Level 1 PRA for external vessel storage tank in whole core refueling in advanced loop-type sodium-cooled fast reactor

Yamano, Hidemasa   ; Naruto, Kenichi*; Kurisaka, Kenichi ; Nishino, Hiroyuki  

Spent fuels are transferred from a reactor core to a spent fuel pool through an external vessel storage tank (EVST) filled with sodium in sodium-cooled fast reactors in Japan. This paper describes identification of dominant accident sequences leading to fuel failure, which was achieved through probabilistic risk assessment for the EVST designed for a next sodium-cooled fast reactor plant system in Japan to improve the EVST design. The safety strategy for the EVST involves whole core refueling-early transfer of all core fuel assemblies into the EVST-assuming a severe situation that results in sodium level reduction leading finally to the top of the reactor core fuel assemblies in a long time. This study introduces the success criteria mitigation along the decay heat decrease over time. Based on the design information, this study has carried out identification of initiating events, event and fault tree analyses, a probability analysis for human error, and quantification of accident sequences. The fuel damage frequency of the EVST was evaluated to be approx. 10/year. This study also quantitatively showed the effectiveness of design improvement.