Probabilistic assessment of the containment performance during severe accidents in PWRs

The aim of this paper is to discuss a way of assessing the external radioactive releases from the NPP Containment System during severe accidents, using information from the probabilistic assessment of core melt accidents (level 1 PSA). The discussion is mainly concerned with the definition of the Containment Accident Scenarios, which include consideration of both the degraded core physical status and the operating status of the Containment Safety Systems. For the discussion, reference is done to the available results of the PUN-PSA that is the level 1 PSA of the Italian PWR Standard Project. The Containment Reference Scenarios were considered as bases for developing the containment event trees, and determining the probabilistic distribution curves of the external radioactive releases. Calculations were made by the AGIP S.p.A. upon a contract with ENEA/DISP. Results of calculations were examined in view of defining probabilistic criteria for the containment performance, being the criteria aimed at minimizing the sanitary consequences and land contamination from external releases during severe accidents with the adoption of practical external protective actions. The paper concludes that satisfaction of the targets could require only minor design changes and consideration of a proper accident management by the operating crew through the use of in-plant emergency procedures