Counting the cost of cracking at EDF’s nuclear reactors in France
Nuclear Engineering, 11 Aug, 22,
The full extent of stress corrosion cracking at EDF’s reactors in France has still to be determined. Nonetheless, lower production as plants are re-examined has come at the worst possible time for the company.
On 15 December 2021 EDF announced that it would temporarily shut down two reactors at the Civaux site. The move came after inspections undertaken as part of as Civaux 1’s 10-yearly in-service inspection revealed defect indications close to welds in pipes that formed part of the of the safety injection system (SIS). This back-up circuit allows borated water to be injected into the reactor core in order to stop the nuclear reaction and to maintain the volume of water in the primary circuit in the event of a loss of primary coolant accident.
The discovery illustrated the mixed blessings of a ‘series’ approach to nuclear build, as EDF decided that it should also investigate and, if needed, address the same problem at other reactors in the N4 series, notably at Chooz, where there are four similar reactors. It began an outage at Chooz 2 on 16 December and at Chooz 1 on 18 December.
At that time EDF said the extended outage at Civaux and the closure at Chooz would cost it about 1TWh in lost generation to the end of 2021. But since then the company has found the problem to be more widespread.
ASN (Autorite´ de Su^rete´ Nucle´aire), France’s nuclear safety authority, said analysis on parts of the pipes removed from Civaux 1 had revealed the presence of cracking resulting from an unexpected stress corrosion phenomenon on the inner face of the piping, close to the weld bead. There was worse news for EDF. The ultrasonic inspection, which had been carried out during the plants’ regular 10-yearly outages, is mainly used to detect cracking caused by thermal fatigue. It is less effective at detecting stress corrosion cracking (SCC). That raised the fear that SCC had been present in reactors that had previously been examined by ultrasound and indications of SCC had wrongly been classified as spurious. The re-examination of Chooz B1 and B2 indicated this was indeed the case and there was SCC that needed to be addressed.
All five of the reactors in the initial group have had to undergo additional checks to determine which areas and systems are affected by the stress corrosion phenomenon.
To make matters worse still, checks at Penly 1, during its third 10-yearly outage, revealed indications on the same pipes, which laboratory analysis showed to be SCC, albeit at a smaller scale than at Civaux 1. Unlike the Chooz and Civaux reactors, Penly is not one of the 1450MWe N4 series but a 1300MWe reactor in an earlier French series.
As a result, EDF has returned to the checks previously conducted on all of its reactors to re-examine the results, searching for indications then thought to be spurious but now seen as potential indications of stress corrosion.
In early May, speaking at an investor meeting after the company published results for the three months to the end of March, Regis Clement, EDF’s Deputy Head of Nuclear Generation, provided an update to investors.
He said inspections and examinations had confirmed stress corrosion in sections of piping at Civaux 1, Chooz 1 and Penly 1, where the affected parts will be removed and replaced. EDF had already begun investigations at Civaux 2 and Chooz 2 and now that has been extended to seven more units – Chinon 3, Cattenom 3, Bugey 3 and 4, Flamanville 1 and 2, and Golfech 1. Of these units, Clement said: “Indications have been found during ultrasound inspection process but we are not yet able to establish whether these are minor flaws in the composition of the steel, traces of thermal fatigue or stress corrosion.” Laboratory tests are under way.
In the end, EDF will inspect all its reactors. It expects that process to be completed by the end of 2023 and largely to be carried out during scheduled maintenance outages. Clement said, “At this time more or less 20% of the fleet is undergoing examination” and EDF expected to have a “high level of requirements” in controlling or remedying the problem.
The overall cost of assessing and remedying the problem cannot yet be fully assessed, ……………………….https://www.neimagazine.com/features/featurecounting-the-cost-of-cracking-9919744/
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