French nuclear outages hit 27.7 GW as heatwave lifts day-ahead 4.4%

French nuclear output faces its steepest heatwave-driven reduction of the summer this Friday, with six reactors projected fully offline and total heat-related cuts reaching 9.4 GW across nine reactors, according to EDF data. Combined with routine maintenance, technical faults, and fuel conservation measures, around 27.7 GW of nuclear capacity will be unavailable, the lowest level since August 2023, LSEG Power Research reports. France normally draws around 70% of its annual electricity from nuclear, so a cut of this scale is not marginal.

The mechanism is regulatory as much as meteorological. French rules cap the heat that plants can discharge into rivers, forcing reactors to throttle output as river water temperatures climb during the fifth heatwave to hit Europe this summer. Two reactors at Gravelines, in northern France, remain offline for several more days after a jellyfish intrusion disrupted cooling intake, separate from the heat-related reductions. Reuters noted that low nuclear output, compounded by weak wind generation, pushed the French day-ahead contract for Friday to its highest level since 23 June.

What this means for UK buyers

The UK is not immune. France remains a net exporter of power, but LSEG analyst Nathalie Gerl points out that average scheduled net exports for August have dropped to about 9.5 GW, roughly 20% below July levels. That has already prompted Britain and Germany to activate coal and gas plants to offset reduced French nuclear exports, tightening the interconnector flows that UK wholesale prices lean on during summer peaks.

For commercial buyers, the practical exposure runs through a handful of channels:

• Day-ahead and prompt power contracts, most sensitive to French export shortfalls
• Flexible or index-linked contracts renewing in August, where volatility could inflate settlement prices
• Sites with high cooling load, facing both higher unit costs and higher demand simultaneously
• Interconnector-dependent supply, as GB balancing leans more on coal and gas back-up
• Fixed-term renewals quoted this week, where forward curves may reflect the current squeeze

August is typically a low-demand month in France, curtailing prices as holiday shutdowns reduce industrial consumption. This year's heatwave has overridden that seasonal pattern, with The Telegraph reporting sustained temperatures of 35°C to 40°C across most regions through Thursday, easing first along the Atlantic coast and Channel on Friday. EDF has also adjusted its maintenance schedule, and nuclear production in June and July actually ran ahead of last year, showing the fleet's underlying resilience even as this week's spike stands out.

Watch how quickly river temperatures ease behind the incoming cooler air from the west, and whether GB balancing costs rise further if French export volumes stay compressed into next week's renewal window.