ASN Report 2020

to the tubes and the internal structures and which can degrade the overall operation of the SG. To prevent or mitigate the clogging effects described above, various solutions can be implemented to limit metal deposits: preventive chemical cleaning or remedial mechanical cleaning (using hydraulic jets), replacement of material (brass by stainless steel or titanium alloy, which are more corrosion-resistant) in certain secondary system exchanger tube bundles, modification of the chemical products used for conditioning of the systems and an increase in the pH of the secondary system. Some of these operations require a license for the discharge of some of the products used. Some chemical cleaning processes are still being tested to confirm that the chemical products utilised are harmless. Replacement of the Steam Generators Since the 1990s, EDF has been running a programme to replace the SGs with the most severely damaged tube bundles, with priority being given to those made of non-heat-treated Inconel 600 alloy (600 MA), and then those made of heat-treated Inconel 600 alloy (600 TT). The campaign to replace SGs with a tube bundle made of 600 MA – some 26 reactors – was completed in 2015 with that of the Blayais NPP reactor 3. It is continuing with replacement of SGs in which the tube bundle is made of 600 TT – that is 26 reactors. Monitoring methods applied to main primary and secondary system Pressure Equipment The Order of 10 November 1999 specifies that the non-destructive testing processes used for in-service monitoring of the Pressure Equipment (PE) of the main primary and secondary systems of nuclear power reactors must be qualified before they are used for the first time. This qualification is granted by a body comprising experts from both inside and outside EDF whose expertise and independence are verified by the French accreditation Committee (Cofrac). Qualification is a means of guaranteeing that the non-destructive testing process actually achieves the anticipated level of performance as described in specifications drawn up beforehand. Owing to the radiological risks associated with radiographic inspection, ultrasound inspections are preferred, provided that they offer equivalent inspection performance. To date, more than 90 non-destructive test processes have been qualified for the in-service inspection programmes. New development and qualification processes to address new needs are in progress. With regard to the Flamanville EPR, virtually all of the test processes for in-service monitoring of the main primary and secondary systems PE were qualified ahead of the Pre-Service Inspection (VCI) of the main primary and secondary systems, corresponding to more than 30 qualified processes specific to the EPR. 2.2.4 Assessment of Pressure Equipment in operation The reactor pressure vessels As part of the preparation for the fourth periodic safety reviews of the 900 MWe reactors (see point 2.10.3 and Notable Events in the introduction to this report), EDF sent ASN a dossier in 2017 substantiating the in-service strength of these reactors after 40 years of operation. The generic approach adopted by EDF consists in conservatively considering the mechanical properties of the vessel experiencing the worst-case irradiation embrittlement for the 900 MWe reactors. EDF carried out fast fracture resistance studies taking account of the changes in the properties of the materials and will carry out inspections to check there are no prejudicial defects in the steel during the ten-yearly outage of each reactor. This generic approach was submitted to the Advisory Committee for Nuclear Pressure Equipment (GPESPN) for its opinion on 20 November 2018, 15 October 2019 and 8 September 2020. The examination concerned the defects analysed, the estim­ ated irradiation ageing of the metal of the vessel, the thermo­ mechanical analyses and the studies assessing the margin with respect to fast fracture of the vessels, the classification of small primary break transients and justification of the level of residual stresses in the circumferential welds of the core shells. The studies carried out conclude that the vessels with no faults are capable of functioning for a further ten years. For the reactor vessels on which previous inspections showed that there were manufacturing flaws, specific studies will be performed before the ten-yearly outage of each of the reactors concerned. This was notably the case for Tricastin NPP reactor 1. Cast elbow assemblies Cast elbow assemblies are piping components installed on PWR main primary systems. They are installed on the hot and cold legs. The cast elbow assemblies installed on the 900 MWe reactors were made of austenitic-ferritic stainless steel. The ferritic phase experiences ageing under the effect of temperature. Certain alloy elements present in the material accentuate this susceptibility to ageing. The result is a deterioration of certain mechanical properties, such as toughness and resistance to ductile tearing. In addition, these elbow assemblies comprise shrinkage clusters or filaments, or solidification cracks, inherent in the static casting manufacturing method, which could, when combined with thermal ageing, increase the risk of fast fracture. EDF has carried out extensive work to learn more about these materials, their ageing kinetics and to assess the fast fracture margins. The dossier produced by EDF was examined by ASN with production of an opinion from the GPESPN on 23 May 2019. Following this analysis, ASN sent EDF requests for additional substantiation of the predicted behaviour of the aged material, identification of the flaws present in the cast elbow assemblies, analysis of the fast fracture margins of the elbow assemblies and in-service monitoring of these components. The principles of the reactor vessels in-service strength demonstration The regulations in force require in particular that the licensee: ཛྷ identify the operating situations with an impact on the vessel; ཛྷ take measures to understand the effect of ageing on the properties of the materials; ཛྷ deploy resources to enable it to ensure sufficiently early detection of defects prejudicial to the integrity of the structure; ཛྷ eliminate all cracks detected or, if this is impossible, provide appropriate specific justification for retaining such a type of defect as-is. ASN Report on the state of nuclear safety and radiation protection in France in 2020 293 10 – THE EDF NUCLEAR POWER PLANTS 10