Manish Kumar

IITB Logo Manish Kumar
Assistant Professor

Department of Civil Engineering
Indian Institute of Technology Bombay, Mumbai

Nuclear Power Plants

USNRC

Pounding in seismically isolated nuclear structures

Here we discuss the possible impact of partially buried structures such as containment and auxiliary units with the moat wall. Please note the underlying assumption here is that provided displacement capacity of isolators is more than seismic gap; otherwise, buckling of elastomeric bearings would be the critical issue at large horizontal displacements rather than impact.
Response of a base isolated structure depends on isolation bearings and flexibility of superstructure. First two participating modes are: 1) Displacement in bearing and, 2) Relative deformation in superstructure.

There are two scenarios for base isolated structures in which impact can occur due to insufficient seismic gap:
1. Impact is mainly due to horizontal displacement in bearing
2.Impact takes place due to combined effect of displacement in bearing and deformation of superstructure, primarily due to displacement overshoot phenomena in top story.

 firstmodesecondmode
Source: CIE625 course notes, SUNY at Buffalo

Which of these scenarios discussed above will dominate, again, depends on relative flexibility of superstructure. Compared to tall buildings, nuclear power plant can be considered sufficiently rigid. So, first impact scenario will govern here. Seismic gap takes the form of moat in isolation of NPP and possible area of impact would lie in the lower height of containment unit. Ideally, stringent code regulation from nuclear community would ensure that “beyond-design-basis” seismic gap is provided around the perimeter of base of structure. However, in extreme case, structure might exhaust its seismic gap capacity and hits the retaining wall of moat. In that case, critical issue for isolation system would be again, tension forces generated in bearings due to overturning moments. Local damage is minimized due to dome shape of containment unit, which distributes the impact force to the whole structure.
If discussed impact scenario is considered in design, isolators supplement with viscous dampers would be a better option than providing any kind of shock absorbers.