Study On Containment And Equipment Classified Of Seismic Category Ⅰ Seismic Margin Assessment Of Nuclear Power Plants

The 3·11 earthquake of Japan caused the Fukushima nuclear accident which aroused the public suspicion and concern of the nuclear safety under severe earthquake.The definition of seismic margin assessment of nuclear power plants beyond the design basis ground motion which will contribute to public understanding of nuclear power plant safety under earthquake event and provide reference for domestic nuclear development and policy-making departments. Currently, the main three evaluation methods of the seismic margin assessment of nuclear power plants are Nuclear Regulatory Commission – seismic margin assessment(NRC-SMA), Electric Power Research Institute – seismic margin assessment(EPRI-SMA) and PSA-based SMA. The system structure component(SSC) obtained by method of success path or the event tree/ fault tree. And then the high confidence low probability failure(HCLPF) of each SSC will be defined based on the conservative deterministic failure margin method of analysis(CDFM) or seismic fragility of analysis method(SFA). However, the above two methods are difficult to obtain the accurate seismic margin assessment of nuclear power plants SSC even if combining with inelastic energy dissipation factor considering material nonlinearity. The development of plastic theory and nonlinear finite element technology in recent years has provided technical support for the seismic margin assessment of nuclear power plants beyond the design basis ground motion. In this paper, based on the seismic margin assessment of the containment and equipment classified of seismic category Ⅰ in nuclear power plants, some specific work and research results are achieved as follows:1. The four design spectrums of typical nuclear power plant including RG1.60 response spectrum, NUREG/CR 0098 response spectrum, GB 50267-97 response bedrock spectrum and hard soil response spectrum were used as a review of ground motion to compare and investigate the results of seismic margin assessment of the containment in China. Results show that the value of seismic margin assessment was relative low than that of the other three methods. Therefore, the review ground motion used for seismic margin assessment in China should select existing strong earthquake motion records. The compared results between seismic fragility of analysis method and conservative deterministic method reveal that the value of the later can reach eighty percent of high confidence low probability failure(HCLPF).2. The sensitivity of structural floor response versus the uncertainty of equipment support structure parameters were studied based on point estimate. Results show that the correspondence relation between the uncertainty coefficient of parameter and the sensitivity of floor spectrum. Therefore, the seismic margin assessment of nuclear equipment should adopt point estimate method combined comprehensively consideringthe influence of structural parameter uncertainty on supporting equipment.3. The applicability of time history analysis method used for seismic qualification of equipment considering parameter uncertainty in the current code was investigated.The compared results of acceleration response spectra between the method of this paper and code method show that the code method is conservative at high and low frequency section, though, partial to unsafe at intermediate frequency section. The corresponding time history curve defined by floor response parameters with a certain amount of guarantee rate is recommended as input time history method when conducts seismic margin assessment of nuclear equipment.4. A set of shaking table tests on a 1:25 scaled nuclear power plant structure was conducted in this paper. And then, the soil- structure interaction(SSI) was discussed.Furthermore, the difference of floor acceleration response under different intensities of ground motion between time domain and frequency domain was investigated. At last,the revised relationship of increment between the input of equipment and calculated top surface of bedrock was provided when conducting seismic performance analysis of equipment.5. The increment relationship of floor response combined with different site and earthquake intensity was obtained based on comprehensive consideration of the effect of parameter uncertainties on the response of the floor. The method of seismic margin assessment on nuclear equipment based on the IDA technology was presented by this paper using the evaluation criteria of limit load. The analysis result of seismic margin assessment on some equipment with I class was obtained using numerical simulation.