ASN Report 2018

2.2.4  –  Assessment of pressure equipment in operation Owing to the difficulties encountered in controlling fouling or in the replacement of some steam generators, the situation of the second containment barrier, that is the reactor coolant system, was given particular attention by ASN in 2018. On the one hand, very high levels of fouling were found in certain steam generators in several reactors, which could be liable to impair their operating safety. These fouling levels are the result of insufficient maintenance to ensure satisfactory cleanness and they highlight the need, for some types of steam generators, to continue to improve the strategy for monitoring and preventing this phenomenon. This situation meant that in 2018, ASN asked for advance chemical cleaning prior to restart on the steam generators of the Dampierre-en-Burly NPP reactor 4. On the other, ASN observes that the last SG replacement operations for the 900 MWe reactors were postponed, notably owing to the numerous deviations in the manufacture of this equipment. These postponements led to operations to make cracked tubes on certain steam generators safe, by plugging or sleeving, until such time as they are replaced. With regard to the 1,300 MWe reactors, the first steam generator replacement was completed in 2018 on the Paluel NPP reactor 2. Finally, with regard to the other equipment on the main primary system, ASN considers that their in-service monitoring, pursuant to the provisions of the Order of 10 November 1999, is carried out appropriately. In 2017, the detection of a new crack on a vessel bottom head penetration in Cattenom NPP reactor 3, for which no change was detected in 2018, nonetheless illustrates the risk of further deterioration associated with the ageing of the facilities and confirms the need to adapt the level of in-service monitoring accordingly and to bring forward the development of repair processes. 2.3  ̶  The containments 2.3.1  –  Monitoring the containments The containments undergo inspections and tests to check their compliance with the safety requirements. Their mechanical performance in particular must guarantee a good degree of reactor building tightness in the event of its internal pressure exceeding atmospheric pressure, which can happen in certain types of accidents. This is why, at the end of construction and then during the ten-yearly inspections, these tests include an inner containment pressure build-up with leak rate measurement. These tests are required by the Order of 7 February 2012 setting the general rules relative to BNIs. 2.3.2  –  Assessment of the condition of the containments • Overall management of the containment function The organisation put into place by EDF for monitoring the activities and systems liable to have an impact on the static and dynamic containment of the facilities remains on the whole satisfactory, even if a complete formal definition is sometimes not yet available. Even if EDF has taken steps to maintain dedicated systems in good condition, improvements are still needed regarding the containment condition of the third barrier and its components, in particular maintenance of the floor drains and the components of the doors contributing to static containment. • Single wall containments with an internal metal liner The ten-yearly tests performed on the 900 MWe reactor containments since 2009 have brought to light no particular problems liable to compromise their operation for a further ten years, with the exception of Bugey NPP reactor 5. This latter is now regularly monitored since an unfavourable change in the leaktightness of its containment was revealed in 2011. This containment was repaired in 2017. The ageing of the 900 MWe reactor containments was examined by ASN with the support of IRSN in 2018 and was presented to the GPR during a session dedicated to ageing. This examination concluded that the cleanness of the outer part of the containments needed to be improved in order to guarantee that there was no stagnant water, debris, moss and other vegetation. • Double-wall containments The test results for the double-wall containments performed during the first ten-yearly outages of the 1,300 MWe reactors detected a rise in the leak rate from the inner wall of some of them, under the combined effect of concrete deformation and loss of pre-stressing of certain tendons that was higher than anticipated in the design. EDF then initiated significant work consisting in using a resin sealant locally to cover the interior surface of the inner wall of the most severely affected 1,300 MWe reactors, but also 1,450 MWe reactors. The tests performed since this work, during the second and the third ten-yearly outages of the 1,300 MWe reactors and the first ten-yearly outages of the 1,450 MWe reactors all complied with the regulation leak rate criteria. In order to ensure that these criteria will continue to be met for the long-term, EDF decided to improve the leaktightness of the inner wall of these reactor buildings by applying the sealant to their outer surface. ASN remains vigilant with regard to changes in the leaktightness of these containments and to maintaining the long-term effectiveness of the coatings. The effectiveness of the containment function of the double-wall system was examined in 2013. ASN concluded that: ∙ ∙ In addition to EDF’s satisfactory monitoring of the condition of the concrete, additional measures to prevent or mitigate the ingress of water from outside must also be envisaged because, given the current state of knowledge, this is the primary means of protecting containments from concrete swelling pathologies. ∙ ∙ EDF must reinforce in-service monitoring and visual inspection of certain containment singularities (sleeves, equipment hatch). ∙ ∙ EDF must give a safety classification to the instrumentation system providing continuous monitoring of the containment leak rate (Sexten) along with in-service monitoring of its correct operation. EDF has begun steps in response to these conclusions. ASN checks that they are correctly implemented, notably by means of inspections. 284  ASN report on the state of nuclear safety and radiation protection in France in 2018 10 – EDF NUCLEAR POWER PLANTS