We hear a fair bit about nuclear plants, especially in France, being forced to curtail their power in the midst of blistering heatwaves. that have struck Europe in 2026. A majority of reports blame this curtailment, correctly, on the record temperatures of rivers, likes that supply cooling water to plant reactors. There is also a tendency imply or to state, incorrectly, that such a heat wave “pushes…temperatures too high to cool reactors safely.”
This isn’t merely wrong. It unfortunately adds yet another piece of misinformation to the realm of anxiety that has held back this noncarbon technology for so many decades. In real world terms, the lost power has never been more than a small fraction (<1.5%) of annual power output.
The reasons for reducing power output, in fact, have nothing to do with human safety. Their legitimacy, instead, comes from commercial and environmental considerations. Another fact: it is not only nuclear plants that need to be turned down or off.
Why Hot Weather Reduces Power Output
Nuclear power uses the process of fission to generate high levels of heat to generate steam. The steam then spins the blades of a turbine connected to a generator. After doing this work, the steam is directed into a condenser, where water from an external source cools it back into liquid form and recycles it back to the reactor core.
Most nuclear plants are therefore located beside rivers, lakes, or coastlines. When the water from any of these sources rises above its normal temperature range, its ability to cool and condense the steam is reduced. This, in turn, has the effect of reducing the efficiency of turbine and thus the generation of electricity.
The magnitude isn’t trivial. As a rule of thumb, plants typically lose on the order of 0.3%-0.5% of cycle efficiency for every 10F (~double that per 0C) cooling water temperature rises above design conditions. The precise sensitivity varies by turbine and condenser design, and by how close the plant already runs to its rated capacity, but the scale is roughly consistent.
But it isn’t just nuclear that is impacted. The same thing happens to any “thermal” power plant that uses heat to produce steam and run a turbine-generator with a water-based cooling system—the Rankine Cycle, as engineers know it. Coal, natural gas, biomass, and steam-cycle concentrated solar power are all affected by the losses in efficiency.
In truth, heat waves stress power grids in a number of ways, producing faults and outages on a variety of scales, small and large. They can cause transmission and distribution lines to swell and sag, transformers to fail, oil and gas in pipelines to expand and increase pressure, and other impacts.
Why, then, does nuclear get all the attention? Mainly because it carries decades of cultural association with risk—it is a politically and symbolically charged technology. A story about “heat wave forces nuclear plant to cut power” fits an existing cognitive template, so to speak, and is bound to gain attention. France has become the poster child for this, as it depends on nuclear for the greatest amount of electricity and forced yet again to “shut down reactors” when a new heat wave arrives.
A counterargument might be that these plants are more visible and generate especially large amounts of power, so when they are curtailed, the effects are more apparent. It is certainly true that recent heatwaves across Europe have led to ordered reductions in output at reactors along the Rhine, Rhône, Danube, and other rivers, when cooling-water availability and regulatory constraints on thermal discharges. But this has also happened to large, GW-scale coal plants across Europe. Nuclear gets the spotlight, but it is only one player in the drama.
If we’re interested in the relevant science, it tells a larger story. Heat waves are a stress test for the entire water-cooled thermal fleet–indeed for the grid as a whole– not a nuclear-specific problem.
Heat Waves Affect Renewables Too
We shouldn’t think, however, that only thermal plants are impacted. Heat waves have different effects on hydro, wind, and solar power with the same overall result: less electricity, less reliably, and, in the latter two cases, equipment that wears out faster.
Heat waves usually arrive alongside drought. Less rainfall and snowmelt mean less water flowing in rivers and into reservoirs, thus less available to generate power. Output falls—sometimes sharply, as when Sichuan Province in China saw hydropower generation cut in half in 2022. The same has happened to some European rivers in 2026 after three major heat waves in the span of just three months.
Solar panels lose efficiency at high temperatures, because heat interferes with how the semiconductor material converts sunlight into electricity—typically a 0.3% to 0.5% drop for every 1oC above the standard testing temperature (typically 25oC, or 77oF). This may not sound like much, but solar begins at a comparatively low efficiency (~15%-23%), so even a a few percent is significant.
Wind turbines can face a triple penalty. Winds die down or even cease during a heat wave in many cases. Such air, moreover, is less dense so carries less energy for the same wind speed; turbines simply generate less power. On top of that, the generator, gearbox, and electronics inside a turbine rely on temperature differences with the outside air to cool themselves, so when the air is already hot, that cooling becomes harder, and turbines are often programmed to automatically reduce output—or shut down entirely—to avoid damage.
Beyond these short-term losses, both technologies age faster because of heat. Repeated exposure to especially high heat accelerates the natural chemical degradation of solar panel materials and can stress electrical connections. With wind turbines, it can degrade the oil, bearings, and insulation, shortening their effective lives.
Again, while nuclear gets the lion’s share of attention, physical science shows that heat waves reduce output and accelerate wear across essentially all forms of power generation.
Climate Change and the Future of Power Generation
Historically, power plants were designed around the climate conditions that engineers expected to encounter. Rivers, lakes, and other water bodies were expected to remain within known, familiar temperature ranges. Summer droughts of unusual severity were considered rare.
Those assumptions are now unreliable. Climate change has increased the frequency and intensity of high heat events, contributing to warmer waters, lower levels in rivers, and longer periods of drought in many regions, Europe and the US among them.
For electricity systems, this has become a growing challenge. The hottest days are also when power consumption is highest because people rely more on air conditioning. Yet those same periods can reduce the ability of some power plants to operate at maximum capacity. This is happening, moreover, against the background of rising demand for electricity overall.
In 2025, the global market for air conditioning was around $137 billion, projected to grow to over $245 billion by the early 2030s (heat pumps use about the same amount of power as air conditioners for cooling but are several times more efficient for heating).
This growth in need for cooling—people, powerplants, data centers, and more—therefore adds to the overall demand for electricity and will continue to do so in decades ahead.
Dealing With The Problem – Incremental Change
To reduce the impacts from heat waves, alternative cooling systems, new power plant designs, and regulatory flexibility have all been put to use. New materials for solar cells and internal cooling and lubricant architectures for wind turbine-generators are in the works.
Adding cooling towers to nuclear and large coal or natural gas facilities is one kind of retrofit. By creating a closed-loop system for condensing the steam back into liquid, it greatly reduces the volume of water drawn from and returned to a river.
Air cooling, using large fans, is another option for thermal plants. Though a proven technology, it is more the exception, mainly confined to water-scarce areas. This is due to lower efficiency than water cooling, higher capital cost, and greater power consumption. Hybrid systems, combining air and water technology, are more useful in some other cases.
Nuclear has other specific alternatives. The new breed of small modular reactors will use much less water individually than a large-sized plant so will have less of an environmental risk. Moreover, the cost disadvantage is for air and hybrid cooling is greatly reduced for these smaller reactors. Beyond this, next generation reactor technologies, several of which are being deployed, use coolants such as helium, molten salt, and liquid metal instead of water.
As for regulatory flexibility, policies exist in many countries that temporarily relax environmentally based limits on power generation when heat waves occur. Such environmental waivers are not arbitrary decisions but legal mechanisms, ordered by local authorities, to prevent grid instability, for example when loss of wind generation would be exacerbated by curtailment of thermal plants.
There’s No Perfect Power System, But Diversity Helps
In the end, media focus on nuclear plants serves as a distraction from the larger problem heat wave events represent for power systems of every type. This matters, because it can warp public perception and support poor policy decision-making.
There is little doubt that in coming decades heat waves will become hotter, more frequent, and longer in duration. In the glare of this reality, electricity demand is growing even as the need for reliability has soared, while grids have been adding larger amounts of less predictable, weather-dependent generation.
All of this argues for a diverse portfolio of sources that can compensate for limitations of individual technologies. As is now widely recognized, nuclear power in a variety of forms will be a necessary part of this.
