A MELCOR Analysis of Fission Product Behaviour during Severe Accident: Influence of Containment Geometry on Airborne Radioactive Aerosols

Sammanfattning

Emergency planning zone (EPZ) boundaries of small modular reactors (SMR) may be reduced compared to large light-water reactors (LLWR) for reasons including their lower fuel inventory and inherent safety features, such as the high surface area-to-volume (A/V) ratio of the reactor containment. The purpose of this thesis is to investigate the influence of the A/V ratio on the source term during a severe accident by comparing the airborne radioactive aerosol mass per unit of power of small and large-scale boiling water reactors (BWR) featuring different A/V ratios. MELCOR severe accident code was used to develop simple models of the BWRX-300 and Economic Simplified Boiling Water Reactor (ESBWR) following a large-break loss of coolant accident (LB-LOCA) using boundary conditions from a Nordic BWR MELCOR model. Results show that at a time of 4 hours, the evacuation time for the precautionary action zone (PAZ), the airborne radioactive aerosol mass per unit of power in the BWRX-300 model was reduced 73% compared to its large-scale counterpart. In all tested cases, the containment with the highest A/V ratio resulted in the highest decontamination factor (DF) for the first 4.5 hours of the accident. The findings of this work can provide insight into the characterization of the source term for SMRs which is a key input in establishing EPZ boundaries. Further modelling of phenomena during the ex-vessel phase is necessary to conclude on findings during the later stages of the accident.