Deterministic and Probabilistic Evaluation of Seismic Loads on RPV Internals including Nonlinear Effects

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 modelling 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 core barrel 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 core barrel and the maximum load on the core barrel flange. This is in agreement with one of the standard assumptions adopted for fragility analysis in the nuclear industry and is confirmed for the core barrel flange in the present analysis.