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模擬燃料デブリ(U,Zr)O$$_{2}$$の過酸化水素水溶液への溶出反応

Dissolution reaction of simulated corium debris, (U,Zr)O$$_{2}$$ in aqueous hydrogen peroxide solution

熊谷 友多   ; 高野 公秀 ; 渡邉 雅之   

Kumagai, Yuta; Takano, Masahide; Watanabe, Masayuki

福島第一原子力発電所の事故後の炉内には、燃料溶融により生じた燃料デブリが留まっている。炉内は強い放射線環境のため、冷却水の放射線分解により過酸化水素が生成する。過酸化水素はウランを水溶性の高い六価に酸化する。そのため、炉内環境で燃料デブリが徐々に溶解することが懸念される。そこで、燃料デブリの冷却水への溶解挙動の推定を目指して、模擬デブリとして(U,Zr)O$$_{2}$$を用いて過酸化水素水溶液中でのウランの溶出を調べた。その結果、(U,Zr)O$$_{2}$$からのウランの溶出は酸化ウラン(IV)(UO$$_{2}$$)に比べて遅いことが分かった。また、UO$$_{2}$$では消費された過酸化水素の約70%がウランの溶出に寄与するのに対して、(U,Zr)O$$_{2}$$では1%程度しか寄与しないことが分かった。これらの結果から、(U,Zr)O$$_{2}$$表面ではウランの酸化に寄与しない過酸化水素の分解反応が進行し、ウランの溶出が抑制されることが示唆された。

In the severe accident at Fukushima, the reactor cores of Units 1, 2 and 3 overheated and largely melted. The molten fuel debris possibly dissolves in water due to the intense ionizing radiations in the reactors. Ionizing radiations decompose water and generate hydrogen peroxide. Hydrogen peroxide may oxidize uranium in the debris from tetravalent to more soluble hexavalent. In order to understand the effect of hydrogen peroxide on dissolution of the fuel debris, leaching of uranium from simulated fuel debris in aqueous hydrogen peroxide solution was studied. The simulated debris used in this study was solid solution of UO$$_{2}$$ and ZrO$$_{2}$$ ((U,Zr)O$$_{2}$$). The leaching experiments were performed on (U,Zr)O$$_{2}$$ and on UO$$_{2}$$ for comparison. The observed leaching rate of uranium from (U,Zr)O$$_{2}$$ was lower than that measured for UO$$_{2}$$. The uranium concentration leached from (U,Zr)O$$_{2}$$ was only about 1 % of consumed hydrogen peroxide, whereas that from UO$$_{2}$$ was as high as 70 % of consumed hydrogen peroxide. The results suggest that dissolution of uranium from (U,Zr)O$$_{2}$$ was inhibited due to decomposition of hydrogen peroxide on (U,Zr)O$$_{2}$$ without oxidation of uranium.

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