Evaluation of severe accident risk during mid-loop operation at Surry unit-1

In the past most probabilistic risk assessments (PRAs) of severe accidents in nuclear power plants have considered initiating events which could potentially lead to core damage and containment failure during normal full power operation. However, recent studies and operational experience during periods while plants were shutdown for maintenance or refueling indicated that potential accidents initiated during low power operation or shutdown conditions could also potentially become important contributors to risk. In 1989, the Nuclear Regulatory Commission (NRC) began an extensive program to assess the risk during low power and shutdown operation. Two plants, Surry (a pressurized water reactor, PWR) and Grand Gulf (a boiling water reactor ,BWR) were selected as the plants to be studied.This paper describes an analysis of accident progression and offsite consequences (level 3 PRA) carried out for the Surry plant. The focus of the level 3 PRA was on mid-loop operation, which is a plant operational state (POS) that can occur while the plant is shutdown for maintenance or refueling. Mid-loop refers to a configuration when the reactor coolant system is lowered to the mid-plane of the hot leg to allow essential maintenance to be performed. This operational state was selected after an initial coarse screening study indicated that reduced inventory during mid-loop operation could pose higher risk than other POSs.