Development of LOCA Response Strategy during Shutdown Operation in Kori 3 and 4 and Ygn 1 and 2 Units

The abnormal transients and accidents during shutdown operation modes of pressurized water reactor, in contrast with those during full power operation modes, have been recently issued due to their potential risk for leading to a severe abnormal condition because the various safety-related systems and equipments may be unavailable. The NRC issued Loss of Reactor Coolant Inventory and Potential Loss of Emergency Mitigation Functions While in a Shutdown Condition, on January 12, 1995, to alert addressees to an incident at the Wolf Creek plant involving the loss of reactor coolant inventory while the reactor was in a hot shutdown condition. Considering those situations, abnormal response mitigation strategies in shutdown operation modes have been developed by evaluating the plant specific thermal hydraulic behavior following LOCA. The objectives of this study are: (1) to verify the effectiveness of abnormal operating procedure (AOP) and emergency core cooling system (ECCS) on reactor safety at shutdown operation, developing the operation strategy at the condition and abnormal operation guideline in the light water reactors; (2) to enhance understanding of the thermal-hydraulic phenomena during abnormal and accident conditions. In this paper, some accident scenarios are analyzed to develop the abnormal operating strategy following LOCA in shutdown operation modes for Kori 3 and 4 and Ygn 1and 2 units. This study shows the adequacy of operator action time to mitigate the LOCA, the thermal hydraulic behavior, reactor inventory distribution and possibility of cold overpressurization after SBLOCA during shutdown operation modes using computer code RELAP5 /MOD3.2