The answers to all your questions about the FA3 RPV

  • 1. What is a nuclear reactor pressure vessel (RPV)?

    The nuclear reactor pressure vessel (RPV) is the piece of equipment within which water from the primary cooling system is heated up in contact with nuclear fuel. The water from the primary system then circulates towards the steam generators to transfer its heat to the water in secondary system, transforming the latter into steam. The steam created in this way is then used to drive a turbine, connected to an alternator to generate electricity.

    The EPR reactor, just like the other reactors of the fleet in operation in France, is a “pressurized water reactor”. In this type of reactor, the water in the primary system does not boil; it is kept in liquid condition, despite the high temperature (~330°C) thanks to the high pressure maintained (~150 bars).

    The reactor pressure vessel is designed accordingly, using steel of 25 cm thickness in its cylindrical section, and weighing a total of around 520 tonnes including its closure head.

  • 2. How are the parts of the RPV manufactured?

    The RPV is made up of several pieces of lightly alloyed steel that are forged, machined and welded together.

    A reactor pressure vessel for an EPR consists of seven main parts:

    • The upper dome
    • The vessel head flange
    • The nozzle shell
    • The two core shells
    • The vessel bottom head flange
    • The lower dome

    Each part is manufactured from a large ingot, weighing from 157 tonnes (for the lower and upper domes) to around 490 tonnes for the nozzle shell.

    These ingots are then forged to the desired shape, before undergoing several heat treatments. Once the heat treatments are complete, the parts are machined and welded together. The entire surface of the RPV is cladded with a 7.5 mm thick double-layer of stainless steel to ensure it is protected against corrosion.

    Throughout the manufacturing process, the materials are subjected to numerous tests. The parts are inspected in their entirety using ultrasound inspection techniques. Samples are taken for testing to allow their mechanical characteristics to be validated. All of the welds joining the parts together are also inspected. Lastly, a final hydrostatic pressure test is carried out in factory to test the capacity of the RPV to withstand a pressure well in excess of the operating pressure.

  • 3. What are the safety challenges associated to the RPV?

    Like all equipment in the primary cooling system, the RPV has to withstand conditions of high pressure and temperature. Together, all these items of equipment form the enclosure which contains the water of the primary cooling system, in other words, the water which has been in contact with the nuclear fuel, and which is likely to carry radioactive elements. The primary system constitutes the second barrier that ensures that the radioactivity remains isolated from the environment under all situations. The first barrier is formed by the sealed cladding into which the fuel pellets are inserted, with the third barrier being formed by the containment buidling, which is a double enclosure in the case of the EPR. Despite the presence of multiple safety barriers, everything possible is done to exclude the possibility of any leak from equipment in the primary system. Utmost importance is in particular attached to demonstrating that no fracture of the RPV is possible.

    The phenomenon to which particularly close attention is paid is that of “fast fracture”. When the water of the primary circuit undergoes are rapid changes of temperature (in the shutdown and restart phases, or possibly in accidental conditions which are taken into consideration to assess the robustness of the facility), the effects of thermal contraction/expansion on a part of this thickness generate high stress levels within the steel. Would a crack be present on the surface of the steel, and would the steel at this location not to be sufficiently strong, the crack could propagate and lead to fracture of the part.

    Thus, during design and manufacturing, checks are carried out to ensure that:

    • the parts of the RPV are exempt of cracks;
    • the conditions to which the RPV will be subjected, under all possible situations, will not generate load levels that could exceed the capacities of the steel;
    • the resistance of the steel to crack propagation, known as toughness to scientists, is sufficient.
  • 4. What do we mean by the toughness of the steel?

    The mechanical property which characterizes the capacity of the steel to resist crack propagation is known as “toughness”. At very low temperature, toughness is low: in this zone, the quantity of energy necessary to propagate a crack which may pre-exist is lower than at higher temperatures. The steel in this temperature zone is said to be “fragile”. When the temperature increases, toughness increases, and even in the presence of a crack, the part when subjected to a load will rather have a tendency to become distorted and a significantly larger quantity of energy will be necessary for the steel to fracture. The steel in this temperature zone is said to be “ductile”.

    It is thus important to check that, at whatever temperatures the steel may be subjected to, the toughness remains sufficient to exclude any risk of fracture in both the ductile zone, and in the fragile zone, in the event that there would be a pre-existing flaw.

  • 5. How does carbon affect the mechanical characteristics of the steel?

    There is a quite clear boundary zone between the fragile and ductile zones, or, in other words, a transition temperature above which the steel shifts from behaving in a fragile way to behaving in a ductile way (see question 4 referring to toughness).

    The presence of a high level of carbon tends to lead to an increase in this transition temperature. Checks should therefore be carried out to ensure that, under whatever operating conditions, the margin with regard to fracture remains sufficient.

  • 6. What is carbon segregation?

    Carbon segregation is a natural phenomenon which is well known to metallurgists; when large ingots of steel are cast, variations in the composition of the material appear upon cooling. The solidification takes places progressively at the core of the ingot. This crystallization occurs at different speeds depending on the chemical compounds, and certain compounds, including carbon, migrate to the core of the part as cooling occurs.

    In general, higher concentrations of carbon will be found in the areas of the part that cooled and solidified last, i.e. at the center and in the upper parts of the ingots, while the bottom of the ingot will display a carbon concentration that will rather be lower than the average for the part.

    This known phenomenon is taken into account when manufacturing parts.

  • 7. What do the regulations require be done to check the adequate strength of the RPV?

    In order to justify the adequate strength of pressure retaining equipment, French regulations in particular require that the mechanical properties of different parts be checked.

    The applicable regulations were updated in 2005, superseding an order dating from 1974 with the "ESPN" (Equipements Sous Pression Nucléaires = Nuclear Pressure Equipment) order.

    The principle applied before and after 2005 is that of technical qualification of the product: the compliance of the product is guaranteed by sticking to a fixed manufacturing program. The manufacturer is thus required to document the manufacturing process precisely, and to conduct tests on sacrificial parts to check their mechanical properties. Once the guarantee has been obtained that the manufacturing process will systematically produce compliant parts, it is possible to move to a "repeat" manufacturing process, where the manufacturer is required to justify that qualified manufacturing program is being followed, and to carry out confirmation tests in zones of predefined parts.

    Practically, prior to 2005, the implementation of the RCC-M industrial code (Règles de Conception et Construction Mécanique = Design and Construction Rules for Mechanical Components) was sufficient to meet the requirements of the 1974 order. It was in particular required to check the mechanical properties:

    • at points of the part that could be subjected to high stresses;
    • taking into account the conditions of use.

    After 2005, and as specified by the ASN in 2011, the requirements changed: the mechanical properties are defined in the administrative order and must be checked at all points of the component.

    In particular, there is a requirement to comply with a minimum impact strength value (bending rupture energy value) for the steel. This mechanical property quantifies the capacity of the steel to withstand an impact. Impact strength was selected as the regulatory measure due to it being a measure which is conservative from the point of view of safety and one that is industrially accessible.

    In 2006 and 2007 when the domes for the RPV of the Flamanville 3 EPR were cast, the procedures for application of the ESPN nuclear pressure equipment regulations were still in the process of being clarified and it was the RCC-M technical qualification process which was applied.

  • 8. What is the nature of the anomalies encountered on the Flamanville 3 RPV and how were they detected?

    Subsequent to the clarification at the end of 2011 of the technical requirements relating to forgings under the ESPN order, in 2012, AREVA NP launched a process to retroactively check the compliance of parts for the Flamanville 3 reactor with the new ESPN regulations.

    For the domes of the Flamanville 3 RPV, AREVA NP proposed to use a similar RPV dome, initially envisaged for an EPR reactor project in the USA, and to carry out the mechanical properties tests on this part.

    After technical exchanges with the ASN on the way in which the tests should be conducted, the tests were carried out in the summer of 2014 and showed impact strength results inferior to the values required by the ESPN order. The subsequent examination showedthat these mechanical properties had been affected by a higher level of carbon segregation than estimated on the exterior of the part.

    The ASN thus concluded that "the technical qualification file presented by AREVA for the Flamanville 3 RPV lower head and closure head domes shows that the risk of heterogeneity due to residual positive segregations has been insufficiently assessed and its consequences poorly quantified". To document the consequences on the actual characteristics of the steel, and in particular on its toughness, AREVA NP and EDF work together in 2015 on an additional test program based on a set of experiments, which was submitted to the ASN for approval in September 2015.

  • 9. What does the justification file contain and what do the test programs proposed by AREVA NP and EDF in 2015 involve?

    In line with research into the phenomenon of fast fracture, the file put forward by AREVA NP to justify the adequate strength of the lower and upper RPV domes was structured in the following way:

    • A first section consisting of an in-depth analysis of manufacturing processes, as well as a series of additional inspections to specify the types and sizes of defects which could potentially be present on the parts. This made it possible to ensure the absence of any  critical defects, in other words cracks of such a size and shape that they might propagate under high stress loads.
    • A second section providing a detailed presentation of the tests to be carried out to measure the physical characteristics of the steels. As these are tests of a destructive nature (the parts are tested until they fracture to measure their ultimate strength), they are carried out on sacrificial parts, initially intended for other projects.
    • A third section justifying the representativeness of these tests for the domes of the Flamanville 3 RPV. AREVA NP was in this way able to demonstrate that the sacrificed parts were indeed representative of the parts for Flamanville 3 (same steel, same manufacturing process, same chemical characteristics in particular).
    • A fourth section with the purpose of comparing the mechanical characteristics measured to the loading conditions resulting from the fast fracture calculations: all normal operation situations, as well as incident and accident situations, were taken into account. This made it possible to confirm that the mechanical properties of the steel from which the domes were made satisfied the necessary mechanical characteristics.

    Before being implemented, this program was validated by the ASN in September 2015, and a supplement to this test program was presented and validated by the ASN in June 2016.

  • 10. Where do the parts which were used for the test program come from?

    Three sacrificial parts weighing 150 tonnes each were used to carry out the test program, and were selected from parts manufactured in an identical way as those for the Flamanville RPV.

    To check the characteristics of the upper domes, a dome that was initially envisaged for the construction of the Hinkley Point EPR in the UK was used, along with a second dome, cast and forged for an EPR project in the USA.

    For the lower dome study, a lower dome intended for an EPR project in the USA was used.

  • 11. Why were these anomalies on the RPV not detected earlier?

    Converging on the scope of the demonstrations and interpreting the requirements of the regulations was a lengthy process and one that involved numerous technical debates. While the ESPN order was first published in 2006, it was only in November 2011 that the requirements for forgings were made clear.

    Having already gone through the process for the technical qualification of the parts under the previous regulations (RCC-M), and with the benefit of the first positive technical elements from the first tests, AREVA NP did not expect to run into any specific difficulties during the 2011-2014 period related to the justification tests conducted a posteriori on the different parts for Flamanville 3.

  • 12. Is there also a risk of encountering other anomalies of this type on other parts, or on other NPPs?

    In order to ensure the absence of any risk related to excessively high carbon segregation on any part whatsoever, AREVA NP has launched an exhaustive review program for forgings manufactured for various nuclear power plants worldwide.

  • 13. Is there any link to the quality problems detected at the Le Creusot plant at the start of 2015?

    These are two distinct issues: the issue of taking carbon segregation into account is not related to the non-compliant practices identified on the Le Creusot site, for which a separate audit is being carried out and a specific improvement plan is being implemented. Within the framework of this audit, the records relating to parts used for the Flamanville 3 EPR have been checked and confirmed.

  • 14. Can the RPV be replaced or repaired?

    At the request of the ASN, AREVA NP and EDF have submitted a description of the operation which would involve removing the RPV from its position within the reactor, in order to replace it.