- 92 - The SSE response spectrum is obtained by conventionally adopting a magnitude which is that of the MHPE plus 0.5 on the Richter scale. Transition from RFS I.2.c (1981) to RFS 2001-01 (2001) The first RFS for determining seismic motion to be considered for the safety of facilities dates from 1981, this is RFS 1.2.c5. It was revised in 2001, becoming known as RFS 2001-016. The RFS revision retained the general approach and added to the previous text by taking account of changes to scientific knowledge and the seismic operating experience feedback from the previous 20 years. The main changes to the RFS concern: the rule for the definition of seismo-tectonic zones in complex fault configurations (fault families). the use of the available correlations (linking magnitude to intensity and to focal distance) best suited to the French context and established on the basis of a range of homogeneous macroseismic data. the notion of fixed spectrum: the fixed spectrum characterising nearby earthquakes has been abandoned in favour of a site spectrum set at 0.1 g with infinite frequency. The RFS revision requires a check that the SSE is higher than a minimum level. This minimum level encompasses a moderate earthquake close to the facility (M=4 at 10 km) and a major event (M=6.6 at 40 km). This minimum level is defined for the two site conditions, both rock and sediment. This approach is in conformity with IAEA's recommendation (Seismic Hazard Evaluation for Nuclear Power Plants, Safety Standards series n° NS-G3-3). Considering this minimum level offers a safety margin and compensates for the lack of data available in low-seismicity regions. The incorporation of seismic operating experience feedback and changing calculation methods: the operating experience feedback from earthquakes in the 1980s showed the significant influence of the surface geological layers, in particular in alluvial zones. These effects, referred to as "site effects" act on the amplitude of the seismic motion, its duration and its frequency. The response spectrum definition was supplemented in the RFS by additional indicators such as strong phase duration, the Arias intensity, the maximum soil speed, etc., which are of use for the designers of structures. Site effects are included by using spectral acceleration attenuation laws, including the complex geometry of sedimentary zones and the geological characteristics of the top thirty metres on the sites (determined by using local instrumental data), which were updated in relation to the previous RFS. Taking account of new and changing knowledge in the field of geology: in the early 1990s, signs of paleo-earthquakes of a magnitude higher than certain events in the SisFrance base were discovered. These earthquakes left geological traces by disrupting geological layers or modifying the landscape. Site design response spectrum (design-basis earthquake - DBE): For the design of each plant series, EDF used a design spectrum encompassing the overall SSE spectrum for each site, using the data and knowledge available at the time. Special steps were taken for sites with seismic characteristics outside the envelope of the standardised plant series (owing to specific local, in particular geological characteristics). Conclusions concerning the adequacy of the Design-Basis Earthquake (DBE) Following a periodic safety review, the changes decided for a plant series are implemented on each reactor, generally on the occasion of the reactor ten-yearly outage inspection. The modifications are thus deployed to the entire plant series over a time-frame that is consistent with the initial time of construction of the corresponding reactors. 5 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 6 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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