Evaluation of Containment Failure Probability by Ex-Vessel Steam Explosion in Japanese LWR Plants

Abstract

The containment failure probability due to ex-vessel steam explosionswas evaluated for Japanese BWR and PWR model plants. A stratified MonteCarlo technique (Latin Hypercube Sampling (LHS)) was applied for theevaluation of steam explosion loads, in which a steam explosionsimulation code JASMINE was used as a physics model. The evaluation wasmade for three scenarios: a steam explosion in the pedestal area or inthe suppression pool of a BWR model plant with a Mark-II containment,and in the reactor cavity of a PWR model plant. The scenario connectingthe generation of steam explosion loads and the containment failure wasassumed to be displacement of the reactor vessel and pipings, andfailure at the penetration in the containment boundary. We evaluated theconditional containment failure probability (CCFP) based on thepreconditions of failure of molten core retention within the reactorvessel, relocation of the core melt into the water pool withoutsignificant interference, and a strong triggering at the time of maximumpremixed mass. The obtained mean and median values of the CCPF were 6.4x10(-2) (mean) and 3.9 x 10(-2) (median) for the BWR suppression poolcase, 2.2x 10(-3) (mean) and 2.8 x 10(-10) (median) for the BWR pedestalcase, and 6.8 x 10(-2) (mean) and 1.4x 10(-2) (median) for the PWRcavity case. The evaluation of CCFPs on the basis of core damage needsconsideration of probabilities for the above-mentioned preconditions.Thus, the CCFPs per core damage should be lower than the values givenabove. The specific values of the probability were most dependent on theassumed range of melt flow rate and fragility curve that involvedconservatism and uncertainty due to simplified scenarios and limitedinformation.