The Nuclear Power Plant (NPP) is designed for two levels of earthquake viz., Operating Basis Earthquake (OBE) and Safe Shutdown Earthquake (SSE). The OBE (S1) level ground motion corresponds to the maximum level of ground motion, which can reasonably be experienced at the site once during the operating life of nuclear power plant with a return period of 100 years. Whereas, the SSE (S2) level ground motion is the maximum potential vibratory ground motion at the site based on the maximum earthquake potential of the (site) region with a return period of 10000 years. The material safety factor (ϒm) for design for concrete and steel considered are 1.5 and 1.15 under OBE and same is 1.3 and 1.0 for SSE. After Fukushima event, i.e., Extreme Earthquake Event (EEE) and subsequent Tsunami, it is required to evaluate the safety of various civil structures in Nuclear Power Plants for Beyond Design Basis Earthquake (BDBE) having return period of more than 10^5 (magnitude 1.67 to 2.5 times of SSE).
My question is that, what partial safety factor for material (both concrete & steel) we should consider for seismic margin under BDBE condition?. For my present work, I have considered a partial safety factor of 1.0 for both concrete and steel under BDBE condition. I need some reference document/code/standard for the same.
Dear Mr Mishra, you may find some basic in IAEA documents such as SSR-2/1 (Rev. 1) Safety of Nuclear Power Plants:Design. However the Agency documents are not mandatory so you need to figure out Design Extension Conditions (DECs) and acceptance criteria on your own.
In my opinion for such rare and severe earthquakes you need to use best estimate values for material properties and also allow for inelastic response of all safety-related structures including the containment (which is not allowed for design combinations including SL-2 earthquake).
Dear Mr Mishra,
Currently no unified approach (or even definition) exist for dealing with beyond-design earthquakes. There are some requirements for stress test in Europe and the US, that maybe used as a cornerstone, but they need to be detailed and complemented for each specific case. The final criteria of course need to be consistent with the Regulator position (if any!) in order to be endorsed.
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