France's Nuclear Fleet Drops to Three-Year Low as Summer Heat Outages Mount
French nuclear capacity averaged 36.1 GW in the week of August 10, a three-year low, as curtailments at Gravelines 1 and St Alban 1 extended into Saturday (2026-08-15).
Preliminary Montel EQ data published on Friday (2026-08-14) showed only 36.1 GW of French nuclear capacity — 57% of EDF's total fleet — available on average during the week of August 10 (2026-08-10), the lowest weekly reading since August 2023.7
Nuclear power supplies roughly 70% of France's electricity mix, leaving the grid with limited headroom when the fleet contracts sharply. Markets reflected the shortfall quickly: French day-ahead power surged as much as 21.8% to 142.5 euros per MWh on Tuesday morning (2026-08-11), per LSEG data cited by Reuters, as nuclear output was expected to be curtailed through the midday peak.6
The week deteriorated in stages. EDF data on Tuesday (2026-08-11) showed heat-related cuts already at 13.2% of the reactor fleet, with that figure projected to climb to 14.4% by Wednesday (2026-08-12), Montel reported. Total availability hit a year-to-date low of 57% by midweek. EDF separately flagged a 7.3 GW output reduction for Wednesday (2026-08-12), equal to 12% of installed capacity.5,6
Friday's (2026-08-14) Montel EQ update showed weekly availability slipping a further 0.3 GW below Thursday's (2026-08-13) expectation. A fresh unplanned outage at Gravelines 1 contributed to the revision. Saturday (2026-08-15) brought further curtailments: a 735 MW reduction at the 1.3 GW St Alban 1 unit, running from midnight to 22:30 CET.7
The constraint is physical. France's inland reactors rely on river water for cooling, and thermal discharge regulations force output cuts when river temperatures climb or flows drop. The heatwave driving the week of August 10 (2026-08-10) curtailments was, by OilPrice.com's count, at least the fifth extremely hot wave to grip France this summer.6
The pattern has repeated across the season. Around July 13 (2026-07-13), EDF halted three reactors and cut output at others as river temperatures rose; Montel reported record nuclear cuts and a Swiss reactor shutdown during that episode. Nuclear generation fell 6.4 GW during that stretch, according to OilPrice.com.6,43
Back in May (2026-05-21), meteorological forecasters had told Montel that frequent cooling rainfall would limit nuclear disruption through the summer. The August three-year low suggests that outlook missed the severity of what followed.1
French nuclear shortfalls lift demand for gas-fired generation across the interconnected European grid. ICE Endex TTF front-month gas closed at €61.38/MWh on August 15 (2026-08-15), with sustained output losses of the scale seen during the week of August 10 (2026-08-10) adding pressure through the gas-to-power channel as gas-fired plant fills the gap left by curtailed reactors.6
EDF's June 2026 (2026-06-03) half-year report outlined a climate adaptation programme for the nuclear fleet alongside targets for rising output. This summer, marked by at least five heatwaves and weekly availability at its worst since August 2023, has stress-tested those plans harder than any published forecast suggested.2
The immediate signal is how quickly river conditions allow EDF to restore curtailed capacity at St Alban 1 and other affected units. Any extension of the Gravelines 1 outage, or fresh declarations elsewhere, would push weekly availability below the three-year low recorded for the week of August 10 (2026-08-10).7