Development of Risk Assessment Methodology of Decay Heat Removal Function Against Natural External Hazards for Sodium-Cooled Fast Reactors: Project Overview and Volcanic PRA Methodology

Abstract

This paper describes mainly volcanic probabilistic risk assessment (PRA) methodology development for sodium-cooled fast reactors in addition to the project overview. In the volcanic PRA, only the effect of volcanic tephra (ash) was taken into account because there is a great distance between a plant site assumed in this study and volcanos. The volcanic ash could potentially clog air filters of air-intakes that are essential for the decay heat removal. The degree of filter clogging can be calculated by atmospheric concentration of ash and tephra fallout duration and also suction flow rate of each component. The atmospheric concentration can be calculated by deposited tephra layer thickness, tephra fallout duration and fallout speed. This study evaluated a volcanic hazard using a combination of tephra fragment size, layer thickness and duration. In this paper, each component functional failure probability was defined as a failure probability of filter replacement obtained by using a grace period to a filter failure limit. Finally, based on an event tree, a core damage frequency was estimated about 3 × 10−6/year in total by multiplying discrete hazard probabilities by conditional decay heat removal failure probabilities. A dominant sequence was led by the loss of decay heat removal system due to the filter clogging after the loss of emergency power supply. A dominant volcanic hazard was 10−2 kg/m3 of atmospheric concentration, 0.1 mm of tephra diameter, 50–75cm of deposited tephra layer thickness, and 1–10 hr of tephra fallout duration.