Jun 20, 2025
Cuts to French Bugey nuclear power are expected to begin on June 25.
High temperatures on the River Rhone are expected to lead to production reductions from nuclear plants along the river, particularly Bugey, as per French utility EdF. A review of potential cuts will take place on June 24, with a detailed announcement to follow if they are confirmed.
France is facing a heatwave and dry conditions, with daily highs in Grenoble predicted to exceed 30°C until early July. The combination of heat and limited rainfall in the Alps raises the likelihood of warmer water and lower flow rates on the Rhone, potentially impacting the country's nuclear fleet. Eight of France's 57 reactors release cooling water directly into the river, and due to environmental regulations, they cannot exceed specific temperature limits. Therefore, if river flow is low and temperatures are high, these units may have to decrease output.
Flow rates from Lake Geneva—where the Rhone exits Switzerland and enters France—have remained between 239-328 m³/s since June 2. Early in June, these rates were within the 25th to 50th percentile of historical averages, but they have since dropped below the 25th percentile, as flows typically rise in June and peak in late June and July.
These low outflows indicate low inflows, with the lake's level managed by the Seujet dam, which keeps it about 1m year-round. The lake typically decreases in level during the first quarter and rises in the second, peaking in June and July. The average outflow temperature was recorded at 20.9°C on Wednesday, placing it between the 75th and 95th percentile for the date.
A treaty between France and Switzerland ensures that the Rhone's flow upstream of Bugey remains above 150 m³/s, with possible water releases from the Emosson reservoir in Switzerland to meet this requirement. Recently, flows at Bugey, including contributions from Lake Geneva and Rhone tributaries, have been well above this threshold.
The thermal limits for Bugey allow a maximum downstream temperature of 26°C and a 5°C difference between upstream and downstream. If the grid operator RTE identifies the plant as critical to supply, this limit can rise to 27°C, allowing only a 1°C temperature difference.
Only two of Bugey's four units discharge cooling water directly into the Rhone, while the other two employ cooling towers, significantly reducing their thermal impact and the extent to which regulations affect them.
If flow rates were at the minimum of 150 m³/s and the upstream temperature was 21°C, the stream could carry away 3.14GW of waste heat before reaching the regulatory limits. This is less than the approximately 3.8GW of waste heat produced by Bugey 2 and 3 operating at full capacity, which is 5.6GW thermal power minus 1.8GW electrical power. Bugey 4 and 5 contribute a minor amount of additional waste heat downstream due to their cooling towers not completely eliminating waste heat.
Both units have had to cease operations at times in the past two years due to heat-related restrictions. The last significant constraint occurred in mid-August when flows from Lake Geneva were considerably warmer, around 24-26°C. At that time, output from Bugey 2 was entirely halted until a drop in water temperature to roughly 20°C by August 18 lifted the constraints. However, forecasted high air temperatures in the region are likely to keep water temperatures elevated in the coming weeks, with daily highs in Sion, Switzerland, expected to be 3-4°C above normal until early July.
Temperatures and flows at downstream reactors without cooling towers—2.6GW Saint Alban and 3.6GW Tricastin—are influenced by other tributary flows that cool the river and enhance its volume after passing Bugey.
France is facing a heatwave and dry conditions, with daily highs in Grenoble predicted to exceed 30°C until early July. The combination of heat and limited rainfall in the Alps raises the likelihood of warmer water and lower flow rates on the Rhone, potentially impacting the country's nuclear fleet. Eight of France's 57 reactors release cooling water directly into the river, and due to environmental regulations, they cannot exceed specific temperature limits. Therefore, if river flow is low and temperatures are high, these units may have to decrease output.
Flow rates from Lake Geneva—where the Rhone exits Switzerland and enters France—have remained between 239-328 m³/s since June 2. Early in June, these rates were within the 25th to 50th percentile of historical averages, but they have since dropped below the 25th percentile, as flows typically rise in June and peak in late June and July.
These low outflows indicate low inflows, with the lake's level managed by the Seujet dam, which keeps it about 1m year-round. The lake typically decreases in level during the first quarter and rises in the second, peaking in June and July. The average outflow temperature was recorded at 20.9°C on Wednesday, placing it between the 75th and 95th percentile for the date.
A treaty between France and Switzerland ensures that the Rhone's flow upstream of Bugey remains above 150 m³/s, with possible water releases from the Emosson reservoir in Switzerland to meet this requirement. Recently, flows at Bugey, including contributions from Lake Geneva and Rhone tributaries, have been well above this threshold.
The thermal limits for Bugey allow a maximum downstream temperature of 26°C and a 5°C difference between upstream and downstream. If the grid operator RTE identifies the plant as critical to supply, this limit can rise to 27°C, allowing only a 1°C temperature difference.
Only two of Bugey's four units discharge cooling water directly into the Rhone, while the other two employ cooling towers, significantly reducing their thermal impact and the extent to which regulations affect them.
If flow rates were at the minimum of 150 m³/s and the upstream temperature was 21°C, the stream could carry away 3.14GW of waste heat before reaching the regulatory limits. This is less than the approximately 3.8GW of waste heat produced by Bugey 2 and 3 operating at full capacity, which is 5.6GW thermal power minus 1.8GW electrical power. Bugey 4 and 5 contribute a minor amount of additional waste heat downstream due to their cooling towers not completely eliminating waste heat.
Both units have had to cease operations at times in the past two years due to heat-related restrictions. The last significant constraint occurred in mid-August when flows from Lake Geneva were considerably warmer, around 24-26°C. At that time, output from Bugey 2 was entirely halted until a drop in water temperature to roughly 20°C by August 18 lifted the constraints. However, forecasted high air temperatures in the region are likely to keep water temperatures elevated in the coming weeks, with daily highs in Sion, Switzerland, expected to be 3-4°C above normal until early July.
Temperatures and flows at downstream reactors without cooling towers—2.6GW Saint Alban and 3.6GW Tricastin—are influenced by other tributary flows that cool the river and enhance its volume after passing Bugey.