Follow-up experimental study on causes of the low-enthalpy failure observed in the reactivity-initiated-accident-simulated test on LWR additive fuels

添加物入りLWR燃料を対象とした反応度事故模擬実験で観察された低破損限界の原因に関する実験研究

三原 武  ; 垣内 一雄 ; 谷口 良徳 ; 宇田川 豊  

Mihara, Takeshi; Kakiuchi, Kazuo; Taniguchi, Yoshinori; Udagawa, Yutaka

Test OS-1, the reactivity-initiated-accident (RIA) -simulated test on 64 GWd/tU high burnup fuel with ADOPT (chromia-and-alumina-doped UO) pellets resulted in a failure at the lowest fuel enthalpy increase among the tests ever performed at the NSRR on high burnup fuels from 40 to 65 GWd/tU. Roles of both fuel pellets and cladding behaviours in this remarkable observation are being investigated. A comparative RIA-simulated test OS-2 was thus performed on undoped fuel that had been base-irradiated in the identical fuel assembly with the OS-1 rod. The transient records acquired during Test OS-2 indicated that the rod survived without fuel failure. Radially projected hydride lengths in the cladding metallic layer were evaluated from the metallograph images observed in the vicinity of the OS-2 test rod and compared with other failure test cases. The comparison suggested that the hydride morphologies affected the low failure limit of the OS-1 rod and also explains the survival of the OS-2 rod, to some extent. Nevertheless, as the OS-2 rod survived 100 J/g higher peak fuel enthalpy than the OS-1 failure limit, further experimental and analytical studies are desired to pursue other possible causes: additional loading specific to ADOPT pellets, difference in the pellet/cladding bonding condition, and so on.