France's nuclear regulator, the Autorité de Sûreté Nucléaire (ASN), has approved the restart of nine of the 12 reactors affected by the carbon concentration anomaly in the steam generator channel heads manufactured by Japan Casting and Forging Corporation (JCFC). The approval, which followed ASN's examination of the results of inspections and technical demonstrations provided by EDF for the 900 MWe reactors, was announced on 12 January.
In June last year, ASN said it had identified 18 French nuclear power reactors operated by EDF - of both 900 MWe and 1450 MWe capacity - whose steam generators could contain high carbon concentrations. Of these, 12 are equipped with channel heads manufactured by JCFC "liable to contain a particularly high carbon concentration". A high carbon content in steel can lead to mechanical properties lower than expected.
The nine that can now be restarted are Bugey 4, Dampierre 3, Fessenheim 1, Gravelines 2 and 4, Saint-Laurent B1, and Tricastin 1, 3 and 4. All of these are 900 MWe units.
The regulator had ordered EDF in October to carry out additional inspections on the steam generator channel heads of certain reactors, within three months.
On 11 January, the company asked that the inspection deadline for unit 2 of the Tricastin plant be postponed by two weeks.
"This request was explained by the risks posed to the electricity grid as a result of the wave of cold weather expected next week," ASN said, adding it considered this postponement to be acceptable from the safety viewpoint. It has set a new deadline of 3 February.
EDF also asked that the inspection deadline set for Civaux 1 be postponed to the end of March. ASN said it is examining data provided by EDF for the 1450 MWe Civaux units 1 and 2, with the assistance of France's Institute for Radiological Protection and Nuclear Safety. On that basis, ASN will analyse the conditions for the restart of unit 2 - on which inspections have been carried out - and will shortly issue a position statement on the request for postponement of these inspections on unit 1.
ASN announced in April 2015 the discovery of an anomaly in the composition of the steel in certain zones of the vessel closure head and the vessel bottom head of the Flamanville EPR reactor. The detection of this anomaly led ASN to ask Areva NP and EDF to "learn all possible lessons from this event".
There are three processes currently under way, ASN said. Firstly, the search for technical anomalies on other EDF reactor components similar to those detected on the Flamanville EPR vessel, which has enabled EDF to identify similar anomalies on the channel heads of certain steam generators. Secondly, manufacturing quality reviews on parts at Areva NP manufacturing plants, which enabled Areva NP to detect irregularities in the manufacturing files from Creusot Forge. Thirdly, initiation of a review of basic nuclear installation licensee monitoring of their contractors and subcontractors, of ASN oversight and of alert mechanisms.
Steam generators are heat exchangers between the water circulating in the reactor's primary coolant circuit - at a temperature of about 350°C and a pressure of 155 bar - and the water in the secondary circuit that supplies steam to the turbines. Their domed lower head is part of the primary circuit and therefore has an important safety role in ensuring cooling water is always available. There are three steam generators in the 900 MWe pressurized water reactors, while the 1450 MWe reactors feature four.
Researched and written
by World Nuclear News