ITER is one of the greatest scientific and technological challenges facing humanity. During the last two decades there have been very important advances in the field of nuclear fusion, and we have well-founded reasons for accepting that this effort is worth it. To be optimistic and anticipate that it will finally work and will be one of the tools that we will have in the future to respond to our energy needs.
In recent months, ITER has been in the spotlight because the French Nuclear Safety Authority (ASN) has asked those responsible for the project to implement a set of corrective measures that ultimately seek to ensure that the nuclear reactor nuclear fusion will work safely. There is no doubt that as much caution as possible needs to be exercised in this context, and both ASN and ITER are proving to be very conservative.
ITER is currently tackling one of the critical phases of the reactor commissioning process: vacuum chamber assembly. In this article we have set out to investigate the great challenges involved, which is why we have spoken with Cristian Casanova, who is the team leader of the vacuum chamber in the area of responsibility of Fusion for Energy (F4E).
This agency is responsible for managing the European contribution to ITER, and Cristian, who is a civil engineer, is responsible for supplying the sectors of the vacuum chamber that are being manufactured in Europe. In fact, the European Union is responsible for providing five sectors, and South Korea the remaining four. Cristian has been linked to ITER for four and a half years and has extensive experience in the field of managing large engineering projects. Also, as we are about to see, he explains himself clearly and in a very didactic way.
These are the great challenges that ITER puts on the table right now
The vacuum chamber is the very heart of this nuclear fusion reactor. This 8,000-ton enclosure is made of stainless steel, although there is also a small amount of boron in its composition (around 2%). Inside, the fusion of deuterium and tritium nuclei takes place, so one of its most important functions is to act as first containment barrier of residual radiation that might not be retained by the mantle (blanket).
The vacuum chamber is hermetically sealed, and its interior preserve high vacuum required for fusion of plasma nuclei to occur. Its toroidal shape contributes to the stabilization of the gas, so that the nuclei rotate at high speed around the central hole of the chamber, but without ever touching the walls of the torus.
On the other hand, the temperature to which this chamber is subjected is very high, so it is necessary to introduce circulating water into a compartment housed between its internal and external walls to cool it and prevent it from reaching its maximum temperature threshold. Cristian explains us what are the main challenges that he and his team are facing during the assembly of this very important element of ITER:
«One of the most relevant challenges is strictly technological. Until now, the European Union has not faced a project with such tight tolerances (there are local tolerances of 0.1%). The vacuum chamber of the reactor has a very complicated shape, it uses plates with thicknesses of up to 60 mm… Everything is very complex. In addition, in the manufacture of the sectors we are using very advanced techniques, such as, for example, the electron beam weldingwhich is welding using an electron beam.
In this photograph one of the engineers from the Walter Tosto company is inspecting one of the poloidal segments of sector 9 of the ITER vacuum chamber. The size of this steel enclosure is colossal.
«The other great challenge that this project implies is managing commitments purchased by contractors. During its set-up we have had the pieces of the vacuum chamber distributed in four or five different workshops in Spain, France, Italy and Germany. We divide each sector into four segments so that it can be manufactured, and then it is assembled in a workshop,” explains Cristian.
«Since there was no company in Europe large enough to take on this effort, it was necessary to create a consortium that could face it, and this strategy entails a very important human and contractual complexity. Fortunately, we have extraordinarily qualified technicians to solve these challenges.”
What the corrective measures imposed by ASN entail
At the end of last February we discovered that the French Nuclear Safety Agency had asked those responsible for ITER to introduce a set of corrective measures in the reactor. F4E explained to us that ASN had not stopped the construction of ITER and that the project was on track, although at this time it was not supposed to undertake any tasks that it was irreversible. Cristian explains in detail what the ASN requirements are and what they should be:
“The ASN requirements are tied to the dimensional tolerances of the vacuum chamber sectors that come from South Korea. It is very difficult to maintain these tight tolerancesAnd while we don’t have full visibility into what’s happened in Korea because it’s the responsibility of another domestic agency, we know they’ve had a hard time with dimensional tolerances.”
“What happens with this is that if these tolerances are not scrupulously respected, the sectors cannot be welded automatically inside the pit in which the vacuum chamber will ultimately reside. When a new sector arrives at the ITER facilities in Cadarache (France), it is worked on for several months to add the instrumentation, the thermal shield and the toroidal magnets. Then it is placed in its final location, inspected and welded to the other sectors of the chamber”, says Cristian.
«ITER can continue to do many things, but it cannot do anything that is irreversible, such as welding two sectors of the vacuum chamber»
«The pre-welding process is carried out automatically using a procedure developed by the Spanish company ENSA (Equipos Nucleares, SA), but the tolerances are very demanding and do not allow us to ignore the slightest deformation. Essentially, what ASN has asked us to do is ensure that any operation we carry out now is reversible.”
“The international organization of ITER can continue to do many things, but it cannot do anything that is irreversible, such as, for example, welding two sectors of the vacuum chamber. This could have an impact on the date in which it was planned to conclude the assembly of the vacuum chamber”, this technician honestly confesses to us.
For ITER, the quality and safety of the fusion reactor come first
Far from being bad news, that both ASN and the agencies directly involved in the construction of ITER adopt a conservative stance to guarantee that the plasma tests will be carried out with the maximum security guarantees. it is very positive. Cristian explains the current state of the vacuum chamber assembly process and what impact ASN’s requirements could have on the itinerary planned by EUROfusion:
“Technical challenges associated with corrective actions can be resolved, although there may be an impact on the assembly dates that were planned”
“Sector number 6, which is of South Korean origin and arrived in August 2020, completed and installed in its final location two months ago. On the other hand, sectors 7 and 8, which are also South Korean, are in different states of readiness, but instrumentation, magnets and a heat shield are currently being added to them.”
“Sector 1, which is the last Korean, will be finished by the end of this year, and corrective measures requested by ASN are being implemented in it. In the first three South Korean sectors, these corrections were not introduced. These sectors will not be able to be welded in their final location until we have the approval of ASN, but what is really important is that the technical challenges involved in corrective measures can be resolvedalthough, logically, there may be an impact on the assembly dates that were planned”, Cristian unambiguously acknowledges.
In this image we can see several technicians undertaking the assembly of sector 5 of the ITER vacuum chamber at the Mangiarotti facilities in Italy. This photograph was taken during the month of May.
“Right now we don’t know when the assembly of the vacuum chamber will be completed because it is necessary to undertake the corrective measures that ASN has requested. In fact, there will be a rescheduling that will seek to solve all this technical problem. In any case, for us quality and safety come first. This is the reason why the sectors of European origin have been delayed. We have invested more time in them to obtain better tolerances”, Cristian assures us with conviction.
Images: Fusion for Energy (F4E)
George is Digismak’s reported cum editor with 13 years of experience in Journalism