JP4260977B2 - Nuclear fuel pellet, method for producing the same, nuclear fuel element and nuclear fuel assembly - Google Patents
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- JP4260977B2 JP4260977B2 JP09970599A JP9970599A JP4260977B2 JP 4260977 B2 JP4260977 B2 JP 4260977B2 JP 09970599 A JP09970599 A JP 09970599A JP 9970599 A JP9970599 A JP 9970599A JP 4260977 B2 JP4260977 B2 JP 4260977B2
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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Description
【０００１】
【発明の属する技術分野】
本発明は、例えば軽水炉や高速増殖炉などの核分裂炉に装荷される核燃料ペレット、核燃料ペレットの製造方法、核燃料要素および核燃料集合体に関するものである。
【０００２】
【従来の技術】
現在商業炉では、出力の安定化を目的として核的毒物であるガドリニウム酸化物が添加されたペレットが使用されている。このガドリニウム酸化物はウラン酸化物と混合された後成形、焼結することにより製造されている。
【０００３】
ガドリニウム酸化物の混合が不十分な場合にはガドリニウム酸化物とウランなどの核燃料酸化物との固溶反応が不十分となり、焼結後のペレットにマイクロクラックが発生したり、結晶粒径が著しく小さくなることが知られている。そこで、ガドリニウム酸化物とウラン酸化物との固溶性改善のために、機械的に混合された混合酸化物に焼結助剤を添加する方法（特開平０５−１１０８８、特開平０１一１９３６９１、特開平０２−２４２１９５号公報）が提案されている。
【０００４】
【発明が解決しようとする課題】
これらの方法によれば、均一な微細組織を持ち、ペレット製造時にマイクロクラックなどの欠陥が発生しない健全なペレットを製造することができる。一方、ウラン酸化物にガドリニウム酸化物を添加し、均一な固溶体とした場合には、ペレットの熱伝導度が低下することが知られている（Journal of Nuclear Materlals l05(1982)201）。
【０００５】
ペレットの熱伝導度が低下すると、原子炉内でのペレット温度が上昇し、結果としてペレットのスウェリングによるペレット被覆管間相互作用（ＰＣＩ）の増大、核分裂生成物の放出率の増加を引き起こすため、ガドリニウム酸化物を添加したペレットの限界出力はガドリニウム酸化物を添加していないペレットの場合よりも低く設定している。
【０００６】
そこで、ガドリニウム酸化物のウラン酸化物への固溶を抑制することによりペレット熱伝導率の低下を防止する方法が提案されている（特開昭５９−９００８２号公報、特開昭６０−２３１１９５号公報、Proc.lnternatlonaIAtomicAgency・ TechnicaICommitteeMeeting on Advances 1n Pellet Technology for lmproved Performance at High Burnup Tokyo Japan (1996)No.2−1、特願平１０−９１１１３号）。
【０００７】
また、特開昭５９−９００８２号公報には、硼化タングステンで被覆したガドリニウム酸化物粒子を添加することにより焼結中のウラン酸化物との接触を避けて固溶させない方法が示されているが、この方法では、タングステンを使用するため、製造コストの増大を招く。
【０００８】
また、特開昭６０−２３１１９５号公報には、１ｍｍ以上のガドリニウム酸化物粒子を添加することにより焼結中のウラン酸化物との接触面積を小さくし固溶反応を抑制することが示されているが、ガドリニウム酸化物の粒径が大きいため、焼結中のガドリニウム酸化物凝集体とウラン酸化物の収縮率が大きく異なり、焼結体内のクラックの発生または焼結後のガドリニウム酸化物／ウラン酸化物界面の空隙の発生を引き起こし、密度、熱伝導度の低下を引き起こす。
【０００９】
PIoc.lnternationaI Atomic Agency TechnlcaI Commlttee Meetlng on Advances in Pellet Technology for lmproved Performance at High Burnup Tokyo Japan (1996)No.2−1には、熱処理を施したガドリニウム酸化物をウラン酸化物に混合し焼結することによりガドリニウム酸化物分散型のペレットを製造する方法が示されているが、この方法により製造されたペレットでは、ガドリニウム酸化物／ウラン酸化物界面に気泡が連結しており、熱伝導率向上効果が小さいと考えられる。
【００１０】
また、特願平１０−９１１１３号では、最大６０ｗｔ％のガドリニウム酸化物を含む高ガドリニウム濃度領域が分散されたペレットが示されているが、低熱伝導率の高濃度ガドリニウム領域が占める体積割合が大きく、顕著な熱伝導率の向上効果が期待できない。
【００１１】
本発明は、上記状況に対処してなされたものであり、その目的とするところは、ペレット内に高濃度のガドリニウム酸化物が分散しており、ペレット内にガドリニウムが均一に固溶したペレットと比較して熱伝導率が高いこの種の核燃料ペレットおよびその製造方法、ならびにそのペレットを用いた核燃料要素および核燃料集合体を提供することにある。
【００１２】
【課題を解決するための手段】
上記目的は、高濃度のガドリニウム酸化物領域が分散した核燃料ペレットにおいて、高濃度ガドリニウム酸化物領域の粒径を最適化すること、および焼結過程において高濃度ガドリニウム酸化物領域の結晶構造がウラン酸化物との固溶反応により単斜晶から立方晶系へ変化しないガドリニウム酸化物濃度とすることにより急激な体積変化を防止し、ペレット製造時の高濃度ガドリニウム酸化物領域とその他の領域との境界における密着性を改善し、高濃度ガドリニウム酸化物領域のガドリニウム濃度を高くすることにより、ペレットの熱伝導率向上効果を大きくすることにより達成される。
【００１３】
具体的には、核燃料酸化物中に高濃度のガドリニウム酸化物領域が分散しており、高濃度ガドリニウム酸化物領域の平均直径が５０〜３００μｍであり、高濃度ガドリニウム酸化物領域のガドリニウム酸化物濃度が６０ｗｔ％以上である核燃料ペレットを用いる。
【００１４】
この濃度範囲における単斜晶の結晶構造を持つ高濃度ガドリニウム酸化物の結晶構造は、Ｕ4０9相を形成しないような条件下にて熱処理および焼結を実施しても変化しないことが確認された。また上記ペレットの製造は、単体にて高焼結密度が得られ、平均粒径が５０〜３００μｍである高濃度のガドリニウム酸化物凝集粒子を作製し、高濃度のガドリニウム酸化物凝集粒子に有機化合物などの１０００℃以下の温度にて分解あるいは蒸発する物質を被覆した後に核燃料酸化物粉末と混合し、成形、焼結することにより達成される。
【００１５】
また、これらの核燃料ペレットを金属製被覆管内に装荷することにより燃料要素を組立て、さらにその燃料要素を燃料集合体を構成する燃料要素の一部あるいは全部に用いることにより達成できる。
【００１６】
すなわち、本発明の核燃料ペレットは、高濃度のガドリニウム酸化物の領域とガドリニウム酸化物を含まない領域とから構成されるため、ペレット内にガドリニウム酸化物が均一に固溶している場合と比較して熱伝導率が高くなる。さらに、高濃度ガドリニウム酸化物領域におけるガドリニウム酸化物濃度を６０ｗｔ％以上とすることにより焼結中のガドリニウム酸化物の核燃料酸化物との固溶反応による単斜晶から立方晶系への結晶構造の変化に起因する急激な体積変化を防止し、その領域の粒径を５０μｍ以上とすることにより、ガドリニウム酸化物の核燃料酸化物への固溶量を小さくし、ガドリニウム酸化物を含まない領域の体積分率を大きくすることができ、ガドリニウム添加によるペレット熱伝導率の低下を小さくすることができる。
【００１７】
また、本発明の製造方法に示されているように、高濃度ガドリニウム酸化物凝集粒子を有機化合物などの１０００℃以下の温度にて分解あるいは蒸発する物質により被覆することにより、成形体の焼結過程での高濃度ガドリニウム酸化物領域とガドリニウム酸化物を含まない領域の収縮率の差異を吸収でき、さらに高濃度ガドリニウム酸化物領域の凝集粒子の平均径を３００μｍ以下とすることにより成形体の焼結過程での高濃度ガドリニウム酸化物領域とガドリニウム酸化物を含まない領域の収縮率の差異をこの被覆層で充分吸収できる量に抑え、結果として両相の境界における密着性が良好となり、焼結に発生する引つ張り応力によるクラックの発生を防止できる。
【００１８】
また、このような核燃料ペレットを燃料要素ならびに燃料集合体に用いることにより、ガドリニウム酸化物が均一に固溶した核燃料ペレットを用いた場合と比較して、ガドリニウム酸化物を含有する燃料要素の限界出力を低く設定できる。
【００１９】
【発明の実施の形態】
〔実施例１〕高純度の酸化ガドリニウム粉末を１４００℃にて熱処理し、単斜晶構造とした後、振動ミルにより充分粉砕することにより一次粒子径がサブミクロンオーダーである高活性の単斜晶酸化ガドリニウム粉末を作製した。その粉末を約ｌｔ／ｃｍ2の圧力にて予備成形し、粉砕、篩い分けにより平均粒径が約２５０μｍである酸化ガドリニウム凝集粒子を得た。
【００２０】
その後、酸化ガドリニウム凝集粒子とステアリン酸亜鉛粉末を粉末混合器を用いて混合し、酸化ガドリニウム凝集粒子の周りにステアリン酸亜鉛を被接した。このときに形成されるステアリン酸亜鉛の膜厚は混合するステアリン酸亜鉛の添加率を制御することにより調整することができる。
【００２１】
この被覆層は、焼結初期および中期において、ウラン酸化物に比べて収縮が遅れる高カドリニウム酸化物とその周りのウラン酸化物との収縮率差に伴い発生する応力を緩和し、焼結後期における両者の密着性を向上する効果を持つ。このステアリン酸亜鉛により被覆された酸化ガドリニウム凝集粒子と酸化ウラン粉末とを酸化ガドリニウムが１０ｗｔ％となる比率にて、凝集粒子が破壊しないように回転容器を用いてソフトに混合し、この混合粉末を２〜３ｔ／ｃｍ2の圧力にて圧縮成形し、酸化ガドリニウム凝集粒子が分散した成形体を作製した。
【００２２】
この成形体を１７５０℃の温度において、Ｈ2０を添加したＨ2とＮ2混合ガス気流を用いて約−３５０ＫＪ／ｍｏｌの酸素ポテンシャルの雰囲気中で４時間焼結した（図１参照）。その結果、得られた焼結体の密度は９６．５％ＴＤ以上の高密度であり、走査型電子顕微鏡観察から酸化ガドリニウムであった部分と酸化ウランであった部分との境界における密着性が良好であり、境界に沿つた大気孔は見られない健全なべレットであることがわかった。
【００２３】
また、本試作ペレットの熱拡散率を評価した結果、ガドリニウム酸化物が酸化ウラン中に均一に固溶したペレットより大きな値を示すことが分かった。
【００２４】
〔応用例１〕上記実施例１では、ガドリニウム酸化物を被覆する物質としてステアリン酸亜鉛を用いたが、成形体の焼結過程において、核燃料酸化物あるいはガドリニウム酸化物の焼結が開始する前に分解あるいは蒸発する他の有機化合物（例えぱステアリン酸、ラウリン酸、べへン酸、インステアリン酸、アミド系化合物等）を用いた場合でも、実施例１と同様の焼結挙動が得られるので、上記実施例と同等の本発明の効果が得られる。
【００２５】
〔応用例２〕上記実施例１では、ガドリニウム酸化物を被覆する物質としてステアリン酸亜鉛を用いた例で説明したが、カーボンブラックをガドリニウム酸化物に被覆する物質として使用した場合も、カーボンブラックがＣ０2として蒸発できる条件にて焼結を実施すれば、ステアリン酸亜鉛と同様に１０００℃以下にて蒸発しその後の焼結挙動に影響を及ぼさないので、実施例１と同様の効果が得られると考えられる。
【００２６】
〔応用例３〕上記実施例１では、ガドリニウム酸化物としてガドリニウム酸化物粉末の凝集粒子を用いた例で説明したが、ガドリニウム酸化物として、理論密度に対する相対密度で９５％ＴＤ以上の密度を持つ焼結球を用い、その周りにウラン酸化物が焼結することによる収縮量に相当する厚さのステアリン酸亜鉛被覆層を形成させた後にウラン酸化物と混合しペレットを製造することにより、ガドリニウム酸化物とウラン酸化物との密着性の良好なペレットが得られるので、実施例１と同様の効果が得られると考えられる。
【００２７】
〔応用例４〕上記実施例１では、高濃度ガドリニウム酸化物凝集粒子として１００％ガドリニウム酸化物粉末を用いた例で説明したが、ガドリニウム酸化物粉末に最大４０ｗｔ％までのウラン酸化物を混合した凝集粒子粉末を用いて、Ｕ4０9相を形成しないような条件下にて凝集粒子の熱処理および焼結を実施しても、凝集粒子の結晶構造は、実施例１と同様に熱処理によって単斜晶となり、焼結によって結晶構造が変化しないことが確認された。
【００２８】
このことから、高濃度ガドリニウム酸化物凝集粒子として最大４０ｗｔ％までのウラン酸化物を混合した凝集粒子粉末を用いても、実施例１と同様の成形、焼結挙動が得られ、同様の焼結密度、焼結体微細構造が得られると考えられる。
【００２９】
〔応用例５〕上記実施例１では、核的毒物としてガドリニウム酸化物を用いたが、Ｇｄと化学的性質が類似するＥｒＤｙ等の他の核的毒物でも同様の効果が期待できる。
【００３０】
〔応用例６〕上記実施例１では、焼結時の雰囲気ガスとして、Ｈ2０を添加したＨ2とＮ2の混合ガスを用いたが、酸化物燃料の焼結挙動が雰囲気の酸素ポテンシャルに依存することを考慮し、同様の酸素ポテンシャルとなるような他の雰囲気ガスを用いても同様の効果が得られると考えられる。
【００３１】
〔応用例７〕上記実施例および応用例により得られた核燃料ペレットを金属製被覆管に装荷し、核燃料要素となすことにより、同出力にて比較した場合に従来のガドリニウム酸化物が核燃料酸化物中に固溶した核燃料ペレットを用いた場合と比較して燃料中心温度を低減できる核燃料要素を得ることができる。したがって、同一燃料中心温度を許容した設計の場合、より高い出力を許容することができる。
【００３２】
〔応用例８〕本発明の核燃料ペレットは、従来から用いられているガドリニウム酸化物添加ペレットと比較して熱伝薄率が高いため、同一の出力履歴にて比較した場合に燃料中心温度が低く、核燃料ペレットからの核分裂生成物の放出率が小さいと考えられる。このような燃料挙動を踏まえ、本発明の核燃料ペレットを装填した金属被覆管のスタック長さに対する自由体積の比を従来よりも小さく設定した燃料要素を用いた燃料集合体を用いることができる。
【００３３】
【発明の効果】
以上説明してきたように本発明によれば、熱伝導率が高くなるので燃料ペレットの中心温度を低減でき、核分裂生成物の放出率低減、原子炉の出力制限の緩和を図ることができる。また、従来と同一の運転条件にて本発明の核燃料ペレットを装填した燃料要素を持つ燃料集合体を使用する場合に、スタック長さに対する自由体積を低減することができる。また、ペレットスウェリング量を低減できるため、燃料ペレットと被覆管との相互作用を軽減でき、燃料の長寿命化が可能となる。
【図面の簡単な説明】
【図１】Ｇｄ2０3粒子が分散したＵ０2焼結体作製の流れを示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a nuclear fuel pellet loaded in a nuclear fission reactor such as a light water reactor or a fast breeder reactor, a method for producing the nuclear fuel pellet, a nuclear fuel element, and a nuclear fuel assembly.
[0002]
[Prior art]
At present, commercial furnaces use pellets added with gadolinium oxide, which is a nuclear poison, for the purpose of stabilizing output. This gadolinium oxide is produced by mixing with uranium oxide, followed by molding and sintering.
[0003]
If the mixing of gadolinium oxide is insufficient, the solid solution reaction between gadolinium oxide and nuclear fuel oxides such as uranium will be insufficient, and microcracks will occur in the sintered pellet, and the crystal grain size will be remarkably large. It is known to become smaller. Therefore, in order to improve the solid solubility between gadolinium oxide and uranium oxide, a method of adding a sintering aid to the mechanically mixed oxide (Japanese Patent Laid-Open No. 05-11088, Japanese Patent Laid-Open No. 01-1193691, (Kaihei 02-242195) is proposed.
[0004]
[Problems to be solved by the invention]
According to these methods, it is possible to produce a healthy pellet having a uniform microstructure and free from defects such as microcracks during the production of the pellet. On the other hand, it is known that when gadolinium oxide is added to uranium oxide to form a uniform solid solution, the thermal conductivity of the pellet is reduced (Journal of Nuclear Materlals l05 (1982) 201).
[0005]
Decreasing the thermal conductivity of the pellet increases the temperature of the pellet in the reactor, resulting in an increase in pellet-pipe-tube interaction (PCI) due to pellet swelling and an increase in the fission product release rate. The limit output of the pellet added with gadolinium oxide is set lower than that of the pellet not added with gadolinium oxide.
[0006]
In view of this, a method has been proposed in which the solid thermal conductivity of gadolinium oxide in the uranium oxide is suppressed to prevent a decrease in pellet thermal conductivity (Japanese Patent Laid-Open Nos. 59-90082 and 60-231195). Publication, Proc.lnternatlonaIAtomicAgency / TechnicaICommitteeMeeting on Advances 1n Pellet Technology for lmproved Performance at High Burnup Tokyo Japan (1996) No.2-1, Japanese Patent Application No. 10-91113).
[0007]
Japanese Patent Application Laid-Open No. 59-90082 discloses a method in which a gadolinium oxide particle coated with tungsten boride is added to avoid contact with the uranium oxide during sintering so as to prevent solid solution. However, since this method uses tungsten, the manufacturing cost increases.
[0008]
Japanese Patent Application Laid-Open No. 60-231195 shows that by adding gadolinium oxide particles of 1 mm or more, the contact area with the uranium oxide during sintering is reduced and the solid solution reaction is suppressed. However, because the gadolinium oxide has a large particle size, the shrinkage ratio of the gadolinium oxide aggregates and uranium oxide during the sintering is greatly different, and cracks are generated in the sintered body or gadolinium oxide / uranium after the sintering. It causes the generation of voids at the oxide interface, causing a decrease in density and thermal conductivity.
[0009]
In PIoc.lnternationaI Atomic Agency TechnlcaI Commlttee Meetlng on Advances in Pellet Technology for lmproved Performance at High Burnup Tokyo Japan (1996) No. 2-1, heat treated gadolinium oxide is mixed with uranium oxide and sintered. Shows a method for producing gadolinium oxide dispersed pellets, but in the pellets produced by this method, bubbles are connected to the gadolinium oxide / uranium oxide interface, and the effect of improving thermal conductivity is exhibited. It is considered small.
[0010]
In Japanese Patent Application No. 10-91113, a pellet in which a high gadolinium concentration region containing a maximum of 60 wt% gadolinium oxide is dispersed is shown, but the volume ratio occupied by the high concentration gadolinium region having a low thermal conductivity is large. Therefore, a significant improvement in thermal conductivity cannot be expected.
[0011]
The present invention has been made in response to the above situation, and the object of the present invention is that a high-concentration gadolinium oxide is dispersed in the pellet, and the gadolinium is uniformly dissolved in the pellet. It is an object of the present invention to provide a nuclear fuel pellet of this type having a higher thermal conductivity and a method for producing the same, and a nuclear fuel element and a nuclear fuel assembly using the pellet.
[0012]
[Means for Solving the Problems]
The above objective is to optimize the particle size of the high-concentration gadolinium oxide region in the nuclear fuel pellet in which the high-concentration gadolinium oxide region is dispersed, and the crystal structure of the high-concentration gadolinium oxide region is uranium oxidized during the sintering process. The concentration of gadolinium oxide that does not change from monoclinic to cubic system due to solid solution reaction with the product prevents sudden volume change, and the boundary between the high concentration gadolinium oxide region and other regions during pellet production This is achieved by improving the effect of improving the thermal conductivity of the pellets by improving the adhesion in the region and increasing the gadolinium concentration in the high concentration gadolinium oxide region.
[0013]
Specifically, the high concentration gadolinium oxide region is dispersed in the nuclear fuel oxide, the average diameter of the high concentration gadolinium oxide region is 50 to 300 μm, and the gadolinium oxide concentration in the high concentration gadolinium oxide region is Use nuclear fuel pellets with a weight of 60 wt% or more.
[0014]
It is confirmed that the crystal structure of high concentration gadolinium oxide having a monoclinic crystal structure in this concentration range does not change even when heat treatment and sintering are performed under conditions that do not form the U 4 0 9 phase. It was done. In addition, the above pellets are produced by producing high-concentration gadolinium oxide aggregated particles having a high sintering density and an average particle size of 50 to 300 μm as a single substance, and adding organic compounds to the high-concentration gadolinium oxide aggregated particles. This is achieved by coating a substance that decomposes or evaporates at a temperature of 1000 ° C. or lower, etc., and then mixing with a nuclear fuel oxide powder, molding and sintering.
[0015]
Further, the fuel element can be assembled by loading these nuclear fuel pellets into a metal cladding tube, and further, the fuel element can be used for a part or all of the fuel elements constituting the fuel assembly.
[0016]
That is, since the nuclear fuel pellet of the present invention is composed of a high concentration gadolinium oxide region and a region not containing gadolinium oxide, it is compared with a case where gadolinium oxide is uniformly dissolved in the pellet. Heat conductivity. Furthermore, by setting the gadolinium oxide concentration in the high-concentration gadolinium oxide region to 60 wt% or more, the crystal structure from monoclinic to cubic system by solid solution reaction of gadolinium oxide during sintering with nuclear fuel oxide is improved. The volume of the region not containing gadolinium oxide is reduced by reducing the volume of gadolinium oxide in the nuclear fuel oxide by preventing the sudden volume change caused by the change and making the particle size of the region 50 μm or more. The fraction can be increased, and the decrease in pellet thermal conductivity due to the addition of gadolinium can be reduced.
[0017]
In addition, as shown in the production method of the present invention, the high-concentration gadolinium oxide aggregated particles are coated with a substance that decomposes or evaporates at a temperature of 1000 ° C. or lower, such as an organic compound, thereby sintering the molded body. The difference in shrinkage between the high-concentration gadolinium oxide region and the region not containing gadolinium oxide during the process can be absorbed, and the average diameter of the aggregated particles in the high-concentration gadolinium oxide region can be reduced to 300 μm or less. The difference in shrinkage between the high-concentration gadolinium oxide region and the gadolinium oxide-free region during the sintering process is suppressed to an amount that can be sufficiently absorbed by this coating layer. As a result, the adhesion at the boundary between the two phases is improved and sintering is performed. It is possible to prevent the occurrence of cracks due to the tensile stress generated in.
[0018]
In addition, by using such nuclear fuel pellets for fuel elements and fuel assemblies, the limit output of fuel elements containing gadolinium oxide compared to the case of using nuclear fuel pellets in which gadolinium oxide is uniformly solid solution is used. Can be set low.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
[Example 1] A high-purity gadolinium oxide powder is heat-treated at 1400 ° C. to form a monoclinic crystal structure, and then sufficiently ground by a vibration mill to obtain a highly active monoclinic crystal having a primary particle size on the order of submicrons. Gadolinium oxide powder was prepared. The powder was preformed at a pressure of about lt / cm 2 , and gadolinium oxide aggregated particles having an average particle size of about 250 μm were obtained by pulverization and sieving.
[0020]
Thereafter, the gadolinium oxide aggregated particles and the zinc stearate powder were mixed using a powder mixer, and the zinc stearate was covered around the gadolinium oxide aggregated particles. The film thickness of the zinc stearate formed at this time can be adjusted by controlling the addition rate of the zinc stearate to be mixed.
[0021]
This coating layer relieves the stress caused by the difference in shrinkage between the high cadmium oxide and the surrounding uranium oxide, which are delayed in shrinkage compared to uranium oxide in the early and middle stages of sintering, and in the later stage of sintering. Has the effect of improving the adhesion between the two. The gadolinium oxide aggregated particles coated with zinc stearate and the uranium oxide powder are softly mixed using a rotating container at a ratio of 10 wt% of gadolinium oxide so that the aggregated particles are not destroyed. Compression molding was performed at a pressure of 2 to 3 t / cm 2 to prepare a molded body in which gadolinium oxide aggregated particles were dispersed.
[0022]
This molded body was sintered at a temperature of 1750 ° C. for 4 hours in an atmosphere having an oxygen potential of about −350 KJ / mol using an H 2 and N 2 mixed gas stream added with H 2 O (see FIG. 1). As a result, the density of the obtained sintered body was 96.5% TD or higher, and the adhesion at the boundary between the portion that was gadolinium oxide and the portion that was uranium oxide was determined by observation with a scanning electron microscope. It was found to be good and a healthy bullet with no air holes along the boundary.
[0023]
Moreover, as a result of evaluating the thermal diffusivity of this prototype pellet, it was found that gadolinium oxide showed a larger value than a pellet in which uranium oxide was uniformly dissolved.
[0024]
[Application Example 1] In Example 1 described above, zinc stearate was used as a material for coating gadolinium oxide, but before sintering of nuclear fuel oxide or gadolinium oxide was started in the sintering process of the compact. Even when other organic compounds that decompose or evaporate (eg, stearic acid, lauric acid, behenic acid, instearic acid, amide compounds, etc.) are used, the same sintering behavior as in Example 1 can be obtained. The effect of the present invention equivalent to the above embodiment can be obtained.
[0025]
[Application Example 2] In Example 1 described above, an example in which zinc stearate is used as a material for coating gadolinium oxide has been described. However, when carbon black is used as a material for coating gadolinium oxide, by carrying out sintering at evaporation can condition as C0 2, does not affect the subsequent sintering behavior was evaporated at less Likewise 1000 ° C. and zinc stearate, the same effects as in example 1 to obtain it is conceivable that.
[0026]
[Application Example 3] In Example 1 described above, an example in which aggregated particles of gadolinium oxide powder are used as gadolinium oxide has been described. However, gadolinium oxide has a density of 95% TD or more relative to the theoretical density. By forming a zinc stearate coating layer with a thickness corresponding to the amount of shrinkage due to sintering of uranium oxide around the sintered sphere, and mixing with uranium oxide to produce pellets, gadolinium oxidation It is considered that the same effect as in Example 1 can be obtained because pellets having good adhesion between the product and uranium oxide can be obtained.
[0027]
[Application Example 4] In Example 1 described above, an example in which 100% gadolinium oxide powder was used as the high-concentration gadolinium oxide agglomerated particles was described. However, up to 40 wt% of uranium oxide was mixed with gadolinium oxide powder. Even when the aggregated particles are heat-treated and sintered under the conditions that do not form the U 4 0 9 phase, the aggregated particles have a crystal structure that is not changed by heat treatment as in Example 1. It became an oblique crystal, and it was confirmed that the crystal structure was not changed by sintering.
[0028]
From this, even if an aggregated particle powder in which up to 40 wt% uranium oxide is mixed is used as the high concentration gadolinium oxide aggregated particles, the same molding and sintering behavior as in Example 1 is obtained, and the same sintering is performed. It is thought that a density and a sintered compact microstructure can be obtained.
[0029]
Application Example 5 In Example 1 above, gadolinium oxide was used as the nuclear poison, but similar effects can be expected with other nuclear poisons such as ErDy, which have similar chemical properties to Gd.
[0030]
[Application Example 6] In Example 1 above, a mixed gas of H 2 and N 2 added with H 2 0 was used as the atmosphere gas at the time of sintering. It is considered that the same effect can be obtained even if another atmospheric gas having a similar oxygen potential is used.
[0031]
[Application Example 7] By loading the nuclear fuel pellets obtained in the above examples and application examples into a metal cladding tube to form a nuclear fuel element, conventional gadolinium oxide can be converted into nuclear fuel oxide when compared at the same output. It is possible to obtain a nuclear fuel element that can reduce the fuel center temperature as compared with the case of using nuclear fuel pellets dissolved therein. Therefore, in the case of a design that allows the same fuel center temperature, a higher output can be allowed.
[0032]
[Application Example 8] Since the nuclear fuel pellet of the present invention has a higher heat transfer rate than the conventionally used gadolinium oxide-added pellet, the fuel center temperature is lower when compared with the same output history. The release rate of fission products from nuclear fuel pellets is thought to be small. Based on such fuel behavior, it is possible to use a fuel assembly using a fuel element in which the ratio of the free volume to the stack length of the metal cladding tube loaded with the nuclear fuel pellet of the present invention is set to be smaller than that of the prior art.
[0033]
【The invention's effect】
As described above, according to the present invention, since the thermal conductivity is increased, the center temperature of the fuel pellet can be reduced, the release rate of the fission products can be reduced, and the output restriction of the reactor can be relaxed. Further, when a fuel assembly having a fuel element loaded with the nuclear fuel pellet of the present invention is used under the same operating conditions as before, the free volume with respect to the stack length can be reduced. Moreover, since the amount of pellet swelling can be reduced, the interaction between the fuel pellet and the cladding tube can be reduced, and the life of the fuel can be extended.
[Brief description of the drawings]
FIG. 1 is a diagram showing a flow of manufacturing a U0 2 sintered body in which Gd 2 0 3 particles are dispersed.
Claims (6)
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