An earthquake with a magnitude of 5.7 occurring at a depth of 7 km right under the reactor.
When it was built in 1970, the reactor was designed to withstand an earthquake corresponding to the criteria defined in the seismic regulations in force at the time (intensity VIII, as per French Seismic Code PS 67). Since then, our knowledge of seismic hazards has evolved, as have the regulations, which have become even more stringent.
In 2004, studies were resumed to define the earthquake characteristics to be taken into consideration and to verify the behaviour of the ILL’s installations under seismic conditions. These studies led to the carrying out of major seismic reinforcement work, which was completed in 2006 at a cost of around 30 million euro. The ILL’s reactor is now designed to withstand an earthquake with a magnitude of 5.7 occurring at a depth of 7 km right under the reactor building.
The approach used is the approach recommended in the Basic Safety Rule on seismic hazards (RFS 2001-01) issued by the French Nuclear Safety Authority (ASN) for evaluating the seismic hazard at the sites of nuclear installations. This approach is broken down into several stages:
- Identifying the earthquakes likely to occur in the vicinity of the installation
This study was assigned to outside experts who used a method which basically involves defining zones in which the probability of an earthquake is identical at every point within the zone. In other words, the seismotectonic zone in which ILL is located is first of all defined, as are the adjacent zones. It is then assumed that if an earthquake has occurred anywhere in the zone, another earthquake may occur at any other place in the same zone. The study is based on knowledge of seismic faults and historical seismicity data, which goes back to the 14th century (see SISFrance database). The most powerful earthquake observed in each zone is then taken as the basis for the next stage. - Defining the earthquake characteristics to be taken into account for designing or reinforcing the installation
The next stage involves “shifting” the earthquakes identified in stage 1) to the most penalising position for the site, i.e. directly under the installation in the case of the maximum earthquake identified in the zone in which the installation is located, and to the edge of the zone as close to the installation as possible in the case of the maximum earthquakes identified in the neighbouring zones. For ILL, 2 earthquakes, known as SMHV (Séismes Maximaux Historiquement Vraisemblables – Maximum Historically Probable Earthquakes), have been identified as relevant:
a. The earthquake of Corrençon (1962) with a magnitude of 5.2 at a depth of 7 km
b. The earthquake of Chamonix (1905) with a magnitude of 5.7, at a distance of 15 km
The magnitude of each SMHV is then increased by 0.5 units in order to define the Safe-Shutdown Earthquake (SSE) in each case, i.e. the maximum earthquake for which the structures, systems, and components important to safety must be designed to withstand and remain functional. - Calculating the ground motions corresponding to these earthquakes
The ground motions are calculated using the method recommended by Basic Safety Rule RFS 2001-01 and taking into account “site effects”, i.e. the amplification effects on ground motions due to the different sediments that fill the Grenoble basin. These calculations produce the “acceleration response spectra” and determine the maximum acceleration generated by the earthquake depending on the ground motion frequency. This calculation demonstrates that the most penalising earthquake to be taken into account is the Corrençon earthquake. Moreover, the acceleration level of the SSE is around 1.5 times greater than that of the SMHV.
Consequently, it was the response spectrum calculated for the Corrençon SSE, i.e. an earthquake with a magnitude of 5.7, occurring directly beneath the installation at a depth of 7 km that was used by earthquake engineers to calculate the reinforcement work needed to ensure that the reactor building can withstand the SSE, while nevertheless taking account of design margins.
After the Fukushima-Daïchi accident France's nuclear safety authorities asked nuclear operators, and reactor operators in particular (EDF, AREVA, CEA and ILL), to base the calculations on an even more serious earthquake (see: Is a more powerful earthquake possible?).