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Framatome’s sub-standard nuclear fuel is threatening the survival of France’s nuclear company EDF

 It is not only in China, in the world’s first operational EPR nuclearreactor, that the fuel produced by EDF’s subsidiary, Framatome, is a problem. In France, in the Ardennes, an unprecedented incident on the nuclear fleet has just occurred in a reactor and potentially concerns ten reactors, to varying degrees of severity.

This nuclear fuel that poisons the life of EDF

by Martin Leers,  Le Journal de l’energie 2nd Aug 2021

Metal guards that enclose the reactor fuel, called cladding, deteriorate too quickly. A problem far from trivial: fuel cladding plays a key role in the safety of nuclear reactors. This “accelerated” corrosion appeared between 2020 and 2021 in one of the two reactors at the Chooz power plant. A fault which currently forces EDF to extend its shutdown since March 2021 and has therefore already cost it more than a hundred million euros.

But the stakes for EDF are much more important than a shutdown of a reactor. The “M5” alloy sheaths, which wear out prematurely in Chooz reactor n ° 2, are fitted to all EPR reactors in France, Finland and China, as well as dozens of other reactors in France and abroad.

Is there a link between this incident in France and that of the leaking ducts of the first EPR reactor in service in the world in Taishan (China)?

Why do these latest generation sheaths wear out prematurely?

A burning question for EDF, which is trying to convince the Nuclear Safety Authority (ASN) and the Institute for Radiation Protection and Nuclear Safety (IRSN) to reuse cladding with questionable reliability in reactors.

How did the problem come to be at the Chooz nuclear power plant?

When the Chooz reactor n ° 2 was shut down in February 2021 to reload the fuel, particles were discovered on the fuel assemblies and in the water of the primary circuit [1] . “Numerous white migrant bodies of a few millimeters were either collected by the anti-debris devices or remained on the assemblies”, explained EDF in an internal letter to ASN, dated July 7, 2021. An abnormal phenomenon . These particles are zirconium oxide which originates from the surface of the fuel cladding. [2] Their presence indicates that the sheaths are degrading. “The consequence of an abnormally high corrosion rate”, clarified EDF in the letter to ASN.

These particles are very friable and cannot cause a loss of tightness in the nuclear fuel, explained Karine Herviou, deputy director general of IRSN, to the Journal de l’énergie .

The fuel claddings are tubes more than 4 meters long and less than 1 centimeter in diameter, very thin (0.6 mm thick) in which the uranium pellets are stacked. These sheaths, commonly called rods, are brought together in assemblies, each made up of 264 sheaths. The core of Chooz nuclear reactor No. 2 contains 205 assemblies. In Chooz reactor n ° 2, the abnormal wear is only in the upper part of certain assemblies.

What consequences for the safety of the nuclear reactor could have the accelerated wear of the fuel rods?

The cladding plays an important role for reactor safety: they form the first barrier between nuclear fuel, containing very dangerous radioactive substances, and the environment. They must prevent radioactivity from spreading in the water circulating in the nuclear core. However, “damage to the surface of certain fuel cladding calls into question the demonstration of the integrity of the fuel in service”, considers ASN. This means that this incident calls into question the parameters which guarantee, in the eyes of ASN, the safety of the reactor in normal operation and during accident scenarios.

“Accelerated corrosion is likely to weaken the cladding and increase the risk of loss of integrity of the rods concerned during accidental transients and therefore lead to rupture of the first barrier”, explains EDF in the internal letter to ASN from July 7, 2021. But EDF does not consider this scenario plausible.

What are the causes of abnormal wear of the fuel rods?

EDF estimated on July 7 that “at this stage of the investigations, no single cause appears at the origin of the phenomenon of accelerated corrosion (…) which rather finds its explanation in a combination of several unfavorable factors”. But the M5 alloy from which the sheaths are made “seems to be the trigger,” notes EDF.

The iron content of the sheaths singled out

It is the low iron content of the cladding alloy which is partly responsible for their degradation. Two production batches for low iron content sheaths have been identified by EDF. The most damaged sheaths come from these lots, which EDF calls “hyper sensitive castings”. But until the February 2021 incident on Chooz reactor no.2, slight variations in iron in the cladding alloy were not considered to be a factor in the degradation of the fuel rods. The variable iron content of fuel cladding does not appear to have been perceived as a problem in France by the fuel assembly manufacturer, Framatome, or by the operator EDF, or by ASN and IRSN.

The sheaths which deteriorate are “in conformity” with the specifications

“The iron content of these batches is within the standards”, explains IRSN. “The products supplied by Framatome comply” with the specifications, “iron was not considered as a characteristic parameter for the behavior of the rods in the core. », Adds ASN.

Following the incident, EDF informed ASN that the iron content will be increased in the cladding which will be used in reactor No. 2 at Chooz from “cycle 20”. Not the next time the reactor is reloaded, but the next.

A “multifactorial” phenomenon

The iron content of pencils is not the only culprit. EDF puts forward other causes to explain the degradation of the fuel cladding. The temperature is higher at the top of the nuclear core in the most powerful reactors, those of 1450 MW, than in the less powerful reactors, those of 1300 MW. It is in this area that the cladding deteriorated in two 1,450 MWe reactors in France. What would happen if low iron cladding was introduced into EPR reactors, even more powerful than the 1450 MW reactors?

Another unfavorable element: the positioning of the fuel assemblies in the reactor vessel. “The corrosion rate depends on the place of an assembly in the reactor core during the first cycle”, particularly in the four most powerful reactors in the French fleet, explains EDF in an internal document. [3]

An unprecedented incident in France but not abroad

If this wear had never been observed in France, it had already occurred on three nuclear reactors in Brazil and Germany, two of which used the same M5 alloy. [4] As at the Chooz power station, the most worn cladding was the one with the lowest iron content. “The phenomenon of accelerated corrosion observed at the end of cycle 18 of Chooz B2 is comparable to other events in Konvoi reactors abroad”, notes EDF in the internal letter to ASN. Therefore, why was this incident not anticipated in France when the nuclear operators and institutions say they maintain a permanent dialogue on safety at the global level?

The nuclear safety experts from the German GRS institute have not been able to fully identify the causes of the corrosion of the cladding on German reactors, adds Karine Herviou of IRSN.

M5 alloy sheaths are fitted to all EPR reactors in France, Finland and China, as well as dozens of other reactors.

Designed to be more corrosion resistant than previous alloys and to improve nuclear fuel efficiency, the M5, manufactured by Framatome, is widely used in nuclear power worldwide.

“A large majority of reactors in France use assemblies with M5 cladding,” explains Karine Herviou of IRSN. Framatome claims its M5 sheaths are used in 96 nuclear reactors around the world, in a 2018 brochure .

The same cladding is used in the EPR reactors at Flamanville (Manche), Olkiluoto in Finland and Taishan in China. M5 alloy sheaths had many leakage problems in the 2000s, says a 2008 IRSN report:

“Between 2001 and 2008, around thirty fuel assembly leaks with M5 alloy cladding were detected. To date, EDF has identified three types of faults causing leaks in fuel rods with M5 alloy cladding. »Defects now corrected……………

10 nuclear reactors in France affected by the cladding defect discovered at the Chooz power plant

For the moment, ten nuclear reactors in France are directly or indirectly affected by the defect in the cladding discovered on the reactor n ° 2 at Chooz.

“To date, seven 1,300 MWe reactors and three 1,450 MWe reactors have at least one rod with a low iron content, in the core or in the management reserve,” ASN told the Journal de l’énergie .

But the inventory of potentially defective rods is still in progress, “even if it should not change”, added ASN. In addition to the two production batches for low iron content sheaths, EDF identified other batches of concern and informed ASN of them in an internal letter. How many ? Mystery.

“An inventory of the iron content of each of the rods of each of the assemblies present in the reactor or in reserve is being drawn up”, specifies ASN…………..

Is EDF’s priority to save fuel even if it means playing stunts with nuclear safety?

EDF is therefore forced to adapt the operation of the two reactors to the defective ducts with “compensatory” measures. EDF proposes that Chooz reactor no. 2 only operate at 92% of its power during its next cycle. For Chooz reactor n ° 1, the operator proposes to reduce load monitoring. [6] “Depending on the elements, EDF could be required, on reactors 1 of Chooz, 1 and 2 of Civaux and 3 of Cattenom to take compensatory measures (either to limit maneuverability or to operate at a drop in power)” , announces the operator in the internal letter of July 7, 2021 to ASN.

Measures that would have a financial impact. The aim, explains ASN, is “not to allow a reactor operating mode where this corrosion acceleration is possible”.

IRSN must deliver its opinion on EDF’s proposals in a few weeks, then ASN will decide.

Why does EDF not give up using potentially defective cladding in reactors? Is it about saving fuel even if it means doing acrobatics with nuclear safety?

Neither EDF nor Framatome answered questions from the Journal de l’énergie . https://journaldelenergie.com/nucleaire/combustible-nucleaire-empoisonne-edf/

August 5, 2021 - Posted by | France, safety

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