France’s nuclear regulator, Autorité de Sûreté Nucléaire (ASN), has approved the conditional restart of 10 reactors that were closed for safety checks by ASN and its technical support organisation, the Institute for Radiological Protection and Nuclear Safety (IRSN). In 2015 ASN asked EDF to look for components in its operating reactors that could be affected by an abnormality in the carbon concentration of steel used in the units’ steam generators. The issue came to attention in April 2015 after ASN confirmed an anomaly in the composition of steel in some areas of the lid and the bottom of the reactor pressure vessel at the EPR under construction in Flamanville, northern France. ASN said this could affect the mechanical properties of the steel and jeopardise the safety of reactors.

This led ASN to ask reactor manufacturer Areva and operator EDF to check other EDF reactor components for similar anomalies. EDF subsequently identified similar anomalies on the lower, primary side cap of a number of steam generators. Areva discovered separately that steel from its Le Creusot forge facility in France and Japan Casting and Forging Corporation (JCFC) might have had carbon concentrations that could lead to anomalies similar those discovered at Flamanville 3.

ASN said on 5 December that it had reviewed information provided by nuclear operator EDF concerning reactors whose lower heads on the primary side of steam generators were manufactured by JCFC. These include 10 900MWe units and two 1,450MWe units. ASN said EDF can restart the ten 900MWe units once the safety case has been confirmed by a programme of further tests. Each reactor restart will still be subject to ASN’s agreement. These include Bugey NPP unit 4, Dampierre 3, Fessenheim 1, Gravelines 2 and  4, Saint-Laurent B1, and Tricastin 1-4. EDF was also asked to undertake out similar checks on two 1,450MWe units  at Civaux NPP and has yet to present its safety case for these units.

ASN has now sent EDF requests concerning "the additional inspections required, the compensatory operational measures to be implemented or reinforced and the additional tests or studies to be performed in the medium term". ASN said: "Provided that its requests are taken into account, the generic demonstrations provided by EDF are acceptable for the 900MWe reactors and can be used on each of the reactors concerned with a view to ASN authorising their restart."