Seismic hazard for design and verification of nuclear installations in France: regulatory context, debated issues and ongoing developments

Deterministic (DSHA) and probabilistic (PSHA) seismic hazard assessments can, and often do, lead to quite different seismic hazard levels. There is a growing need in France for decision makers to understand the origin of these differences. The focus of this paper is to present the current status in the development of a formalized propagation of uncertainties based on the French deterministic seismic nuclear safety rule (RFS 2001-01, 2001). Along the lines of PSHA approaches, the method consists in the construction of a Logic Tree (LT). Preliminary comparisons between DSHA-LT and PSHA-LT averaged over 30 nuclear sites located in metropolitan France show that, if the resulting uncertainty in the hazard spectrum may be comparable, the median hazard spectral values may actually differ. Not surprisingly, the resulting general tendency is that in low seismicity zones, DSHA tends to predict higher median spectral values compared to PSHA_RT=1000yr. In higher seismicity zones, comparable hazard levels are obtained on average between DSHA_MHPE (Maximum Historically Probable Earthquake) and PSHA_RT=1000yr, but extrapolation of PSHA results at 10,000- year return periods may have a tendency to exceed DSHA_MHPE+0.5 values. A site-specific analysis including all the constraints imposed by the RFS 2001-01 (2001) (minimal spectral response, paleoseismic spectral response and site-effects) will be necessary for a complete DSHA/PSHA comparison.