ASN Report 2020

This conformity assessment concerns the equipment intended for the new nuclear facilities (more than 200 equipment items are concerned on the Flamanville EPR) 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 takes the form of 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 inspection bodies or organisations are currently approved by ASN to assess NPE compliance: Apave SA, Bureau Veritas Exploitation, Vinçotte International and the inspection body of the EDF users. In 2020, the approved organisations carried out 2,750 inspections on the design and manufacture of the NPE intended for the Flamanville EPR and 5,000 inspections for the replacement NPE intended for the NPP reactors in operation. These inspections are performed under ASN supervision. 2.2.2 Assessment of the design and manufacturing of Nuclear Pressure Equipment Actions focusing on detection and processing of deviations and implementation of improvement plans in the fabrication plants 2020 saw extensive mobilisation of the resources of the manu­ facturer Framatome for continued examination of the deviations detected, in particular those which had affected the post- weld heat treatment of the connecting welds for replacement SG components, produced at the Framatome Saint-Marcel plant. 2020 was also marked by the continued implementation of the improvement plan for the Creusot Forge plant, which notably includes reinforcing the safety culture, improved management of the industrial tools and consolidation of technical skills. In the light of the results obtained, ASN gave a favourable opinion for the resumption of manufacturing of the shells intended for the SG replacement programme. Framatome will be required to maintain efforts to ensure that a long-term, robust and efficient organisation tailored to the safety issues is maintained within the Creusot plant. ASN will maintain particular surveillance of this plant. Fault in implementation of a post-weld heat treatment process during manufacture of the Framatome Steam Generators 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 firstly to remove the stresses resulting from the welding, and secondly to avoid 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 Nuclear Power Plants (NPPs) to assemble components or install the Steam Generators (SGs), 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. During each reactor outage, the welds concerned are specifically checked (thickness measurements and defect search). ASN checks the justifications provided by EDF for each reactor before it is restarted. At the same time, EDF has set up a programme of detailed characterisation based on mock-ups and material tests. ASN rated this event level 1 on the International Nuclear and Radiological Event Scale (INES scale). In addition, equipment currently being manufactured by Framatome is also concerned: 22 SGs intended for reactors in operation, as well as the SGs, pressuriser and secondary system lines for the Flamanville Evolutionary Power Reactor (EPR). Framatome is defining the appropriate treatment strategies for each of the equipment items concerned. This includes repair studies, test mock-ups and digital simulation studies to assess the impact of the deviations on the required mechanical properties. ASN questioned the other manufacturers of large equipment items (Westinghouse and MHI), in order to check that they correctly implement the post-weld heat treatment processes. Final joint Heat treatment by muffles equipped with electrical resistors Joint between shells Head/Plate joint ASN Report on the state of nuclear safety and radiation protection in France in 2020 291 10 – THE EDF NUCLEAR POWER PLANTS 10