Currently, 25 of the 56 reactors of the French nuclear fleet are still shut down, a situation that has lasted since spring 2022. This situation is mainly caused by the recent detection of stress corrosion cracking in several reactors, but not only!
September 2021, discovery of stress corrosion cracking in one of the reactors at the Civaux plant. Since then, many reactors have been shut down to test whether corrosioncorrosion was present in the same places. Of the 56 reactors in the French nuclear fleet, 27 were shut down this summer, between ten-year inspections, stops for recharging and this famous stress corrosion cracking which caused 12 reactors to stop for checks. Today many reactors have not yet restarted, 25 are still shut down, but a dozen should restart by the beginning of December 2022. What are the causes of all these unavailability of reactors? OlivierOlivier Dubois, deputy director of safety expertise at the Institute for Radiation Protection and Nuclear Safety (IRSN), agreed to answer Futura’s questions on this matter.
“This is an unprecedented situation, which has several factors”
It is during sunny days that the shutdowns for maintenance and nuclear fueling traditionally begin: after the reactor is shut down and its gradual cooling down for a few daysthe fuelfuel The spent fuel is fed into a cooling pan at the power plant site while the new fuel is fed into the tank. In total, this process takes at least a month and up to six months if the work is planned simultaneously. Because of this, some reactors are not restarted yet, for further checks and repairs.
” Recharging stops are often made in early spring, because electricity needs are less than full winterwinter », explains Olivier Dubois. ” They then follow during the summer, when the need for electricity is minimal. »
“But currently it is an unprecedented situation and that has several factors. About half of the reactors are shut down., says Olivier Dubois. ” The main reason is the stress corrosion cracking discovered on some circuits, but there are also the planned ten-year inspections of the 900 MWe (electric megawatts) reactors as well as those of some 1,300 MWe reactors. »
The expansion of 900 MWe reactors takes time
The purpose of these ten-year inspections is to check each element of the reactor, in order to extend or not its ten-year life. ” EDF carries out a large battery of tests, which IRSN and ASN then verify. The checks range from the anchoring of the pipes to the behavior in the event of earthquakeearthquaketo the control of the tank, which is a non-replaceable component”, explains O. Dubois.
Since nuclear fission reactions occur in the vessel, it is designed to withstand significant irradiation and must be particularly robust. It is also subject to high conditions of pressurepressure and temperature, with a temperature of the water circulating around the fuel of about 300°C and a pressure 155 times that of theatmosphereatmosphere ! ” EDF uses an In Service Inspection Machine (MIS), which will check that there are no faults in the vessel or that there are no changes in known faults. Some tanks have been known to have defects since their manufacture: closed, invisible crackseyeeye bare, but present under the tank lining. They correspond to an area where the grains of the metal are not attached to each other.adds O. Dubois.
As 900 Mwe reactors reach 40 years of operation, EDF’s goal is to extend this period to 50, or even 60 years. “One of the major changes to the 900 Mwe reactors is the inclusion of a corium (molten core) stabilizer, in case ofaccidentaccident similar to that of The three mile island which took place in 1979. We want to ensure that, in the event of perforation of the vessel by the corium, it can spread correctly over an area large enough to cool and not perforate the foundation slab in concreteconcrete of the reactor”, says O. Dubois. But the changes associated with the 4And ten-year inspections of 900 MWe reactors require many months: some have already benefited from the changes, while EDF’s goal is that “They all passed the 10-year inspection by 2027, then it will be the turn of the latest 1,300 MWe reactors. »
The discovery of stress corrosion cracking changed the game
If the ten-year inspections are planned and calibrated to optimize the operation of the nuclear fleet, corrosion has changed the situation, especially after the effects ofepidemicepidemic of Covid. But what does it correspond to? ” It was detected during the decennial inspection of Civaux 1, thanks to the checks of ultrasoundultrasound which allow to check the presence of defects in the pipes at the level of the welds”explains O. Dubois.
Stress corrosion cracking is the aa damage MaterialMaterial under the effect of its chemical environment. “Where EDF found stress corrosion cracks, the pipes were inside stainless steelstainless steeltherefore the experts did not expect it a prioricontinues O. Dubois. The slits are located at the level of the safety injection circuit, which allows the reactor to continue to cool in the event of a primary circuit failure: more precisely, at its junction with the primary circuit. »
As Olivier Dubois explains, the corrosion was most likely activated precisely because of this proximity to the primary circuit. Indeed, the temperature is around 300°C, which can activate corrosion. ” The defects are located near the welds, directly at the piping level. They are not linked to the age of the reactor, as they are present in particular on the most recent reactors of the nuclear fleet, the N4, but probably linked to the mechanical load exerted on the welds. » The operator EDF has announced repairs to the reactors affected, but the development of which could take from a few months to several years at the level of the French nuclear fleet and depending on the list of reactors affected which will be drawn up once all the checks have been completed.