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On 28th May 2015, reactor 1 in the Cattenom  NPP was kept at a nuclear power close of 2% of its rated power, after completing the restart tests which followed its outage for maintenance and partial refuelling [Figure n°1]. The reactor was cooled by the steam generators. At 13h25, the reactor protection system triggered a series of automatic measures which automatically shut down the reactor as a result of a GCTa  system valve being blocked completely open. Deterioration of the reactor’s thermohydraulic parameters led the licensee to trigger the on-site emergency plan at 14h08.
ASN was immediately notified and activated its emergency organisation without delay. The ASN Strasbourg division placed a staff member at the disposal of the Moselle Prefect’s office, to provide it with technical support. A team of nuclear safety inspectors went to the Cattenom NPP emergency management centre.
On 29th May 2015, ASN performed an inspection on the Cattenom site.
Description of the event
Illustrations of successive thermohydraulic states of the Cattenom reactor 1 during the event of 28th May 2015
The inadvertent opening of the turbine bypass system vent valve (GCT 021 VV) installed on the steam generator (SG 1) steam line led to a rapid rise in the steam flow in this SG and a drop in its pressure and level [Figure n°2]. This led to excess cooling of the reactor coolant system and contraction of the reactor coolant, measured by the rapid fall of the water level and of the pressure in the pressuriser [Figure n°3]. When the low water level was detected in SG1, with excessive cooling of the reactor coolant system, the protection system automatically shut down the reactor, isolated the containment and activated safety injection  .
In accordance with the operating rules, reactor shutdown to a safe state was initiated. The GCTa line was thus isolated, by closing the steam isolation valve (GCT 011 VV) located upstream of the faulty blowdown valve. Cooling was then provided by the three unaffected steam generators, until pressure and temperature conditions were reached in the reactor coolant system such as to allow connection of the reactor at shutdown cooling system. These conditions were reached at 18h00 [Figure n° 4].
The event caused a sudden drop in pressure and level in SG 1 and may have damaged  the internal structures of this steam generator.
The information recorded by the measurement channels continuously monitoring the radioactivity of the secondary system revealed no rise in activity liable to result from damage to a tube of the steam generator affected. No increase in radioactivity was measured on the site or in the nuclear power plant’s environment
The event which occurred on 28th May 2015 is a design-basis accident considered in the design of the NPP reactors.
The loss of integrity of the secondary system is a reactivity accident: it leads to excess cooling of the reactor coolant system, which causes a rise in the nuclear power of the reactor and finally a rise in the temperature of the fuel, liable to lead to its significant deterioration if the protection and safeguard systems were to fail.
With regard to the potential consequences for individuals and the environment, this accident led to the release of steam into the environment from the secondary system, as happens during the reactor start-up and shutdown phases. Conservation of the integrity of the reactor coolant system is a major issue. In the event of rupture of one or more tubes on the SG affected, the radioactive contamination of the reactor coolant system can then be released into the environment. In a situation such as this, the reactor control strategy would have led to the pressure in the reactor coolant system being reduced more rapidly, to cancel out the leak between the reactor coolant and secondary systems.
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ASN activated its emergency center in Montrouge with the technical support of the IRSN emergency center. ASN Strasbourg division immediately placed a staff member at the disposal of the Metz Prefect’s office, while a team of nuclear safety inspectors went to the Cattenom NPP’s emergency management center. On 29th May 2015, ASN performed an inspection on the Cattenom site.
The results of the first investigations reveal a malfunction of the positioner on valve GCT 021 VV [Photo 1]. The role of the positioner is to adjust the opening and closure of the valve according to the setpoints sent by the reactor the opening and closure of the valve according to the set points sent by the reactor command control system.
The inspectors examined the maintenance, adjustment and testing of this device. This examination failed to reveal the root causes of the positioner malfunction. They noted that one of its electronic components was flagged as being defective by the self-diagnostic system.
Le positionneur en cause avait été remplacé lors de l’arrêt pour maintenance du réacteur à la suite d’un premier dysfonctionnement. Le nouveau dysfonctionnement à l’origine de l’événement du 28 mai 2015 pose donc la question de la fiabilité intrinsèque de la vanne GCT 021 VV et de ses accessoires, alors que cet équipement doit répondre à des exigences de haute fiabilité pour assurer notamment sa fonction de protection contre les surpressions des circuits secondaires principaux.
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Given the above and in addition to the measures taken by EDF, ASN asked EDF to carry out the following prior to restart of Cattenom reactor 1:
check on SG 1 to ensure that there is no damage to the internal structures and that there is no foreign material,justify that:
the high-reliability requirement concerning the GCT 021 VV equipment is not compromised by the malfunctions observed on it
the interventions following the event of 28th May 2015 enable the availability of this equipment to be guaranteed for the forthcoming cycle.
In addition, given the potentially generic nature of the anomaly which caused this event, ASN asked EDF to list any malfunctions of this equipment which have affected the reactors in operation since 2012 and to specify the origins.