This is how the assembly of the fusion reactor goes after the demands of the Nuclear Safety Authority

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At the end of last February the alarms went off. At that time, an official communication sent several weeks earlier by Bernard Doroszczuk, who is the head of the French Nuclear Safety Authority (ASN), to Bernard Bigot, the director general of ITER, came to light, in which he asked do not carry out assembly of the first two sectors of the vacuum chamber inside the pit in which the reactor will be housed.

The vacuum chamber is a gigantic stainless steel enclosure (it will weigh 8,000 tons when completed), and inside which the fusion of deuterium and tritium nuclei takes place. One of its most important functions is to act as first containment barrier of the residual radiation that will not be able to be retained by the mantle that covers its interior. It is clear that it is a crucial element of the nuclear fusion reactor, so its assembly must be extremely painstaking.

The first information that saw the light argued that Doroszczuk’s request had been motivated by the identification of a structural deformation in the two sectors of the vacuum chamber. This alteration seemed to have been caused during the manipulation of the sectors in the factories in South Korea and Italy in which they were produced, and, according to the head of ASN, it had to be corrected before carrying out their assembly in their final location. .

This step is irreversible, so it is vital that the vacuum chamber sectors are scrupulously matched to specification to ensure that the radiation will be contained as ITER’s designers intended. When we heard this news, Fusion for Energy (F4E), which is the European Union organization responsible for managing Europe’s contribution to ITER, contacted us to offer us your version from what has occured. And it does not reflect a reality as dramatic as the one we could perceive at first.

According to F4E, the alteration of the sectors of the vacuum chamber is of a technical nature, and not the result of an accident during handling. In addition, again according to this organization closely linked to ITER, the ASN letter has triggered the constitution of a working group that pursues resolve requests of the French Nuclear Safety Authority to go ahead with the assembly of the experimental nuclear fusion reactor.

Almost a month has passed since our conversation with those responsible for F4E, so we have contacted them again to find out what state ITER is currently in and if the assembly of the nuclear fusion reactor is really progressing. Our interlocutor has been Aris Apollonatosthe communication manager of the administration department of F4E, and what he has told us is very interesting because it reflects not only the current state of ITER; it also reveals some of the challenges that this project has ahead.

Nuclear fusion reactor assembly continues

A few days ago, in the middle of this month of March, ITER technicians completed the installation at its final location in the center column which, among other functions, acts as a structural support to which the sectors of the vacuum chamber are fixed. In the cover image of this article you can see it housed inside the enclosure where the nuclear fusion reactor will be installed. It is certainly good news that this important component is already installed; however, Aris offers us background information to tell us how the project is going:

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«The assembly continues for all the actions considered reversible, such as, for example, the installation of the central column inside the reactor well tokamak. The assembly work on the coils that must correct the power of the magnetic field also continues, and in approximately a month the first module of the central solenoid will enter the assembly phase. In addition, we continue to manufacture several essential components, such as toroidal magnets. One of them arrived last week from Japan, and another made in Europe will arrive in April.


The ITER technicians continue to work on the installation of the eighteen correction coils that are responsible for adjusting the magnetic cage generated by the three large reactor magnets with extreme precision.

The photographs illustrating this article show that ITER assembly is indeed going ahead. However, it is important that we do not overlook the fact that Aris tells us that what continues is the installation of the elements whose implementation is considered reversible. The installation of the vacuum chamber sectors inside the reactor pit is an irreversible step, which is why ASN has asked those responsible for ITER not to undertake this phase of the project until they are completely sure that each of the sectors scrupulously meets design specifications.

ITER defends that the demands of ASN do not threaten its calendar

One of the consequences that the requirements whose compliance is being demanded by ASN from ITER could have a priori is that the engineering effort involved triggers an inevitable delay in some of the reactor assembly phases. Aris explains to us if the itinerary defined by EUROfusion may or may not be threatened for the additional work that is being necessary to undertake to guarantee the viability of the project:

“The montage of tokamak started on July 28, 2020. In fact, there are already parts installed inside the machine. The welding work of the two sectors cannot proceed until ASN looks at the additional documentation, but these months of delay do not prevent the assembly of other parts from advancing because we work in parallel.”

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On February 28, a few days after we met the requests that ASN has done to ITER, the latter organization published a statement which is in line with the statements that Aris has shared with us:

«[…] The ASN requirements have not had any impact on ITER assembly planning so far. The subassembly of the first sector has now been completed, and will be transferred to the pit of the tokamak during the next few weeks. In addition, the manufacture of the elements that will allow the subassembly of the second sector has already begun, but it will take several months to be completed and even longer until it can be integrated into the reactor pit. […]».

It sounds reassuring. However, Aris gives us one more clue that reveals some of the solutions What is being proposed by the working group created by ITER to respond to the additional requirements of ASN:

“We have identified a technique for welding the two sectors of the vacuum chamber that aims to resolve the dimensional distortion that occurs at the interface of these components. In any case, the ITER organization considers that even with these additional ASN requirements, the planned schedule for assembly can be maintained because the welding was to be carried out during the month of July, as confirmed Bernard Bigotthe general director of ITER».

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The cryostat is a massive 29 x 29 meter stainless steel chamber. It is responsible for providing the high vacuum necessary for the conditions required for the fusion of the deuterium and tritium nuclei to occur inside. One of its sections weighs 665 tons, and it arrived at ITER in October 2022.

Again, the ITER and F4E explanations sound good. Even so, we could not miss the opportunity to ask those responsible for this project if it is possible that the demands of ASN, which, on the other hand, are indisputably relevant, cause the delay of the first tests with plasma, which are planned for the year 2025. In fact, that is the year in which, if all goes well, ITER should complete the assembly of the nuclear fusion reactor. This is what Aris explained to us:

“During this year the ITER Council will decide on the project calendar after having analyzed the impact of various factors, such as the COVID-19 pandemic, the unforeseen events that are taking place in the field of logistics and transport, the current supply chain challenges, etc.”

It seems reasonable. Let’s cross our fingers that everything goes well and this project continues its course without further delay. Or assuming minimal delays. After all, at the current juncture, nuclear fusion is one of our best assets for providing an effective response to our energy needs.

And furthermore, it should allow us to do so assuming minimal environmental impact. Of course, in the short term the most important thing is that ITER technicians offer ASN the necessary solutions to ensure viability of this very important project.

Pictures | ITER

At the end of last February the alarms went off. At that time, an official communication sent several weeks earlier…

At the end of last February the alarms went off. At that time, an official communication sent several weeks earlier…

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