Reduction of accident occurrence frequencies by taking safety measures for defense-in-depth level 4 in a sodium-cooled fast reactor

Abstract

The effectiveness evaluation of safety measures against severe accident is necessary for restart of experimental sodium-cooled fast reactor Joyo in Japan. These safety measures correspond to those in defense-in-depth (DiD) level 4. In the previous study, an internal event level-1 probabilistic risk assessment (PRA) at power was performed to calculate frequencies of the accident sequences of failure of safety measures in DiD levels 1 to 3, to identify dominant accident sequence groups, and to identify dominant accident sequence for selecting important accident sequences in each accident sequence group, which are needed for implementing the effectiveness evaluation of safety measures in DiD level 4. Based on this, the present study implemented internal event level-1 PRA at power to show quantitatively reduction of those occurrence frequency by the safety measure in the DiD level 4. As a result, the frequency of each accident sequence group decreased significantly, and the total frequency of the accident sequence groups decreased to about 1×10^-6 /reactor-year, which is about 1/1000 times the one estimated in the previous study. The protected loss of heat sink was the largest contributor in all the accident groups, and a dominant accident sequence in each accident group was also identified in this study.