Nuclear generation down slightly

Installed nuclear capacity ended the year unchanged at 63.1 GW, which represents about 47% of total French capacity (135.3 GW). Output from nuclear plants contracted by 3.5% (13.7 TWh) and accounted for 70.6% of total power production in France, the lowest level since 1989. This decrease in output was attributable chiefly to lower nuclear plant availability.

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Nuclear plant unavailability increased

Average nuclear power plant unavailability rose slightly in 2019, to 17.7 GW compared with 16.3 GW in 2018.

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Nuclear plant unavailability in 2019

Nuclear power accounts for a large share of electricity generation in France (more than 70% of the total in 2019). Specific factors must therefore be taken into account in operating the power system, including a real dependence on the performances of the nuclear fleet. Not counting unavailability for technical or regulatory reasons, France’s nuclear power plants generate “base-load” electricity, meaning they run year-round and produce a fairly stable quantity of electricity.

Since 2015, the EU transparency regulation has required that operators of any type of generating unit report the unavailability of 100 MW or more, once it is known or scheduled, up to three years ahead of time.

French nuclear power plants are designed to remain in service for at least 40 years. Maintenance is scheduled over this period with stoppages of reactors at regular intervals. Two types of unavailability must be reported to the EU transparency platform.


Approximately every 12 or 18 months, depending on the technical series, each reactor is shut down for around a month to replace the fuel in the core, or for several months to replace all the fuel and conduct heavier maintenance. And every ten years, reactors undergo detailed and in-depth ten-year inspections, notably to inspect key components (reactor vessel, primary circuit, steam generators, containment building, etc.).

Operators do their best to schedule these shutdowns outside the winter months, but given the size of the fleet and the heavy constraints that exist (notably regulatory and industrial), outages must be spread out over time with some occurring in winter.

Unavailability forecasts may be adjusted, usually extended, due to complications or events that arise during the outage. Another type of planned unavailability occurs when maximum power available is lowered temporarily (typically after a long outage when the ramp-up takes several days).


Unplanned unavailability usually occurs following a technical failure or due to regulatory constraints that force the reactor to reduce power or even shut down.

In such instances, the supply-demand balance (mainly in winter) may be affected by the nuclear plant maintenance schedule set by the operator, but more often it will be by unplanned outages resulting from the extension of planned outages, specific weather or environmental conditions, social movements, or decisions taken by the Nuclear Safety Authority (Autorité de Sureté nucléaire –  ASN).

Nuclear power plant unavailability increased in 2019

In 2019, the average availability of the nuclear fleet decreased slightly to 45.4 GW from 46.8 GW in 2018.

The grey on the graph below shows unavailability between 2015 and 2018. The yellow corresponds to actual unavailability in 2019 that was planned and the red to unplanned unavailability, also in 2019.

Most of the unavailability reported corresponded to planned outages. It is naturally higher during the months when weather is mildest, this to ensure higher availability in winter, when demand for electricity increases.

Highlights of 2019 included:

18 to 21 January:

A number of technical difficulties affected the nuclear fleet in the second half of January, driving availability down well below historical averages. This put the system on high alert, notably regarding supply to the Grand Ouest region in France.

24 and 25 July:

On 24 and 25 July, during the second heatwave period in France, six nuclear power plants lowered their power to comply with environmental restrictions (Saint-Alban, Bugey, Dampierre, Golfech, Tricastin and Blayais).
The first round of feedback on periods of heatwaves allowed RTE to show that output declines at all types of generation plants (nuclear, hydropower, wind, solar, gas) during heatwaves. This is the case for nuclear power plants, as the operator may be required, for environmental reasons, to stop or reduce production at certain sites to prevent excessively high water temperatures in rivers. When a nuclear plant is next to a river, it draws water to cool the steam in the secondary loop that drives the turbines, and then releases it back into the river (unless the plant has a cooling tower). The water discharged must be below a certain temperature to avoid altering the ecological balance of the river. Water intake may also be constrained by minimum flows. In sum, nuclear power plants have a set of limitations that reflect their specific geographic, technical and environmental characteristics.
It should be noted that in 2018, four reactors on the Rhône river had to be shut down for short periods.


11 November:

A 5.4-magnitude earthquake was recorded in the Ardèche region, near the city of Teil.
In the wake of that event, the Cruas nuclear power plant stopped production at reactors 2, 3 and 4 for inspections. This outage reduced available capacity by 2,700 MW.
Operators will typically stop reactors when an earthquake of this magnitude occurs to determine whether the event had any impact on the equipment.


Month of December:

Unavailability increased in December 2019, and was on average 6 GW higher than the average for the past four years.

Though three generating units at Cruas were brought back online in the second week of December, several sites were affected by strikes and technical difficulties.

At many facilities, resumption of production was pushed back to 2020, later than the initially planned date.

As was reported in additional analyses conducted and published following RTE’s 2018 Generation Adequacy Report and forecasts for the winter of 2019, the risk of voltage collapse in the North-Western quarter of France does exist. In the absence of sufficient local generation capacity, the routing of electricity over long distances causes voltage to decrease, at times steeply if consumption in the region is high.
RTE remains particularly vigilant with regard to the availability of all generation capacity during the 2019 winter to limit the risk of voltage collapse, especially with the delayed restart of the Flamanville reactors.

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