ASN Report 2017

339 ASN report on the state of nuclear safety and radiation protection in France in 2017 Chapter 12  - EDF Nuclear Power Plants 2.2.3 Monitoring the operation of pressure equipment The reactor Main Primary and Secondary Systems (CPP and CS) operate at high temperature and high pressure and contribute to the containment of the radioactive substances, to cooling and to controlling reactivity. The monitoring of the operation of these systems is regulated by the Order of 10th November 1999 relative to the monitoring of the operation of the Main Primary and the Main Secondary Systems of nuclear pressurised water reactors mentioned in point 3.6 of chapter 3. These systems are thus monitored and periodically maintained by EDF. This monitoring is itself checked by ASN. These systems are subject to periodic re-qualification every ten years, comprising a complete inspection of the systems involving non-destructive examinations, pressurised hydrotesting and verification of the good condition and proper operation of the over-pressure protection accessories. Nickel-based alloy areas Several parts of pressurised water reactors are made with nickel-based alloy. The use of this type of alloy is justified by its resistance to generalised or pitting corrosion. However, in reactor operating conditions, one of the alloys adopted, Inconel 600, proved to be susceptible to stress corrosion. This particular phenomenon occurs when there are high levels of mechanical stress. It can lead to the appearance of cracks, as observed on the SG tubes in the early 1980s or, more recently in 2011, on a vessel bottom head penetration in Gravelines reactor 1 and in 2016 on a vessel bottom penetration in Cattenom reactor 3. These cracks require that the licensee repair the zones concerned or isolate the part of the system concerned. At the request of ASN, EDF adopted an overall monitoring and maintenance approach for the areas concerned. Several parts of the main primary system made of Inconel 600 alloy are thus subject to special monitoring. For each of them, the in-service monitoring programme, defined and updated annually by the licensee, is submitted to ASN, which ensures that the performance and frequency of the checks carried out by EDF are satisfactory and able to detect the deteriorations in question. The strength of reactor pressure vessels The reactor pressure vessel is an essential component of a pressurised water reactor and contains the reactor core and its instrumentation. For the 900 MWe reactors, the vessel is 14 m high, 4 m in diameter, 20 cm thick and weighs 330 tonnes. For the EPR, currently under construction, the vessel is 15 m high, 4.90 m in diameter, 25 cm thick and weighs 510 tonnes. 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 monitoring of the state of the reactor pressure vessel is essential for two reasons: ཛྷ ཛྷ The vessel is a component for which replacement is not envisaged, owing to both technical feasibility and cost. ཛྷ ཛྷ Monitoring also 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. In operation, the vessel’s metal slowly becomes brittle under the effect of the neutrons from the fission reaction in the core. This embrittlement makes the vessel particularly sensitive to pressurised thermal shocks or to sudden pressure surges when cold. This susceptibility is also aggravated when technological defects are present, which is the case for some of the reactor vessels that have manufacturing defects under their stainless steel liner. FOCUS Technical anomalies linked to carbon segregations in certain Steam Generator (SG) channel heads Following the detection of the Flamanville EPR reactor pressure vessel anomaly (see point 2.11.2), EDF informed ASN that the channel heads of SGs fitted to 18 reactors, manufactured by the Creusot Forge plant and by Japan Casting and Forging Corporation (JCFC), were also concerned by the carbon segregation problem. All of the checks carried out by EDF, in particular those specified by ASN on 18th October 2016, required the shutdown of five reactors and were completed at the beginning of 2017. They enabled EDF to demonstrate that there was no risk of fracture of the channel heads of the 46 SGs concerned. The conservative hypotheses adopted by EDF in its fracture strength calculations, led it to modify the operating conditions of the 18 reactors concerned. These modifications are implemented pending confirmation of the design hypotheses which should be provided by a vast test programme currently being carried out on the channel heads representative of the components in service on the French reactors. FUNDAMENTALS The principles of demonstrating the resistance of reactor vessels The regulations in force require in particular that the licensee: ཛྷ ཛྷ identify the operating situations with an impact on the equipment; ཛྷ ཛྷ take measures to understand the effect of ageing on the properties of the materials; ཛྷ ཛྷ take steps enabling it to ensure sufficiently early detection of defects prejudicial to the integrity of the structure; ཛྷ ཛྷ eliminate all cracks detected or, if this is impossible, provide appropriate specific justification for retaining such a type of defect as-is.