EPR Reactor: Information Letter n°24: Monitoring the construction of the Flamanville-3 EPR reactor: the notable actions of ASN in 2022 and early 2023

Published on 28/08/2023 at 14:31

Information notice

In 2022 and in the first half of 2023, ASN continued with its monitoring of the construction of the Flamanville EPR reactor. It was particularly vigilant with regard to the treatment of the deviations detected on the main primary and secondary system welds and to the correct operation of the filtration function of the safety injection system (SIS). It continued to examine the installation start-up tests and the correct operation of the primary system protection valves. In addition, ASN launched a programme in 2022 to check the completion of the installation and is continuing its assessment of the equipment quality review carried out by EDF and of preparations for operation. In a resolution of 5 May 2023, ASN authorised use of the Flamanville EPR reactor vessel head until the reactor’s first refuelling outage. In addition, from 5 June to 15 September 2023, ASN makes the file associated with the EPR reactor commissioning application submitted by EDF available to the public. During this period, the public are asked to submit their comments on the ASN website.

Anomalies detected in the main secondary system welds

In 2017, EDF informed ASN of anomalies that had occurred during welding of the main steam letdown pipes (VVP system) of the Flamanville EPR reactor. These pipes are covered by "break preclusion" criteria, which imply tightened design, manufacturing and in-service monitoring requirements such that it can be considered, with a high degree of confidence, that a break of these pipes is extremely improbable. ASN's inspections revealed a lack of proficiency in the welding operations performed on the VVP pipes and deficiencies in EDF's monitoring of its outside contractors (see EPR Newsletter No. 21).

In response to the conclusions of the investigations carried out by ASN on the anomalies affecting the pipe welds on the main secondary systems presented to the advisory committee for nuclear pressure equipment (GP ESPN) in April and June 2019, EDF has since 2020 deployed its weld treatment strategy. The treatment strategy adopted for the containment penetration welds consists in complete repair of the welds from the inside, using a robot developed by the Westinghouse company. For the other welds on the main steam lines, the treatment strategy adopted is also to repair the welds to ensure conformity.

All of these operations are currently being completed and hydrotesting – part of the final check on correct performance of these weld repair operations – has also started and will continue until August 2023.

Through EDF, Framatome and Bureau Veritas inspection campaigns, ASN ensures that all of these activities are checked and considers that the rigorousness employed by the various stakeholders ensures that a high level of weld quality is achieved.

Anomaly on three main primary system nozzles

On 3 March 2021, EDF notified ASN of a significant event concerning a design anomaly on three nozzles of the main primary system of the reactor, the rupture of which is not covered by the reactor’s safety case because their diameter is too large.

The solution proposed by EDF is to install a retaining collar around each nozzle concerned. In the event of rupture of the nozzle set-in weld, this collar would limit the size of the resulting break. The consequences of this break would then be covered by the reactor’s existing safety studies.

ASN considered that the solution proposed by EDF is in principle acceptable on condition that EDF provides a demonstration of the effectiveness of the arrangement, and the required data concerning the design, manufacturing and operating requirements applicable to the collars and the quality of the set-in welds around which these collars would be installed. These data are currently being examined by ASN.

This solution is subject to an in-service inspection of these three welds based on the principle of defence in depth, in order to provide a lasting guarantee that there are no defects liable to compromise the mechanical strength of these welds.

Inspection de revue du chantier de l'EPR à Flamanville - mai 2023 - @EDF Photothèque

In 2022, ASN carried out an on-site inspection regarding the quality control of the set-in welds. In the light of this examination, ASN considers that the checks defined and performed on the welds are such as to guarantee the quality of the production of these welds.

ASN also carried out another on-site inspection in 2022 regarding the manufacture and installation of the retaining collars. Based on this examination, ASN considers that the preparation for and performance of these operations, along with the corresponding surveillance by EDF, are satisfactory.

Anomaly regarding the correct operation of thefiltration function of the safety injection system (SIS) in recirculation

The safety injection system (SIS) can operate in “recirculation”: this function consists in filtering the water that has accumulated in the bottom of the reactor building in an accident situation and reinjecting it into the reactor core to help cool the fuel assemblies.

Tests performed by EDF have shown that if too many fibres pass through the filter, this can compromise the capacity to cool the fuel assemblies. In order to resolve this problem, EDF decided to reduce the size of the filter mesh on the filters. The initial results of tests with the new mesh size showed a significant reduction in the quantity of fibres passing through the filters. However, following new tests, EDF identified the fact that the glue contained in certain tapes and sleeves, providing fire protection on certain cables and openings in the reactor building represented a significant filter clogging threat. Given the small quantity of glue needed to clog the filters, EDF stated that it would remove the fire protection systems containing this glue.  

These actions imply that, for the Flamanville EPR reactor commissioning authorisation application, EDF must revise the fire risk case and the effectiveness of the filtration function. ASN’s examination of these dossiers is currently being finalised.

In 2022, ASN carried out an on-site inspection to monitor the glue removal work. In the light of this examination, ASN considered that an effective glue removal method had been developed and then implemented, but that improvements were still needed, notably with regard to managing the cleanness of these worksites. The effectiveness of the improvements proposed by EDF was checked during an on-site inspection in 2023 on the completion of the SIS system.

Examination of correct operation of the primary system protection valves

The three valves, called the pressuriser safety valves, installed on independent lines and located at the top of the primary system pressuriser:

  • protect the primary system from over-pressure;
  • allow safe depressurisation of the primary system until safety injection system connection conditions are reached;
  • enable heat to be removed from the core, for certain types of breaks on this system.

The pressuriser safety valves shall also be capable of closing safely after being opened, in order to avoid a loss of coolant accident. Finally, they are part of the “hardened safety core” of safety measures: the ability of the pressuriser safety valves to perform their functions reliably is thus particularly important for the safety of the Flamanville EPR reactor.

As part of its examination of the Flamanville EPR reactor commissioning authorisation application, ASN is currently continuing with its review of the detailed design of the pressuriser safety valves, with the assistance of IRSN. Recommendations and requests for firm undertakings were issued and concern the inspection of the valve components, preventive maintenance, notably with the provisions for cleaning regarding all maintenance operations, as well as particular operating rules in the event of an anomaly.

Continuation of investigations regarding other subjects concerning nuclear pressure equipment

The continued investigations regarding subjects concerning the nuclear pressure equipment were presented at two sessions of the Advisory Committee for nuclear pressure equipment (GP ESPN) on 30 November 2022 and on 20 and 21 June 2023.

In addition to the subjects previously covered concerning repairs to the welds on the main secondary systems, treatment of the anomaly regarding the three main primary system welds and the primary system protection valves, the progress of the investigations presented to the Advisory Committee concerned:

  • addressing the operating experience feedback from the stress corrosion damage detected on the EDF NPP fleet and all the local thermohydraulic phenomena which could lead to fatigue or corrosion damage;
  • post-weld heat treatment;
  • generic defects on the “NOREM” material valve coatings;
  • analysis of the impact of irregularities detected at certain suppliers.

Even if all these investigations have not yet been completed, the measures needed to do so have been clearly identified and ASN will send Framatome and EDF requests for formal undertakings and action plans which will enable them to be completed.

Check on start-up tests and conservation of equipment

Examination of the results of the start-up tests

The start-up tests are designed to guarantee the conformity of the installation with the safety case presented in the commissioning authorisation application file.

Two months before the envisaged date for commissioning of the EPR, EDF is required to communicate the results of the tests performed, in accordance with the binding requirement [INB167-50-1] of resolution 2013-DC-0347 of 7 May 2013. These results include data enabling the validity of the results and the representativeness of the test conditions to be assessed, along with:

  • the list of start-up tests still to be performed before commissioning;
  • the list of start-up tests already performed, the results of which would not allow commissioning to take place (along with the necessary remedial measures).

ASN and EDF agreed to bring forward the examination of these results on the basis of the tests already performed. Thus, in June 2020, EDF sent ASN a preliminary version of the results of the start-up tests, which is regularly updated.

Jointly with IRSN, ASN drew up a two-part investigation programme:

  • The first part of the investigation consists of an examination of the results of the start-up tests submitted in June 2020, along with the corresponding updates. This examination, which was started at the end of 2020, will continue until commissioning of the installation, in order to incorporate the updates arising from the progress of the start-up tests programme;
  • the second part of this investigation consists of an examination of a number of particular technical points, most of which are anomalies detected during the start-up tests and for which an in-depth examination of how they are addressed by EDF would appear to be necessary. A specific file is submitted by EDF to ASN for each of the points identified and each file undergoes a specific examination. The vast majority of the particular technical points have been investigated and have now been cleared.

These examinations are required prior to ASN’s issue of the installation commissioning authorisation.

At the same time, ASN is continuing to monitor the start-up tests in progress. ASN notably defined the specific procedures for monitoring the overall tests phase scheduled prior to loading.

Continued validity of start-up test results and conservation of equipment

Given the current delays on the Flamanville EPR reactor construction site, numerous start-up tests were performed well before commissioning of the installation. This situation has led EDF to shut down numerous equipment items pending the end of the worksite. The sometimes lengthy period between the tests and commissioning requires that the licensee adopt a strategy to demonstrate that the results of the start-up tests already performed are still valid.

EDF presented a range of processes helping to guarantee the continued validity of the test results. ASN checks the performance of these processes by means of targeted inspections during the course of the start-up tests.

At the same time, EDF presented a strategy aiming to guarantee the conditions for the long-term conservation of the equipment installed on the site. This strategy was evaluated by ASN and its effective implementation was checked by means of specific inspections. In the second half of 2022, an inspection was carried out on the site and concluded that the conservation measures implemented by EDF were satisfactory. ASN will pay particular attention to the inspection programme implemented by EDF when equipment is removed from conservation for start-up of the reactor.

Completion of the installation

In 2022, ASN began a programme to check the completion of the installation, with four inspections on this topic in 2022 and early 2023, along with an in-depth inspection carried out in May 2023. During these checks, ASN found that numerous activities still had to be carried out to finalise preparation of the installations (treatment of anomalies, certain start-up tests, several equipment modifications along with finishing work).

ASN asked EDF to present it with a periodic progress report regarding completion of the installations. ASN found that EDF had implemented a dedicated organisation and taken appropriate corrective actions in response to its requests. Nonetheless, it draws EDF’s attention to the fact that considerable work still needed to be done prior to commissioning of the reactor, in order to demonstrate the conformity of the installation.

Performance of an equipment quality review

The numerous anomalies identified during the production of the main secondary system welds, along with the anomalies discovered during ASN inspections or dealt with as significant events since the start of the project, led ASN in 2018 to determine that EDF's monitoring of its contractors was deficient. This finding led ASN to ask EDF to perform a quality review of the Flamanville EPR reactor equipment.

EDF conducted additional inspections as of 2018 through a review programme, which was amended in late 2018 to take account of ASN's request to extend the review to cover a wider range of equipment and subcontractors. In response to ASN’s additional requests made during the inspections of 27 June 2019, 21 July 2021 and 18 October 2021, EDF changed its programme by adding documentation or equipment inspections. The progress of this programme is periodically presented to ASN and a final report should be transmitted to ASN in the summer of 2023.

These additional inspections include the performance of new interpretations of main primary system weld radiographs. These activities were inspected on 21 July 2021, during which the inspectors noted the rigorousness and professionalism of the staff in charge of these inspections, which thus helps provide robust guarantees as to the quality of the production of these welds.

These additional checks also include documentary and in-situ checks on the main safety-important equipment. These activities underwent an inspection on 18 October 2021, which concluded that the checks performed were appropriate, although a few improvements were needed and additional checks were required.

The equipment quality review also includes an in-depth examination by EDF of the conformity of the main primary piping with the “break preclusion” baseline requirements. The guarantee of the quality of production of this equipment, required in accordance with this baseline, notably includes the additional inspections, the results of which proved to be conforming.

Continued examination of the commissioning authorisation application file  

A new commissioning authorisation application was filed by EDF on 4 June 2021. This application takes the place of the initial application of 19 March 2015 and comprises a complete update of the file appended to the initial application. This file thus contains the safety analysis report, the general operating rules, a waste management study, a decommissioning plan, an on-site emergency plan and an update of the initial impact assessment with a view to performing an environmental assessment of reactor commissioning. The EDF file is available to the public from 5 June to 15 September 2023.

ASN’s examination of the commissioning authorisation application is currently ongoing, in particular regarding aspects relating to the general operating rules, instrumentation & control, the operation of the primary system protection valves and the correct operation of the filtration function of the safety injection system (SIS) in recirculation, along with the operating experience feedback from the EPR reactors in Taishan (China).

The reactor commissioning authorisation can only be issued on condition that the overall examination of the file underpinning the application is completed. In addition, and on this basis, the installation’s conformity with the applicable technical and organisational baseline requirements shall be established, notably by examining the results of the requalification tests and the reactor start-up tests.

International operating experience feedback follow-up

ASN participates in several international bodies and regularly organises meetings with the nuclear regulators concerned by the various EPR projects around the world, which enables it to collect feedback on the construction, tests and operation of this reactor.

ASN closely examines the operating experience feedback from the EPR reactors in operation and regularly checks that EDF duly integrates international operating experience feedback.

Preparedness of the teams who will be operating the reactor

The ASN inspectors examined the definition and gradual implementation of the various organisations planned for the operations phase, management of staff skills as well as production of the future documentation, in order to check that the teams will be ready to fully assume their responsibilities.

In 2022, ASN carried out six inspections on the preparation for operation of the reactor, with respect to a number of topics (preparedness for emergency situations, fire risk and earthquake risk control, organisation of worker radiation protection, control of environmental risks and detrimental effects, commissioning of nuclear pressure equipment). In 2023, 15 ASN inspectors and 11 IRSN experts took part in a one-week in-depth inspection devoted to this subject and to completion of the installation. These inspections showed that the organisational measures were implemented satisfactorily but that numerous actions (operational application of the requirements, training and familiarisation of the teams with the operating practices, completion of the installation of certain systems) are still required before commissioning of the reactor.

To find out more:

Published on 22/10/2021 at 09:59

Design flaw on nozzles on the Flamanville EPR: ASN has no objection to the solution proposed

ASN considers that the solution proposed by EDF is in principle acceptable. ASN will issue a final ruling once EDF has provided the required data concerning the demonstration of the effectiveness of the arrangement, the design, manufacturing and operating requirements applicable to the collars and the quality of the set-in welds around which these collars would be installed.

Published on 17/05/2023


Décision n° 2023-DC-0760 de l’ASN du 16 mai 2023

Décision n° 2023-DC-0760 de l’Autorité de sûreté nucléaire du 16 mai 2023 modifiant la décision n° 2018-DC-0643 de l’Autorité de sûreté nucléaire du 9 octobre 2018 autorisant la mise en service et l’utilisation de la cuve du réacteur EPR de la centrale nucléaire de Flamanville (INB n° 167) ;

Published on 31/05/2023 at 14:44

ASN authorises use of the Flamanville EPR reactor vessel closure head

The commissioning and operation of the Flamanville EPR reactor pressure vessel were authorised by an ASN resolution in 2018. This resolution took into account a manufacturing anomaly in the lower dome and the vessel closure head and limited the use of the vessel closure head to 31 December 2024.


Reference texts

Inspection follow-up letters


Date of last update : 05/09/2023