of operation of the MPS and the MSS of nuclear PWRs. These systems are thus the subject of monitoring and periodic maintenance by EDF. These systems are subject to periodic re-qualification every ten 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. The licensee is required to keep and update the regulatory reference files required by the above-mentioned Order of 10 November 1999 with regard to MPS and MSS monitoring. These files consist of design, manufacture, overpressure pro– tection files, materials files, in-service observations and, as applicable, deviations processing files. The licensee is required to update these files as often as necessary and on the occasion of the periodic requalifications. Owing to the standardised nature of the French NPP reactors, EDF can perform a generic update of these files. The safety implications of some 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 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 reactions in the core. This embrittlement more particularly makes the vessel more susceptible to thermal shocks under pressure, or to sudden DEVIATION IN FRAMATOME’S USE OF POST-WELD HEAT TREATMENT PROCESSES DURING THE MANUFACTURE OF NUCLEAR PRESSURE EQUIPMENT Assembling components by welding creates mechanical stresses in the welded areas. To reduce these stresses, the manufacturer applies a Post-Weld Heat Treatment (PWHT), which consists in heating the material for several hours to a temperature of several hundred degrees. This heating can be carried out on the complete part in a furnace if the size of the part so permits, or locally by using heating devices such as electrical heating elements. The treatment temperature and duration must be controlled, in order to remove the stresses resulting from the welding without altering the mechanical properties of the material. In 2019, the manufacturer Framatome brought to light the fact that certain processes used in its Saint-Marcel plant, or in the NPPs for SG assembly, had led to insufficient control of the temperatures around the circumferences of the treated welds. This deviation concerns 177 of the 192 SGs installed in EDF’s reactors in operation. EDF justified the continued integrity of the equipment concerned, by drawing on the results of tests performed on representative mock-ups, on material test coupons and on numerical temperature prediction models. The equipment currently being manufactured is also concerned by this deviation. This involves 22 SGs intended for reactors in operation, as well as the SGs, pressuriser and secondary system lines for the Flamanville EPR reactor. Framatome is defining the appropriate treatment strategies for each of the equipment items concerned. These include repair studies, test mock-ups and digital simulation studies to assess the impact of the deviations on the required mechanical properties when repairs cannot be carried out. In 2021, during the additional investigations it was carrying out, Framatome brought to light residual stresses of an unexpected level, generated during the implementation of these heat treatments, even if performed correctly. For the equipment in service, Framatome deployed a characterisation programme which, on the basis of experimental measurements and digital simulation, was able to assess the level of these stresses and their impact on the mechanical strength of the equipment. ASN is examining the specific justifications provided by EDF for the welds concerned by the deviation. For all the heat treated welds, ASN also asked EDF to conduct an analysis of the potential risks. ASN is also examining the justifications provided by Framatome for the equipment currently being manufactured. It also questioned other manufacturers of large equipment (Westinghouse and MHI), so that they could examine whether the post-weld heat treatment processes they use also generate similar effects. Final joint Heat treatment by muffles equipped with electrical resistors Joint between shells Head/Plate joint 286 ASN Report on the state of nuclear safety and radiation protection in France in 2021 10 – THE EDF NUCLEAR POWER PLANTS
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