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Options of principles of fuel debris criticality control in Fukushima Daiichi reactors

福島第一原子力発電所で生じた燃料デブリの臨界管理方針の検討

外池 幸太郎; 曽野 浩樹; 梅田 幹; 山根 祐一; 久語 輝彦; 須山 賢也

Tonoike, Kotaro; Sono, Hiroki; Umeda, Miki; Yamane, Yuichi; Kugo, Teruhiko; Suyama, Kenya

福島第一原子力発電所事故で生じた燃料デブリの性状は、観察や測定による確認に至っておらず、今なお不明である。原子炉格納容器からは漏水が続いており、燃料デブリは中性子毒物を含まない水で冷却されている。放射性Xeガスの濃度監視では再臨界の兆候は見られないが、燃料デブリの未臨界担保はできていない状況である。本発表ではこれらの状況、及び燃料デブリの基本的な臨界特性を踏まえ、今後とるべき臨界管理の方針を議論する。

In the Three Mile Island Unit 2 reactor accident, a large amount of fuel debris was formed whose criticality condition is unknown except the possible highest $$^{235}$$U/U enrichment. The fuel debris had to be cooled and shielded by water in which the minimum critical mass is much smaller than the total mass of fuel debris. To overcome this uncertain situation, the coolant water was borated with sufficient concentration to secure the subcritical condition. The situation is more severe in the damaged reactors of Fukushima Daiichi Nuclear Power Station, where the coolant water flow is practically "once through". Boron must be endlessly added to the water to secure the subcritical condition of the fuel debris, which is not feasible. The water is not borated relying on the circumstantial evidence that the xenon gas monitoring in the containment vessels does not show a sign of criticality. The criticality condition of fuel debris may worsen due to the gradual drop of its temperature, or the change of its geometry by aftershocks or the retrieval work, that may lead the criticality. To avoid criticality and its severe consequences, a certain principle of criticality control must be established. There may be options, such as prevention of the criticality by coolant water boration or by neutronic monitoring, prevention of the severe consequences by intervention measures against criticality, etc. Every option has merits and demerits that must be adequately evaluated toward selection of the best principle.

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