Deterministic and Probabilistic Evaluation of Seismic Loads on Reactor Pressure Vessel Internals Including Nonlinear Effects

Abstract

The paper presents analytical results for the dynamic analysis of the control rod drive mechanisms (CRDM) and selected reactor pressure vessel (RPV) internals, i.e., core barrel (CB) upper flange and upper core plate centering, as well as resulting stresses. The model for the dynamic analysis of the RPV Internals with CESHOCK consists of springs and lumped masses, combined with beam elements. The model is taking into account material nonlinearities, sliding, friction, and gap/impact effects inside the RPV. Refined finite element modeling of contact regions is used to calibrate the force–displacement relationship used in the dynamic model. Both deterministic and probabilistic analyses are presented. The excitations are based on upstream time history (TH) analyses with a coupled model of the nuclear steam supply system (NSSS) and the reactor building. Variability of ground motion parameters (shear modulus and damping) and building parameters (Young's modulus and damping) is taken into account. The deterministic analyses confirm that the CB upper flange is the most highly stressed part of the RPV internals in case of seismic excitations. The probabilistic analyses show that there is a robust correlation between the spectral acceleration at the fundamental frequency of the CB and the maximum load on the CB flange. This is in agreement with one of the standard assumptions adopted for fragility analysis in the nuclear industry and is confirmed for the CB flange in the present analysis.