Seismic isolation has been proven as an effective strategy to protect critical facilities including Nuclear Power Plants (NPPs) from the damaging effects of horizontal earthquake ground shaking. The increased flexibility and resulting elongation of the natural vibration period of the structure leads to significant reductions in acceleration and forces transmitted to the structure above the isolation level at the expense of large displacements in the isolation system.  These large displacements need to be accommodated by the isolation bearings while sustaining the weight of the structure above. Further, the isolated structure requires a large horizontal clearance at the basement level that is often limited by a moat wall that can also function as a hard stop for the isolation system. In the case of beyond design basis earthquake, there is a potential for impact of the isolated structure to the hard stop or failure of the bearings that can be a significant safety concern. Through a combination of numerical and experimental studies, the stability of seismic isolation bearings and the effects of moat wall impact on the response of seismically isolated NPPs is evaluated. Experimental research incudes hybrid simulation with testing of full scale seismic isolation bearings under combined three-dimensional seismic loads and shake table testing to evaluate pounding forces.  The experimental data is being used to develop models able to capture the behavior of nuclear power plants for beyond design basis shaking.