Following the Fukushima Daiichi accident, the Western European Nuclear Regulators Association (WENRA) updated in 2014 the safety reference levels (SRL) for existing reactors, introducing a new chapter specific to natural hazards.

In 2015, in preparation for the 4th periodic safety review of its 900 MW units, EDF aimed at meeting these new reference levels. While many of them were already satisfied for a long time by EDF (for example: Identification of natural hazards, Site specific natural hazard screening and assessment, Protection against design basis events), several of them were new objectives:

- T4.2: The exceedance frequencies of design basis events shall be low enough to ensure a high degree of protection with respect to natural hazards. A common target value of frequency, not higher than 10−4 per annum, shall be used for each design basis event. Where it is not possible to calculate these probabilities with an acceptable degree of certainty, an event shall be chosen and justified to reach an equivalent level of safety.

- T6.1: Events that are more severe than the design basis events shall be identified as part of DEC analysis.

This article focuses on the first objective that is WENRA RL T4.2.

Estimating a 10−4 Return Level for natural hazards is generally based on the application of the statistical Extreme Value Theory (EVT). In case of lack of reliable data or intermittent phenomenon, it is difficult to estimate such a level.

With regard to the intensity of natural hazard to be used to define the protections, EDF has developed an approach distinguishing 3 types of hazards:

- Those for which 10−4 level is definable, as earthquake, external flooding and tornado. For these hazards, the facilities are already protected against this level of hazard.

- Those for which the 10−4 level is evaluated indirectly, such as cold temperatures, warm temperatures, and high winds. For those, EDF defined a “WENRA hazard”, which complements the Design Basis Hazard, and verified the capacity of the facilities to cope with it. This hazard is determined on the basis of a value with a “reasonably quantifiable” frequency of occurrence (typically a 100-year return period), to which EDF then adds a margin to target a level of risk that can reach a 10−4 level. The method of quantification of this margin crosses different approaches (mainly the gap between the observed records and statistical extrapolation)

- Those for which the 10−4 level is considered not relevant, such as lightning or snow. For lightning, the robustness is ensured on the one hand by taking into account for the Design Basis lightning the highest level of the standard AFNOR NF EN 62305-1 and on the other hand by the protection of the hardened safety core equipments against an extreme lightning level. For snow, protection is based on the normative context with margins for some sites. The robustness of the structures and the organizational arrangements make it possible to cope with snow levels higher than those used for the design basis.

In conclusion, the capacity of the EDF 900 MW NPPs to cope with high level of natural hazard (equivalent to decamillennial events) is being verified through the 4th periodic safety reviews, in compliance with WENRA reference level T4.2.

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