Record decline in nuclear generation

In 2020, installed nuclear generation capacity declined for the first time since 2009, dropping from 63.1 GW to 61.4 GW. This followed the permanent closure of the last two 900 MW reactors at the Fessenheim power plant, which was disconnected from the grid on 29 June. Commissioned in 1977, it was the oldest nuclear power plant in France. The closure reduced the share of nuclear power in the French mix to 45.1 %.

Nuclear generation contracted by 11.6 % year-on-year, to 44 TWh, the lowest level on record since 1993. This represented 67.1 % of total electricity generated in France for the year. The decrease in generation was attributable to the closure of Fessenheim and, more importantly, to a lesser availability of nuclear power plants and to the health crisis. The year-on-year decline in nuclear generation directly linked to the Covid crisis is estimated at 34 TWh (see section on nuclear power plant unavailability below).


Nuclear power plant unavailability up sharply


Data from the ENTSO-E Transparency Platform
Most of the plant unavailability times were reported as scheduled outages. Unavailability was naturally higher during the warmer months of the year, this to guarantee greater availability in winter, when demand rises.

Longer maintenance periods

Average nuclear power plant unavailability rose sharply in 2020, to 22.3 GW from 17.8 GW in 2019, contributing to a steep drop in nuclear generation.

Unavailability reached its highest point for the year on 7 July 2020, with an average of 37.9 GW unavailable that day, or more than 60 % of installed capacity. This was the result of maintenance cycles overlapping at numerous French nuclear power plants. Indeed, maintenance work was extended on much of the French nuclear fleet in 2020 as a direct consequence of the health crisis, which slowed maintenance efforts. Some reactors were also stopped in the second and third quarters of 2020 to save fuel and thus guarantee maximum availability in winter, when demand can surge

The impact of the crisis was significant

EDF’s January 2020 forecast for unavailability across its entire nuclear fleet over the year was 11 GW lower than the actual figure. Its forecast did not take into account unscheduled unavailability, variations in maximum available power, extended shutdowns or other planned stoppages not foreseen in January 2020. One year prior, in January 2019, EDF’s full-year unavailability forecast was 7.2 GW below the actual figure.
Comparing 2020 to 2019, when plant operating conditions were closer to normal, the difference represents an average of 3.9 GW (11 – 7.2 = 3.9 GW).
It is possible to deduce that operation in degraded mode, under the constraints created by the health crisis in 2020, increased average unavailability by 3.9 GW relative to 2019, equivalent to about 34 TWh.

Closer look

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Closer look

Different reasons why nuclear power plants may be unavailable

Nuclear power plants account for a large share of electricity generation in France (about 67 % of the total in 2020). Specific factors must therefore be taken into account in operating the power system, including a real dependence on the performances of the nuclear fleet.

Nuclear power plants generate “base-load” electricity, meaning they are designed to operate for most of the year. Since variable costs are low, these plants produce power at their maximum capacity almost all the time, except when they are unavailable or when electricity prices fall to very low levels.

Since 2015, the EU transparency regulation has required that operators of any type of generating unit report the unavailability of 100 MW of capacity or more, once it is known or scheduled, up to three years ahead of time.
Maintenance on nuclear power plants is scheduled 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 part of the core, or for several months to replace all the fuel and conduct heavier maintenance. And once a decade, reactors undergo detailed and in-depth ten-year inspections, notably to inspect key components (reactor vessel, primary circuit, steam generators, containment building, etc.). They also specify any maintenance work required to improve safety at facilities and extend their service life (subject to approval by French nuclear safety authority ASN).

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

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


Unplanned unavailability usually occurs following a technical failure or when a regulatory constraint forces the operator to reduce power or even shut down a reactor.

The supply-demand balance may thus be affected (mainly in winter) by nuclear power plant maintenance schedules set by operators, but more often it will be impacted by unplanned outages resulting from the extension of planned outages, specific weather or environmental conditions, social movements, or decisions taken by French nuclear safety authority ASN.

Environmental constraints

The first round of feedback on heatwaves allowed RTE to show that output declined at all types of generation plants (nuclear, hydropower, wind, solar, gas) during these periods. This is the case for nuclear power plants, as operators may be required, for environmental reasons, to stop or reduce production at certain sites to keep water temperatures in rivers within the range allowed. Nuclear power plants located near rivers draw water from them to cool the steam in the secondary loop, which spins the turbine blades, and then release it back into the river (unless the plants have cooling towers). 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 operate with a set of limitations that reflect their specific geographic, technical and environmental characteristics.

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