The high temperature warning has come early this year but will affect fewer nuclear power plants.
High temperatures could halve nuclear power production at plants along France's Rhone River this week.
Output restrictions are expected at two nuclear plants in eastern France due to high temperature forecasts, nuclear operator EDF said. It comes several days ahead of a similar warning that was made last year but will affect fewer plants.
The hot weather is likely to halve the available power supply from the 3.6 GW Bugey plant from 13 July and the 2.6 GW Saint Alban plant from 16 July, the operator said.
However, production will be at least 1.8 GW at Bugey and 1.3 GW at Saint Alban to meet grid requirements, and may change according to grid needs, the operator said.
Will high temperatures increase energy costs?
Kpler analyst Emeric de Vigan said the restrictions were likely to have little effect on output in practice. Cuts are likely only at the weekend or midday when solar output was at its peak so the impact on power prices would be slim.
He said the situation would need monitoring in the coming weeks, however, noting it was unusually early in the summer for such restrictions to be imposed.
Water temperatures at the Bugey plant already eclipsed the initial threshold for restrictions on 9 July. They are currently forecast to peak next week and then drop again, Refinitiv data showed.
"France is currently net exporting large amounts of power – single nuclear units' supply restrictions will not have the same effect as last year," Refinitiv analyst Nathalie Gerl said.
The Garonne River in southern France has the highest potential for critical levels of warming, but its Golfech plant is currently offline for maintenance until mid-August, the data showed.
"(The restrictions were) to be expected and it will probably occur more often," Greenpeace campaigner Roger Spautz said.
"The authorities must stick to existing regulations for water discharges. Otherwise, the ecosystems will be even more affected," he added.
What is EDF doing to adapt to the heat?
EDF is committed to adapting its facilities to climate change, a company spokesperson said. They added that since 2000, losses due to high river temperatures have represented an average drop of just 0.3 per cent of annual power production.
The nuclear operator said previously a study it conducted showed last year's higher temperatures had no impact on biodiversity.
However, a study conducted by French nuclear watchdog ASN saw a slight increase in algae and plankton growth around the Bugey plant during the heat wave. Fish populations were also affected through the autumn at the Saint Alban plant.
The watchdog stipulated it is currently impossible to distinguish the impact of the raised limits compared to the other ecological effects of the heat wave, but it is continuing to monitor the river biome.