Study On The Influence Of Environmental Factors On Real-time Risk Monitor Of Passive Nuclear Power Plants

Probabilistic safety assessment(PSA)has played an increasingly important role in nuclear power safety in recent years.PSA-based Risk Monitor(RM)can provide time-varying risk information to nuclear power operation and management personnel according to the power plant configuration and provide a reference for power plant operation decisions.The accuracy and efficiency of transforming the baseline PSA model into a risk monitoring model are directly related to the accuracy and timeliness of risk monitoring and analysis results.The processing of dynamic events plays an essential role in model transformation.This paper analyzes the influence of environmental factors on the risk monitoring of passive power plants in dynamic events.This paper gives the analysis process of risk assessment considering environmental factors,and the AP1000 loss of off-site electrical accident(LOOP)is analyzed.The influence of environmental factors on the initial event and mitigation system was considered,respectively,and the environmental factors were divided according to seasonal temperature changes.The frequency of LOOP and the failure probability of the passive containment cooling system(PCS)were calculated using the fault tree model under different environmental factors.Under hightemperature weather in summer,the frequency of the initial event is 1.1 times the reference condition,and the failure probability of the PCS system is 1.85 times the reference condition.Under cold conditions in winter,the frequency of the initial event is 2.13 times the reference condition,and the failure probability of the PCS system is 0.12 times the reference condition.The dynamic factors under different environmental factors are given.In Risk Monitor,the selection of dynamic factors can complete the correction of the occurrence frequency and system failure probability.The event tree/fault tree model under different environmental factors was constructed by Risk Spectrum software,and the core damage frequency(CDF)was obtained by analyzing LOOP events.The results showed that the CDF value was 1.28E-07/a under high-temperature conditions in summer,1.62E-08/a under cold conditions in winter,1.34E-08/a under suitable conditions in spring and autumn,and the annual benchmark CDF value was 7.58E-08/a.There are significant differences in CDF values between different environmental factors and between other environmental factors and the yearly baseline.It can be seen that the impact of various environmental factors on the risk assessment results cannot be ignored.Taking the changes in environmental factors into the risk monitoring model can reflect the risk level of the power plant more honestly and accurately.Through the analysis of accident sequence and minimum cut set under different environmental factors,the basic event that has the most significant influence on LOOP accidents is the main external network fault during the maintenance of the auxiliary external network.In mitigation systems,functional failures of PCS systems affect multiple minimum cut sets in configuration 1.Therefore,further strengthening the security of the leading external power network during the maintenance of the auxiliary external power network can reduce the frequency of initial LOOP events.Meanwhile,the maintenance of the additional external power network should be avoided in summer under high-temperature conditions,which will also reduce the influence of the high failure probability of the PCS system on the safety of the power plant in summer.In addition,this paper analyzes the △CDF generated during emergency diesel generator maintenance under different environmental factors.It shows that the risk increment of test maintenance activities in spring and autumn is minimal,which provides suggestions for arranging test maintenance activities in power plants.