(Mediapart) Nuclear: a wear problem threatens nearly half of the reactors

Nuclear: a wear problem threatens nearly half of the reactors (PDF attached french article)

Nearly half of the nuclear reactors are threatened by a fuel cladding wear phenomenon, according to an internal document that EDF has acquired Mediapart. The electrician has launched a real tussle with the Nuclear Safety Agency on the subject.

Nearly half of the nuclear reactors in France is threatened by a wear phenomenon that affects fuel cladding, corrosion victims with repeated use, according to an internal document that EDF has acquired Mediapart. Authority of safety of nuclear power (ASN) envisages measures operating restriction to prevent the risk of rupture of the metal casings. Their behavior is essential for the functioning of nuclear power plants as fuel cladding provide the first safety barrier system. But these provisions are challenged by the group that considers too restrictive. The ASN must render an opinion on this subject in the coming days.

The document which has survived from the anonymous, is a tracking form Technical Alerts nuclear fleet, prepared by the Operations Engineering (UNITE) nuclear generation division (DPN) Electrician unit. It covers the week ending December 13, 2013 (see below).


From page 7, you can read several paragraphs on a corrosion problem on the Zircaloy cladding, metal alloy zirconium present in 25 reactors National Park - which has 58 in all, spread across 19 plants. This material is presented as "more susceptible to corrosion" as newer products, also used in the French plants. It is therefore subject to special monitoring, guided by the established models What the nuclear safety experts. Specifically, 13 slices are involved 1,300 megawatts (MW), among the most powerful, and 12 900 MW, one of the oldest.

Schéma sur le fonctionnement d'un réacteur nucléaire (Wikipedia). Cliquer pour agrandir
Diagram of the operation of a nuclear reactor (Wikipedia).
Click to enlarge
The problem is that the operator has found that the corrosion extended to deeper than expected on some duct thicknesses. So it had to change its reference model. But these new measures are rejected by the ASN considers that they are "not acceptable", as can be read in this document. Consequently, the authority to "consider the establishment of" operating restrictions on fuel "in order to limit radiation, thus reducing the thickness of maximum cladding corrosion."

In other words, it may request a precautionary EDF to use less fuel and its time to replace them more often with new equipment. Exactly the opposite of what is today the electrician, which lengthens the period of use of fuels, increasingly loaded uranium 235 to increase its nuclear production.

"On the razor's edge"

Coupe métallographique d'un crayon de combustible issu d'un réacteur (©IRSN). Metallographic section of a fuel rod from a reactor (© SNRIs).
A showdown begins, and we discover the intensity over the paragraphs. EDF proposes raising the maximum corrosion threshold to 108 microns, or 0.108 millimeter. A trifle? Not really concerned ducts do not measure more than 0.57 millimeter, or even five times the thickness corroded "It's a lot, it is a problem.. This means that between a fifth and a sixth part of the sheath is "Monique Sené analysis physicist, founder of the Group for scientific information on nuclear energy (GSIEN).

But meanwhile, the ASN intends to impose more drastic restriction of 0.1 millimeter. A view rejected by EDF, who openly expresses, "this value is the historical landmark value unrelated to any physical justification." Above all, such a limit would "drastically reduce lengths of campaign", that is to say, periods of use of fuel in the reactors 13 1300 MW. The group may therefore need to review thoroughly the management of uranium fuel at the price of substantial additional costs.

Typically, a fuel rod remains four years in a reactor, which corresponds to the three "rings" of EDF typology. They are then removed during maintenance shutdowns, to be returned to treatment Areva La Hague plant. In the heart of the reactors, new and older assemblages are mixed, according to scientific calculations neutron (see below examples of these "managements"). Longer fuels are used less often they should be changed, and thus stop the reactors. It is a measure of budgetary savings.

Exemple de plan d'assemblage dans un cœur de réacteur, mêlant combustibles neufs et anciens. Example of assembly plan a heart reactor, mixing old and new fuels.
But this is not all. Car Accident, corrosion sheaths could aggravate the situation. Normally, nuclear reactors are controlled by control rods. They determine its power rising or dropping into the machine. If one of them suddenly pops as a result of too much pressure in the system, it is the beginning of a potentially serious accident - he did is never produced such until here in France. The temperature inside the reactor and then greatly increases very quickly. The fuel rods are subjected to a shock treatment and can be deformed. Sheaths corroded might break from 0.08 mm (80 microns) corrosion alert ASN, who beyond, "the safety demonstration of holding the pencil rod ejection is not established."

Photographies de tronçons de zircaloy 4, à différents stades d'oxydation (©IRSN). Cliquer pour agrandir Photographs of sections of Zircaloy 4 at different oxidation stages (© IRSN). Click to enlarge
But then again, EDF is resisting and considers that "the value of 80 microns is reached during the second half of the cycle (between 60 and 80% of the natural length of campaign), this limit can not become a constraint on the length of campaign. " In other words, since it is known that the corrosion thickness reaches this before the end of the planned duration of use, we can not refrain from crossing the threshold. In response, the ASN calls EDF a safety demonstration "acceptable." And could accept a slow operation, "base", that is to say without changing the reactor power during the second round of fuel use. "This is an operation on the razor's edge, Monique analysis senna, accident and ejection of clusters, there is no leeway. It's annoying to work without a net. "We swim in full gray area.

The ASN must give its opinion on this controversial topic in the coming days. But the problem could last five to six more years until 2019 or 2020, when the entire stock of Zircaloy sheaths should be passed. "The heart of the discussion now is to determine how thick corrosion is tolerable, told Mediapart Houdré Thomas, director of nuclear power plants to control the ASN; "is technically examine this subject, which is not entirely trivial. It's complicated. "

More specifically, the "discussions focus on the fact establish compensatory measures compared to a situation where doubts about the acceptability of the corrosion could have. It is not completely obvious. " Several possibilities are considered: change fuel assemblies, reactor power block, and thus the amount of nuclear electricity, remove all ducts Zircaloy park. The latter option would mechanically stop all reactors involved, the time to change uranium rods. It should also find new solutions for the storage and treatment of corroded assemblies. Unable to change or repair the fuel cladding, too radioactive to be approached. We must keep all or change any.

"Challenging the safety demonstration"

Intérieur de la cuve d'un réacteur nucléaire. Inside the vessel of a nuclear reactor.
This problem is the subject of intense discussions between EDF, ASN and its network of experts from IRSN (Institute for Radiological Protection and Nuclear Safety) for fifteen years. The development of the corrosion of Zircaloy has been modeled, and even an experimental program was established to provide a tool for the simulation of damage in the event of ejection ducts cluster. Zirconium is popular with manufacturers of nuclear components because it absorbs very little neutron and thereby facilitates the process of chain reactions. Its default is to corrode faster.

In the past, IRSN estimated that the maximum thickness of corrosion were dumped by EDF. In 2011, as noted in a report (read it here) that the maximum thickness of 115 microns reach calculated oxide or 0.115 millimeter. That is to say, even more than the maximum threshold EDF is now trying to accept. These thicknesses exceed the design criteria previously selected by EDF, and "could jeopardize the safety demonstration" (see this document, p. 2) . When experts analyzed 2,800 fuel assemblies unloaded slices park, 119 were "leaky". In June 2011, two months after the Fukushima accident, these studies are restarted from the feedback from the use of fuels between 2003 and 2009. The novelty today is the recent actions reveal a larger than expected corrosion process.

While EDF puts pressure on the executive for extending the life of nuclear power plants beyond 40 years the revelations on the wear of fuel cladding creep new pebbles in the shoes of the electrician. Over the years, the equipment is used, leaving fuiter radionuclides inside and outside the reactor building. Yet, in theory, the fuel cladding must be as insurmountable as the Great Wall of China. They provide the first safety barrier designed to prevent the spread of radioactive elements outside the reactor. In fact, cases of cladding failure have been observed, including the Cattenom (Lorraine). Over time, a rate of cladding failure between 1 and 2% was allowed.

"Flying a reactor is not simple, it must take into account all the corrosion," says physicist Monique Sené. If a sheath ruptures, radioactive materials spread in the primary circuit of the reactor. "It is pure non-stop but it is not possible to retain the gaseous products," she says. By dint, pipes and even parts of the reactor building may be contaminated. Personal find themselves at risk of radiation, especially during maintenance operations, the most dangerous.

For EDF, "we can not talk about wrestling with the ASN, it is a technical issue which is the subject of exchanges of work." But the group recognizes that "the issue of corrosion of certain ducts is under discussion."

Far from being anecdotal, the problem of wear sleeves fits the heart of the current EDF dilemma, torn between the demands of security and economic imperatives. Given the large number of tranches (25) managing this problem will necessarily have an impact on the operation of the park, and therefore the amount of nuclear electricity produced.