Why the heatwave isn’t halting nuclear power (despite the French case)

Every summer, as reliably as the heatwave, the same refrain is heard: nuclear power stations don’t work in the heat. But is that really the case? Let’s start by saying that all thermal power stations (coal-fired, gas-fired, nuclear, geothermal and solar-thermal) need to condense the steam at the turbine outlet. This requires water, drawn from a river or the sea, which is returned in its entirety, slightly warmed. In the absence of abundant water, this open-cycle system can be replaced by evaporative cooling towers, which require far less water and of which only a small percentage evaporates.

So what does the heatwave have to do with the operation of a nuclear power station? It has been very hot recently in France and elsewhere. Of the 18 French power stations (57 reactors in total), five are located on the coast and use seawater. The other 13 are situated along major rivers (the Rhône, Loire, Garonne, Seine and Meuse). However, if the river temperature is already elevated due to specific environmental conditions, the authorities may require a reduction in power output in order to proportionately limit the rise in the temperature of the discharged water, up to the extreme measure of ordering a complete shutdown, in exceptional cases, to avoid disrupting the river’s ecosystem.

On 23 June, power reductions were therefore ordered for seven reactors along the Rhône and the Seine, and two reactors were shut down on the Garonne, as the temperature upstream of the power station was 28 degrees – a threshold considered to be the absolute limit. The result was a 5 per cent reduction in nuclear output, according to RTE, the French grid operator. As this phenomenon has so far only occurred on a few days each year, the operators do not consider it cost-effective to modify the cooling system to increase the water intake or to switch to a hybrid system with evaporative cooling towers, as is already the case in other European countries.

Will this issue also affect future Italian nuclear power stations? Very little. Because most of them will be located on the coast and along the River Po, and we will be able to design the cooling system correctly – just as we did 50 years ago (at Caorso, the water was returned ‘heated’ by three degrees) – so that it is suited to the new climatic conditions. This, too, is adaptation.

Finally, it should be noted that adverse weather conditions have a much greater impact on renewable energy sources. In the first five months of this year, national hydroelectric generation was 30 per cent lower than in the first five months of 2024. At the time of writing (25 June, 13:00), Italian electricity demand stands at a record high of 57.5 GW (largely due to air conditioners operating at full capacity), while light wind conditions mean that wind farms are generating only 1.1 GW (8 per cent of installed capacity), and the 46 GW of photovoltaic capacity is producing slightly more than half of its rated output. Fortunately, the often-criticized French nuclear reactors are supplying us (directly and via Switzerland) with a full 6 GW. Without this input, temperatures in offices and homes would be significantly higher.

Giuseppe Zollino, Head of Energy at Azione