ASN Report 2018

Risk of blockage of control rods in the 1,300 MWe reactors On 14 February 2018, EDF notified ASN of a significant safety event involving a risk of blockage of control rod clusters owing to significant wear of the thermal sleeves on the vessel closure heads of Belleville-sur-Loire NPP reactor 2 and Saint-Alban/Saint-Maurice NPP reactor 2. ASN rated this event level 1 on the INES scale. The nuclear power reactor vessel closure heads are fitted with thermal sleeves where they are penetrated by the control rod clusters. The control rod clusters are groups of mobile rods joined together and containing a neutron absorbing material. They are used to control the reactor’s power level when inserted into the reactor core and thus stop the nuclear reaction in the event of an incident or accident situation. The significant wear of several thermal sleeves led to the rupture of their upper part, which then formed a metal ring. This ring then blocked the movement of one of the control rod clusters. A blockage of this type had been observed by EDF on 3 November 2017 on the Saint‑Alban NPP reactor 2, on 5 and 13 December 2017 on the Belleville-sur-Loire NPP reactor 2 and on 17 August 2018 on the Nogent-sur-Seine NPP reactor 1. The checks and any repairs require shutdown of the reactor. Since this wear has been detected, EDF checks the thermal sleeves on all its reactors. The investigations carried out showed that twenty of EDF’s 1,300 MWe reactors are particularly susceptible to being affected by this significant wear. They will be inspected no later than during their next refuelling outage, with the last ones being scheduled for the first quarter of 2019. For the 1,300 MWe reactors not yet inspected, ASN asked EDF to carry out monthly control rod drop tests and to take operational measures to ensure reactor scram even if several control rod clusters are blocked. ASN checked the investigations carried out by EDF and the implementation of the provisions it requested. Reactor pressure vessel ASN report on the state of nuclear safety and radiation protection in France in 2018  295 10 – EDF NUCLEAR POWER PLANTS 10