Stepwise Probabilistic and Deterministic Safety Analyses of Loss of Coolant Accident under Shutdown Conditions

Abstract

A risk of loss of coolant accidents (LOCA) under shutdown condition is evaluated using a stepwise probabilistic and deterministic approach. The method consists of two stepwise analyses. One is the evaluation of 95 %-confidence core damage frequency of a large LOCA under shutdown states using probabilistic safety assessment (PSA) to determine plausible break size under shutdown condition. This is followed by the second step of thermal-hydraulic analysis of loss of coolant accident in terms of required operator response time to mitigate the shutdown LOCA. The probabilistic safety analysis applied for a three-loop pressurized water reactors provides the result that the 95 %-confidence core damage frequency of large LOCA under shutdown states is below 8 % of that under full power operation, and this justifies existing full power LOCA analysis would be sufficient in terms of risk to demonstrate the performance of the existing emergency core cooling system (ECCS). On this basis, plausible break size under shutdown states is determined to be 6 inches and thermal-hydraulic safety analyses of small break LOCAs of the size under a limiting shutdown state of the said plants are performed by applying more realistic assumptions of the model such as loop seal clearing restriction. However, initial plant conditions such as pressure and temperature are selected to be limiting with respect to break flow and core uncovery. It is demonstrated that the cladding failure is not expected provided that an operator establishes a flow from one high head safety injection pump within 15 minutes after the recognition of appropriate symptoms of LOCA.