Heatwave: nuclear reactors adapt to extreme events

A heat wave will cross France in the coming days. The mercury could reach or even exceed 40°C. In this context, certain nuclear reactors, like all production tools, could see their production reduced to protect the ecosystems of rivers. EDF has been working on these phenomena for years. On average, they represent a loss of production of around 0.4% per year. 

With Météo France announcing a possible heat wave in France this week, the political debate on nuclear power is heating up over the question of the operation of nuclear reactors in extreme heat or drought situations. These situations are likely to occur more and more often with climate change. Some see this as evidence of the “intermittency” of nuclear power. According to RTE, from 2015 to 2020, the unavailability of nuclear reactors for climatic reasons amounted to an average of 1.4 TWh, i.e., less than 0.4% of annual nuclear production.

Regulatory constraints to preserve biodiversity

Generally speaking, a nuclear power plant, like any thermal power plant (coal, gas), needs water to operate and cool its various circuits. Therefore, the installations are near the sea, a river, or a stream. They withdraw a certain amount of water from the sea, river, or stream and then return most of it to the environment (about 98%, source: water information center). These withdrawals and discharges are strictly controlled by regulatory constraints set by the ASN site by site. Reactor shutdowns or power reductions are not due to a lack of water but to a concern to withdraw less water or limit the temperature increase for environmental reasons, mainly to protect biodiversity.

Episodes of heat and drought can impact the temperature and flow of certain rivers, even upstream of the power plants. To comply with the regulations, EDF may have to reduce the production of specific units from time to time. For example, on May 10th, EDF had to reduce its output at the Blayais power plant by 100 MW to meet the regulatory temperature limit of 30 degrees for winter thermal discharges into the Gironde. Note that this limit increases to 36.5 degrees for the summer from May 15th. In the same way, EDF announced, as is its legal obligation towards the network manager and the market, a probable reduction in production at reactor n°1 of the Saint-Alban Saint-Maurice power station, located on the Rhône. At this power plant, the regulations set a maximum temperature rise of 3°C in the Rhône, with a maximum downstream temperature of 28°C from May 16th to September 30th, to avoid any consequences for aquatic flora and fauna. The power plants located by the sea (14 of the 56 reactors) are little, if any, affected. The water is colder and more abundant, and the discharges have a little thermal impact beyond the area near the discharge site.

These reductions in production remain very small, not only in relation to annual output but also in relation to other causes of unavailability or modulation. (cf. RTE report of March 2021)

According to RTE, these decreases concerned 9 of the 14 river power plants between 2015 and 2020 and are induced either by high temperatures or low flows. According to a presentation by RTE in March 2021 (WG1 Climate meeting #4), the simultaneously unavailable power peaks amounted to between 1 and 6GW. They had no impact on French security of supply, as they occurred during the summer period when electricity consumption, of the order of 50GW, is much lower than the winter peak consumption (of the order of 80GW). The summer period is also generally the time the operator chooses to carry out maintenance and refuel nuclear reactors.

Increasingly high-profile episodes

Production shortfalls are highly publicized and are included in debates on the future of nuclear power. This was the case, for example, for the drop in production at Le Blayais on May 10th, even though it represented less than 1% of the nuclear production on that day.

On the same day, wind and onshore generation showed variations in the 50-100% magnitude output, but there was no media coverage. It is important to note that, in general, the increased frequency and duration of hot events will affect the power system, including consumption, the various means of production, and the transmission and distribution networks.

The most significant case of such an episode was California’s power cuts imposed in August 2020 [1]. These blackouts resulted from heavy use of air conditioning, unplanned unavailability of some power plants, limited electricity imports from neighboring states, and insufficient solar and wind generation. This has led to an imbalance between electricity production and consumption. It is worth noting that the Diablo Canyon nuclear power plant usually operated at total capacity throughout the heat wave. There are many examples worldwide where nuclear power plants can be adapted to work in very hot or water-starved conditions. The Barakah plant In the United Arab Emirates has two 1.4GW reactors in operation in 2020 and 2021. The 4GW Palo Verde plant in the United States, located in the Arizona desert, is cooled with wastewater from neighboring cities, including the city of Phoenix. Similarly, Spain, where only one of the seven reactors is on the coast, has shown minimal downtime in recent years.

What is the outlook for 2050?

In its study “Energy Futures 2050”, RTE points out that the main risk to the security of electricity supply will remain concentrated on cold winter waves. Nevertheless, it notes that “the increase in drought situations at the end of the summer and in the autumn could also lead to periods of tension for the electricity supply-demand balance, all the more so if these droughts are combined with periods of low wind .”RTE emphasizes the need to manage hydro stock differently, as the filling of dams due to snow melt will be earlier in the year, and late droughts will be more frequent in the early autumn.

It notes that existing nuclear power plants on the river will be more regularly affected by high heat and drought periods. The volumes of “lost” energy, which could involve significant one-off power losses, will remain small on an annual scale. The sensitivity of the new nuclear reactors to these climatic hazards could be minimized by favoring specific sites (on the seashore or the banks of rivers with low constraints in terms of flow rates and threshold temperature) and thanks to air coolers for future plants on the banks of rivers. ■