The hot weather is likely to halve the available power supply from the 3.6 gigawatt (GW) Bugey plant and the 2.6 GW Saint Alban plant from July 13 and July 16 respectively, 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.
Kpler analyst Emeric de Vigan said the restrictions were likely to have little effect on output in practice, with cuts likely only at the weekend or midday when solar output was at its peak, so that the impact on power prices would be slim.
He said the situation would need monitoring in 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 on July 9 where restrictions are possible, and 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 warming to critical levels, but the 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.
EDF is committed to adapting its facilities to climate change, a company spokesperson said, adding that since 2000, losses due to high river temperatures have represented an average drop of just 0.3% of annual 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 algal and plankton growth around the Bugey plant during the heat wave, while fish populations were 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.
