Aging nuclear power plants: how do we ensure their proper functioning?

Today, approximately three quarters of French electricity is produced thanks toenergy nuclear. As the Earth slowly warms firethe energetic transition advocated of course involves a decarbonization of energy. It is therefore necessary to eliminate the fossil fuels : coal, gas and oil. For this, France has been relying on atomic energy since the 1960s.

According to the French Nuclear Energy Society (SFEN), thenuclear energy emits very little greenhouse gas (GES), with a carbon footprint about 6 g of CO₂ equivalent per kilowatt hour. A balance sheet comparable to that of theHydro-electric power or wind power. But it is nonetheless a non-renewable energy: it uses materials raw materials whose resources are limited. Indeed, the combustible is composed ofuranium which, even if it is partially recycled, remains in limited quantity on Earth. According to Orano, “Known uranium resources represent 100 years of global consumption and up to 250 years if estimated resources are included. »

Aging reactors, almost 40 years old

In addition to the fuel limits, that of the power plant lifetime. Originally designed for a ” duration 40 year life » according to the Institute for Radiation Protection and Nuclear Safety (IRSN), they are improved during their lifetime, depending on the “Feedback and evolving safety requirements”. This maintenance can allow them to operate longer than their original design intended.

In France, the power stations are checked regularly, “particularly through scheduled or unannounced inspections by ASN, in which IRSN technical experts often take part, and during ten-yearly inspectionsExplain Olivier Dubois, Deputy Director of Safety Expertise at IRSN. Every ten years, after technical expertise by the IRSN, the ASN (note, Nuclear Safety Authority) decides on the state of the plant, and defines the requirements so that it can operate for another ten years”.

Currently, the 900 MWe reactors are in the line of fire: “For the 900 MWe reactors, the objective is to ensure that they can last up to 50 years, as EDF wishes. ASN made its requirements in 2021 so that their extension is possible, EDF’s objective is that they will have almost all completed their ten-year inspection by 2027”continues Olivier Dubois.

During these ten-yearly inspections, various detectors and sensors are placed on either side of the reactor and the circuits that surround it: for example, certain cracks are checked with eddy currents. “The current will interact with the metal, and if a crack is present then it will be less conductive: the received signal will be different. » The control techniques are varied (ultrasound, x-rays…) and adapted to the component to be scanned and the types of faults sought.

Constant adaptation through feedback

This is followed by the technical requirements recommended by IRSN and imposed by ASN, with the aim of maintaining, or even improving, the plant safety level whose life is extended. “Many major changes have already been made, in particular to strengthen the resistance to natural disasters following theaccident of Fukushima: ultimate water sources have been put in place, designed to withstand floods or some earthquakes. These are either wells, fetching water directly from the groundwater, or flexible tarpaulins, installed on the ground. Ultimate emergency diesel generators have been installed. Secure buildings called local crisis centers are being constructionin order to house the local response teams in the event of a nuclear accident”explains the Deputy Director of Safety Expertise at the IRSN, Mr. Dubois.

But, sometimes the changes are actually major replacements. The generators of vapor for example (often written as “GV”), extract the heat which comes from the heart, and have already been replaced: this type of work takes a few months, knowing that a GV is over 20 meters high! “They are not so much subject to irradiation but they have other weaknesses: in particular the tubes of the steam generators are quite thin, and it is through them that the heat passes. On the oldest reactors, almost all the steam generators have been replaced, this is an operation that EDF performs once in the life of a reactor. »

Nuclear accidents shape safety

Because if relatively little experience feedback was available at the time of the construction of power plants on this type of reactor, nuclear incidents and accidents, serious or not, have changed the situation. In particular the accident of Three Mile Island (TMI) in 1979, during which the fuel melted! It triggered an awareness of the risks of merger of the heart, and the importance of the human factor in addition to the technical aspects. But the most significant event remains of course Chernobyl which, unlike TMI, led to major radioactive releases into the environment! After Chernobyl, “the Ines scale (note: International Nuclear Event Scale) was set up to help people understand the gravity nuclear incidents or accidents”explains Mr. Dubois.

Events, the consequences of which turned out to be minor but significant in terms of safety, also occurred in France: “During the storm of 1999, the Blayais website was flooded. Three reactors were initially in operation and one shut down after refueling. Systems for normal operation were not impacted, but some systems for backup were unavailable. Fortunately, there were no serious consequences, and the incident was rated 2 on Ines’ scale. Following this, all the dikes were raised to take storms like this into account.”describes Olivier Dubois.

The tank, a non-replaceable element

But, among all the elements checked by the operator EDF, under the supervision of ASN and IRSN, the vessel is perhaps the most scrutinized in detail: it is this which houses the fuel, and therefore, all the heat creation that keeps the plant running. The fissions which take place there cause a constant irradiation of the interior walls of the tank.

This is why it is designed with special care, resulting in technical requirements that are much higher than for most other components. It must also withstand the temperature conditions imposed by the design of the reactor, i.e. some 300 degrees and 155 times the atmospheric pressure !

The tank must, moreover, be manufactured with almost perfect homogeneity in order to better resist in the event of a crack. It is also necessary to limit the irradiation on all the zones of the tank. It is a lack of homogeneity that has caused the discussion around the EPR tank of Flamanville: a anomaly was detected in 2014 following checks on the bottom and the head of the vessel. An excessive carbon concentration (0.32% instead of 0.22%) was observed locally, reducing the toughness of the vessel, ie its ability to resist the propagation of a crack. Finally, ASN concluded in 2017 on the possibility of using the vessel as it is, and on the need to replace the lid fairly quickly. the Flamanville reactor should be commissioned in 2023.