How the heat wave affects French nuclear power plants

This is one of the consequences of the scorching temperatures announced for the weekend. EDF could reduce the production of two of its nuclear power plants: that of Bugey (Ain) from August 19 and that of Tricastin (Drôme), from August 20. A production alert directly linked to the technical particularities and the location of these two production units. The environmental regulatory framework, strict in France, applies. The operation of each plant is governed by decisions of the nuclear safety authority (ASN) on the basis of an impact study. The regulations set limits in normal climatic conditions, and when they are reached, production is reduced.

Since 2000, operating losses of production for environmental reasons represent, on average, 0.3% of the annual production of the nuclear fleet. 2003 remains the record. Since this year, marked by a particularly long heat wave, a "very hot" plan has been deployed on EDF's nuclear fleet and integrated into the large fairing program - a very thorough overhaul of the power plants, allowing their lifespan to be extended. . Cooling units, more powerful fans, materials that better withstand high temperatures are added. Nevertheless, by 2050, due to global warming, this percentage is likely to increase while remaining within a range of around 1.5% of production which could be stopped for climatic reasons.

To understand the reasons for the reduction in production, or even the shutdown of nuclear power plants in the event of high temperatures, it is necessary to bear in mind their mode of operation and their location. The Bugey plant is located on the banks of the Rhône and that of Tricastin on the Canal de Donzère-Mondragon, a derivation of the Rhône. The water is used to cool the power plant circuits. This system is also common to the various production units, whether nuclear, gas or coal. In the case of nuclear power, there are three water circuits, compared to two for the others. Either way, a cold source is needed.

Two methods are used by EDF to cool the power stations. The first consists of passing water from the river - or the sea - through a circuit. All the water is then returned and immediately available for other uses. But the warming is substantial, around 4 to 5 degrees at the exit. In periods of high heat, this is where the shoe pinches. When the water in the river - or the canal - is already warmer than normal, adding a few degrees can be harmful for the fauna and flora. Very strict regulations set temperature thresholds beyond which EDF must reduce or suspend its electricity production for these riverside units.

Other plants are equipped with cooling towers. The water is then distributed in droplets in the exchange body of the power plant. In reaction with the ambient air, these droplets cool the production unit, like a fogger. The heating for the river is then only a few tenths of a degree, but part of the water used is evaporated. EDF is nevertheless working to avoid this loss. MIT (the prestigious University of Massachusetts) has developed a solution to recondense it and recover some of the water vapor: infinite cooling. EDF should test this innovation in Bugey, to determine its adaptability on its sites.

In summer, EDF's water needs are generally lower. The group positions the shutdowns of its power plants for maintenance during the hottest months, when electricity consumption is generally 50% lower than in the middle of winter. According to figures published by the Ministry of Energy Transition, 51% of water withdrawals in France are made for cooling power plants, but it is largely returned and immediately available for other uses. Consumption itself, which is evaporated, amounts to 12%, far behind agriculture, 58% and drinking water, 26%.

The procedure is well established. “The temperature and flow of the rivers on which nuclear power plants are located can lead, in order to comply with the regulations relating to thermal discharges, to reduce the production of certain power plants. Nuclear reactors are able to optimize production while respecting regulatory limits. There is no security risk”, explains EDF.