The same text every summer: Why do French nuclear power plants reduce output during a heat wave?

Current heat wave that hit Western Europe has again sparked a wave of superficial media headlines about how “French nuclear power plants shut down due to high temperatures.” Data from the grid operator RTE and EDF show that nuclear output in France has temporarily decreased by about 4.1 gigawatts (about 7 percent of total demand), and that the Golfech 2 unit on the Garonne River has been temporarily disconnected from the grid.

However, behind these figures there is not a technical problem in the plants themselves, nor a safety risk to the reactors, but the application of environmental laws that protect the river ecosystem.

Technologically safe, but ecologically constrained

Nuclear power plants are designed to operate at extremely high external temperatures, and the heat does not affect their functional safety. The problem that arises during summer is solely a matter of thermal load on rivers.

Nuclear plants that use the so-called open cooling system take water from the river, use it to cool condensers, and then return it to the river, slightly warmed (usually by a few degrees). When external temperatures are extreme—as these days when temperatures in France exceeded 44°C—the rivers themselves heat up naturally.

French environmental regulations strictly limit the maximum allowable river water temperature downstream of the plant (for the Garonne this is about 28°C) in order to protect living beings, primarily fish and river flora. When the river itself reaches that critical point, EDF is legally obliged to reduce reactor power or shut it down completely, so that the returned water does not further raise the river's temperature and endanger the biosphere. Therefore reactors are not shut down because they “cannot operate,” but to prevent overheating of the rivers.

Which nuclear plants do not have this problem?

This seasonal limitation is not a universal flaw of nuclear technology, but a locality-specific issue tied to the chosen cooling system. The problem with summer power reductions does not affect three types of nuclear power plants:

1. Nuclear plants on seas and oceans: coastal plants do not have this problem because sea basins are so vast that the warmer water the plant returns has no measurable impact on the temperature of the surrounding body of water.
2. Nuclear plants beside large lakes: Similar to the sea, large bodies of water have enormous thermal capacity that neutralizes the ecological risks of overheating.
3. Nuclear plants with closed cooling systems (cooling towers): plants that have massive hyperbolic cooling towers use significantly less river water. They do not return heat to the river, but release it as water vapor into the atmosphere (which are the recognizable white clouds above the towers). Because they do not warm the river flow, they can operate at full power even during the hottest heat waves.

The problem of the entire thermal energy sector

Although nuclear plants attract the most media attention, the physical laws and ecological cooling constraints apply equally to all thermal energy sources. High temperatures and low water levels affect hydroelectric plants, as well as gas- and coal-fired power plants.

Thus, for example, during this same heat wave, five gas-fired power plants in the United Kingdom had to reduce their output by a total of 2.5 gigawatts because the hot air reduced the efficiency of their cooling systems.

In times when climate change brings increasingly frequent summer peaks in consumption due to air conditioners, power systems will have to adapt. For the nuclear sector this does not mean changing the reactor technology itself, but investments in modernizing cooling systems and moving to closed cycles with towers at those river locations that are most exposed to summer heat.

What we are interested in at this moment is the situation with data centers (because they consume large amounts of water to cool processors) and whether we will see some media coverage on that topic.

S.A.