- 91 - Methodology used to evaluate the Design Basis Earthquake (DBE) The conformity of the basic nuclear installations with the regulations is periodically checked every ten years, on the occasion of the periodic safety reviews. These reviews are the opportunity to perform an in-depth, detailed conformity examination, to reassess the SSE levels in the light of the most recent data and new knowledge, to reexamine equipment for which seismic resistance is required, to take account of changes in the field of paraseismic engineering and to make the corresponding necessary improvements to the facilities. The seismic motions corresponding to the SSE are established on the basis of a RFS, which has itself evolved to take account of new data and knowledge. The regulatory requirements: RFS 1.2.c and 2001-01: A deterministic approach is used to define the seismic hazard to be considered in the design of the facilities. The general approach to characterising the seismic hazard follows 3 steps: geological and seismic characterisation of the region, to identify zones with homogeneous characteristics, definition of one or more reference earthquakes, calculation of the seismic motion at each site. The approach is, for each site, to look for an earthquake encompassing the known historical earthquakes in the most penalising epicentre positions (in terms of MSK intensity, representative of surface effects) while remaining compatible with geological and seismic data. The whole of France is covered by seismotectonic zoning. Information on past earthquakes was obtained from the interpretation of historical archives describing the damage caused, characterising 1,000 years of seismicity (the SisFrance database contains about 10,000 documents describing more than 6,000 events, and 100,000 observation points), plus a catalogue of instrumental measurements taken since the 1960s (CEA/LDG database). Definition of the MHPE The Maximum Historically Probable Earthquakes (MHPE) are the earthquake or earthquakes which, for the site concerned, produce the highest intensities, bearing in mind that: the historical earthquakes of the tectonic domain to which the site belongs are considered as being capable of reoccurring under the site, the historical earthquakes belonging to a neighbouring tectonic domain are considered as being capable of occurring at the point in this domain closest to the site. The intensity of an earthquake cannot be directly used in the design of a facility. Earthquakes are described by their response spectrum (given by the zero period acceleration value, expressed in "g"). For this, it is necessary to determine the magnitude and the focal depth of the historical events. For each MHPE, a Safe Shutdown Earthquake (SSE) is deduced by means of a simple relationship in terms of MSK4 intensity on the site: Definition of the SSE The intensity of the SSE on the MSK scale is conventionally defined by: ISSE= IMHPE +1 The MSK scale was determined such that a one-degree increase corresponds overall to a doubling in the motion parameter. 4 The Medvedev-Sponheuer-Karnik scale (also called MSK scale) is a scale measuring the intensity of an earthquake.
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