Probability distribution of ex-vessel steam explosion loads considering influences of water level and trigger timing

Abstract

The probability distribution of steam explosion loads in terms of uncertainties in the model parameters and initial/boundary conditions was examined with an ex-vessel steam explosion scenario in a typical PWR reactor cavity geometry. Six uncertainty parameters, the melt inlet diameter and velocity, initial melt temperature, water pool depth, melt droplet diameter during premixing and the triggering time, were chosen and modeled by probability profiles. Input data sets generated by the Latin-Hypercube Sampling (LHS) were fed to the steam explosion simulation code, JASMINE, and probability profiles of the loads were obtained. The triggering was basically assumed at the first peak of the premixed mass (PPM), that was defined as molten corium mass in the low void fraction zone (less than 0.75). The delay time from PPM was treated as an uncertainty variable. The kinetic energy and impulse on the bottom were chosen as the load indexes. Six LHS sets were produced in 2 groups, one with triggering always given at PPM and another with varied triggering time. The former showed larger loads and narrower distributions of them. The loads were strongly correlated with the premixed mass as an intermediate variable, which depended primarily on the melt jet inlet diameter. The water depth showed nearly linear impact on the potential load. The mean energy conversion ratio based on the premixed mass in all the cases fell in relatively narrow range around 4%.