According to grid operator RTE, 45 nuclear reactors were in operation in France on Friday, generating up to 45,360 megawatts, the highest level since Feb. 11 last year but about 12 percent below the average of the past five years.

However, the surge in nuclear power output in France is likely to be short-lived, as maintenance this month could push output to historic lows for the time of year. Nuclear power in France is crucial for Europe's energy security at a time when Russian gas supplies have largely stopped.

Grid operator RTE previously said eight reactors would be under maintenance this month. And with temperatures dropping and Germany's wind power generation set to fall sharply in the coming days, coal and gas use is likely to increase.

Nuclear power generation in France fell to a 30-year low last year as several reactors went offline. Carbon emissions in the European Union rose 3 per cent last year, according to Ember, despite record new photovoltaic capacity being installed.

Improved nuclear power generation in France, along with increased hydropower and renewables, is expected to help Europe cut its fossil-fuel generation by 43 percent this year, according to the report.