Fuel-cladding chemical interaction in MOX fuel rods irradiated to high burnup in an advanced thermal reactor

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田中 康介  ; 前田 宏治  ; 佐々木 新治  ; 生澤 佳久   ; 安部 智之

Tanaka, Kosuke; Maeda, Koji; Sasaki, Shinji; Ikusawa, Yoshihisa; Abe, Tomoyuki

新型転換炉原型炉「ふげん」でペレットピーク燃焼度47.5GWd/tまで照射したMOX燃料E09において、被覆管内面に着目した組織観察及び元素分布測定を実施した。被覆管内面における反応層厚さについては、同程度の燃焼度のUO燃料における測定値と概ね等しい結果が得られた。また、局所的に燃焼度が増加するPuスポット部においても、その有意な増加は認められず、周辺マトリックス部における値と明確な違いは観察されなかった。被覆管内面にはボンディング層の発現が認められ、同層の形態、存在する元素、発現照射条件は、FPガス放出の大きいBWR用UO燃料における結果と類似していた。

The performance of MOX fuel irradiated in the advanced thermal reactor, FUGEN, to the burnup of 47.5 GWd/t, was investigated by using a telescotpe, optical microscope, SEM and EPMA. Observations focused on elucidating the corrosion behavior of the cladding inner surface. A reaction layer was observed at burnups higher than about 35 GWd/t. The relationship between the thickness of the reaction layer and burnup was similar to that reported in the literature for conventional UO fuel and other MOX fuels. The existence of a plutonium spot near the outer surface of the fuel pellet had no significant effect on the thickness of the reaction layer. A bonding layer was observed on the cladding inner surface. Its morphology and elemental distributions were not so different from those in BWR UO fuel pins irradiated to high burnup, in which the fission gas release rate is high. In addition, the dependences of bonding layer formation on the burnup and linear heat rating were similar to results of UO2 fuel pins. It was, thus, suggested that the bonding layer formation mechanism was similar in both UO2 and MOX fuel pins.