Considerations for modeling of base isolated nuclear power plants subjected to beyond design basis shaking

Abstract

Modeling of the seismic isolation system plays a key role in simulating the seismic response of base isolated nuclear power plants (NPP), including estimation of peak isolator displacement and potential to exceed the clearance to stop. Under beyond design basis shaking, seismic isolators such as lead rubber bearings (LRB) may undergo large displacements and exhibit complex behavior including cyclic heating causing strength degradation of the lead and strain hardening in the rubber at large strains. Displacement demands on the isolation system can be limited by a stop such as a moat wall to prevent bearing failure, with the impact to the moat having potential to increase the transfer of forces to the superstructure. Towards a more accurate seismic assessment of NPP subjected to beyond design basis shaking, a model of a seismically isolated NPP is developed with advanced modeling capabilities for LRB isolators and simulation of moat wall impact. The bearing models capture the cyclic behavior of LRB as observed in measured experimental data from full-scale bearings designed for NPP applications. The moat wall model considers a concrete retaining wall with backfill soil and contact elements developed based on experimental data and high-fidelity Finite Element Method (FEM) simulations. The simulation tools are used to evaluate the consequences of bearing modelling and impact to moat walls on the seismic performance of NPP and further develop effective mitigation measures for the seismic protection of NPP. As a result of the studies presented here, a simplified design methodology to estimate impact response parameters including moat wall deformation due to impact, impact velocities, and forces is proposed.