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 expe– riences 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 properties of this cast steel and reduce 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 the licensee 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 MPS 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 the licensee, is submitted to ASN, which checks that the performance and frequency of the checks carried out by EDF 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. To minimise the fouling 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. Since the 1990s, EDF has been running a programme to replace the SGs with the most severely degraded tube bundles. The SG replacement campaign for 26 reactors with non-heat treated Inconel 600 alloy tube bundles has been completed. It is continuing with replacement of SGs on the 26 reactors in which the tube bundle is made of heat treated Inconel 600. 2.2.4 Assessment of Pressure Equipment in operation The reactor pressure vessels ASN issues reports following the inspections made during each ten-yearly outage on the primary systems, the reactor pressure vessels in particular, which undergo numerous checks and a hydro-test during these outages. STRESS CORROSION DETECTED ON THE PIPES OF SEVERAL REACTORS During the ultrasounds inspections carried out during the second ten-yearly outage on the Civaux NPP reactor 1, indications(1) were found on the welds of the elbows on the line connecting the safety injection system to the reactor’s Main Primary System. EDF decided to shut down Civaux NPP reactor 2 so that these inspections of the areas could be carried out ahead of schedule, as the previous inspections dated from 2012. The results of these inspections confirmed the presence of indications similar to those of reactor 1. The parts of the pipes concerned on reactor 1 were cut for metallurgical analysis in the laboratory and revealed the presence of cracking resulting from a stress corrosion phenomenon. Given the unexpected origin of the cracks found, EDF decided to shut down the reactors of a design similar to that of the Chooz NPP, in order to conduct further inspections in addition to those performed in 2019 and 2020 during their ten-yearly outage. These examinations revealed indications. Indications were also detected during the third ten-yearly outage of Penly reactor 1. At the beginning of 2022, EDF continued its investigations in order to characterise the factors that caused this phenomenon and identify the reactors and the areas potentially concerned. With the technical support of IRSN, ASN is closely following these investigations and the resulting conclusions. For the latest information on the subject: asn.fr, headings “L’ASN informe”, “Actualités”. 1. An indication is a signal (typically an echo for ultrasonic inspections) revealing the possible presence of a defect in the material being inspected. ASN Report on the state of nuclear safety and radiation protection in France in 2021 287 10 – THE EDF NUCLEAR POWER PLANTS 08 07 13 04 10 06 12 14 03 09 05 11 02 AP 01
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