2.2.2 Assessment of the design and manufacturing of Nuclear Pressure Equipment ASN assesses the regulatory compliance of the NPEs most important for safety, referred to as “level N1”, corresponding to the reactor pressure vessel, the SGs, the pressuriser, the reactor coolant pumps, the piping, notably that of the Main Primary (MPS) and Secondary (MSS) Systems, as well as the safety valves. This conformity assessment concerns the equipment intended for the new nuclear facilities (more than 200 equipment items for the Flamanville EPR reactor) and the spare equipment intended for nuclear facilities already in service (notably the replacement SGs). ASN can be assisted in this task by organisations that it approves. These latter can be mandated by ASN with performance of some of the inspections on the “level N1” equipment and are tasked with assessing the regulatory compliance of the NPE less important for safety, said to be “level N2 or N3”. The oversight by ASN and the approved organisations is carried out at the different stages of the design and manufacture of the NPE. It involves an examination of the technical documentation of each equipment item and inspections in the workshops of the manufacturers, as well as at their suppliers and subcontractors. Four organisations or bodies are currently approved by ASN to assess NPE compliance: Apave Exploitation France, Bureau Veritas Exploitation, Vinçotte International and the inspection body of the EDF users. In 2023, with regard to NPE design and manufacture, the approved organisations carried out about 5,000 checks on the NPE intended for the Flamanville EPR reactor and about 5,200 checks on the replacement NPE intended for the NPP reactors in operation. These checks are performed under ASN supervision. In 2023, Framatome reinforced its quality improvement actions in its three plants, as part of the start of the EPR 2 programme. Framatome and EDF continued to deploy an approach to monitor the most sensitive industrial processes, such as the welding and heat treatment processes, along with supplier approval, evaluation and surveillance. Through its inspections, ASN assessed the results of these actions. It underlines the scale of the programmes implemented. Similarly, Westinghouse continued to apply its quality system and internal surveillance improvement plan in its SG manufacturing plant in Italy. As a result of the progress made, ASN considers that the particular surveillance in place in this plant can be lifted. ASN also finds that the approved organisations, the manufacturers and the licensees are currently deploying organisations and resources in their structures for preventing and detecting fraud. Although progress has been observed in this respect, the actual implementation of these action plans still needs to be continued by all stakeholders in the sector. 2.2.3 Operation of Nuclear Pressure Equipment The reactor 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 MPS and the MSS of PWRs. These systems are thus the subject of monitoring and periodic maintenance by EDF. These systems are subject to periodic requalification every ten years, which comprises a complete inspection of the systems involving non-destructive examinations, pressurised hydro-testing and verification of the good condition and good operation of the over-pressure protection accessories. The licensee is required to keep and update files on the design, manufacture, overpressure protection, materials, findings made during operation and, as applicable, processing of deviations, as often as necessary and at the time of the periodic requalifications. Some of the safety issues of the components of the primary or secondary systems are detailed below. The reactor pressure vessels The reactor pressure vessel is an essential component of a PWR and contains the reactor core and its instrumentation. In normal operating conditions, the vessel is entirely filled with water, at a pressure of 155 bars 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 reactions 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 also aggravated by the presence of technological flaws, which is the case for some vessels with manufacturing defects under their stainless steel liner. Cast elbow assemblies The MPS of some of the reactors of the French fleet comprises several austenitic-ferritic stainless steel cast elbow assemblies. The ferritic phase of this steel experiences ageing under the effect of temperature. Certain alloy elements present in the material aggravate this ageing sensitivity, notably on the 900 MWe reactors and the first 1,300 MWe reactors. The result is a deterioration of certain mechanical properties, such as toughness and resistance to ductile tearing. The elbows also comprise flaws inherent in the static casting manufacturing method. The effects of thermal ageing lessen the fast fracture resistance margins in the presence of defects. EDF has carried out extensive work to learn more about these materials, their ageing kinetics and to assess the fast fracture margins. Nickel-based alloy zones Several parts of the PWRs are made of nickel-based alloys, owing to its generalised or pitting corrosion resistance. However, in the reactor operating conditions, one of the alloys chosen, Inconel 600, has proven to be susceptible to the Stress Corrosion (SC) phenomenon. 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 led EDF to repair the zones concerned or isolate the part of the system concerned. ASN Report on the state of nuclear safety and radiation protection in France in 2023 295 • 10 • The EDF Nuclear Power Plants 10 05 15 08 11 04 14 06 07 13 AP 03 02 09 12 01
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