Comparative Analysis of Timeliness and Reliability of OBE Alarm Strategies in Nuclear Power Plants

Based on 1642 records of 31 earthquakes in China from 2003 to 2015, the timeliness and reliability of OBE alarm strategies used in 2^nd and 3^rd GNP were compared and analyzed. The results showed that, for the same ground motion, when OBE was beyond limit for both 2^nd and 3^rdGNP, the OBE alarm of 3^rd GNP were later than that of 2^nd GNP, with an average delay of 20 seconds. Peak ground acceleration could be as high as 0.7 g before 3^rd GNP alarmed. OBE over-limit determination performed using data of 108 stations from the region of Ⅵ intensity of 4 destructive earthquakes showed that the proportion of stations exceeded limit among 3^rd GNP, 2^ndGNP（CGN）, and 2^nd GNP（CNNC） were 41%, 63% and 76%. It indicated that due to the aim of lowering the rate of unnecessary shutdown caused by near-field earthquakes with small magnitude and large amplitude pulses and earthquakes with small amplitude and long duration ground motions, two-parameter decision method was applied in threshold selection, thus the timeliness and reliability of 3^rd GNP was reduced, which is insecure for important nuclear facilities.