PRELIMINARY EVALUATION OF THE RELIABILITY OF GEN II OR III REACTORS IN BDBE CONDITIONS

The dramatic consequence of the 9 magnitude Fukushima earthquake highlighted and confirmed that the existing Gen II/III reactors 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 earthquakes, tsunamis, hurricanes, flooding, etc. In the proposed study, the attention has been focused on the evaluation of the seismic loading bearing capability of the overall nuclear structures, systems and components (SSCs) to withstand an exceptional seismic event, characterized by a magnitude well beyond the design basis value, known as BDBE (e.g. 2007 Niigataken Chuetsu-Oki or the recent 2011 Tohoku-Taiheiyou-Oki earthquakes). Therefore to evaluate / re-evaluate the safety performances of already operating (or future) nuclear plants, in agreement with the “stress tests” suggestions, presently foreseen by the European and International Associations, a comprehensive risk assessment representing the dynamic behaviour of NPPs, when facing a BDBE situation, has been carried out adopting a deterministic approach with -reference, as an example, to the particular case of an intermediate near term deployment (INTD) NPP. To attain this intent, quite refined FEM models were set up and implemented considering suitable materials behaviour and constitutive laws for the reactor materials, taking into also account the aging effects. The obtained results were used to appropriately check mainly the considered NPP containment strength reserve. The obtained results seemed to confirm the overall containment reliability even if with minor upgrading actions in operating procedures and design characteristics