High temperatures could lead to production restrictions at EDF's Golfech nuclear plant on the Garonne river in southern France starting on Wednesday, including the stoppage of the reactor 2, French grid operator RTE said.
Golfech's two reactors each produce 1.3 gigawatts (GW) of electricity.
A similar notice had been issued last week for possible output cuts at Golfech from Tuesday, before RTE later withdrew the warning.
Temperatures in the region are expected to reach 30 degrees Celsuis on Wednesday, continuing the week-long heat wave that has also led to increased river temperatures.
EDF's use of water is regulated by law to protect plant and animal life. It is obliged to reduce output during hot weather when water temperatures rise, or when river levels and the flow rate are low.
The current heatwave is not forecast to stretch beyond the weekend, with temperatures beginning to recede on Thursday, data from Meteo France showed.
