Selection of pressurized thermal shock (PTS) transients to include in PTS risk analyses

摘要

Pressurized thermal shock (TPS)-risk-significant events have not typically been included in the probabilistic risk analyses (PRAs) of nuclear power plants (e.g. those used to perform the individual plant evaluations (IPEs)). This paper describes the process used to identify the PTS-risk-significant events to be added to the PRAs of plants being studied in an on-going Nuclear Regulatory Commission (NRC) PTS risk evaluation. The process requires consideration of the five necessary features of PTS events: (1) and (2) fast neutron embrittlement and a crack or flaw in the reactor pressure vessel; (3) rapid cooling of the primary system; (4) a sustained low temperature (< approx 176.7 deg C {350 deg F} for vessels with reference temperature for the nil-ductility transition (RT_(NDT)) less than 132.2 deg C {270 deg F}); and (5) repressurization (or maintenance of high primary system pressure). Without the presence of all five of these features, no event will pose a significant PTS-related risk. This paper assumes the presence of the first two (materials-related) features, and discussed the remaining three (transient-related) features. To date, no radically new initiators or event sequences have been identified. However, since this study considers the added operational difficulties that may be caused by the loss of support systems and the effects on the operators of more complex initiation events, it is believed that compared to previous studies, more realistic PTS risk results will be obtained.