Fragility Analysis And Study On The Main Influencing Factors Of Earthquake Risk Of Nuclear Power Plant

As an extremely destructive natural disaster,earthquakes have great uncertainty and randomness.There are two methods for risk assessment of nuclear power plants under earthquake conditions:seismic probabilistic safety assessment(SPSA)method and seismic margin assessment(SMA)method.Taking the AP1000 nuclear power plant as the research object,this paper introduces the SPSA method in detail.Seismic fragility analysis is an important step of SPSA.This paper introduces the calculation method and the variables involved in seismic fragility analysis of structures and equipment in detail.Based on this,the fragility analysis of the core makeup tank(CMT)is performed Obtained its fragility curves and calculated the high confidence of low probability of failure(HCLPF)value is 0.59g which is significantly higher than that of safety shutdown earthquake(SSE),which indicats that the CMT has good seismic performance.The sensitivity analysis of fragility parameter errors shows that the change of βR has little influence on the conditional failure probability and HCLPF value of CMT.It can simplify the consideration of some randomness and make the fragility analysis more concise.In this paper,the PSA models of internal initiating events in the AP1000 nuclear power plant are established,and its core damage frequency(CDF)value is calculated to be 2.63E-07/(reactor year).Through the analysis of the minimum cut set,it is found that the relatively large impact on the CDF is mainly due to the occurrence of the initiating event-the breakage of the safety injection line and the loss of function of in containment refueling water tank(IRWST).At the same time,the common cause failure(CCF)of the same kind of equipments is also an important factor leading to core damage.Subsequently,based on the PSA model of internal initiating events,the seismic PSA models of the AP1000 nuclear power plant are established.Taking the seismic hazard curves as the earthquake input,the frequency of different intensity earthquakes can be obtained by Monte Carlo sampling method,and the CDF value of nuclear power plant under different earthquake intensity is obtained finally.The results show that the CDF value increases gradually with the increase of earthquake intensity,and when the earthquake intensity is at a low level,its growth rate is relatively slow that because of the design of structures,components and systems of the nuclear power plant has a certain seismic capacity.When it exceeds 0.6g,a large number of components in the power plant fail,the system cannot perform its safety function,resulting in the rapidly increase of CDF value.Meanwhile,it is found that IRWST is more sensitive to earthquake,and the failure under earthquake is also the main cause of core damage.The cliff edge effect is of great significance to the safety analysis of nuclear power plants,which is the small change of a parameter in a power plant can lead to a disproportionate increase in the consequences.In this paper,four groups of different seismic hazard curves are selected to analyze the influence of the inflection point of hazard curves on CDF.The results show that whether the slope of seismic hazard curves changes suddenly at low or high earthquake intensity,the corresponding CDF value of nuclear power plant will decrease,and then with the increase of earthquake intensity,the CDF value will continue to increase.Therefore,the inflection point of seismic hazard curves has no negative impact on the risk of nuclear power plant.At the same time,during the process of analysis,it is found that the discretization degree of the seismic hazard curves as the input of the earthquake has little influence on the results,so when dealing with such problems,the discretization degree of the hazard curves can be appropriately reduced to simplify the calculation.