Seismic resistance flaw on the anchors of the auxiliary systems of the backup diesel generating sets for the EDF reactors

In 2017, EDF notified a significant safety event concerning the failure to demonstrate the seismic resistance of the civil engineering anchors of the auxiliary systems of the backup diesel generating sets for its 1300 MWe nuclear power reactors. ASN had rated this event level 2 on the INES scale.

The event covers both design problems, which are generic to all the reactors concerned, and local problems relating to the poor condition or poor assembly of the anchors. This was the subject of ASN information notices on 20 June 2017, 30 October 2017 and 19 January 2018. At that time, it concerned 26 reactors of 900 and 1300 MWe.

On 3 April 2018 and then on 16 November 2018, following analysis of the results of additional inspections, EDF informed ASN that the seismic resistance flaws concerned several other 900 MWe reactors.

ASN thus rated this significant event level 2 on the INES scale for an additional 11 reactors: reactors 1 and 2 of the Le Blayais NPP, 1 to 6 of the Gravelines NPP, 1 and 2 of the Saint-Laurent NPP and 2 of the Chinon NPP. This event was also rated level 1 on the INES scale for reactors 3 and 4 of the Le Blayais NPP, 2 of the Dampierre NPP, 1 of the Cruas NPP, 1, 3 and 4 of the Chinon NPP and 1, 2, 3 and 4 of the Tricastin NPP, because they are also affected by this deviation, but to a lesser extent.

The additional inspections also revealed new seismic resistance flaws for the reactors of the Paluel, Saint-Alban, Belleville sur Loire NPPs and for reactor 2 of the Nogent sur Seine NPP. For these reactors, the flaws are of the same type as those which were already covered by the above-mentioned information notices.

Each of the reactors in the French NPPs has two backup diesel generators. This equipment provides a redundant electrical power supply to certain safety systems in the event of the loss of off-site electrical power, more particularly in the wake of an earthquake. The emergency diesel generator sets comprise a generator, a diesel engine and auxiliary systems (cooling, pre-lubrication, etc.). If off-site electrical power is lost as a result of an earthquake, the operation of the emergency diesel generator sets could no longer be guaranteed as a result of the failure of their auxiliary systems.

On all of the reactors concerned by this event, the work carried out to reinforce the anchors of the backup diesel generating sets has now been completed. A number of inspections are however still required, more particularly during the reactor outages.

During the course of these inspections, ASN checked that the reinforcement work on the anchors concerned had been correctly carried out.