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      The hazard standards are reviewed site by site during each periodic safety review. Major events determine doctrine changes and the resulting modifications, such as the Blayais flood or the Fukushima Daiichi accident (one part of the stress tests concerned the robustness of the facilities to extreme natural hazards). Margins relating to extreme weather events are verified every five years. A specific analysis is performed when an exceptional event occurs.

      episode de pluie extreme en france

      The post-Fukushima Daiichi assessments and the new standards issued by the Western European Nuclear Regulators Association (WENRA) have modified the doctrine by taking into account more extreme and rarer climatic events. In this way, we are looking for levels of climatic hazards and floods corresponding to return periods of 10,000 years. This is all beneficial to nuclear safety.

      In my eyes, some design rules need to be better defined. For instance, should low-probability events (e.g. a 10,000-year return period) be combined with hypothetical accidents? It seems logical to combine an extreme hot weather event with the total loss of grid, as these two events can be related, but it is perhaps less so to combine it with 4th category transients.

      Climate change will obviously change the standards (see below).

      A more embodied approach

      Although there are still cases where the main reason for a study is in defence to a question from the French nuclear safety authority (ASN), based on my interviews with central engineering staff, I gained a sense of better ownership in the area of natural hazards. The ADAPT programme initiated by the Nuclear & Conventional Fleet Directorate (DPNT) for adapting to climate change has been a catalyst. Everyone’s perception of climate issues, illustrated by experience and widely reported by the media, also helps to raise awareness. Though some absurdities remain (decimal points in temperature predictions in 30 years’ time, e.g. 45.7°C, or some tornado protection structures), they remind us of the importance of going to sites before designing modifications and of keeping an engineering judgement.

      I found ownership levels generally better in the plants I visited. I met some plant engineering teams that were very engaged and proactive. At other plants, non-shift operations specialists competently monitor river temperatures, prepare operational responses and coordinate with EDF Hydro and neighbouring sites. Reviews are organised several months ahead of summer to be ready to deal with temperature peaks.

      I also see greater investment in the material condition of heat sinks (see Chapter 2). Reinforcement of the facilities considerably improves the protection and nuclear safety of sites, while furthering staff ownership. However, the level of awareness and organisation in this field still varies too much from one site to another. Plant operation must recognise the importance of understanding and integrating climatic conditions.

      Blockage caused by dead leaves

      A massive influx of dead leaves during heavy rainfall blocked a pipe on the system used to clean the pumping station strainers. This blockage caused the water level to rise in the overflow system, which caused internal flooding of the pumping station galleries, without any impact on nuclear safety. The FARN’s pumping equipment and fire-fighters were called in to supplement the site’s own means. The analysis must identify all the consequences.

      The new sets of rules and requirements relating to equipment that contribute to protection against natural hazards (RASA) are receiving a mixed reception, and even creating a certain passion. Considered by some as an example of the increasing complexity of the general operating rules (RGE) (10,000 items of equipment classified as important for nuclear safety in the RASA), and even described as monstrous, they are recognised by others as a step forward in risk awareness and rigour. Their principle is hardly questionable: list all the equipment that helps protect the plant against natural hazards and apply the existing requirements. However, care must be taken not to model their application on that of the technical specifications (accumulated limiting conditions, timescales for controlled shutdowns, multiplication of safety-important equipment and activities, etc.) as it could turn them into a bureaucratic machine. The goal is to ensure that this equipment is known, carefully managed, and that staff feel a sense of ownership and responsibility.

      Remarkable skills but a fragmented, complex organisation

      Having visited the teams of experts in the field (R&D, LNHE, DTG, DT, CNEPE, DIPDE, UNIE, CTO), I can say that the Group’s skills are first rate. They have been steadily built up over decades and, in addition to the outstanding theoretical knowledge, these skills are rooted in experimentation (e.g. the National Hydraulics & Environment Laboratory (LNHE)) and in operational practices. The Climate Services Department in R&D, created in 2014, has created a network of these groups. The close ties between the nuclear division and EDF Hydro are invaluable. External cooperation is strong. The General Technical Division (DTG) at EDF Hydro plays an essential role in supporting the fleet and nuclear projects; it is important to adapt its resources to the increasing challenges of the nuclear sector.


      The competences are outstanding, but the organisation remains complex and responsibilities are fragmented, creating the risk of missing the overall strategy, postponing changes, or encouraging organisational silence. There is a risk that experts will limit themselves to the question they have been asked and will not raise issues outside the scope of their study.

      I believe it necessary to appoint a single point of responsibility who is accountable for the management of those risks, from doctrine to operational implementation. This strategy should cover the fleet and new-build projects. Discussions between entities should focus more on risks and their management, rather than just responding to a particular milestone, requirement or request.

      Though the UK experienced new momentum in 2023, the resources are stretched and the issue calls for a stronger management drive.

      Flooding poses a serious threat of provoking a common-mode failure. This is the main lesson learned from Fukushima Daiichi where the ingress of water into the reactor buildings rendered inoperable the safety systems and control & instrumentation systems. The Blayais event was a first warning and led to strengthening the standards for site flooding protection. When the Missouri River burst its banks in 2012, there was no impact on the Fort-Calhoun site thanks to mobile protection arrangements that were deployed in time.

      fort calhoun crue du Missouri en 2012

      Much has been accomplished since, the embankments at Blayais have been raised again and extended, a flood defence wall now protects the Gravelines plant, the embankment at Tricastin has been reinforced (see 2018 report), etc. The standards take into account a 1000-year flood or sea level, increased by 15%.

      The nuclear industry defines design-basis floods by extrapolating historical statistics; EDF Hydro models them by combining sets of catchment characteristics with a series of plausible weather events (SCHADEX method). This method has the advantage of representing the physical phenomena and incorporating more events, thus making the assessments even more robust. It also models water spreading further upstream, which is sometimes unaccounted for in historical data and can moderate certain floods. I note that the DPNT is planning a test case with this method.

      The engineering teams believe that the fleet is protected by robust margins. It is still worth remaining curious and prudent, always questioning if there are any events or combinations of events that could take us by surprise, in order to anticipate and prepare for a flood of an unprecedented size.


      This climate and hydrology simulation method is used to estimate plausible extreme floods in a catchment area according to its topography, characteristics and the return period in question.

      Developed by EDF, it has been the reference method for designing dam spillways since 2007. SCHADEX uses two components: a probabilistic rainfall model and a hydrological model that simulates the effect of these rainfall events for all possible hydraulic conditions in the catchment area (dry or soaked soils, snow height and density, water table levels, lake water levels, etc.).

      Palo Verde centrale americaine en plein desert Arizona

      EDF has excellent skills in this field, having started R&D on climate change in the early 1990s, the day after the IPCC published its first report on the subject. It is a solid foundation on which to base its strategies and responses.

      It is also good to see that studies to prepare the fifth ten-yearly inspection outages (VD5) mainly focus on plant responses to climate change (see Chapter 5), and that the DPNT has incorporated them into a long- term perspective beyond 10 years between VD5 and VD6. The EPR2 project defines the conditions in the initial design scope and identifies possible changes that could allow the reactor, during its operational lifetime, to adapt to climate change as observed and predicted. The development of a global strategy at the DPNT under the ADAPT programme is commendable; it covers all non-nuclear-safety factors of plant operation, including water resource management, by considering them in their geographic, social and economic environment.

      Will we run out of water?

      Most of the water from rivers flows into the sea. A nuclear reactor operating with a closed system (cooling towers) consumes less than 1 m3/s of river water through evaporation, while those operating in an open circuit return all their cooling water to it. It is worth remembering that the average flow rate at the mouth of the Rhone River is 1700 m3/s while the Loire River is 900 m3/s.

      Generation limits imposed on reactors during summer are not due to the lack of water, but the limits on the water temperature as defined in the ministerial orders on discharge limits to protect flora and fauna. These limits generally affect less than 0.5% of EDF’s generation. Estimates predict that this figure could increase up to 1.5%, which is still low. The ministerial orders could be reviewed where they cite an absolute temperature limit, rather than the actual increase in temperature measurements upstream and downstream of the reactor, as biotopes adapt to an overall increase in the temperature of water courses. This approach seems feasible judging on the mass of data and accumulated knowledge since the start-up of the plants. This is how special exemptions were able to be granted in 2022; monitoring has confirmed the absence of any consequences on the environment.

      Estimates foresee a reduction in the average flow rate of rivers in summer due to more infrequent rainfall, reduced snowfall, and increase in evapotranspiration from vegetation. A lack of water in rivers would affect generation well before nuclear safety: power generation requires removing two-thirds of the reactor’s nominal thermal power, while ensuring nuclear safety during shutdown only requires removing a small percentage. EDF R&D and engineering are investigating ways of reducing water consumption in the long term, which is encouraging. It is positive that the DPNT and the DIPNN travelled to benchmark the Palo Verde nuclear power plant, which, in the Arizona desert, is cooled by wastewater from the town of Phoenix. It is equally interested in other solutions deployed at the Trillo and Almaraz plants in Spain. In France, the ultimate heat sink at the Civaux plant is, since its start-up, designed for the extremely low water levels of the Vienne River.

      The ultimate heat sink at Civaux

      The Vienne River has a very low flow rate in summer, so a specific ultimate heat sink was designed from the onset of the Civaux plant. Large reservoirs guarantee the cooling of safety systems via cooling towers and give the plant a minimum of ten days’ self-sufficiency without having to pump water from the river. Much smaller in size than those used for generation, these cooling towers have forced ventilation. With redundancy and electrically backed-up by diesel generators, the whole system is safety-classified.

      EDF Hydro’s support can ensure an upstream flow and temperature that enable reactors to be kept in production during heatwaves. EDF Hydro therefore assumes the global role of optimising the management of water sources. Other than generating electricity, the hydroelectric structures along the Durance-Verdon river basins provide potable water in the region, protection against flooding of the Durance River, irrigation for farming, and water levels that enable tourist activities in the lakes of Serre-Ponçon and Sainte-Croix. In regions where nuclear power is produced, management of the upstream heat sink, support for reactors from hydroelectric resources, and integrated management of water use are all public service requirements that will only increase with climate change. This is why it seems essential to maintain close ties between EDF nuclear and hydroelectric departments in the scope of current or future reorganisations. The management of cross-border rivers will also call for the constant involvement of our diplomatic staff.

      In the nuclear safety cases, there is only one site that requires hydro support in case of low river water levels. It would be advisable to check whether climate change will affect this number. The severe low-water periods do indeed need to be considered in order to confirm whether the safety flows could be called into question on certain watercourses. Analysis of this issue has begun, with the DTG publishing a first report. The river version of the EPR2 may, depending on the river flow, incorporate an ultimate heat sink similar to that at Civaux.

      Climate change can also modify biotopes and increase the risk of significant quantities of material flow into the heat sinks and block filters: jellyfish, juvenile fish, seaweed, etc. Improvements have been made: monitoring the level and pressure differential downstream of the rotary-drum filters, design changes to screen rakes, tracking of fauna, agreements with fishermen, etc. These improvements should be pursued as climate-related events continue to occur.

      Will there be too much water?

      The rise in sea levels as predicted in the different IPCC scenarios is easy to calculate. In addition to being predictable, sea levels rise slowly: we can therefore prepare the plants in advance.

      Experts do not consider that climate change is obviously likely to raise the risk of major river flooding. Nevertheless, we must remain cautious and analyse the projections in detail. This is the purpose of the programme to assess extreme flooding under climate change that the DTG recently initiated, with the DPNT, DIPNN and R&D also involved.

      I believe we should pay particular attention to severe local weather phenomena. A very conservative postulation of extremely heavy rainfall was taken into account after Fukushima Daiichi: 20 cm in two hours. Since then, violent weather events have been observed and rainfall records are regularly broken. It is not certain that heavy rainfall will be more probable in the future (more blocking anticyclones), but it can be more violent (a hotter sea generates air masses that carry more moisture). The rainfall events known to occur in the Cevennes area are now covering a larger region. I call for a re-examination of the assumptions in this area. Safety case studies take into account the complete blockage of the rainwater drainage systems; it would be worth renewing attention to the condition of these systems to improve margins. Depending on the results of these studies, the height of the flood defences around nuclear islands may need to be checked.

      In the UK where all the nuclear power plants are located on the coast, considerable work was carried out in the wake of Fukushima Daiichi to assess the risk of sea flooding and protect the facilities, e.g. raising the height of flood defences, using mobile dam-boards, etc. Nuclear Operations has restarted these studies with focus placed on the combination of rain and wind, which is important.

      Managing heat waves

      From a nuclear safety perspective, in the event of a heatwave, the following must be kept below a specific limit:

      • The water temperature of the cooling system that removes decay heat from the fuel and cools equipment by means of heat exchangers
      • The air temperature of rooms containing safety-significant equipment, by means of ventilation and air-conditioning, backed up by diesel generators.

      It must also be checked that the temperature conditions are safe for the operational staff.

      An acceptable temperature limit for river water is defined in the relevant safety cases and compared with the maximum expected temperatures. If they become excessively high, the first response will be to increase the cooling surface area or flow rate of the heat exchangers. This is one of the real nuclear safety topics of climate change.

      As far as air temperature is concerned, in France the sizing of chillers and diesels is based on modelling the temperature of each room as a function of that of the outside air, the characteristics of the air conditioning system and the heat dissipated in the buildings by pumps, electrical panels, control systems, residual power, etc. The studies are conservative: for instance, all the equipment is assumed to operate at the same time and at full power. The UK uses a more empirical method based on measuring the temperature in the buildings.

      In France, the heatwave in 2003 revealed some design limits. As a result, a ‘hot weather’ plan regularly updated, has proved its effectiveness in 2021 and 2022 (see 2022 report). The approach will have to be continued and expanded, particularly during the fifth ten- yearly inspection outages (VD5) on the 900 MWe series. Records are regularly beaten, especially in the north-west of France. To date, the heatwave temperature limits retained for the VD4s have not been called into question. Temperatures of 50°C are considered for the fleet (VD5 900) and the EPR2, intended to cover a return period of 10,000 years taking into account climate change.

      The UK experienced unprecedented and unexpected temperatures in the summer of 2022, reaching up to 36°C even in the north of the country. Although none of the plants had any problems, the effectiveness of the seasonal readiness plans was questioned. It became apparent that this type of event needed to be incorporated into nuclear safety cases. A nuclear safety assessment taking into account a temperature of 36°C was carried out. The assessment essentially recommends the preventive shutdown of reactors in this case. It is important to take the process further than this provisional report, whether in terms of the analysis methods, the temperatures considered (40°C), the actions to take, or the solutions chosen. This analysis needs to be extended to cover other unforeseen weather events or combinations of events.

      Other than reinforcing the capacity of chiller units, diesel generators and heat exchangers, which is necessary but is still a ‘more of the same’, I suggest considering:

      • The resistance of equipment to high temperatures: true resistance, or even the ultimate resistance, of existing equipment and the specification of new equipment
      • The means of limiting heat dissipation by switching off non- essential equipment or reducing consumption of new equipment, or reducing absorbed heat by painting buildings white, for example
      • The margins applied in calculations
      • Events that could be assumed not to occur in combination with extreme temperatures
      • The additional margin ultimately gained through preventive reactor

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