ASN Report 2018

In operation, the vessel’s metal gradually 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. ASN regularly examines the evidence to substantiate the in-service resistance of the vessels transmitted by EDF, to ensure that it is sufficiently conservative. In mid-2016, EDF more specifically sent ASN a file substantiating the in-service resistance of the 900 MWe reactor vessels after 40 years of operation and this is currently being reviewed by ASN. A first part of this file was submitted to the GPESPN for its opinion on 20 November 2018 and the second part should also be submitted during the course of 2019. • Maintenance and replacement of steam generators The SGs comprise two parts, one of which is a part of the primary system and the other a part of the secondary system. The integrity of the main steam generator components, more specifically the tubes making up the tube bundle, is monitored. This is because any damage to the tube bundle (corrosion, wear, cracking, etc.) can lead to a leak from the primary system to the secondary system. A steam generator tube rupture would lead to bypassing of the reactor containment, which is the third containment barrier. Steam generators are therefore the subject of a special in-service monitoring programme, established by EDF, reviewed periodically and examined by ASN. After inspection, tubes that are too badly damaged are plugged to remove them from service. • Clogging of the tubes and internals of the secondary part of the steam generators Over time, the SGs tend to become clogged with corrosion products from the secondary system exchangers. This leads to a build-up of soft or hard sludge at the bottom of the SGs, fouling of the tube walls and clogging of the tube bundle tube support plates. The corrosion products form a layer of magnetite on the surface of the internals. On the tubes, the layer of deposits (fouling) reduces the heat exchange capacity. In 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 steam generator. To prevent or mitigate the fouling effects described above, various solutions can be used to minimise metal deposits: preventive chemical cleaning or remedial mechanical cleaning (using hydraulic jets), material replacement (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 licensing for the discharge of some of the products used. Some chemical cleaning processes are still being tested to demonstrate that the chemical products utilised are harmless. In particular, the identification of a corrosion risk on reactors which had undergone such cleaning in 2016 led ASN to request the implementation of specific maintenance measures, more particularly non-destructive examinations of the zones potentially exposed to this risk. • Replacement of steam generators Since the 1990s, EDF has been running a programme to replace the steam generators with the most heavily degraded tube bundles, with priority being given to those made from Inconel 600 without heat treatment (600 MA) and then those made from Inconel 600 with heat treatment (600 TT). The replacement campaign for SGs with a tube bundle made of 600 MA (26 reactors) was completed in 2015 on reactor 3 at the Le Blayais NPP. It is being continued with the replacement of SGs with 600 TT tube bundles (26 reactors). In 2018, replacement of the SGs of the Paluel NPP reactor 2 was completed. • Monitoring methods applied to main primary and secondary system pressure equipment The Order of 10 November 1999 specifies that the non- destructive testing processes used for in-service monitoring of the pressure equipment of the main primary and secondary systems of nuclear power reactors must be qualified before they are used for the first time. This qualification is granted by a body comprising experts from both inside and outside EDF whose expertise and independence are verified by the French accreditation committee. Qualification is a means of guaranteeing that the non- destructive testing process actually achieves the anticipated level of performance as described in specifications drawn up beforehand. Owing to the radiological risks associated with radiographic inspection, ultrasound inspections are preferred, provided that they offer equivalent inspection performance. To date, more than 90 non-destructive test processes have been qualified for the in-service inspection programmes. New development and qualification processes to meet new needs are currently being drawn up. With regard to the Flamanville EPR, virtually all of the test processes for in-service monitoring of the pressure equipment of the main primary and secondary systems were qualified ahead of the Pre-Service Inspection (VCI) of the main primary system and the main secondary systems, corresponding to more than 30 qualified processes specific to the EPR.  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. ASN report on the state of nuclear safety and radiation protection in France in 2018  283 10 – EDF NUCLEAR POWER PLANTS 10