Is a decline in the flow of the Rhone a threat to nuclear power plants? (1/3)

He is the “River-King”. However, even he must bow to the melting of the Swiss giant that bears his name: the Rhône Glacier, north of the canton of Valais. According to a Water Agency report published in March 2023, the drama going on there is responsible for a rapidly significant decline in river flows. In 2050, its low flow rate – the lowest average annual level of a river by which floods are measured – will be reduced by an average of 20%. However, the Rhône today feeds one million people, provides water to more than two million and alone produces “A quarter of the country’s electricity production”, There are at least 19 hydroelectric power stations and four nuclear power stations on its coasts. They’re the ones we talk about the most… but is that really fair?

Water is essential for the operation of power plants.

In France, the reactors of our nuclear power plants are pressurized water reactors which provide 70.6% of the country’s electricity production. To function, water taken from an external source (river, sea) feeds three independent circuits that sequentially produce large amounts of thermal energy in the reactor, convert it into mechanical energy and cool the system. . Reactor Backup. The first two circuits are “closed” circuits: water “goes into the circle” with no way out. Therefore they consume less water.

Open or closed cooling circuit: you have to choose

The cooling circuit is more demanding. This is what we are interested in. These are of two types: open circuit and closed circuit. In open circuit power plants, chilled water taken from an external source cools the backup system, then is completely released into the water source. This is, for example, the case of the Bugy, Tricastin and Saint-Alban power plants – a power plant where an average of 57m³/second is taken from the Rhône.

“The counterpart is that if electricity production is significant then the induced warming in the environment can be quite significant, Cecil Laugier, EDF’s director of environment and foresight, expressed anger at a press conference given by the company last May on global warming. In practice, for our nuclear power plants, this causes an average of 4° to 5° warming in the environment. ,

Closed circuit plants take very little water (about 2 m³/second). After passing through the cooling circuit, the water is taken to the cooling tower where the cold air call system lowers its temperature. Part of the water falls as droplets, while the rest escapes through the chimney: this is the famous plume of water vapor, the epigraphic image of power plants. This process has the advantage of rejecting cold water into the source, but it does not allow everything to return: out of 2 m³ of cold water, about 0.7 m³ evaporates, so we recover about 1 m³ of water. . This cooling circuit prevents the external water from overheating.

In short: without water, nuclear power plants cannot operate; And without a guarantee that the water released will not raise the temperature of the source beyond a certain limit, no operation is possible, as the law requires. It therefore appears that the decline in low flow rates of the Rhône does not bode well for the cooling circuits of our power plants, which are supplied with water from the river…