Effects of shutdown period extension on core damage frequency

The main objective of this study is to investigate the reduction possibilities of the CDF of LPSD conditions in a typical Westinghouse type PWR. For this, a comprehensive LPSD PSA model has been developed for a typical nuclear power plant (NPP). Total LPSD CDF is estimated to be 2.27E−06/year, assuming refueling outage as shutdown representative. To decrease it, CDF of LPSD conditions is re-evaluated for sensitivity case with shorter shutdown period. The results represent 13% decrease in total CDF of LPSD conditions. The contribution of CDF of the most dominant POSs changes from 66.24 to 10.45%, for POS 9 and POS 7, to 70.20 and 1.99%, respectively. The study also shows the proportionality of the change in percent contributions of POS_CDF in total CDF of LPSD conditions with that of POS duration in total duration of LPSD conditions. In this regards, percent change in CDF of POS 7 and POS 5 whose durations undergoes −4.76 and 3.31%, respectively, is −83.38 and 65.5%. Changes in CDF of POSs are also compared. Change in CDF of POSs is proportional to change in POS duration. However, for POS 9 and POS 7, with the higher primary contribution in total CDF of LPSD conditions, the percent change is more conspicuous. The study also shows the proportionality of the change in percent contributions of POS_CDF in total CDF of LPSD conditions with that of POS duration in total duration of LPSD conditions. Therefore, it is concluded that shortening of POSs with higher contributions (yet might not with longer duration) in total duration of LPSD conditions, leads to more decrease in total CDF of LPSD conditions. The results are useful for outage planning and management decision-making.