Development of reliability evaluation methodology on natural circulation heat removal in level-1 PSA for Japan sodium-cooled fast reactor

Yamano, Hidemasa  ; Sakai, Takaaki; Kurisaka, Kenichi 

This paper provided a failure probability of the natural circulation heat removal function that is expected to actuate only one direct reactor auxiliary cooling system assuming the loss of two primary reactor auxiliary cooling systems. The probability was obtained in the following steps. At first, a reference case was defined through parametric analyses for a realistic evaluation in the PSA. After selecting key uncertainty parameters according to the PIRT, sensitivity analyses were carried out to develop a response surface. Based on this response surface, Monte Carlo calculations were performed assuming appropriate probability distributions of uncertainty parameters, leading to an exceedance probability in terms of temperature difference from the reference case result. In this study, the failure probability was determined above a safety criterion of the coolant boundary temperature. The failure probability of the reference evaluation was very low.