According to the Periodic Safety Review Process, the safety level is re-assessed every ten years, considering national and international operational feedback, evolution of knowledge and best available practices. Protection against natural hazards is part of this safety level re-assessment. In the current global change context, climate change impact has to be integrated in external natural hazards estimations, such as climatic hazards or external flooding. EDF has consequently implemented a climate watch approach. Undertaken approximately every 5 years, roughly in line with the publication of the assessment reports of the Intergovernmental Panel on Climate Change (IPCC) and with the update of safety licensing basis during Periodic Safety Reviews, this approach is intended to: - revisit the climatic hazards which present a plausible or certain upward trend, and could lead to an increased reference hazard level, - monitor the reach of target levels which should trigger a thorough analysis (concept of Major Climate Event) to ensure the robustness of the reference hazard level between two periodic reviews. This climate watch approach is developed in partnership with the scientific community and is based on the following activities: - compile and analyze datasets on hazards that are subject to changes with climate change (observed and modelled time series), - develop knowledge of associated climatic phenomena (models, projections). The application of this approach is presented in two steps: - the key implications of the last climate watch exercise carried out in 2015, which identified climatic hazards whose evolution is unfavorable and is plausible or certain for the sites of EDF NPPs: ○ High air and water temperatures (for the “heat wave” hazard) ○ Sea level (for the “external flooding” hazard for coastal or estuary sites) ○ Drought or « low flow » hazard for fluvial sites; - the results obtained for the 900 MW units, for which EDF started the 4th periodic safety review in 2019. Such an approach, which is closely linked to periodic reviews, ensures the robustness of nuclear power plants to the climatic hazards through the consideration of the updated hazard levels.

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.

The field of protection against external natural hazards (eg.: rare and severe hazards) has regularly evolved since the design of the first NPPs (Nuclear Power Plants) to take into account the experience feedback. Following the Fukushima Daiichi accident in March 2011, consideration of rare and severe natural hazards has considerably increased in the international context. Taking rare and severe natural hazards into account is a challenge for operating nuclear reactors and a major issue for the design of new nuclear reactors. In Europe, considering lessons learnt from the Fukushima Daiichi accident, European safety authorities released new reference levels in the framework of WENRA 2013 (Western European Nuclear Regulators Association) standards for new reactors [1] to address external hazards more severe than the design basis hazards. Considering this input, the French and UK nuclear regulators have released specific guidelines (Guide No. 22 related to design of new pressurized water reactors [2] for France and ONR Safety Assessment Principles SAPs [3] for the UK) to describe how to apply those principles in their national context. To comply with those different guidelines, EDF has developed different approaches, called Beyond Design Basis (BDB) approaches, related to rare and severe natural hazards issue in the French and UK context for nuclear new build projects. Those two approaches are presented in the present technical paper with the following structure: - safety objectives; - hazards to consider; - SSCs (Structures, Systems, and Components) required to meet safety objectives; - study rules and assumptions; - analysis of deterministic or probabilistic nature, thereby including the following: ○ analysis of available margins (margin between 10−4 per annum exceedance frequency of hazard site level or equivalent level of safety and the chosen Design Basis Hazard level also called ‘inherent margin’); ○ Fukushima Daiichi accident Operating Experience feedback; ○ probabilistic safety analyses. This technical paper highlights common characteristics and differences between the two approaches considering the French and UK regulatory contexts.

Over the years, power plants have been hit by numerous severe weather events (storm, flood, heat wave...). EDF (Electricity of France) and ASN (Nuclear Safety Authority) want to assess the future impact of severe weather events on the power plants. Furthermore, recent research on storms estimates more accurate wind speed return values than before. For this reason, the severe wind value is an important parameter to quantify on a NPP (Nuclear Power Plant) site, in order to verify if the protection measures are sufficient or, if necessary, to design adequate protection. To cope with those objectives, wind flow behavior around a PWR (Pressurized Water Reactor) nuclear power plant is studied. The goal of this work is to check that there is no exceeding local wind speed relative to the wind entering the site. The severe winds are characterized locally near the buildings in terms of location and amplitude. Different kind of topology for the nuclear power plant sites are studied in the project: near a cliff, in a plain or in a basin. In our study, the CFD (Computational Fluid Dynamics) open source tool Code_Saturne developed at EDF-R&D is used to simulate the wind over a French PWR site located in nearly flat terrain in a plain. The 3D mesh includes buildings of the site. Several wind directions corresponding to the prevailing winds are studied. Two wind speeds corresponding to wind speed return values are studied (eg: the inlet wind speed is 25 m/s at 10 meter high for a return period of 50 years). Furthermore, several locations selected near buildings are studied carefully. Swirling flows have been viewed between buildings. Analysis of the results shows that the wind speed near the buildings does not exceed the wind speed at the entrance of the domain for the three directions studied except near the cooling towers and above buildings. However, this result should not be generalized to other PWR sites due to the specificities of each site such as relief, buildings position, buildings size, roughness, wind rose... This methodology could be applied at other nuclear power plant sites.