High temperatures on the River Rhone are "likely" to drive production cuts from nuclear plants along the river, especially Bugey, according to French utility EdF.
The potential cuts will be reviewed on 24 June, and a specific publication will detail the cuts if they are confirmed, EdF said.
France is about to see a combination of a heatwave and dry spell, with daily highs in Grenoble forecast to remain mostly above 30°C until the beginning of July.
The combination of heat and a lack of rain in the Alps has increased the chance of hotter water and lower flows on the Rhone, which could have a knock-on effect on France's nuclear fleet. Eight of the country's 57 reactors discharge their cooling water directly into the river, and environmental regulations limit the temperature at which they can do this, meaning if the river's flow is low and temperatures high, the units can be forced to reduce output.
Flows out of Lake Geneva — where the Rhone leaves Switzerland and enters France — have remained at 239-328 m³/s since 2 June. Earlier in the month this was within the 25th to 50th percentile of historical values, but is now slightly below the 25th percentile, as flows typically increase in June to peak at the end of the month and into July.
The low outflows are reflective of low inflows, as the lake's level is regulated by the Seujet dam at the lake's exit to the Rhone, which keeps its level about 1m all year. The lake's level typically falls in the first quarter and rises in the second to a peak in June and July, and the level has been creeping up over June in line with the historical trend.
The outflow's average temperature of 20.9°C on Wednesday was between the 75th and 95th percentile for the date.
A convention between France and Switzerland guarantees that the flow of the Rhone upstream of Bugey, the first nuclear plant on the river, is to remain above 150 m³/s, with the potential for release of water from the Emosson reservoir in Switzerland to the Arve River to achieve this. In recent days, flows at Bugey — combining outflows from Lake Geneva and tributaries of the Rhone, which have brought more than 100 m³/s — have been well above this minimum.
And thermal limits for Bugey are set at a maximum temperature downstream of the plant of 26°C and a maximum temperature difference between upstream and downstream of 5°C. If grid operator RTE determines that the plant is necessary to maintain supply, this maximum limit can increase to 27°C, but with only a 1°C temperature difference allowed.
Only two of Bugey's four units discharge their cooling water directly into the Rhone, with the other two using cooling towers, which sharply reduce their thermal impact on the river, and so the extent to which the regulations affect them.
If water flows were at their minimum of 150 m³/s, and at a temperature of 21°C upstream of the plant, this stream would have the capacity to carry 3.14GW of waste heat away from the plant before hitting the regulatory limit of 5°C of temperature difference and 26°C maximum temperature downstream. This is less than the roughly 3.8GW of waste heat generated by Bugey 2 and 3 operating at full power, including their thermal power of 5.6GW less their electrical power of 1.8GW. Bugey 4 and 5 contribute a small amount of extra waste heat downstream, as their cooling towers do not completely eliminate waste heat.
Both units have had to stop at moments in the past two years because of heat-related constraints. In the last period when Bugey was constrained, in mid-August, flows leaving Lake Geneva were significantly hotter than now, roughly 24-26°C. At that point, output from Bugey 2 was reduced to zero, before a sharp fall in water temperature to roughly 20°C by 18 August allowed constraints to be lifted. But air temperatures in the region are forecast to remain elevated for the next two weeks, which will boost water temperatures over time. Daily highs in Sion, Switzerland, are seen at 3-4°C above the norm through to the beginning of July.
Temperatures and flows at reactors further downstream which do not possess cooling towers — 2.6GW Saint Alban and 3.6GW Tricastin — are affected by other flows on tributaries to the Rhone, which cool the river down and increase its volume after passing Bugey.
