ASN Report 2017

340 ASN report on the state of nuclear safety and radiation protection in France in 2017 Chapter 12  - EDF Nuclear Power Plants ASN regularly examines the evidence to substantiate the in-service resistance of the vessels transmitted by EDF, to ensure that it is sufficiently conservative. In mid-2016, EDF more specifically sent ASN a file substantiating the in-service resistance of the 900 MWe reactor vessels after 40 years of operation and this is currently being reviewed by ASN. This file will be submitted to the GPESPN for its opinion during the course of 2018. Maintenance and replacement of steam generators The SGs comprise two parts, one of which is a part of the primary system and the other a part of the secondary system. The integrity of the main steam generator components, more specifically the tubes making up the tube bundle, is monitored. This is because any damage to the tube bundle (corrosion, wear, cracking, etc.) can lead to a leak from the primary system to the secondary system. A steam generator tube rupture would lead to bypassing of the reactor containment, which is the third containment barrier. Steam generators are therefore the subject of a special in-service monitoring programme, established by EDF, reviewed periodically and examined by ASN. After inspection, tubes that are too badly damaged are plugged to remove them from service. Clogging of the tubes and internals of the secondary part of the steam generators Over time, the SGs tend to become clogged with corrosion products from the secondary system exchangers. This leads to a build-up of soft or hard sludge at the bottom of the SGs, fouling of the tube walls and clogging of the tube bundle tube support plates. The corrosion products form a layer of magnetite on the surface of the internals. On the tubes, the layer of deposits (fouling) reduces the heat exchange capacity. In the tube support plates, the deposits prevent the free circulation of the water-steam mixture (clogging), which creates a risk of damage to the tubes and the internal FOCUS Fall by a steam generator of the Paluel NPP reactor 2 During the Paluel NPP reactor 2 outage (since May 2015) for its third ten-yearly outage inspection, a steam generator fell while being handled on 31st March 2016. The reactor vessel was completely emptied of its fuel at the time of the event. Nine persons were present inside the building at the time of the fall and one person suffered a chest injury. The assessments carried out by EDF on the condition of the installation revealed a certain amount of damage to the metal liner of the reactor building cavity, electrical and mechanical equipment and the civil engineering structures. However, as concerns either the concrete or the metal liner, only the cavity required extensive repairs which should be completed at the beginning of 2018. The general assessment methodology and the cavity repairs were examined by ASN, with the support of IRSN. A new lifting beam was designed and manufactured, enabling replacement SGs to be brought into and installed in the reactor building at the end of 2017. EDF notified a significant safety event on 1st April 2016. The use of a lifting beam added to the polar crane for the 1,300 MWe reactors, which is different from that of the 900 MWe reactors and which comprises a design flaw, is among the main causes identified by EDF. This event also reveals flaws in the monitoring and decision-making process on the part of EDF, with regard to the contractor in charge of replacing the SGs. Since the SG fell, ASN has carried out about fifteen inspections on the follow-up to this event in order to check the various operations carried out. It is continuing its monitoring of the operations in preparation for restart of the installation scheduled for 2018, notably with regard to the hydrotesting of the main primary system. Finally, by an Order of 26th January 2017 and further to the opinion of ASN, the Minister responsible for Nuclear Safety prolonged the period beyond which shutdown is considered to be final.