ASN Report 2020

2. Oversight of nuclear safety 2.1  Fuel 2.1.1 Changes to fuel and fuel management in the reactor In order to increase the availability and performance of the reactors in operation, EDF and the nuclear fuel manufacturers are developing improvements to be made to the fuels and to how they are used in the reactors. EDF has standardised its fuel management methods. ASN ensures that each change to fuel management undergoes a specific safety demonstration for the reactors concerned. Any change in the fuel or its management must first be examined by ASN and may not be implemented without its consent. As fuel behaviour is a key element in the safety of the core in a normal or accident operating situation, its reliability is crucial. Thus, the leaktightness of the cladding of the fuel rods, tens of thousands of which are present in each core and which constitute the first containment barrier, receives particularly close attention. In normal operation, leaktightness is monitored by EDF through permanent measurement of the activity of the radionuclides contained in the primary system. Any increase in this activity beyond predetermined thresholds is a sign of a loss of leaktightness in the fuel assemblies. During each shutdown, EDF must look for and identify the assemblies containing leaking rods, which must not then be reloaded. If the activity of the primary system becomes too high, the General Operating Rules (RGE) require shutdown of the reactor before the end of its normal cycle. ASN ensures that EDF looks for and analyses the causes of the loss of leaktightness observed, notably by examining the leaking rods in order to determine the origin of the failures and prevent them from reoccurring. The preventive and corrective measures may concern the design of the rods and assemblies, their manufacture or the reactor operating conditions. In addition, the conditions of fuel assembly handling, of core loading and unloading, as well as preventing the presence of foreign objects in the systems and pools are also covered by operating specifications, some of which contribute to the safety case and for which EDF’s compliance is spot-checked by ASN during inspections. ASN also carries out inspections to check the nature of EDF’s monitoring of its fuel suppliers. Finally, ASN periodically consults the Advisory Committee for Nuclear Reactors (GPR) concerning the lessons learned from fuel operating experience feedback. 2.1.2 Assessment of the condition of the fuel and its management in the reactor ASN considers that in 2020, EDF’s management of the integrity of the first barrier, that is the fuel rod cladding, was on the whole satisfactory for all the NPPs. The progress observed in 2019 with regard to the risk of foreign material entering the primary system, which could then damage the first containment barrier, continued in 2020. The number of reactors with one or more assemblies with cladding defects was similar to the previous year. ASN will be attentive to the results of the investigations carried out by EDF on the fuel assemblies concerned and to ensuring that lessons are learned in order to improve their design, manufacture and operation. In dealing with the obsolescence of the sipping machines in the fuel buildings, in which the assemblies are checked, ASN will also be attentive to the correct performance of all the maintenance operations performed on this equipment. This attention will be maintained until the deployment of new mobile sipping machines currently being designed. As in 2019, ASN observes that few events were reported during the fuel handling operations in 2020. ASN continues to pay particular attention to correct application of the improvement measures implemented on all the sites following the snagging of a fuel assembly during the Tricastin unloading operations in 2019. By comparison with 2019, ASN observes that in 2020, fewer reactors carried out a long power increase after refuelling, requiring a modification of their operating baseline requirements. The reactors concerned operated for a prolonged period of time at intermediate power, which increases the risk of rupture of the first barrier in certain accident situations. These extended power increase durations were, in the cases encountered in 2020, caused by incidents on certain secondary system equipment. ASN considers that EDF must ensure that its facilities are available, more specifically the secondary system, before carrying out the divergence and power increase transients. At unloading of Paluel NPP reactor 2 at the end of 2019, EDF observed the presence of a corrosion deposit on more than one third of the core fuel assemblies. This deposit was observed on a French NPP reactor for the first time and is due to the replacement of the Steam Generators during the previous outage. This operation led to a high concentration of corrosion products in the primary system. These corrosion products, which are usually neutralised and removed, were deposited on the core, notably owing to a first slow power increase after refuelling. Several analyses aimed at characterising the phenomenon were carried out in 2020 and will continue in 2021. With regard to the fabrication of fuel assemblies, ASN is maintaining its vigilance following the anomalies encountered in 2017 on MOX fuel (presence of large-sized plutonium enriched islands) which reoccurred in 2019, despite the steps taken in Orano’s Melox plant. EDF’s notification of a significant event regarding the rising neutron flux phenomenon at the bottom and top of the fissile column of MOX fuel assemblies led ASN to ask the licensee to adopt compensatory measures in 2018, pending the deployment of a change to the design of these assemblies. Finally, in 2020, a generic anomaly on the fuel assemblies designed by Westinghouse was brought to light. One of the grids of these assemblies experienced damage during operation, generating migrating objects. Pending additional technical data and the replacement of these grids by reinforced grids for the next assemblies, ASN asked EDF to perform additional tests on the reactors concerned, to ensure that the safety functions remain available. 2.2  Nuclear Pressure Equipment 2.2.1 Monitoring of the design and manufacturing conformity of Nuclear Pressure Equipment ASN assesses the regulatory compliance of the Nuclear Pressure Equipment (NPE) most important for safety, referred to as “level N1”, corresponding to the reactor pressure vessel, the SGs, the pressuriser, the primary circuit pumps, the piping, as well as the safety valves. These regulations are a guarantee of their safety. They are defined by a European Directive on NPE and are supplemented by requirements specific to NPE. 290 ASN Report on the state of nuclear safety and radiation protection in France in 2020 10 – THE EDF NUCLEAR POWER PLANTS