Fuel pin behavior up to cladding failure under pulse-type transient overpower in the CABRI-FAST and CABRI-RAFT experiments

CABRI-FAST及びCABRI-RAFT実験におけるパルス型過出力試験下での被覆管破損前燃料挙動

深野 義隆  ; 小野田 雄一 ; 佐藤 一憲 

Fukano, Yoshitaka; Onoda, Yuichi; Sato, Ikken

In the CABRI-FAST and CABRI-RAFT programs within a collaboration with the IRSN and FZK, five pulse-type transient overpower tests were performed in order to study fuel pin behavior and failure condition in the Unprotected Loss-of-Flow (ULOF) accident. In these tests, two types of low-smear-density fuels irradiated in the French Phenix reactor at different burn-up levels were used so that an experimental database extension from the former CABRI-1 and CABRI-2 programs can be obtained. Pin failure took place in three of these tests giving information on the failure threshold. In two tests, no pin failure took place and useful information related to the transient fuel behavior up to failure and failure mechanism was obtained. These test results were interpreted through detailed analysis of experimental data and PAPAS-2S code calculations. In these calculations, pretransient fuel characteristics obtained from the sibling fuels were reflected, such that the uncertainty of the boundary condition can be minimized. Through the comparison among these tests and formerly existing CABRI tests, generalized understanding on the transient fuel behavior was obtained. It was concluded that the low-smear-density fuel mitigates cavity pressurization, thereby enhancing the margin-to-failure. It was also understood that this failure-threshold enhancing capability is dependent on the type of transient.