From their joint analysis of the files submitted by Areva NP, IRSN (the French technical support organisation), and the Nuclear Pressure Equipment Department (DEP) of ASN (the French nuclear safety authority) conclude that if the serviceability of the closure head and lower head of the Flamanville 3 reactor pressure vessel is demonstrated, in-service inspections must be implemented. At this stage, the feasibility of these inspections appears to have been acquired for the lower head. The same is not true for the closure head.

At the end of 2014, Areva NP discovered a manufacturing anomaly in the closure head and lower head of the Flamanville 3 reactor pressure vessel. This anomaly undermines certain mechanical characteristics of the steel of these components, in particular its toughness, i.e. its ability to withstand crack propagation in the event of a pre-existing flaw.

In 2015, Areva NP proposed an approach to demonstrate the sufficiency of the steel's toughnessfor the closure head and lower head of the pressure vessel. This approach was examined jointly by IRSN and the Nuclear Pressure Equipment Department (DEP) of ASN, and reviewed by the Advisory Committee for nuclear pressure equipment (GP ESPN) on 30 September 2015. ASN then took position and considered acceptable, in principle, the demonstration approach proposed by Areva NP.

At the end of 2016, Areva NP transmitted its analysis of the consequences of the anomaly of the closure head and lower head of the Flamanville EPR reactor pressure vessel. Areva NP concludes that the two components are serviceable.

This analysis was the subject of a joint ASN DEP - IRSN assessment, the conclusions of which were presented on 26 and 27 June 2017 to the GP ESPN. From their instruction, ASN DEP and IRSN conclude that if Areva NP has demonstrated the serviceability of the closure head and lower head of the pressure vessel of the EPR reactor in Flamanville, in-service inspections must be implemented to periodically check this equipment during the operation of the installation (expected operating life of 60 years).

At this stage, the feasibility of these inspections appears to have been acquired for the lower head. The same is not true for the closure head: failing to implement these inspections, ASN DEP and IRSN consider that the replacement of this closure head should be carried out a few years away.

More information on the assessment of the consequences of the anomaly

IRSN’s information report of 28 June 2017: Note on the assessment of the closure head and lower head of the Flamanville 3 EPR reactor pressure vessel

ASN DEP-IRSN joint report of 15 June 2017: Analysis of the consequences of the anomaly in the Flamanville EPR reactor pressure vessel head domes on their serviceability

More information on the analysis of the demonstration approach

ASN DEP-IRSN joint report of 15 September 2015: Analysis of the procedure proposed by AREVA to prove adequate toughness of the domes of the Flamanville 3 EPR reactor pressure vessel lower head and closure head

Opinion of the Advisory Committee for nuclear pressure equipment  (GP ESPN) of 30 September 2015: Opinion concerning the procedure proposed by AREVA to demonstrate the adequate toughness of the domes of the Flamanville 3 EPR reactor pressure vessel (RPV) bottom head and closure head

Position statement of ASN: Letter of 14 December 2015 from ASN to Areva

Position statement of ASN: Interim review of the approach to demonstrate the adequate toughness of the reactor pressure vessel upper and lower domes - 26 September 2016

On 30 November 2016, IRSN submitted to ASN its assessment of the risks of fracture in steam generators (SG’s) with abnormally high levels of carbon in the steel that makes up their channel head.

The local carbon levels measured in these channel heads are in the region of 0.4 % compared to the maximum expected value of 0.22%. This anomaly meant that a re-examination of the risks of fracture of those steam generators was required since the mechanical properties of the steel have been altered.

The assessment relates to the SG’s in 900 MWe reactors whose channel heads were manufactured by the company Japan Casting and Forging Corporation (JCFC).

The evaluation of the risks of failure in the SG’s consists of examining the risks of fracture initiation resulting from a known or postulated crack in the steel. Thus, IRSN evaluated in particular the extent of the cracks postulated by EDF, the loads that give rise to stresses likely to open the postulated cracks, i.e. the thermal shocks that can affect the SG’s under all reactor operating conditions, as well as the mechanical properties of the steel, taking account of the preliminary data provided by EDF for the steel showing 0.4 % carbon levels.

The evaluation conducted by IRSN has led it to conclude that there is no risk of fracture for the SG’s manufactured by JCFC and fitted in the 900 MWe reactors covered by the assessment (Bugey 4, Dampierre 3, Fessenheim 1, Gravelines 2 and 4, Saint-Laurent-des-Eaux B1, Tricastin 1, 2, 3 and 4), with the exception of three reactors (Bugey 4, Fessenheim 1 and Tricastin 4) subject to the recommendations of IRSN relating to limiting the extent of the potential thermal shocks and the results of the inspections requested by ASN. For the three aforementioned reactors, the study still needs to be finalised and submitted by EDF.

In order to reaffirm its assessment of the mechanical properties of the steels showing high carbon levels, IRSN has called on the assistance of its Belgian expert assessor counterpart, BEL-V, which is a member of the European ETSON network. ASN and IRSN also went to Japan to visit the manufacturer JCFC in order to better understand the cause of the excessive carbon levels observed.

IRSN assessment has been published on its website and presented to the press during a joint conference with ASN on 5 December 2016. It has been be presented to the High Committee for Transparency and Information on Nuclear Safety (HCTISN) at its meeting on 6 December 2016.

Moreover, in conjunction with the National Association of Local Information Commissions and Committees (ANCCLI) and ASN, a meeting has been organised for February 2017 to hold an exchange of views with civil society stakeholders as an extension to the technical dialogue launched with ANCCLI, the Flamanville Local Information Commission and ASN on the topic of similar anomalies affecting the reactor vessel at the EPR in Flamanville.

Download IRSN information notice of 5 December 2016: IRSN assessment of the safety of reactors equipped with steam generators whose channel heads contain an abnormally high level of carbon

In a new report, IRSN presents the main characteristics of passive safety systems and outlines the main difficulties associated with assessing the performance and reliability of passive safety systems for nuclear reactors, as well as priority research areas to be developed in order to overcome these difficulties.

Pressurized water reactors currently operating in France are equipped with active safety systems requiring a power source, such as an electrical power supply, and also include passive safety features (nuclear fission reaction control and shutdown rods, hydrogen recombiners, etc.).

Certain nuclear reactor designs currently under construction or development make more extensive use of passive safety systems in order to bring the reactor to a safe shutdown state and maintain this state for a long period of time without need for human intervention and with limited reliance on support functions.

IRSN considerations to date on passive safety systems have led to the identification of a number of intrinsic difficulties, particularly concerning the performance and reliability assessment of such systems.

Further research is required in order to properly assess the performance and reliability of passive safety systems to be implemented in new reactor designs. Initial considerations have already been identified as part of IRSN's scientific strategy, with emphasis on necessary understanding of physical phenomena influencing the operation of passive safety systems, necessary simulation capabilities for such phenomena, and necessary testing for validation of simulation software.

IRSN pursues this research within the framework of joint actions with foreign organisations so as to ensure fruitful exchanges and benefit from available knowledge.

Download IRSN report on passive safety systems for nuclear reactors (PDF)

In an article published on 8th June 2015, the French online journal Mediapart mentions “a confidential report” from IRSN about the safety valves on the European Pressurized Reactor (EPR) under construction at the Flamanville nuclear power plant, and another report by IRSN on the manufacturing defects in the lower and upper head of the reactor vessel.

IRSN examines the design of the EPR reactor for many years. The technical instruction conducted by IRSN is assessed on the basis of documents submitted by EDF: these documents lead to technical exchanges between IRSN, EDF, the designer AREVA and the French nuclear safety authority (ASN). These discussions are not public. At the end of its examination, IRSN transmits the conclusions of its expertise to ASN in the form of a “review”. After the decision of the French safety authority, IRSN’s review may be published on its website. As part of the instruction of the EPR reactor under construction in Flamanville, IRSN has already submitted about 150 reviews on many subjects (equipment, accident studies ...).

In 2014, IRSN began the review of the pressurizer safety valves design of the EPR: the pressurizer's role is to maintain a given pressure in the reactor coolant system, and is equipped with three safety valves in order to ensure protection against overpressure. The examination performed by IRSN aimed to ensure that these safety valves meet the requirements according to the high reliability expected from this equipment for the reactor coolant protection (opening pressure, closing pressure…).In this context, IRSN had technical meetings with EDF, and witnessed the qualification tests performed in France for the safety valves: these tests in particular revealed an unexpected behavior, including a failure to open at the expected pressure.

At a meeting in February 2015, IRSN presented to ASN the status of its examination, including the analysis of risks of failure of the safety valves and the results of ongoing qualification tests. The report mentioned by Mediapart is actually a “working document” (slides), prepared for the meeting, which reports IRSN questions at that time. The technical instruction of the pressurizer safety valves design of the EPR is still ongoing and still lead to technical exchanges with EDF. IRSN expects to transmit the conclusions of its examination to ASN during the summer of 2015. These conclusions may include recommendations for additional or corrective actions.

The article also mentions a report by IRSN on the manufacturing defects in the lower and upper head of the Flamanville EPR reactor vessel. This information comes from a first IRSN review transmitted to ASN in early April 2015. The instruction of this subject will continue on the basis of additional elements submitted by AREVA, elements which now require thorough analysis by IRSN, with the assistance of external experts if necessary.

Download this information note in PDF

As part of the third ten-year inspection of 1,300 MWe reactors, IRSN has published a summary (in French) of its assessment of actions taken by EDF regarding severe accidents, on the basis of which ASN has issued a position in letters addressed to EDF on November 25, 2013 and January 20, 2014.

Rather logically, in connection with the third ten-year inspection of these reactors and with respect to severe accidents, EDF is proposing to focus on improving radioactive materials containment and on reducing the probability of scenarios likely to lead to early or significant radioactive releases.

While finding the facility modifications proposed by EDF to be pertinent in principle, IRSN's assessment identified several potential avenues for improvement, in particular concerning EDF’s planned measures to eliminate any possibility of a steam explosion in the event of a reactor vessel breach following a core meltdown.

In the field of level 2 probabilistic safety assessments, used to assess the probability of occurrence of an undesirable event and its potential consequences in terms of radioactive releases to the environment, IRSN noted that EDF's initiative was generally consistent with the practices of other countries. However, the Institute also found that certain risks – such as hydrogen combustion in the space between the two superposed containment buildings (a specificity of these reactors) – were not taken into account and considered that additional studies were needed to give a more realistic grasp of certain environmental release risks. For the operator, the challenge is to demonstrate that its proposals help achieve the lowest level of risk possible, given the intrinsic technological characteristics of these reactors.

More information in French: IRSN's summary of assessment of actions taken by EDF regarding severe accidents

Created in 2006, the European Technical Safety Organisations Network ETSON [1] aims to contribute to the harmonization of nuclear safety in Europe and beyond. Today, the network publishes four technical guides to promote and disseminate best practices for the assessment of nuclear safety.

Since the launch of the EUROSAFE initiative in 1999, the French institute IRSN, the German GRS and the Belgian Bel V (formerly AVN) pursued the objective of advancing the harmonization of nuclear safety in Europe by comparing their methods to assess safety. In 2004, they published a "Guide for Safety Evaluation" with practical methods for performing common safety assessments.

Pursuing this approach, the European Technical Safety Organisations Network ETSON releases today four technical guides to promote and disseminate best practices for the assessment of nuclear safety at European and global level. The preparation of these guides relied on twelve thematic working groups in which experts have analyzed the different methods of evaluation of nuclear safety implemented in European countries.

A general document (Safety Assessment Guide) defines the scope of the methodology used in Europe during safety assessments, presents different requirements for the assessment of nuclear safety and also the technical elements required to complete an evaluation. It is supplemented by the first three thematic technical guides: Deterministic severe accidents analysis, Event review and precursor analysis, Human and organizational factors in nuclear facilities design and modification processes.

Each ETSON members will appropriate the harmonised methods described in the TSAGs, decline them in their countries for their own safety studies assessment and, then, share the experience feedback.

Download the ETSON guides (PDF files) on the ETSON website www.etson.eu​

• Safety Assessment Guide
• Technical Safety Assessment Guide : Event review and precursor analysis
• Technical Safety Asessment Guide : Human and organisational factors in nuclear facilities design and modification processes
• Technical Safety Assessment Guide : Deterministic severe accidents analysis

Note:

1- The ETSON network currently comprises eight members (IRSN - France, GRS - Germany, Bel V – Belgium, UJV - Czech Republic, VTT – Finland, LEI – Lithuania, VUJE – Slovakia, PSI - Switzerland) and 3 associate members (JNES – Japan, SSTC - Ukraine, SEC NRS – Russia)

Following the accident that occurred on the Fukushima Daiichi nuclear power plant on 11th March 2011, the French prime minister asked the French nuclear safety authority (ASN) to carry out an audit on the safety of French nuclear facilities.

On 5th May 2011, ASN required the French nuclear operators to perform Complementary Safety Assessments (CSAs) of their facilities. The CSAs evaluate the capacity of French nuclear facilities to withstand extreme situations beyond design basis assumptions. In 2011, these evaluations included the power reactors in operation or under construction, as well as certain nuclear facilities considered by ASN to be priority.

The operators submitted their CSAs reports to ASN on 15th September 2011. IRSN’s review and of these reports was transmitted to ASN and to the Nuclear Safety Advisory Committees on 4th November 2011.

Based on IRSN's critical assessment, the Nuclear Safety Advisory Committees met the 8th, 9th and 10th November 2011 to review these Complementary Safety Assessments and the relevance of the improvements proposed by the operators to enhance the safety of their facilities in the event of extreme situations (earthquake, flood, loss of electrical power supply, loss of heat sinks).

On 17th November 2011, during a conference jointly organised by ASN and IRSN, IRSN presented its analysis and conclusions to the press and made public its CSAs report. And on 3rd January 2012, ASN’s report on the CSAs was submitted to the Prime Minister and made public.

The review conducted by IRSN of the CSAs reports submitted by operators revealed three main observations:

1. A small number of nuclear facilities have compliance gaps in terms of requirements, which weaken their safety within the framework of events taken into account for their design. Corrective actions are already in progress and are to be accelerated.
2. Developments in knowledge must lead to some requirements being re-examined in advance. This is especially the case with the integration of earthquakes, for which knowledge has advanced massively over the last years.
3. The issue of the behaviour of nuclear facilities in the event of a conceivable extreme natural disaster is raised since, in the current situation, this could have unavoidable consequences, ending up in core meltdown and radioactive release.

To confront these exceptional - but nonetheless conceivable - scenarios, IRSN recommends adopting an additional safety requirement level, entitled “hardened safety core”, which would guarantee that the vital basic functions of nuclear facilities are sustained over several days, thus enabling off-site resources to intervene.

More information:

IRSN information notice of 17th November 2011: IRSN analysis and conclusions following the expert review of the Complementary Safety Assessments submitted to ASN by the operators at the request of the Prime Minister after the Fukushima accident

Technical summary of IRSN review of CSAs report: Post-Fukushima Complementary Safety Assessments: behaviour of French nuclear facilities in the event of extreme situations and relevance of the proposed improvements