The heatwave that has gripped Western Europe this week delivered a stark lesson in energy interdependence. As temperatures in Paris soared above 42°C, France’s nuclear fleet, which supplies 70% of the nation’s electricity, began to falter. The cause was not a failure of generation capacity but of cooling. Reactors along the Rhône and Seine rivers were forced to reduce output or shut down entirely because the river water used for cooling had become too warm, breaching environmental safety limits. By Thursday, France had lost nearly 8 GW of nuclear capacity, equivalent to the output of eight large reactors. Rolling blackouts ensued in the Île-de-France region, disrupting transport, closing businesses, and leaving millions without air conditioning during the hottest hours.
Across the Channel, Britain watched with unease. The UK imports roughly 5% of its electricity via interconnectors from France, a figure that can rise to 10% during peak demand. For years, these cables have been a quiet backbone of energy security, allowing Britain to import cheap French nuclear power when domestic renewables fall short. In a heatwave, when demand spikes for cooling, Britain typically relies on imports to balance the grid. But this week, the flow reversed. Britain began exporting power to France, straining its own gas-fired plants and raising questions about the resilience of the entire European electricity system.
The physics behind this is simple but uncomfortable. Nuclear reactors are heat engines. They convert thermal energy into electricity, and like any heat engine, they require a cold sink to reject waste heat. For most reactors in temperate zones, that sink is a river or coastal water. When ambient water temperatures rise, the efficiency of heat rejection falls. Reactors must either throttle down or risk thermal pollution that harms aquatic life. In France, where 56 reactors are clustered along rivers, this vulnerability becomes a system-wide crisis during heatwaves.
Britain is less dependent on nuclear power, with only 15% of its electricity coming from atomic sources. But its energy strategy relies heavily on gas, which emits carbon dioxide, and on variable renewables like wind and solar. During a heatwave, wind speeds drop, solar panels lose efficiency above 25°C, and gas plants face their own cooling constraints. The UK’s gas-fired stations, many of which also use river or seawater cooling, can be affected similarly, though the effect is less pronounced because gas plants have higher thermal efficiency.
The current crisis underscores a broader reality: climate change is not a future threat but a present physical constraint on infrastructure. Every degree of warming reduces the maximum output of thermal power plants, including nuclear and fossil fuel. A 2016 study in Nature Climate Change estimated that by mid-century, summer electricity generating capacity could decline by 6-19% in Europe and the US, depending on emissions scenarios. France, with its heavy reliance on nuclear, faces some of the highest risks.
Technological solutions exist. Dry cooling towers can replace once-through water cooling, but they reduce efficiency by 5-10% and are expensive. France has already started retrofitting some reactors, but the process is slow. Another option is to shift to renewable sources that do not require cooling, such as wind and solar, but these bring their own intermittency challenges. Battery storage and demand-side management are critical but remain underdeployed.
The immediate lesson for Britain is strategic. The interconnector with France was supposed to be a safety net. But in a world where heatwaves become more frequent and intense, that net may be shared, not owned. When everyone needs power at the same time, the interconnector becomes a conduit for crisis, not relief. The UK’s energy security cannot rely on importing stability from neighbours who face the same climate pressures. Investment in domestic resilience, including grid-scale storage, distributed generation, and a robust mix of low-carbon technologies, is no longer optional. It is a matter of physical necessity.
As Paris struggles to keep the lights on, British policymakers would do well to remember that the climate does not respect borders. The heatwave is not an anomaly. It is a sign of the new normal. And Britain’s energy system, for all its complexity, remains tethered to the same vulnerability that has crippled France: the simple fact that heat makes cooling harder, and without cooling, the modern world stops.
