SEISMIC ROBUSTNESS OF REACTOR TRIP VIA CONTROL ROD INSERTION AT INCREASED SEISMIC HAZARD ESTIMATES

摘要

A comprehensive seismic safety demonstration of the reactor trip for the 3-loop PWR (pressure water reactor) at the Gosgen NPP (nuclear power plant) is presented. The demonstration addresses an increased seismic hazard estimation resulting from the probabilistic seismic hazard analysis for Swiss NPP sites (PEGASOS). The analysis focusses on the following relevant failure modes: (i) excessive inelastic deformation of the fuel assemblies due to spacer grid buckling, (ii) excessive deformation of the control rod pressure tubes, (iii) excessive relative displacement between RPV (reactor pressure vessel) internals, and (iv) damage of the RPV internals and the CRDMs (control rod drive mechanisms). A staggered approach has been followed in the evaluation of the robustness. In a first step, the robustness of the reactor trip has been evaluated based on existing design documents, resulting in an estimate of the HCLPF (High Confidence of Low Probability of Failure) capacity based on the CDFM (conservative deterministic failure margin) method. The second step involved a full scope probabilistic dynamic reanalysis of the entire analysis chain, consisting of a SASSI-model of the reactor building, an ANSYS-model of the RPV including the internals, fuel assemblies and CRDMs, and a dedicated impact model of the fuel assemblies using the proprietary simulation code KWUSTOSS. Based on this reanalysis, fragility curves were developed using the separation of variables method. The main conclusions of the study consist in (i) validation of the conservative CDFM-based HCLPF capacities via a fully featured, Latin Hypercube Sampling based probabilistic dynamic analysis, and (ii) quantitative evidence that inelastic deformation of the spacer grids implies significantly reduced response variabilities in the corresponding fragility analysis.