Analysis of PWR SBO sequences with RCP passive thermal shutdown seals

The operation of recently implanted low-leakage seals after Fukushima has altered the analysis of classical PWR Station Blackout (SBO) sequences , as Seal Loss of Coolant Accident (SLOCA) is no longer one of the dominant factors in the accident progression . An analysis of different management strategies in non-SLOCA sequences has been performed by means of the Integrated Safety Assessment (ISA) methodology using the SCAIS-MAAP model of a 3-Loop PWR Westinghouse design. Through the use of the Damage Domain concept(i.e. the region of the uncertain crew actuation times or physical parameters space where each damage limit is exceeded for each sequence), the times for reaching different damage limits are obtained. Results evidence the positive impact of low-leakage seals, which greatly increase the margin to core uncoveryand reduce core damage frequency. Results also allow concluding that an SBO is dominated, namely by the Auxiliary Feed-Water (AFW) mass flow(turning blind AFW management into an essential procedure), SLOCA (in case the new low-leakage seals fail or they are not present), an excessive AFW mass flow (leading to Turbine-Driven Pump failure) and the DC failure time (losing control valves and the instrumentation).