ASN Report 2020

The manufacturer Westinghouse continued to apply its improve­ ment plan in its SG manufacturing plant in Italy, with regard to the internal monitoring quality system. The conditions were thus defined for lifting the reinforced surveillance in place. Processing of the irregularities reported at the end of 2018 by the supplier of special alloys and steels, Aubert & Duval, is also continuing. The investigations carried out have not yet identified any consequences for the safety of the facilities. At the same time, the organisations, manufacturers and licensees are developing an organisation and the corresponding resources within their own structures, in order to prevent and detect the risk of fraud. Although progress has been observed, improvements are still needed in the implementation of the technical procedures. Reinforcing justification of the design of NPE ASN has regularly observed that the justifications and demon­ strations provided by the manufacturers with regard to the regulations applicable to NPE, notably in terms of the good design of this equipment, are unsatisfactory. The industrial firms, EDF and Framatome in particular, therefore took fundamental measures as of the first half of 2015 to change their practices and bring them into line with the regulatory requirements. ASN monitored these actions, most of which were carried out within the framework of the French Association for NSSS Design, Construction and Monitoring Rules (AFCEN) and involved the majority of the profession. ASN considers this approach to be a positive one and, for most of the problems identified in 2015, considers that the AFCEN guides and methods published are appropriate. This approach was repeated for the years 2019 to 2022, so that the profession continues to make progress on certain topics and in order to learn the lessons from the initial applications of the guides and methods created and the deviations observed during ongoing manufacturing. 2.2.3 Monitoring the operation of Pressure Equipment The reactor Main Primary and Secondary Systems (MPS and MSS), which contribute to the containment of the radioactive substances, to cooling and to controlling reactivity, operate at high temperature and high pressure. The monitoring of the operation of these systems is regulated by the Order of 10 November 1999 relative to the monitoring of operation of the main primary system and the main secondary systems of nuclear pressurized water reactors. These systems are thus the subject of monitoring and periodic maintenance by EDF. This monitoring is itself checked by ASN. These systems are subject to periodic re-qualification every 10 years, which comprises a complete inspection of the systems involving non-destructive examinations, pressurised hydrotesting and verification of the good condition and good operation of the over-pressure protection accessories. Nickel-based alloy zones Several parts of the PWRs are made of nickel-based alloy. This type of alloy is chosen for its resistance to generalised or pitting corrosion. However, in the reactor operating conditions, one of the alloys chosen, Inconel 600, has proven to be susceptible to stress corrosion. This particular phenomenon occurs in the presence of significant mechanical stresses. It can lead to the appearance of cracks, as observed on certain SG tubes in the early 1980s or, more recently in 2011, on a vessel bottom head penetration in Gravelines NPP reactor 1 and in 2016 on a vessel bottom head penetration in Cattenom NPP reactor 3. These cracks require that the licensee repair the zones concerned or isolate the part of the system concerned. At the request of ASN, EDF adopted an overall approach to monitoring and maintenance for the zones concerned. Several zones of the main primary system made of Inconel 600 alloy are thus subject to specific monitoring. For each of them, the in-service monitoring programme, defined and updated annually by the licensee, is submitted to ASN, which checks that the performance and frequency of the checks carried out by EDF are satisfactory and able to detect the deteriorations in question. The strength of reactor pressure vessels The reactor pressure vessel is an essential component of a PWR and contains the reactor core and its instrumentation. For the 900 MWe reactors, the vessel is 14 meters (m) high, 4 m in diameter, 20 centimeters (cm) thick and weighs 330 tonnes (t). For the EPR, currently under construction at Flamanville, the vessel is 15 m high, 4.90 m in diameter, 25 cm thick and weighs 510 t. In normal operating conditions, the vessel is entirely filled with water, at a pressure of 155 bar and a temperature of 300°C. It is made of ferritic steel, with a stainless steel inner liner. Regular inspection of the condition of the vessel is essential for two reasons: ∙ The vessel is a component for which replacement is not envisaged, owing to both technical feasibility and cost. ∙ Monitoring contributes to the break preclusion approach adopted for this equipment. This approach is based on particularly stringent design, manufacturing and in-service inspection provisions in order to guarantee its strength throughout the life of the reactor, including in the event of an accident. During operation, the vessel’s metal slowly becomes brittle, under the effect of the neutrons from the fission reaction in the core. This embrittlement more particularly makes the vessel more susceptible to thermal shocks under pressure, or to sudden pressure rises when cold. This susceptibility is aggravated by the presence of technological flaws, which is the case for some vessels with manufacturing defects under their stainless steel liner. ASN regularly examines the evidence to substantiate the in-service resistance of the vessels transmitted by EDF, to ensure that it is sufficiently conservative. Maintenance and replacement of Steam Generators The SGs comprise two parts, one of which is a part of the main primary system and the other a part of the main secondary system. The integrity of the main components of the SGs is monitored, more specifically the tubes making up the tube bundle. This is because any damage to the tube bundle (corrosion, wear, cracking, etc.) can lead to a primary system leak to the secondary system. Rupture of one of the tube bundles would lead to bypassing of the reactor containment, which is the third containment barrier. The SGs are the subject of a specific in-service monitoring programme, defined by EDF and periodically revised and examined by ASN. Following the inspections, those tubes which 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. The layer of deposits (fouling) that forms on the tubes reduces the heat exchange capacity. On the tube support plates, the deposits prevent the free circulation of the water-steam mixture (clogging), which creates a risk of damage 292 ASN Report on the state of nuclear safety and radiation protection in France in 2020 10 – THE EDF NUCLEAR POWER PLANTS