy metal) were selected from the elements shown in Table 2 that showed a remarkable effect on the combustion start temperature.
Each of the added compounds was nitrate, and was attached to a graphite sample as an aqueous solution and dried after burning in the same manner as in the procedure of Experiment (I), and the catalytic effect in air combustion was investigated.
[0036]
The experimental results are shown in Table 3.
[0037]
[Table 3]

[0038]
In Table 3, the result of the sample to which distilled water was added and dried was the upper stage, and the combustion start temperature was 850 ° C. (similar to Experiment (I)).
On the other hand, in the sample to which the compound was added, even when K and Pb were added alone or in combination, the combustion start temperature decreased.
Further, regarding the addition amount, the combustion start temperature rises when the addition amount exceeds a certain value, and it seems that the catalyst effect is high when the addition amount is about 1% or less.
[0039]
Therefore, in order to oxidize and burn graphite for reactors, it is not necessary to add a large amount of elements such as K and Pb, and these compounds can be used alone or in combination with an amount of about 1% or less based on the weight of graphite. It was found that the optimum catalytic effect can be obtained.
[0040]
(III) Experiments on the effect of added compounds on combustion in high-concentration oxygen
From the above patent No. 3051859, it is clear that the combustion start temperature of graphite is lower in high-concentration oxygen than in air, has a higher combustion rate, and can be oxidized and burned in a block shape.
In this invention, the sample which added the compound using the pure carbon piece was created, it burned in high concentration oxygen, and compared with the combustion in the air.
The experimental method is the same as in the experiments (I) and (II), and the results are shown in Table 4.
[0041]
[Table 4]

[0042]
By comparing Table 3 and Table 4, even when compounds are added, as described in Japanese Patent No. 3051859, when burned in higher concentration oxygen than when burned in air (Table 3) It was confirmed that (Table 4) lowers the combustion start temperature.
[0043]
(IV) Relationship between combustion temperature and combustion rate
In Table 4, when attention is paid to the “burning rate” calculated from the time required for the graphite to burn out, it can be seen that the burning rate is decreased in the case where the compound is added compared to the case where the compound is not added. Recognize. In other words, when the compound is added, it takes a longer time to burn out the graphite than when the compound is not added.
In order to elucidate this point, the inventor obtained a TG-DTA chart in the case where Pb was added in the same manner as in Experiment (I) (FIG. 2).
[0044]
From FIG. 2, it can be seen that several exothermic reaction peaks are observed in DTA in the oxidation reaction of graphite to which lead is added.
This is thought to be because lead is reduced and carbonized during the graphite oxidation reaction and loses its catalytic action, but is oxidized again to become a combustion catalyst.
[0045]
Here, the inventor pays attention to the result when Pb and K are added in combination, and it has been found that it helps to prevent Pb from being deactivated during the combustion of graphite.
In Table 4, the experiment No. 45, when Pb was added at 0.97% and K was added at 0.80%, the combustion rate was decreased even though the combustion start temperature was decreased by 228 ° C and the combustion end temperature was decreased by 164 ° C. 4.20mg / min is almost as fast as 4.31mg / min when no compound is added.
This is a completely surprising result in view of the common sense of temperature dependence that “the chemical reaction rate increases rapidly as the temperature increases”.
[0046]
That is, when a catalyst in which Pb and K are combined is used, first, the effect of lowering the reaction start temperature of Pb is exhibited, and graphite begins to oxidize at 552 ° C., which is 228 ° C. lower than that without a catalyst. In the middle of the reaction, the catalyst deactivation is prevented by K, and the oxidation reaction proceeds smoothly. The oxidation reaction ends at 652 ° C., which is 164 ° C. lower than the non-catalytic combustion end temperature of 40 (816 ° C.), and the graphite is burned out. Furthermore, the burning rate is the same as that of the experiment No. It was 4.20 mg / min, which is almost the same as that of 40 without catalyst (4.31 mg / min).
[0047]
In Table 4, the experiment No. 45 and no. 46, it can be seen that there is an appropriate amount of catalyst added (as in combustion in air) in the amount of catalyst added. No. with catalyst addition rate exceeding 1%. 46 is No. 46. Compared to 45, the catalytic effect was reduced in all points of combustion start temperature, combustion end temperature, and combustion speed.
[0048]
Considering the above facts, it is considered that the heavy metal effective for lowering the temperature can be combined with the alkali metal effective for lowering the temperature at the same time to control the combustion start / combustion end / combustion speed.
Further, an alkali metal such as Rb, Cs, Na, Li or the like is added instead of K, or Bi, Pd, V, Ag, Cr, Ce, Mo, Cu, Sr, Rh, Cd, Fe, W, Ca, Sb, platinum group elements, rare earth elements, etc. can be added instead of Pb.
Furthermore, it is presumed that it is effective to add two or more of the above elements in combination from the characteristics of each element.
[0049]
【Example】
An embodiment of the present invention will be described with reference to FIGS.
The graphite 1 used in the nuclear reactor is a material subjected to oxidative combustion in the present invention. Although detailed illustration is omitted, it is a columnar body having a substantially regular hexagonal cross section, and has a circular hole into which the fuel body is inserted at the center. In the nuclear reactor, in order to form a large block moderator by combining hexagonal columns of various lengths, each hexagonal column is provided with linear irregularities in the longitudinal direction. The dimensions of the individual constituent blocks are a diagonal length of about 24 cm, a length of about 40 to 85 cm, and a weight of about 35 to 70 kg.
[0050]
In addition to these, graphite bodies other than those that comprise fuel sleeves or specially designed graphite bodies for reactor functions are used in nuclear reactors. Then, even if it is somewhat broken, it is collectively called block-like graphite in the method of the present invention.
In addition, a piece of graphite that is inconvenient to handle as a single block, even if it is an integrated shape using packaging material such as plastic, paper, or wood that does not interfere with combustion, It is included in the concept of block graphite.
[0051]
The graphite block 1 is moved to the catalyst solution coating device 2, where the catalyst solution 2a is added by the catalyst solution dissolution and storage device 3 that previously dissolves and adjusts the concentration of the catalyst.
Various methods of applying the catalyst solution 2a can be selected. As an example, a method of immersing the graphite block 1 in the catalyst solution 2a, painting with a brush, or spraying as shown in FIG. . Among them, the spraying method is the simplest method in that the quantitative determination of the catalyst solution 2a is easy.
Needless to say, spraying may be performed using a pneumatic system in addition to the pump pressure system.
[0052]
Furthermore, when the additive elements are used in combination, there is no particular difference in configuring the present invention, whether they are added in advance in a combined solution or sequentially added separately.
[0053]
The surfactant may be mixed in the catalyst solution 2a of the storage device 3 by the stirring means 3a, or may be added and applied in advance prior to the addition of the catalyst.
[0054]
Next, the graphite block 1 coated with the catalyst solution 2a is sent to the catalyst drying device 4 and dried to remove the solvent in the catalyst solution.
This drying method may be naturally dried or separately dried, and various selections are possible.
Further, when the combustion furnace starts to operate, the entire system becomes excessively heated, and it is also conceivable to use recovered heat.
[0055]
If no special drying means is used and the amount of water in the solvent is not large, it is considered that there is no problem because the water is dried and evaporated by heat in the furnace. Care must be taken because the catalyst may sag and eventually the catalyst may be unevenly distributed.
However, the presence or absence of the catalyst drying device 4 is not necessarily a condition required for configuring the present invention.
[0056]
A high-temperature gas is sent from the drying heat source 5 to the catalyst drying apparatus 4 shown in FIGS. 3 and 4.
Further, when the drying device is an electric heating system, electric power is supplied from the heat source 5.
The heat source 5 can be recovered and used by exchanging the heat of chemical reaction directly or indirectly after the combustion furnace is operated.
Further, the wet gas 6 a generated by separating the water in the solvent by the drying device 4 is exhausted by the dry exhaust device 6. As long as the heat source 5 has no radioactivity, the wet gas 6a usually does not contain a radioactive substance and can be exhausted to the outside.
[0057]
In this way, the graphite block 1 to which the catalyst has been added is then sent from the confidential graphite charging device 7 to the combustion furnace 8.
The combustion furnace 8 is supplied with oxygen 10a from a high-concentration oxygen generator 10 to burn the graphite block 1.
As this high-concentration oxygen generator 10, a device using a pressure swing adsorption method (PSA) using molecular sieve, a device using a permselective membrane, a device using separation from liquid air, or the like is used.
[0058]
Further, at the start of oxidation combustion, as described above, since a part of the graphite block 1 in the furnace must be heated to the combustion start temperature, the auxiliary combustion fuel 9 is burned in the furnace and the temperature is increased.
Reference numeral 11 denotes a residual ash discharge lid.
[0059]
The gas obtained by combustion in the combustion furnace 8 is sent to the gas cooler 12 for the purpose of lowering the temperature.
The gas cooler 12 is effectively a boiler, cooling water 13 is transferred through combustion gas and heat transfer tubes, the temperature of the gas is reduced, and the water becomes steam or hot water 14.
The combustion gas whose temperature has been lowered is easily processed by the subsequent dust collection and gas processor 15.
The gas processor 15 collectively represents a set of devices used for the purpose of purifying fly ash in these gases and reducing gaseous impurities.
[0060]
In addition, there is a fan 16 for operating the entire system including the combustion furnace at a negative pressure to compensate for the pressure loss of each facility and connection part. This can be installed in front of dust collection or gas treatment equipment for the same purpose.
The combustion gas 17 leaving the fan 16 is processed in each case.
The combustion gas 17 contains some nitrogen, argon, etc. in addition to carbon monoxide and carbon dioxide, and must be separated as necessary. Also, if environmental conditions are allowed, when directly released to the atmosphere or injected into the oceanofThere is a possibility.
[0061]
【The invention's effect】
Since the present invention is configured as described above, radioactively contaminated graphite (hereinafter referred to as atomic graphite) such as graphite for nuclear reactors is used.FurnaceIt is possible to control the oxidation start temperature and the oxidation rate while keeping the block shape of graphite for the oxidation. The advantages of this control are described below.
i) Effectiveness when lowering the oxidation start temperature
When the nuclear reactor graphite is burned in the block shape, preheating is performed as described above. At this stage, when the oxidation starts at a low temperature, the operation of the oxidation facility is easily started.
If oxidative combustion is started even partially, then the combustion reaches the entire combustion furnace, and oxidative combustion can occur over a wide range.
[0062]
Moreover, when the oxidation reaction temperature is low, the ratio of carbon monoxide generated during oxidation decreases, and the composition concentration of carbon dioxide increases. Thus, when it is desired to increase the concentration of carbon dioxide in the generated gas, it is effective to lower the combustion temperature. Further, by adding the catalyst in combination, it is possible to prevent a partial stop of the oxidation reaction that occurs when the combustion temperature is lowered too much, and it is possible to continue the oxidation stably.
[0063]
Further, when the combustion temperature is lowered, the heat resistance design of the oxidation reaction furnace becomes very easy, and it becomes possible to use a refractory material that is inexpensive and easy to handle. It is also possible to plan water-cooled metal walls.
When the temperature in the furnace is low, there is no concern about melting of combustion ash, preventing wear of the furnace body and refractory material, and no corrosive molten ash sticking to the heat exchanger pipes. High temperature corrosion of metallic materials such as pipes can be prevented. As a result, the operation and maintenance of the combustion furnace and the gas cooler are facilitated. The handling of combustion residual ash is also simple because it does not melt and adhere.
In addition, when the gas obtained in the post-combustion process is compressed and liquefied, it is necessary to burn harmful carbon monoxide with an afterburner and convert it to carbon dioxide gas. This process can be omitted and the process can be simplified. .
[0064]
ii) Effectiveness when raising the oxidation start temperature
As the oxidation reaction of graphite is performed at a higher temperature, the generation ratio of carbon monoxide becomes higher. When considering the separation and concentration of carbon 14 which is a radioisotope in the subsequent process, carbon monoxide as a gas to be supplied to the separation process contains a smaller amount of oxygen than carbon dioxide. Since the ratio increases, carbon monoxide is more advantageous in isotopic carbon separation efficiency no matter what method is used for isotope separation.
[0065]
In addition, even when high-temperature oxidation is performed, initial ignition of graphite in the furnace is performed in order to start operation at a low temperature and then increase the combustion temperature when starting operation of the graphite combustion furnace. A combination of attaching a low temperature combustion catalyst to a part of the graphite and a high temperature combustion catalyst to the other parts of graphite is also effective in operation.
[0066]
As described above, according to the present invention, as a method for oxidizing the radioactive graphite used in the nuclear reactor, according to the invention of the catalyst technology, the design structure of the combustion furnace, the operating conditions of the combustion furnace, and maintenance of the combustion furnace and incidental equipment -Comprehensive evaluation of the composition of generated gas and subsequent processes, etc., and combustion start temperature / combustion speed can be selected in a wide range.
[Brief description of the drawings]
FIG. 1 is a TG-DTA chart for measuring a combustion start temperature and a combustion end temperature in a graphite combustion experiment of the present invention.
FIG. 2 is a diagram showing an embodiment of the present invention, and is a TG-DTA chart showing an oxidation reaction of graphite using a lead catalyst.
FIG. 3 is a chart showing an embodiment of the present invention.
FIG. 4 is a longitudinal sectional view showing an embodiment of the present invention.
[Explanation of symbols]
1 Graphite
2 Catalyst solution coating device
2a Catalyst solution
3 Catalyst solution dissolution storage device
3a Stirrer for catalyst solution
4 Catalyst dryer
5 Heat source for drying
6 Drying exhaust system
6a Wet gas
7 Confidential graphite charging device
8 Combustion furnace
9 Fuel for auxiliary combustion
10 High concentration oxygen generator
10a oxygen
11 Residual ash discharge lid
12 Gas cooler
13 Cooling water
14 Hot water
15 Gas processing machine
16 fans
17 Combustion gas

Claims (11)

  A used graphite block in its original shape is put into a combustion furnace, and the same shape as when used in a nuclear reactor where oxygen is supplied from a high-concentration oxygen generator into the combustion furnace and burned. In the method for treating a graphite block, a catalyst element compound for controlling the oxidation reaction start temperature and / or the oxidation reaction continuation temperature is formed on the surface of the graphite block before the graphite block having the shape is put into the combustion furnace. The graphite block coated with the catalyst element compound and applied with the coating device is charged into the combustion furnace with an airtight graphite charging device and oxygen is supplied from the high-concentration oxygen generator to burn it. Whether the oxidation reaction start temperature and / or the oxidation reaction continuation temperature is controlled by the identification of the catalytic element compound and the amount of addition of the catalyst element compound. The method of oxidative combustion of the potential graphite.   2. The method for controlling oxidative combustion of graphite according to claim 1, wherein the oxygen concentration of oxygen supplied from the high concentration oxygen generator is not less than the oxygen concentration in the air and not more than 100% oxygen.   An alkali metal for controlling the oxidation reaction initiation temperature and / or the oxidation reaction continuation temperature to be lower than that in the case of oxidative combustion without adding the catalytic element compound, and / or 3. The graphite according to claim 1, wherein the graphite is a compound selected from compounds containing a heavy metal element, and the addition amount thereof is 0.01% to 1% on a weight basis with respect to the graphite block. Control method of oxidation combustion.   The method for controlling oxidative combustion of graphite according to claim 3, wherein the alkali metal contains lithium, sodium, potassium, rubidium, and cesium.   Heavy metals are lead, bismuth, palladium, vanadium, silver, chromium, cerium, molybdenum, cobalt, copper, strontium, rhodium, cadmium, iron, tungsten, calcium, antimony, platinum group elements, rare earth elements The method for controlling oxidation combustion of graphite according to claim 3 or 4.   6. The oxidation combustion of graphite according to claim 3, wherein the catalyst compound containing an alkali metal or heavy metal element is a composite of two or more compounds containing an alkali metal or heavy metal element. Control method.   6. The method for controlling oxidative combustion of graphite according to claim 3, wherein the catalyst element compound is a combination of a heavy metal element compound and an alkali metal compound. The addition amount of the catalyst compound is 0.01% to 1% on a weight basis as a whole with respect to the graphite used as a catalyst element in a nuclear reactor, or 0 on the surface area basis on the surface of the graphite. The method for controlling oxidative combustion of graphite according to any one of claims 3, 4, 5, 6, and 7, characterized in that it is 0.05 g / m ² to 5 g / m ² . A surfactant for making the surface of the graphite block easy to wet is added to the catalyst compound, or a surfactant for making the surface of the graphite block easy to wet is preliminarily applied to the surface of the graphite block. The method for controlling oxidation combustion of graphite according to claim 1 .   When the surfactant for easily wetting the surface of the graphite block contains an alkali metal, the alkali metal content is included in the range of the alkali metal content in the catalyst compound according to claim 8. The method for controlling oxidation combustion of graphite according to claim 9. Phosphorus, aluminum, magnesium for controlling the catalytic element compound added to the surface of the graphite block to be higher than the oxidation reaction start temperature and / or the oxidation reaction continuation temperature higher than in the case of oxidative combustion without adding the catalytic element compound, The method for controlling oxidative combustion of graphite according to claim 1 , wherein the method is a compound selected from compounds containing boron, niobium, and manganese elements.

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