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 reviewed by ASN, which verifies that the performance and frequency of the checks put into place 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 this fouling, 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 must be authorised beforehand, because they imply discharges of some of the products used. Since the 1990s, EDF has been running a programme to replace those 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 reactors in which the tube bundle is made of heat treated Inconel 600 (22 reactors are still concerned). 2.2.4 Assessment of operation of Nuclear Pressure Equipment Monitoring the operation of reactor main primary and secondary systems ASN considers that EDF’s surveillance of the operation of the MPP and MPS, which constitute the reactors’ second containment barrier, remains a point warranting particular attention. 2023 was again particularly marked by the SC phenomenon affecting the auxiliary lines of these systems (see box next page). This phenomenon illustrates the point that unexpected degradation modes can appear on these systems, including in the absence of international OEF on similar phenomena. In 2016, the carbon segregation anomaly in the Flamanville EPR reactor vessel revealed that the SG channel heads on 17 EDF reactors were also concerned by the issue. These channel heads were manufactured by the Creusot Forge plant and the Japan Casting and Forging Corporation (JCFC). Following this discovery, EDF carried out non-destructive tests on the channel heads concerned, notably those prescribed by ASN on 18 October 2016. These inspections were completed in early 2017. They demonstrated that the reactors concerned could be kept in service. In addition, a wide-ranging programme of tests was carried out between 2017 and 2021 on channel heads representative of the components in operation on the French reactors, in order to confirm the design hypotheses. ASN convened the Advisory Committee of Experts for Nuclear Pressure Equipment (GPESPN) on 17 November 2023 for a ruling on EDF’s conclusions, the results of the thermohydraulic, chemical and metallurgical analyses and the mechanical tests carried out. These data indicate that the mechanical properties of the steel are not affected by the carbon segregation in the channel heads manufactured by JCFC. With regard to the channel heads manufactured by the Creusot Forge plant, the hypotheses adopted by EDF regarding a reduction in the mechanical properties, to take account of the presence of segregation, were considered to be sufficiently penalising. These calculations confirmed the serviceability of these channel heads. ANOMALIES LINKED TO CARBON SEGREGATIONS IN CERTAIN STEAM GENERATOR CHANNEL HEADS The risk of fast fracture of a reactor pressure vessel depends on the joint presence of three factors: the presence of a flaw (such as a crack), thermomechanical loading and insufficient mechanical strength of the material. The analysis of the fast fracture risk on the reactor pressure vessels therefore comprises the following steps: • a determination of the dimensions of the flaws to be studied; • evaluation of the characteristics of the material, taking account of steel embrittlement under the effect of irradiation; • evaluation of the loadings liable to initiate the flaw, in all reactor operating situations (normal or accident). The analysis approach to the risk of fast fracture consists in comparing the peak fault load with the mechanical strength of the irradiated material. Safety coefficients are considered at this stage. This approach is applied, considering both the existing flaws detected and a hypothetical flaw, corresponding to the largest crack that cannot be detected by the non-destructive tests performed and positioned at the location where the material is the most embrittled by irradiation. THE PRINCIPLES OF THE REACTOR VESSELS IN-SERVICE STRENGTH DEMONSTRATION 296 ASN Report on the state of nuclear safety and radiation protection in France in 2023 • 10 • The EDF Nuclear Power Plants
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