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Thermal conductivity change in high burnup MOX fuel pellet

高燃焼度MOXペレットの熱伝導率変化

中村 仁一; 天谷 政樹; 永瀬 文久; 更田 豊志

Nakamura, Jinichi; Amaya, Masaki; Nagase, Fumihisa; Fuketa, Toyoshi

高燃焼度MOX燃料棒とUO$$_{2}$$燃料棒が商用炉BWRで照射された燃料棒から再加工された。試験燃料棒には、燃料中心温度計,燃料棒伸び計,内圧計が取り付けられ、ノルウェーのハルデン炉(HBWR)で再照射された。MOX燃料棒とUO$$_{2}$$燃料棒の燃焼度はそれぞれ84GWd/tHMtと72GWd/tに達した。燃料中心温度は、照射試験中連続して測定された。高燃焼度燃料の熱伝導率変化を測定温度と燃料挙動解析コードFEMAXI-6による計算データを比較することにより評価した。比較結果からMOX燃料の熱伝導率が約80GWd/tの高燃焼度においてはUO$$_{2}$$の熱伝導率と同程度であることが示唆された。熱伝導率に影響を与える固溶性FPや照射欠陥の蓄積効果が高燃焼度領域で支配的になるため、Pu原子の不純物効果が燃焼度の増加とともに次第に減少すると考えられる。

High burnup MOX and UO$$_{2}$$ test rods were prepared from the fuel rods irradiated in commercial BWRs. Each test rod was equipped with a fuel center thermocouple, and was re-irradiated in the Halden boiling water reactor (HBWR) in Norway. The burnups of MOX and UO$$_{2}$$ test rods reached about 84 GWd/tHM and 72 GWd/t, respectively. Thermal conductivity change in high burnup fuel was evaluated from the comparison between the measured fuel temperature and the data calculated by using the fuel analysis code, FEMAXI-6. The comparison results suggested that the thermal conductivity of MOX fuel pellet is comparable to that of UO$$_{2}$$ fuel pellet in the high burnup region around 80 GWd/t. It is probable that the impurity effect of Pu atom gradually diminishes with increasing burnup because other factors which affect pellet thermal conductivity, such as the accumulation effect of soluble fission products and irradiation-induced defects in crystal lattice, become dominant in high burnup region.

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