The nuclear power plants (NPP) in Switzerland have to review their plant safety against the updated seismic hazard (uniform hazard Spectra, from 10−3/a to 10−7/a– denoted as ENSI-2015)—by perform- ing both deterministic and probabilistic safety analyses. For this purpose, a method for direct estimation of seismic safety margins based on nonlinear soil-structure analysis was developed and applied for systems and components located inside the reactor building of NPP goesgen. The method is based on the evaluation of safety factors derived from scaling of response spectra for different hazard levels. For this purpose, two sets of deterministic in-structure floor response spectra (ISrS) were developed tak- ing into account soil-structure interaction (SSI). The first set consists of the new linear elastic ISrS for the reference review level Earthquake (uhS, frequency of exceedance of 10−4/a, PgA=0.41g (mean value), called NESK3 in Swiss national regulations). The second one consists of the calculated nonlin- ear ISrS computed for the Safety margin review level Earthquake (uhS, frequency of exceedance of 10−5/a, PgA=0.71g (mean value), SmrlE). For obtaining the ISrS an equivalent linear-elastic 3D finite element model of the reactor building and the associated soil column (Soil structure analysis (SSI) in frequency domain, SASSI (ElFD), for NESK3) as well as a nonlinear model (SSI in time-domain using lSDyNA, for the SmrlE (NlTD)) were developed. Soil nonlinearity for the latter was incorpo- rated through a hysteretic plasticity model whose shear response is dependent on soil effective pressure. To calibrate the plasticity model, gravel’s shear-stiffness degradation-curve was modified to produce shear strength values consistent with the laboratory-measured friction angle. It is demonstrated that the direct estimation of safety margins by nonlinear soil-structure analysis yields more realistic results than extrapolations common to standard fragility analysis methods.
Fragility Analysis, Nonlinear Soil-Structure Analysis, Seismic Safety Margins.
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