Effects of Moat Wall Impact on the Seismic Response of Base Isolated Nuclear Power Plants

Abstract eng:
Seismic isolation can be an effective strategy to protect critical facilities including Nuclear Power Plants (NPPs) from the damaging effects of horizontal earthquake ground shaking. The increased flexibility at the base 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. To accommodate these displacements, the isolated structure requires a large horizontal clearance or moat at the basement level. The surrounding moat wall around the clearance is intended to function as a hard stop to limit isolation system displacements. In the case of an extreme earthquake, there exists the potential for impact of the isolated structure to the moat wall. This impact is of significant concern especially in NPPs due to the potential for increased transfer of forces and amplification in response of the structural system, piping and other contents. However, the consequences of impact or factors important to mitigate its effects are not very well understood. To address this concern, this study examines simple NPP models to capture the impact forces and the effects of impact on the response of seismically isolated NPPs. Characteristics of the isolation hardware and hard stop that minimize the effects of impact are also examined by considering variable distances to the hard stop and properties of the moat wall and isolation system. The amplification in response is reported for acceleration and floor spectral accelerations at different points along the height of a simplified model of a containment structure. Results of these studies indicate the moat wall can have significant penetration, and thus not fully limit displacement, while having significant increases in accelerations throughout the height of the NPP model.

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