Consideration on rationalization of reactor safety systems; Reliability assessment of decay heat removal systems in commercialized sodium cooled FBR concepts

not registered

For commercialization of FBR (Fast Breeder Reactor), the reactor safety systems are needed not only to have necessary and enough reliability but also to decrease the amount of materials in order that the FBR has economical competitiveness against LWR (Light Water Reactor) and another electrical power supply systems. ln this study, reliability assessment, which calculates the occurrence frequencies of PLOHS (Protected Loss Of Heat Sink) sequences, was performed for three kinds of large size sodium cooled fast reactors with decreased number of loops and of support systems examined in the Feasibility Studies on Commercialized FBR System. The realistic evaluation was performed using the failure rate data of components based on the domestic LWR operating experience. The result is: (1)AIl of the three kinds of commercialized FBR concepts are expected to achieve the frequencies of PLOHS sequences caused by internal events of the plant under 10/ry, assuming that common-mode-failure is excluded. ln addition, the dominating cause of the coincidence of the incidents and the information that improves reliability of decay heat removal systems are summarized for each concept. (2)In order to evaluate design margin, the reliability assessment was performed in the case that the capacity of natural circulation cooling was reinforced from 100%/33 loops to 50%3 loops or fiom 25%4 loops to 100%/34 loops easing to succeed decay heat removal. ln that case, it is confirmed that the frequencies of PLOHS sequences decrease by about one order of magnitude.