By Forrest Crellin

PARIS, Aug 13 (Reuters) - Europe's fifth heatwave this summer is expected to trim French nuclear power production by about 15% of the nuclear fleet's total capacity on Friday, with ‌six reactors expected to be fully offline, data from operator EDF showed.

Temperatures in Europe have risen to ‌unprecedented levels after successive heatwaves this summer, causing water shortages, wildfires, and some deaths.

Total nuclear curtailments are expected to reach 9.4 gigawatts ​at the peak on Friday across nine reactors, EDF data showed.

The heat-related outages combined with the regular maintenance schedule, technical issues and fuel saving are expected to put some 27.7 GW of nuclear power offline on Friday, the lowest level of nuclear availability since August 2023, data from LSEG Power Research showed.

Low nuclear production paired with limited wind output ‌lifted power prices, with the French day-ahead ⁠contract for Friday the highest since June 23, the previous peak this summer, up 4.4% at €154.50 ($178.08) per megawatt hour, LSEG data showed.

Power prices are traditionally much lower in August ⁠as it is a major holiday month in France, limiting demand.

The nuclear shortfall also ripples across borders, forcing countries such as Britain and Germany to turn on coal and gas plants to replace reduced exported nuclear power.

Even with the low ​nuclear ​availability, France continues to be a net exporter of electricity ​but is sending less power than average, LSEG ‌analyst Nathalie Gerl said.

Average scheduled net exports for August have fallen to around 9.5 GW, approximately 20% lower than in July, which means European neighbours are still receiving a quite substantial power flow from France and the reliance on French export stability is high, she said.

France relies on nuclear power for about 70% of its annual electricity production. French rules limit how much heat plants can discharge into rivers for environmental protection, forcing reactors to ‌cut output when water temperatures rise during heatwaves.

Thursday is expected ​to be the peak of the current heatwave, with temperatures between ​35 degrees Celsius (95 degrees Fahrenheit) and 40 C ​in most regions of the country, Meteo-France said.

Temperatures are expected to stay warm overnight, with ‌some cooler air coming from the west and ​bringing relief on Friday to ​areas along the Atlantic and English Channel first, while the rest of the country remains warm, Meteo-France said.

Despite the continued outages, nuclear power production in France increased in June and July compared to the ​previous year as EDF has reworked ‌its nuclear fleet maintenance schedule.

Two of the jellyfish-hit nuclear reactors at the Gravelines plant in the ​north of the country are expected to remain offline for several more days.

(1 = 0.8676 euros)

(Reporting by ​Forrest Crellin; Editing by Sharon Singleton and Keith Weir)