The buildings and equipment in NPPs which are important for safety are designed to withstand earthquakes of an intensity higher than all the earthquakes that have already occurred in the vicinity of the site, plus an additional safety margin. The rules for dealing with the seismic risk are regularly updated in order to take account of new data with retroactive application on a case by case basis during the periodic safety reviews. The conformity reviews are also an opportunity for detailed checks. Although when compared with other countries France is not particularly seismic, considerable efforts are devoted to this subject by EDF and close attention is given by the ASN.

Updating of the design rules

After the 2001 update of the basic safety rule covering how to determine the seismic risk for the safety of surface BNIs (RFS 2001-01), the ASN in 2005 continued its work to update the RFS dealing with the construction rules to be used to protect against the effects of an earthquake (RFS V.2.g). The current rule dates from 1985 and the new data available in this area must be taken into account.

This new "seismic design" guide will detail the main steps in the design of the civil engineering structures with regard to earthquakes, from a statement of the basic principles underpinning this design, to determination of the spectra to be used for sizing of the equipment anchored to the civil engineering part. It will apply to all surface BNIs. A draft guide will be discussed at a meeting of the Advisory Committees in 2006.

Seismic design reviews

Within the framework of the current periodic safety reviews, the seismic design review in particular consists in updating the level of the earthquake to be taken into account, under application of the above-mentioned RFS 2001-01.

For the 30-year periodic safety review on the 900 MWe reactors, the ASN asked EDF to review in particular the seismic design of the electrical buildings of the CPY series of reactors (Gravelines, Saint-Laurent-des-Eaux, Dampierre, Cruas, Tricastin, Chinon). To date, these studies have shown no need to reinforce the buildings. For the reactors of the CP0 series, the ASN asked EDF to study the seismic design of the nuclear island buildings and the turbine hall.

With regard to the 20 year safety review for the reactors of the 1300 MWe series, EDF reviewed the seismic stability of the turbine halls in the P'4 type reactors (Cattenom, Nogent-sur-Seine, Belleville, Golfech, Penly) and the strength of the civil engineering structures in the electrical and the safeguard auxiliaries buildings in the P4 type reactor (Flamanville, Paluel, Saint-Alban). These studies revealed that the original design would guarantee that these reactors could withstand the design basis earthquake reassessed in accordance with RFS 2001-01.