ICONE19-44124 Development of PIRT for Fast Reactor under Natural Circulation Decay Heat Removal Operations

Toward the commercialization of fast reactors, a design study of Japan Sodium-cooled Fast Reactor (JSFR) is being performed. In this design study, fully natural circulation system is adopted as the decay heat removal system to satisfy the requirements of economical competitiveness and higher reliability. We have been developing an evaluation method of core hot spot which can be applied to natural circulation decay heat removal. Since the method is currently based on conservative assumptions and data, there is room for further rationalization of the safety margin which can be achieved by conducting best estimate analyses with confidence and with quantified uncertainty of results. This paper describes a development of PIRT (Phenomena Identification and Ranking Table) for JSFR under natural circulation decay heat removal operations as the first step to improve the evaluation method. In this study, we chose the highest coolant temperature in the core as the safety parameter of JSFR under natural circulation decay heat removal operations and divided the time region into two periods: one of which is from scram to the moment when the secondary peak of the coolant temperature appears just after the end of flow coast-down and the other period is after the secondary peak to the moment when the balance between the total decay heat removal and the core heat generation has been reached. Then we chose important physical phenomena occurring in the primary cooling system and decay heat removal system of JSFR in each time region, and also picked up the uncertainties which should be considered when evaluating these phenomena. Finally the sensitivity analyses of the uncertainties were performed. Uncertainties come from input parameters, physical modeling, and numerical methods employed. In this study, the input uncertainties and modeling uncertainties were assessed. As a result, the highly influential uncertainties were identified which are related to important physical phenomena of the core hot spot evaluation for JSFR under natural circulation decay heat removal operations.