Seismic Probabilistic Risk Analysis For Nuclear Power Plant Containments Based On Site-specific Hazard And Target Spectra

After the Japanese Fukushima nuclear accident in the year of 2011,seismic safety capacities of nuclear power plants(NPPs)have been highly paid attention to.There are mainly two methodologies for conducting seismic safety evaluation including seismic probabilistic risk assessment(SPRA)and seismic margin assessment(SMA),in which SPRA is an evaluation methodology from the full probability view.For dealing with the impact of the Japanese Fukushima nuclear accident,the nuclear safety department of China required that seismic safety capacities of the domestic NPPs should be evaluated.However,the research on SPRA method has only just begun in China,which needs urgently be conducted.Based on the above research background,respectively taking a nuclear containment as the object studied and taking a nuclear power plant site in South China as the target site in this paper,the research on SPRA method is conducted.The main contents of this paper are as follows:(1)Based on the seismicity characteristics in China,Chinese probabilistic seismic hazard analysis(c-PSHA)and Chinese seismic hazard disaggregation(c-SHD)procedures based on Monte Carlo simulation(MCS)were developed,and c-PSHA and c-SHD for a nuclear power plant site in South China were conducted using the developed procedures.The results show that the procedures developed in this paper are of good convergence and accuracy under 400 simulations for 50000 years of seismicity data.(2)Based on the seismicity characteristics in China and the theory of vector probabilistic seismic hazard analysis,the procedures of Chinese vector probabilistic seismic hazard analysis(c-VPSHA)and Chinese vector seismic hazard disaggregation(c-VSHD)based on MCS were developed.Based on the developed procedures,c-VPSHA and c-VSHD for a nuclear power plant site in South China were conducted.Chinese conditional probabilistic seismic hazard analysis(c-CPSHA)theory was summarized,and procedures of c-CPSHA based on MCS were developed.Based on the developed procedures,c-CPSHA for a nuclear power plant site in South China was conducted.The results show that vector probabilistic seismic hazard analysis(VPSHA)results reduce the conservatism of scalar seismic hazard curves,as the intensity measure(IM)correlation is considered for VPSHA.The secondary IM could be predicted based on principal IM using conditional probabilistic seismic hazard analysis.(3)The concept of the single variable earthquake return period was summaried,and the single variable earthquake return period was analyzed for a nuclear power plant site in South China.On the basis of the single variable earthquake return period concept of earthquake engineering field,the concepts of two-variable earthquake return period and conditional earthquake return period were firstly proposed and the relationship among three earthquake return period concepts were analyzed.Finally,two-variable earthquake return period and conditional earthquake return period were analyzed for a nuclear power plant site in South China.The results show that the two-variable earthquake return period is equal or greater than the larger one of two single variable earthquake return periods,and the conditional earthquake return period is the ratio of two-variable earthquake return period to single variable earthquake return period.(4)Based on scalar probabilistic seismic hazard analysis and disaggregation results,uniform hazard spectrum(UHS),conditional mean spectrum(CMS),conditional spectrum and uniform risk spectrum could be generated;Based on vector probabilistic seismic hazard analysis and diaggregation results,simplified generalized conditional mean spectrum-I(s-GCMS-I)and simplified generalized conditional spectrum-I(s-GCS-I)could be generated,and simplified generalized conditional mean spectrum-II(s-GCMS-II)and simplified generalized conditional spectrum-II(s-GCS-II)with vector hazard level could be generated.Based on the conditional probabilistic seismic hazard analysis results,conditional uniform hazard spectrum(CUHS)could be generated.Based on the above site-specific conditional spectrum and generalized conditional spectrum,ground motion sets were selected and modified using the greedy optimization technique and taking NGA-West2 as alternative ground motion database.The results show that UHS is more conservative,and the site-specific spectra considering spectral shape correlation coefficient are more reasonable under normal conditions.The variability of seismic response of the containmet is largest using the ground motion records,which is selected taking conditional spectra at the second translational period of the containment.(5)Theoretical basis of seismic fragility model considering epistemic uncertainty was summarized.The fragility model formula with confidence was firstly derived from the view of “uncertainty of fragility”.The relationship of two types of high confidence of low probability of failure(HCLPF)values was analyzed.Finally,using safety factor method,the fragility curve and HCLPF values of a nuclear containment were obtained based on two data sources including analytical fragility data and empirical fragility data.The results show that HCLPF values based on UHS and URS are smaller than CMS;For the structures sensitive to the first period,results based on GCMS-I and CUHS compared with UHS could be more conservative;The standard deviation based on incremental dynamic analysis is larger than safety factor method,while the median values based on these two methods are close to each other.HCLPF values based on Method II is smalled than that of Method I,so Method I is more conservative compared with Method II.(6)The formulas of two types of seismic risk analytical models considering epistemic uncertainties were summarized,and the confidence function of the mean seismic risk model was derived.The confidence levels of mean seismic risk model at different sites with different hazard levels were analyzed.Finally,based on the seismic risk analytical model formulas considering epistemic uncertainty,seismic risk levels of a nuclear containment were obtained considering seismic hazard levels of the target site and seismic fragility analysis results of a nuclear containment.The results show that the confidence of the mean seismic risk model is not high;The risk results taking Sa(0.07s)as IM are smaller than the results taking Sa(0.24s)as IM,so seismic risk results of the containment would be underestimated if Sa at non-first mode period is taken as IM;The seismic risk results based on CMS are smaller than UHS,URS,CUHS,GCMS-I and GCMS-II,so the risk results based on CMS might be not conservative;The seismic risk results based on multi modal response spectrum analysis are smaller than that based on incremental dynamic analysis;Seismic risk levels of the nuclear containment under all site-specific spectra generated in this paper are low.