ASN gives a level 2 rating on the INES scale to an event leading to a risk of loss of heat sink for 29 nuclear reactors operated by EDF.

ASN gives a level 2 rating on the INES scale to a significant safety event regarding a risk of the loss of heat sink for the reactors in the Belleville-sur-Loire, Cattenom, Chinon, Cruas, Dampierre-en-Burly, Golfech, Nogent-sur-Seine, Paluel, Saint-Alban and Saint-Laurent-des-Eaux NPPs. 29^[1] 900 MWe and 1300 MWe reactors are concerned by this event.

The reactor heat sink could be lost owing to the unavailability of the pumps of the reactor essential service water system (SEC^[2]) as a result of internal flooding following an earthquake-induced rupture of the piping supplying water to the fire protection network (JPP system) and the raw water filtration network (SFI or CFI systems).

The insufficient earthquake resistance of a JPP pipe was initially detected by EDF in the Belleville-sur-Loire NPP. Additional investigations requested by ASN and performed by EDF in early June 2017 revealed that several sections of these pipes were degraded, with thicknesses less than the minimum thickness required for earthquake resistance. This degradation is the result of corrosion which may have developed because of a lack of appropriate preventive maintenance. This event was provisionally rated level 1 on the INES scale on 2^nd August 2017.

EDF then took thickness measurements on piping sections of other systems (SFI and CFI) situated in the same areas as the JPP pipes, from early July to the end of September 2017, on all the EDF NPP reactors potentially concerned. Following this measurement campaign and then the earthquake resistance analysis of the piping concerned, EDF declared on 10^th October 2017 that 20 reactors were concerned by a risk of total loss of heat sink (loss of both trains^[3] of the SEC system). This event is therefore rated level 2 on the INES scale.

Nine other reactors are for their part concerned by a risk of partial loss of heat sink (loss of one SEC system train), which is a situation rated level 0 on the INES scale.

Repairs have been initiated on the JPP, SFI or CFI piping, so that there is a secure SEC train for all the reactors concerned. The repairs to ensure total availability of the SEC system in the event of an earthquake must be initiated as early as possible for the reactors currently in operation and before restart of those that are currently shut down.

More particularly during the course of its inspections, ASN ensures that the repairs are carried out satisfactorily.