Application of risk informed safety margin characterization to the analysis of a pressurized water reactor

The sequence Core Damage Frequency of a pressurized water reactor has been quantified in three initiating events that are Medium Break Loss of Coolant Accident, Small Break Loss of Coolant Accident and Steam Generator Tube Rupture, following a realistic methodology called risk informed safety margin characteristic. The surrogate plant analyzed in the study is a typical pressurized water reactor. The plant adopted two Westinghouse Three-Loop Pressurized Water Reactors with rated thermal power of 2,830 MWt. The phenomenon identification and ranking table is applied for uncertainty analysis. The mitigation actions as described in plant specific Probabilistic Risk Assessment include cooldown and depressurization, emergency cooldown and depressurization, high head safety injection, high head safety recirculation, low head safety recirculation and Refueling Water Storage Tank replenishment. These mitigation actions are analyzed by thermal hydraulic system analysis code RELAP5-3D to determine the successfulness of the actions. The uncertainties of input parameters of the plant conditions are included, and the time of mitigation action executed is treated as one of the input uncertainties. The results of realistic methodology show a decrease in Core Damage Frequency for all three analyzed events in comparison with conventional methodology. The differences between three initiating events are also discussed.