Benefits of advanced non-linear modeling in assessing the seismic safety margin of NPP structures

The evolution of safety standards due to evolution of knowledge, aging effect of installations, or feedback from recent events such as the Fukushima accident, require a continuous assessment of seismic resistance of structures. As a consequence more and more investigations conclude to deficiencies that in some cases lead to structural retrofitting. However, the experience from various studies, confirmed by observations made on site with structures exposed to severe earthquakes indicate that the effective resistance of structures is not always well assessed by classical linear elastic analysis. The purpose of this communication is to illustrate the benefits of applying appropriate methodologies based on advanced computational modeling, by considering various phenomena such as soil-structure interaction (linear and nonlinear), geometrical nonlinearity (uplift, collision), material nonlinearity (concrete cracking, rebar yielding, damping). These features allow performing reliable best estimate analyses by predicting the apparition of local degradations and their evolution to reach the ultimate capacity of a structure. Also essential to determine reliable fragility curves for other type of analysis such as seismic probabilistic risk assessment (SPRA). This paper, which is essentially an extension of previous works (Voldoire, 2006), presents the application of several methodologies elaborated to take into account different phenomena to the analyses of the SMART structure and of an actual nuclear facility.