Level-1 internal event PRA of a PWR during midloop operation

Traditionally, probabilistic risk assessments (PRAS) of severe accidents in nuclear power plants have considered initiating events potentially occurring only during full-power operation. Some previous screening analyses that were performed for other modes of operation suggested that risks during those modes were small relative to full-power operation. However, more recent studies and operational experience have implied that accidents during low power and shutdown could be significant contributors to risk. During 1989, the U.S. Nuclear Regulatory Commission (NRC) initiated an extensive program to carefully examine the potential risks during low-power and shutdown operations. The program includes two parallel projects being performed by Brookhaven National Laboratory and Sandia National Laboratories. Two plants, Surry (pressurized water reactor) and Grand Gulf (boiling water reactor), were selected as the plants to be studied. The objectives of the program are to assess the risks of severe accidents initiated during plant operational states other than full-power operation and to compare the estimated core damage frequencies, important accident sequences, and other qualitative and quantitative results with those accidents initiated during full-power operation as assessed in NUREG-1150. The scope of the program includes that of a level-3 PRA. This paper presents the approach utilized in the level-1 PRA for the Surry plant and discusses the results obtained