Seismically induced uplift effects on nuclear power plants. Part I: Containment building rocking spectra

Current nuclear regulatory codes specify design considerations for extreme seismic scenarios, focusing primarily on the response of the containment structure of a nuclear power plant. However, in current state-of-practice and in most seismic regulations worldwide, the consideration of soil-structure interaction and potential development of geometrically nonlinear effects, such as rocking and sliding with uplift, is not taken into consideration. To explore this issue, a refined 3D finite element model of a typical nuclear power plant containment structure is developed, comprising solid elements for the soil and foundation, plus shell elements for the structure. The aim is identification of foundation-soil separation phenomena under a suite of ground motions with distinct frequency content. At first, harmonic excitations are used, for both cases of stiff sand and rock subsoil profiles, leading to rocking spectra that depict the displacement demand in connection with nonlinear separation. Clear influence zones can be distinguished, especially in the low frequency bands for the stiff sand case. Next, three subsets of 30 ground motion records are carefully selected and grouped in ensembles according to their frequency content, normalized to a PGA of 0.36 g, which corresponds to the highest design acceleration in Europe. Ground motions with low mean frequency content are observed to lead to the onset of geometrically nonlinear phenomena, along with a higher displacement demand. The interplay between ground motion characteristics, dynamic properties of the containment structure and stiffness of the soil is also highlighted. More specifically, it is shown that stiff containment structures on soft soils are more prone to foundation uplift. This possibility is often neglected in design codes and the consequence is that under certain circumstances, damage may be caused to the internal power generation equipment.