On 27 September, 2010, the French anti-nuclear network Sortir du nucléaire received internal EDF documents, showing that the design and manufacture of the vessel closure head for the EPR in Flamanville could, in theory lead to a Chernobyl-type accident. Several EDF documents show that the number of welds and the type of steel used in some parts of the reactor vessel may cause leaks. EDF considers that the leaks may, in turn, develop into a Chernobyl-type of accident. The type of steel and welds used are part of the emergency shutdown system of the EPR and cover 89 points of entry into the reactor vessel.
The documents demonstrate that EDF engineers have designed parts of the vessel closure head for the EPR that not only endanger safety but also knowingly violate French law (namely violations of the decree of 12 December 2005 on nuclear pressure equipment) relating to nuclear facilities under pressure.
For Sortir du nucléaire, the conclusion is obvious: in spite of all these issues, EDF persists in a policy that sacrifices security for profits. In view of the catastrophic consequences of an accident, this attitude is unimaginable and unforgivable.
Sortir du Nucleaire is working hard to get all the technical documents translated into English, but summaries are already available. Although much has to be investigated before final conclusions can be drawn, we support Sortir du Nucleaire in exactly this call; let the French safety authorities give full disclosure of all documents and let independent specialists research the issue and come to conclusions. If there is no reason for fear it is in the interest of the French authorities and EDF to follow this route, otherwise there is a clear public interest for full disclosure. EDF has confirmed that the documents are genuine but have also already said that they see no problem; they have taken the theoretic problem into account while building the EPR.
Summary of documents highlighting EPR weaknesses
The EDF documents reveal the weaknesses in the design and the manufacture of the control rod drive mechanism (CRDM) casing. This complex mechanism enables the emergency shutdown system of the reactor to be activated. The casing for each mechanism is connected to the closure head of the reactor's pressure vessel and contributes to the leak-tightness of the vessel up to a pressure of 155 bars. If one of the casing is weakened, the whole of the reactor's pressure vessel becomes vulnerable. Sortir du nucleaire comes to three main conclusions
1. Weakness in the welding of the CRDM casing: 4 welds rather than 1
EDF has opted to use 4 welds for the control rod drive mechanism (CRDM) casing of the EPR, whereas only one weld was used for the casing of the CRDM of the 58 French nuclear reactors, in order to minimize the risk of leakage.[i] These four welds constitute a breach of the 12 December 2005 decree on nuclear pressure equipment; the decree states in Appendix 1 (3.3) that "socket welded connections are forbidden."[ii]
However, they are being used for the casings of the CRDM. Yet the EDF is fully aware of the regulations, as it refers to the French regulations having set "a limit on the number of welds."[iii]
A greater number of welds increases the risk of failure of the leak-tightness of the CRDM casing, and this in turn greatly increases the risk of control rod cluster ejection. The consequences of such a failure would be a loss of primary coolant and a real risk of reactor core fusion. According to the EDF's head of nuclear fuels, a control rod cluster ejection can cause a Chernobyl-type accident.[iv]
The risk of rupture of any of the mechanisms' casing in the head of the EPR pressure vessel is multiplied by the number of mechanisms penetrating the vessel head (89), in other words there are 89 weakness sites.
2. Weakness in the stainless steel used in the CRDM casing: a steel which doesn't stand the test of time
The central part of the CRDM casing used for the EPR will be made of martensitic stainless steel which becomes brittle when exposed to heat. This type of stainless steel can fracture without warning, a well-known fact.[v]
In view of its fragility, martensitic stainless steel is not suitable for pressurized equipment in the main primary circuit of a nuclear reactor. And yet this is what EDF plans to do, in full knowledge of the risks: the EDF document points out that "small errors of temperature or functioning time have a big impact on the behavior of these hardened steels".[vi]
This is the second time that there is a breach of regulations for equipment that is crucial to the safety of the EPR. This is a breach of the 12 December 2005 decree on nuclear pressure equipment which stipulates that "the ratio between elastic limit at ambient temperature and resistance to traction at ambient temperature must not exceed 0.85 for martensitic steels".[vii]
The decree states that these are "essential safety requirements for nuclear pressure equipment". EDF engineers are fully aware of this: "Using this type of steel for pressurized equipment in the main primary circuit has always been prohibited in any nuclear reactor. Its use for EPR mechanisms has therefore come under scrutiny, particularly since this steel does not meet the NPE (Nuclear Pressure Equipment) criteria stipulating that the ratio between elastic limit at ambient temperature and resistance to traction at ambient temperature must not exceed 0.85.[viii]
The use of this type of stainless steel increases the risk of sudden rupture of the CRDM casing and control rod cluster ejection. Such a rupture would cause a loss of primary coolant and a real risk of fusion of the nuclear core. According to the EDF's head of nuclear fuels, a control rod cluster ejection can cause a Chernobyl-type accident.[ix]
As said, the risk of fracture of the stainless steel casing of one of the mechanisms in the head of the EPR pressure vessel is multiplied by the number of mechanisms penetrating the vessel head (89), in other words there are 89 weakness sites.
3. Weakness due to the lack of mechanism preventing control rod cluster ejection
The welding weaknesses of the CRDM and the type of steel used in their casing increase the risk of ejection of the control rod cluster. According to a memo written by EDF's head of nuclear fuels in 2001,[x] ejection of the control rod cluster could cause a Chernobyl-type accident: "The Chernobyl accident in 1986 was due to uncontrolled reactivity, leading to core melt and explosion. Until then, only a few calculations had taken into account this type of accident. The Three Miles Island accident (sic) had already raised this problem. At the time, I took part in an intercompany working group looking at this issue, to carry out a risk analysis of such an accident for our PWRs (Pressurized Water Reactor). Reactivity accidents could occur when the reactor is running at full power. A rupture in the winch or the vessel head could cause one or several of the control rod clusters to be ejected."[xi]
What follows reveals that a Chernobyl-type reactivity accident could happen in any French nuclear reactor: "During such an accident, the fuel close to the ejected control rod will suddenly become very reactive. It is likely to reach very high reactivity values. This power excursion may cause the rupture of the casing, and a fuel pellet explosion, with uranium dispersing into the main circuit water. This could be followed by a steam explosion. If not controlled, a steam explosion produces a huge amount of energy likely to rupture the pressure vessel."[xii]
Finally, according to the same document, a locking device for the rod cluster control ejection would limit the risk of reactivity accident[xiii]. Yet not locking device for rod cluster control ejection has been planned for the EPR[xiv].
EDF's head of nuclear fuels suggests at the end of his memo: "Ideally, we should try not to take into account this type of accident when planning future reactors"[xv].
Notes:
[i] Doc n°2 Synthèse des choix de conception des mécanismes de commande, 5.1. Modification et contrôle des soudures p.11-12, F.Odier, EDF-SEPTEN, (08.12.2008).
[ii] Arrêté du 12 décembre 2005 relatif aux équipements sous pression nucléaires
http://www.legifrance.gouv.fr/affichTexte.do?cidTexte=JORFTEXT0000004539...
[iii] Doc n°2 Synthèse des choix de conception des mécanismes de commande, 5.1 Modification et contrôle des soudures p.11-12, F.Odier, EDF-SEPTEN, (08.12.2008).
[iv] Doc n°3 Management des activités Physique des Coeurs et Combustibles, p.112 EDF-SEPTEN, A. Berthet (20.12.2001).
[v] "Suite à des constats sur site de fragilisations et de ruptures brutales de tiges de vanne en aciers inoxydables martensitiques", Doc n°4 Note de synthèse sur le vieillissement des aciers martensitiques, III.1, p.9, EDF-Direction Production Ingénierie (08.08.06).
[vi] Doc n°4 Note de synthèse sur le vieillissement des aciers martensitiques, III.3, p.11, EDF-Direction Production Ingénierie (08.08.06).
[vii]Arrêté du 12 décembre 2005 relatif aux équipements sous pression nucléaires, annexe 1, point 4. http://www.legifrance.gouv.fr/affichTexte.do?cidTexte=JORFTEXT0000004539...
[viii] Doc n°2 Synthèse des choix de conception des mécanismes de commande, 5.4, p.12-13, EDF-SEPTEN (08.12.2008).
[ix] Doc n°3 Management des activités Physique des Coeurs et Combustibles, p.112 EDF-SEPTEN, André Berthet (20.12.2001). [x] Doc n°3 Management des activités Physique des Coeurs et Combustibles, p.112 EDF-SEPTEN, André Berthet (20.12.2001).
[xi] Id.
[xii] Id.
[xiii] Doc n°3 Management des activités Physique des Coeurs et Combustibles, p.115 EDF-SEPTEN, A. Berthet (20.12.2001).
[xiv] L'EPR sous pression, p.4, document anonyme reçu en septembre 2010.
[xv] Doc n°3 Management des activités Physique des Coeurs et Combustibles, p.115 EDF-SEPTEN, A. Berthet (20.12.2001).
List of EDF documents:
- Document 1: L'EPR sous pression (EPR under pressure), p.4, anonymous document received in September 2010.
- Document 2 : Synthèse des choix de conception des mécanismes de commande, F.Odier, EDF-SEPTEN, (08.12.2008).
- Document 3 : Management des activités Physique des Coeurs et Combustibles, EDF-SEPTEN, A. Berthet (20.12.2001).
- Document 4 : Note de synthèse sur le vieillissement des aciers martensitiques, III.1, p.9, EDF-Direction Production Ingénierie (08.08.06).
Link to EDF documentation and detailed analysis (in French): http://www.sortirdunucleaire.org/dossiers/EPR-revelations2.html
More information about all problems of EPR: http://www.sortirdunucleaire.org/dossiers/EPR.html
Source and contact: Sortir du Nucleaire, 9 rue Dumenge, 69317 LYON cedex 04, France.
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web: www.sortirdunucleaire.fr