Significant safety event rated level 2 on the INES scale during operations to drain the reactor 2 primary system in the Golfech NPP

On 11 October 2019, EDF reported a significant safety event to ASN concerning non-compliance with the general operating rules during operations to drain the reactor 2 main primary system on the Golfech NPP.

On 8 October 2019, the reactor was being shut down so that part of its fuel could be renewed. A field operator went to the reactor building to open the pressuriser vent, in accordance with procedures. This operator was interrupted during the course of his work and the vent was in fact not actually opened. Assuming that the vent had been opened, the operators in the control room began the scheduled primary system drainage operations, although with vent closed, leading to depressurisation of the system. In this configuration, the water level measurements in the primary system are no longer representative of the actual situation.

Eight hours later, an operator saw that the water level in the primary system was not changing as expected. After analysis, the operating team interrupted drainage of the system and sent a field operator to the reactor building to check the position of the pressuriser vent. Finding it to be closed, they requested that it be opened, although without first carrying out the steps required by the general operating rules.

This opening led to uncontrolled water movements in the primary system and a drop in the water level. The licensee then made up the water level in the primary system.

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The decisions made and the steps taken by the licensee were hasty following the discovery of non-opening of the vent, with no preliminary assessment of the actual and potential impacts and with the reactor in a configuration non-compliant with the general operating rules. The subsequent analysis performed by the licensee, at the request of ASN, showed that cooling of the fuel assemblies in the reactor pressure vessel was maintained during the event.

This event took place a few days before an in-depth inspection carried out on the Golfech site by 13 ASN inspectors and 11 IRSN experts. This inspection was an opportunity for ASN more particularly to check the steps taken by EDF to ensure the safety of continued reactor shutdown operations and reinforce the monitoring of control activities following this event.

ASN also asked EDF to assess the consequences of depressurisation on the primary system equipment, which led to additional inspections being performed on the facilities. Their results were analysed by ASN as part of its examination of the reactor 2 restart approval request, which was granted on 21 November 2019.

Non-compliance with the general operating rules during primary system drainage operations had no consequences for persons or the environment.

Owing to the degraded safety functions and the potential consequences for nuclear safety, notably linked to errors in the management of the event and the monitoring of the operation activities, as well as to the fact that insufficient lessons had been learned from operating experience feedback, the event is rated level 2 on the INES scale (International Nuclear Event Scale, ranked from 0 to 7 in ascending order of severity).