Dynamic Analysis And Seismic Safety Assessment Of Nuclear Power Plant Containment Structures Subjected To Mainshock-aftershock Sequences

The nuclear power in China has entered a rapid development period,and a series of nuclear power plants has been built in our country.In the emergency disaster,once the nuclear accident happens,it will cause the destructive damage to environment and society.The containment structure is the last barrier to prevent nuclear leakage,and the seismic safety margin of this structure will directly affect the safety of the whole system.In the Fukushima nuclear accident,the containment structures suffered multiple earthquakes,and aftershocks can increase the seismic hazard and cause a greater structural damage.To ensure the absolute safety of the nuclear power plant,it is necessary to consider the influence of all possible adverse factors on the safety margin of nuclear power plants.Although great progress has been made in seismic design and evaluation of nuclear structures,the impact of mainshock-aftershocks on the safety margin of containment structures has not been taken into account,which is unfavorable to the safety of nuclear power plants.Therefore,studying the dynamic response and seismic performance of containment structures under mainshock-aftershocks,and proposing the seismic safety assessment method for seismic sequences,is important to guarantee the normal operation of nuclear power plants and is also an important subject that need to be solved in the construction of nuclear power plants.In this paper,the dynamic response and seismic safety assessment of containment structures subjected mainshock-aftershocks are studied systematically,which include the dynamic response of containment structures under mainshock-aftershocks,the determination of the seismic capacity of damaged containment structures,the mainshock-aftershock fragility analysis of containment structures,and the seismic safety assessment of containment structures considering the effect of aftershocks.The main research contents are as follows:(1)This study establishes the three-dimensional numerical model of the containment structure,and verify the validity of the numerical simulation method;The nonlinear time-history analysis of the containment structure is carried out using the selected mainshock-aftershock ground motions,and the obtained dynamic analysis results under various seismic intensities are compared.The effects of aftershock characteristics such as duration on the seismic performance of the post-mainshock containment structures are analyzed,and the effect of near-fault and far-fault seismic sequences on dynamic responses of the containment is also studied.The results show that aftershocks can cause the additional damage to the containment structure,and the aftershock characteristics and initial damage states have a significant impact on the dynamic response and accumulative damage of the containment structure.(2)To investigate the failure process and damage states of the containment structure,the IDA analysis is conducted on the structure under mainshocks;By comparing the dispersion of various IDA curves,the preferable damage measure and intensity measure for the containment structure are selected.To study the dynamic characteristics of the damage containment structures,the static cyclic analysis and IDA analysis are performed on containment structures with different initial damage states,respectively.The IDA analysis under mainshock-aftershocks is carried out on the containment structures,and bi-dimensional limit state functions are proposed based on the multi-dimensional limit state theory.(3)A new fragility analysis method under mainshock-aftershocks is proposed,which can effectively consider the effect of post-mainshock damage states on the residual seismic capacity when calculating the exceedance probability.The fragility analysis of the containment structure is performed using the proposed method,and the technical basis for aspects of this approach,like the construction of joint seismic demand models and the calculation of exceedance probability,is provided.The fragility curves generated by different fragility analysis methods are compared,and the influence of correlation parameters of limit-state functions and the impact of near-fault and far-fault seismic sequences on fragility curves are studied.(4)Based on the total probability theory,a seismic assessment method considering the aftershock hazard is proposed,and the seismic risk evaluation of the containment structure and internal equipment is conducted by this method.The seismic hazard analyses of mainshocks and aftershocks are carried out,and fragility curves of the containment structure under mainshocks and the conditional fragility curves of the post-mainshock structure under aftershocks are calculated.The results show that the seismic hazard of aftershocks has a significant effect on the annual exceedance probability.(5)To consider the uncertainty of the structural state under seismic sequences and the time-dependent aftershock seismic hazard,a Markov process-based seismic assessment method is proposed,and the seismic safety assessments of the containment structure and the system in post-mainshock and pre-mainshock environment are carried out using the proposed method.The fragility curves and seismic hazard curves are used to calculate the probability of state transition,and the Markov matrices are constructed to consider the uncertainty of structural state and the increased seismic hazard.Finally,the limit-state transition probability of the containment structure and system in post-mainshock and pre-mainshock environment is obtained.The results show that the increased seismic risk caused by aftershocks should be considered in the seismic safety assessment of nuclear power plants,which can make the structure maintain the expected performance level during seismic sequences and ensure the seismic safety of nuclear power plants.