- 237 - 2. Earthquake This chapter presents the main elements of the CSA reports relative to earthquakes submitted by the licensees, and the conclusions of ASN after examining them. The following points were addressed in succession for each facility or group of facilities: Design of the facility; Measures for protecting the facilities against the seismic risk; Conformity of the facilities with the current frame of reference; Evaluation of safety margins; Envisaged measures to reinforce facility robustness to the seismic risk. After examining the reports, ASN considered that improvement measures were required for certain facilities. It will therefore demand these measures, in some cases through formal decisions taken by the ASN Commission, which become legally enforceable requirements. 2.1 Design of the facilities ASN requires that the basic nuclear installations (BNI) be designed to withstand an earthquake of greater intensity than the maximum historical earthquake observed in the last thousand years in the area in which the facilities are located. To this end the licensees are required to define a design-basis seismic hazard. The rule for determining the seismic hazard is defined in a Fundamental Safety Rule (RFS). The purpose of the RFS's is to set out the regulatory objectives and describe, where applicable, the practices ASN considers satisfactory. They are revised periodically to integrate developments of knowledge in the subject. The first RFS on the subject, called RFS I.2.c2, dates from 1981 and was revised in 2001 and became RFS 2001-013. These RFS's are also used to verify the design of facilities in operation during the periodic safety assessments, and for defining reinforcement measures where necessary. These rules set two earthquake levels, namely the Maximum Historically Probable Earthquake (MHPE) and the Safe Shutdown Earthquake (SSE), which is the earthquake intensity used to verify that the earthquake finally considered by the licensee for the design of its facility (design-basis earthquake or DBE) complies with the requirement. The frame of reference for new facilities or for the periodic safety reviews is therefore the RFS 2001-01, relative to determining the seismic risk for the safety of basic nuclear installations with the exception of long-term radioactive waste disposals. For some facilities however, particular rules were taken into consideration in their design and have not been reassessed in the periodic safety reviews. In such cases they are specified for each facility concerned. 2.2 Experimental reactors 2.2.1 Reactors operated by the CEA (Osiris, JHR, Phénix) Osiris Facility design and conformity In 1963, as this site was situated in a tectonically stable zone, it could be considered to be "aseismic". No particular measures were required with respect to earthquakes. In 2004 a study by the CEA led to the defining of characteristic earthquakes for the site of intensity V (MHPE level set at 0.04 g) and intensity VI (SSE level set at 0.08 g). RFS 2001-01 led to a minimum fixed spectrum set at 0.1 g being defined for the Saclay site. 2 RFS I.2.c of 1st October 1981 relative to the determining of seismic movements to be considered for the safety of facilities 3 RFS 2001-01 of 31st May 2001 relative to the determining of the seismic risk for the safety of above-ground basic nuclear installations.
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