- 88 - which constitutes input data for the calculation of structures and equipment, but also on the basis of developments in paraseismic engineering. In addition, seismic operating experience feedback (both nuclear and non-nuclear) and the construction robustness studies are also sources for evaluating seismic conformity. 2.1.1 Seismic level for which the facilities are designed The approach used to define the seismic loads to be considered in the design of the facilities is a deterministic one: it is postulated that any earthquake known in the region of the site (taking account of historical observations over a period of about 1,000 years) is liable to reoccur with the same characteristics in the position most unfavourable to the facility, while remaining compatible with the geological and seismic data; from this, the intensity of the "Maximum Historically Probable Earthquake" (MHPE) is deduced; as part of the safety approach and to take account of uncertainty surrounding the data and the available knowledge, a degree of intensity is arbitrarily added to the MHPE to define the SSE; the installation is then designed to withstand a hazard level at least equivalent to that of the SSE; reactor safe shutdown, fuel cooling and containment of radioactive products must be guaranteed for this type of earthquake; this approach also takes account of soil effects and paleo-earthquakes1. Given the standardisation of the nuclear reactors operated in France, EDF has introduced the notion of the Design-Basis Earthquake (DBE): this is the envelope spectrum of the various SSE spectra associated with the different sites of the same plant series. Moreover, a basic safety rule (RFS - see § 2.1.2) defines acceptable methods for determining all the movements to which the "seismic-classified" civil engineering structures are subjected, based on the seismic motion considered and the corresponding load levels, in order to allow design and verification: of the civil engineering strength of these structures subjected to the loads resulting from earthquakes and other actions combined with earthquakes; of the correct behaviour and performance of the equipment in the facility. Characteristics of the Design-Basis Earthquake (DBE) ASN requires that basic nuclear installations be designed to withstand an earthquake higher than the maximum earthquake that has occurred during the last thousand years in the area in which they are sited. The licensees are therefore required to define an earthquake for design purposes. The rule for determining this earthquake is defined in a RFS. The RFS defined by ASN are in particular designed to explain the regulatory objectives and, as applicable, describe the practices considered by ASN to be satisfactory. They are periodically reviewed to take account of changing knowledge and new information. The first RFS on the subject, RFS 1.2.c2, dates from 1981. It was revised in 2001, which this revision being known as RFS 2001-013. These RFS are also used to check the design of the installations in operation on the occasion of the periodic safety reviews, with reinforcements defined as and when necessary. These rules define two seismic levels, the MHPE and the SSE, which is that used to check that the earthquake finally adopted by the licensee in the design of its facility (DBE) is in conformity with the requirement. 1 Paleo-earthquake: earthquake which left traces of deformation in the surface geological layers 2 RFS 1.2.c of 1st October 1981 concerning the determination of the seismic motion to be taken into account for the safety of the facilities 3 RFS 2001-01 of 31st May 2001 concerning the determination of the seismic risk for the safety of surface basic nuclear installations.
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