ASN takes stock of two level-2 incidents relative to the earthquake resistance of the EDF reactor emergency diesel generator sets
In 2017, EDF notified two events significant for safety which occurred on the emergency diesel generator sets of its nuclear power reactors. ASN rated these two events level 2 on the INES scale.
Each of the 900 MWe and 1300 MWe reactors of the French NPP fleet is backed up by two emergency diesel generator sets. These generator sets provide a redundant electrical power supply to certain safety systems in the event of loss of off-site electrical power, particularly further to an earthquake. The emergency diesel generator sets comprise a generator, a diesel engine and auxiliary systems (cooling, pre-lubrication systems, etc.). If off-site electrical power is lost as a result of an earthquake, operation of the emergency diesel generator sets could possibly no longer be guaranteed due to failure of their auxiliary systems.
Anchoring of the emergency diesel generator auxiliary systems
A first significant event for safety concerns the failure to demonstrate the earthquake resistance of the anchors in the civil engineering of the emergency diesel generator set auxiliary systems. It covers both design problems which are generic to all the reactors concerned and local problems relating to the poor condition or poor installation of the anchors. This event was rated level 2 on the INES scale.
This event formed the subject of ASN information notices of 20th June 2017 and 30th October 2017. At the time it concerned twenty four (24) 900 and 1300 MWe reactors.
On 27th December 2017 EDF notified ASN that reactors 3 and 4 of the Bugey NPP were also concerned, bringing to 26 the number of reactors concerned by this event1.
All the 1300 MWe and 900 MWe reactors concerned by this event have undergone work to reinforce the anchoring of the emergency diesel generator set auxiliary systems, with the exception of reactor 2 of the Fessenheim NPP, which is currently shut down and for which ASN has prescribed completion of the repairs before the end of January 2018. ASN has verified through inspections that this work has effectively been carried out.
Corrosion of the surge tanks
A second significant event for safety concerns the failure to demonstrate the earthquake resistance of the surge tanks2 of the emergency diesel generator sets due to deficiencies associated with corrosion. These deficiencies result in particular from insufficient maintenance of these items of equipment. Identified by EDF at the Penly NPP in July 2017, they formed the subject of a generic significant event notification to ASN on 9th November 2017 for 7 reactors in the 1300 MWe series.
The corroded surge tanks of the reactors concerned were repaired or replaced between August and October 2017.
ASN rates the events as follows:
- level 2 on the INES scale for the earthquake resistance deficiencies of the surge tanks of the two emergency diesel generator sets of reactors 1 and 2 of the Paluel NPP, given that both redundant generator sets are affected;
- level 1 on the INES scale for the earthquake resistance deficiencies of the surge tanks of one emergency diesel generator set of the Nogent 1, Penly 1, Belleville 2, Cattenom 2 and Paluel 3, reactors, given that only one generator set out of the two is affected.
Read ASN resolution of 26th October 2017 [in french]
Read the information notice of 21th June 2017
1. The reactors in question are those of the Belleville, Bugey, Cattenom, Fessenheim, Flamanville, Golfech, Nogent, Paluel, Penly and Saint-Alban NPPs.
2. The cooling of the emergency diesel generator sets is ensured by several forced water circulation cooling systems. The surge tanks in these systems are auxiliary systems that absorb the variation in water volume when it expands under the effect of the heat produced when the motor is operating.