Loss of Coolant Accident Analysis by RELAP5/MOD3.3 in Support to Human Reliability Analysis

Human Reliability Analysis (HRA) was developed as the need to describe incorrect human actions in the context of Probabilistic Safety Analysis (PSA). In the past simplified computer codes has been used for PSA. However, the recently released PSA standard recommends the use of best-estimate codes to improve the quality of a PSA. Therefore the aim of this study was to estimate the operator action success criteria time window in the case of small or medium loss of coolant accident (LOCA), which was needed for updated HRA. The specified time windows are important for HRA because in the case of long time windows the recovery operator action may be considered. The RELAP5/MOD3.3 best-estimate computer code and the qualified RELAP5 input model representing a two-loop pressurized water reactor, Westinghouse type, were used for the analysis. Analyses of LOCAs with different break sizes were performed. The results showed that larger breaks depressurize (through the break) in any case below the accumulator injection setpoint after some time and AFW is not needed. For smaller breaks it was determined which break size first heat-up above the core damage limit. For this break parametric studies were performed with different delays of AFW in order to determine the time window. Depending on the secondary pressure the SG PORVs automatically open and close. The results showed that another operator action is also needed for success, i.e. manual full opening of one steam generator PORV. With these two actions the primary pressure was depressurized in due time to prevent core heatup. It was determined that more than one hour is available for operator actions. As the experience of operators with plant simulator shows that actual time for performing the event is 1-10 minutes, this gives additional time for possible recovery action.(author)