Seismic Safety Margin of an Isolated SMR Reactor Under Severe Earthquake

The dramatic consequence of the magnitude 9.0 earthquake in Fukushima Daiichi nuclear power plant, reactors 1, 2, 3 and 4, highlighted and confirmed that the existing and the future nuclear installations should be designed to be highly secure and capable to withstand a wide range of internal and external extreme loads, such as pressure, aircraft crash and, of course, earthquakes. The aim of this paper is the evaluation the seismic behavior of an innovative SMR hit by an exceptional seismic event, characterized by a magnitude well beyond the design basis value (e.g. also 2007 Niigataken Chuetsu-Oki or 2010 Chile earthquakes), in order to understand the true state of the SSCs in terms of their required safety functions and capacity, and, as a result, to be able to assess correctly the seismic safety margin of the considered installation. In this context, it has been also considered the adoption of the highly attractive strategy of the seismic isolation to increase the reliability or safety margin of the nuclear safety relevant structures, during and after the seismic event, with the aim of avoiding or mitigating the related structural damaging effects. To the purpose a rather refined numerical methodology was employed and several three-dimensional models (FEM approach) of the SMR reactor containment and its safety relevant structures were set up and used in the performed analyses, taking also into account a suitable materials behaviour and constitutive laws for both the reactor materials and the isolators. In addition the real behaviour and characteristics of isolators, experimentally determined, have been used as input in the carried out simulations. The obtained results were used to appropriately check mainly the NPP containment strength reserve and the isolators safety factor.