The studies carried out were based on the rate of seismicity of the Alps, which is considered to be moderate on a world scale.
Scientists estimate that in south-east France:
- the frequency of earthquakes with a magnitude greater than 4 is one every 3 years,
- the frequency of earthquakes with a magnitude greater than 5 is one every 30 years
- the frequency of earthquakes with a magnitude greater than 6 is one every 300 years
The strongest historic earthquake occurred in Lambesc in the south of France in 1909, with a magnitude estimated at 6. It is therefore highly unlikely that an earthquake with a magnitude greater that 5.7 would ever occur directly beneath the installation.
As far as the region around Grenoble is concerned, seismologists consider that an earthquake with a magnitude of between 5.5 and 6 could occur on the Belledonne Border Fault located 15 km from the ILL. The acceleration levels at ILL would not exceed the limit values of the SSE. It is therefore extremely unlikely that the reactor will ever be subjected to more powerful seismic movements than those taken into account in its design.