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 MSS of 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 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. 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 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 a reactor comprises several austenitic-ferritic stainless steel cast elbow assemblies. The ferritic phase 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 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 led EDF to 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 EDF, is submitted to ASN, which checks that the performance and frequency of the checks carried out are satisfactory and able to detect the deteriorations in question. The Steam Generators The SGs comprise two parts, one of which is a part of the MPS and the other a part of the MSS. 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. Over time, the SGs tend to become clogged with corrosion products from the secondary system exchangers. The layer of deposits of corrosion products (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 to the tubes and the internal structures and which can degrade the overall operation of the SG. LEAK DURING HYDRO-TESTING OF CIVAUX REACTOR 1 During the ten-yearly outages, the primary system undergoes periodic requalification which notably includes a hydrostatic pressure test. During the hydrostatic pressure test on Civaux reactor 1 on 2 November 2022, a major event occurred at 190 bar and 98°C during the pressure rise. Rapid depressurisation was observed, with a loss of 40 bar in just one second, followed by a drop of more than 10 bar per minute. This situation was caused by the ejection of a removable tube from the In-core Instrumentation System (RIC). An element of the retaining device specifically installed for the hydro-tests was missing from this tube. This event led EDF to reinforce its checks and its monitoring, to safeguard the next hydro-tests on the reactor primary systems. Reactor safety was not compromised by this leak. This is because, during the hydro-test, no fuel assemblies were present in the reactor vessel. 292 ASN Report on the state of nuclear safety and radiation protection in France in 2022 • 10 • The EDF Nuclear Power Plants 10
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