French Report ” Nuclear Power Plant Flexibility at EDF” via

January 16, 2021

In early 2019, four French EDF scientists wrote a 22 page report on load following in French nuclear reactors. The English version was first published on April 1 2020 but this has only recently been brought to my attention (ie mid Jan 2021).

This report is instructive and worrying, and requires careful reading. In essence, it discusses how French nuclear engineers have managed to retrofit and configure France’s reactors so that they can follow the diurnal loads increasingly required by France’s electricity needs.

It should be borne in mind that EDF’s 58 nuclear reactors are very old and past their sell-by dates. Most are between 30 and 40 years old with an average age of 33 years in 2018.

Some background is necessary to explain why this report was written. French reactors have been operating since the 1980s. Since their gross output has usually exceeded French domestic requirements, especially at night, much is exported to France’s neighbours ie UK, Belgium, Netherlands, Germany, Italy and Spain. Large amounts were until recently also sent to large pumped storage schemes in Switzerland at night. These transfers have been at a considerable financial loss to EDF and the French Treasury as the prices for such supplies are understood to be low. In addition, during the day, France imports significant amounts of electricity- mainly from the renewables in Germany.


These data and forecasts present serious problems for EDF’s nuclear reactors. In a nutshell, they mean that EDF needs to reduce their nuclear electricity outputs to make space for increasing RE supplies. The obvious method is to shut down its older reactors, but EDF has resisted this so far, apart from the Fessenheim station.

Instead, EDF’s engineers have opted to reduce reactor outputs by adapting them to follow electrical load. This is not stated in the report, but that is essentially what has been happening. For about 30 years, French engineers have been introducing novel techniques – in particular “grey” control rods and boric acid regimes – so that their reactors could be ramped up and down to follow daily diurnal loads.

Readers may be aware that nuclear utilities in all other nuclear countries do not do this as it is considered unsafe. Ramping up and down is restricted to approximately yearly episodes during refuelling and maintenance. Daily ramping up and down certainly does not occur in UK or US reactors.

The reason is as follows. The most dangerous time in the operating regimes of nuclear reactors is when they are powering up or powering down, because the balances of fast neutrons, slow neutrons, xenon levels and boric acid concentrations have to be strictly controlled during these episodes. Ramping up and down is usually done in UK and US reactors slowly over two or three days. To get these tricky balances wrong, as happened at Chernobyl in 1986, is to court disaster.


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