Seismic Isolation of Nuclear Power Plants using Sliding Isolation Bearings

PhD Student: Manish Kumar

Advisors: Andrew Whittaker and Michael Constantinou

Abstract: Seismic base isolation is a viable strategy for protecting nuclear power plants (NPPs) from extreme earthquake shaking. This study focuses on seismic isolation using single concave Friction Pendulumâ„¢ (FP) bearings.

Friction at the sliding surface of an FP bearing changes continuously during earthquake-induced sliding depending on sliding velocity, axial pressure and temperature at the sliding surface. The velocity- and pressure-dependencies are coupled. Temperature is a function of the histories of coefficient of friction, sliding velocity and axial pressure. A model to describe the evolution of coefficient of friction is proposed, verified and validated.

Seismic hazard for a seismically isolated NPP is defined using a uniform hazard response spectrum (UHRS) at mean annual frequencies of exceedance (MAFEs) of 10-4 and 10-5. The UHRS at the MAFE of 10-4 is increased by a design factor (≥ 1) for conventional (fixed-base) nuclear structure to achieve a target annual frequency of unacceptable performance. Risk calculations are performed to compute design factors for seismically isolated NPPs. The MAFE of core damage for isolated NPPs, with and without consideration of a hard stop, at sites of eight nuclear facilities across the United States, are calculated and discussed. Strategies to further reduce risk are identified.

Response-history analyses using different models of seismically isolated NPPs are performed to understand the importance of choice of friction model and inclusion of vertical ground motion in response prediction. A friction model should include the heating effects; velocity- and pressuredependencies are not important. Isolation-system displacements can be computed using a macro model comprising a single FP bearing. Floor spectra at different locations of NPPs should be computed using a detailed model of NPP with vertical ground motion included in analysis.