Simple method for risk assessment of nuclear power plant refueling outages

This paper discusses the method used for probabilistic risk assessment (PRA) studies of the Monticello Nuclear Power Plant's 1993 and 1994 refueling outages. The method made full use of work already completed as part of the United States Nuclear Regulatory Commission's request for at-power risk studies known as individual plant examinations (IPEs) and therefore required less resources than many other techniques for shut-down risk management. The four plant models used to examine various plant outage states are described and discussed. The models are (1) reactor during manual shut-down, (2) reactor in cold shut-down with head installed, (3) reactor with head off and (4) fuel storage pool. These models were manipulated before the refueling outage began using the computer programs SETS and CAFTA to simulate different decay heat levels and various components out of service during time segments of the outage. This was used to identify higher risk periods during the outage and to sensitize operators and other personnel to the important equipment and activities during the daily briefings during the outage. Several techniques are discussed in this paper, including (1) methods to modify an existing PRA model for lower decay heat conditions, (2) editing existing cutsets to simulate various equipment availability states and (3) the use of assumptions to simplify the analysis. (orig.)