The ITER full-W divertor design goes to great lengths to make sure that there is no possibility—on any of the many thousands of high heat flux handling elements—of an edge sticking up (for example, as a result of mechanical misalignment) that could overheat and begin to melt under the relentless bombardment these components receive during high power operation. However ITER’s size means that it will have the capacity to reach a value of stored energy in the plasma more than a factor of 10 higher than the largest currently operating tokamak, JET (EU). When some of this energy is released in a rapid burst (for example due to very transient magnetohydrodynamic events such as ELMs), some melting is possible—even if all edges have been hidden by clever design.
On the other side of the CEA fence, in Cadarache, sits a large tokamak which played an important role in the definition of ITER. Tore Supra, a CEA-Euratom device which began operating in 1988, was the first tokamak to successfully implement superconducting magnets and actively-cooled plasma-facing components.
Over the past twenty-four years, Tore Supra has explored the physics of long-duration plasma pulses, reaching a record of 6.5 minutes in December 2003.
In 2000-2002, Tore Supra was equipped with a new carbon-carbon fibre (CFC) „limiter” — the equivalent of the divertor in ITER — capable of withstanding an ITER-relevant heat load of 10 MW per square metre.
This project, named CIEL for Composants Internes Et Limiteurs, demonstrated that, while CFC performs very well in terms of power handling and compatibility with the plasma, its use results in substantial erosion caused by the physico-chemical reactions between the carbon of the limiter and the hydrogen (deuterium) in the plasma. Further experiments in JET have confirmed these observations.
Now, there are not many options when it comes to choosing the material of a divertor. Fifty years of experience in tokamak technology have narrowed them to two: it’s either CFC or tungsten, their respective advantages or disadvantages depending on the plasma regimes they are exposed to. (More here).
In ITER, it was originally planned to begin operations with a CFC divertor and replace it with a tungsten one before the start of nuclear operation (deuterium + tritium) in 2026. After years of discussions, panels and reviews, a new plan was established and ITER is now considering doing without the first-phase CFC divertor.
Indeed, substantial cost reductions would be achieved by installing a tungsten divertor right from the start and operate it well into the nuclear phase. This solution would also provide for an early training, during the non-nuclear phase of ITER operation, on how to operate with a tungsten divertor.
The ITER Members, however, have not yet reached a unanimous position on this issue.
Whatever ITER decides eventually, the tungsten option must be explored and this is what Tore Supra’s WEST project (W Environment in Steady-state Tokamak, where „W” is the chemical symbol of tungsten) is about.
„ITER success is CEA’s top priority,” says Alain Bécoulet, the Head of CEA-IRFM (Institut de Recherche sur la Fusion Magnétique) which operates Tore Supra. „By installing an ITER-like full tungsten divertor in Tore Supra, we can turn our platform into a test-bench on ITER critical path. We can thus contribute to reducing the risk and to saving time and money for ITER. WEST is not something we would add to Tore Supra like we did with CIEL. It’s more like Tore Supra becomes WEST to serve ITER.”
The CIEL project provided IRFM with a strong experience in cooperating with the industry. Adapting Tore Supra to accommodate a full tungsten divertor — 500 components with a total of 15,000 tungsten tiles — is a challenge the Institute is ready to take on. (All carbon will have to be taken out of the device; in-vacuum vessel magnetic coils will need to be installed in order to modify the plasma shape from circular to „D-shaped” and heating systems will have to be adapted to the new configuration.)
The formal decision to go WEST is due to be taken by CEA at the end of 2012; Bécoulet is optimistic: partners are showing interest and „customers” other than ITER appear eager to utilize the future test bench as well. „All fusion machines, present and projected,” he says „are expected to go tungsten.”
Bringing a timely answer to ITER interrogations means that Tore Supra, which Bécoulet calls „a technological jewel”, should prepare to go WEST early in 2013 and be ready for the first experiments in 2015.
Click here to view an animation of the WEST project.
On 29 July, a new milestone was reached in the licensing process of ITER. A little more than one month after being notified that our proposals on the Tokamak’s operational conditions and design fulfilled the French safety requirements, we have now received from the Autorité de Sûreté Nucléaire (ASN) the draft of the Décret d’Autorisation de Création — the final green light from the French Authorities to create our installation.
We are currently analyzing this draft and we will soon send back our comments to ASN. Then, a discussion will be organized with a college of ASN experts and at long last the final decree will be published — hopefully before the end of the year.
This is a lengthy, complex, demanding — sometimes frustrating… — process. But I must say it is also a very good process. ITER is the first fusion installation that will receive a full nuclear licence. And this is very important, not only for us here at ITER but for the whole worldwide fusion community.
We have always claimed that fusion is safe and in the past two years, we went through an exceptionally strict and challenging process to demonstrate that it is indeed. Now an independent body of experts, with a deserved reputation for being among the „toughest” in the world, is in the process of validating our claim. And again, this is a first: no fusion installation, not even JET or TFTR which, at one point implemented deuterium + tritium fusion, went through this process.
Twenty-seven years have passed since President Reagan and Secretary Gorbatchev met in Geneva and laid the ground for the project of an international experimental fusion reactor „for the benefit of all mankind”.
We all feel a deep satisfaction in seeing these 27 years of hard work and dedication now converging into a decision that, in many ways, is historical.
On the 20th and 21st of August several meetings took place at Rokkasho (Japan) between the CODAC teams in charge of the machine protection and interlocks of ITER and the International Fusion Materials Irradiation Facility (IFMIF) team.
IFMIF is one of the projects of the Broader Approach Agreement between Japan and Europe, which was signed to support ITER and achieve an early realization of Fusion Energy for peaceful purpose. In particular, IFMIF must present results in parallel with ITER operation since these will allow the design of DEMO by qualifying the materials capable to withstand the neutron flux that a commercial nuclear nusion reactor will undergo.
The aim of the meetings was to establish a first contact between the controls groups of both „brother” organisations focusing on the development of the machine protection systems. The sessions started with a seminar by Antonio Vergara (ITER) summarising his experience on the design, implementation and commissioning of machine protection systems for high energy physics accelerators like the Large Hadron Collider at CERN and how the lessons learnt can be applied to ITER and IFMIF interlocks. The presentation was followed by a series of meetings organised by the IFMIF/ Engineering Validation and Engineering Design Activities (EVEDA) Project Leader, Juan Knaster.
IFMIF plant will bombard suitable materials reaching more than 20 displacements per atom (dpa)/year (this value means that in average an atom has been displaced from its lattice 20 times per year). This would allow to obtain within a few years of operation the expected 150 dpa at the end of life of a commercial reactor; and with neutrons at an energy spectrum around the 14 MeV (typical of a Deuterium-Tritium nuclear fusion).
The neutron flux will be obtained by accelerating at 40 MeV two parallel beams of 125 mA Deuteron current and make them collide onto a Liquid Lithium screen. The Accelerator validation will be achieved by the installation, commissioning and operation of the Linear IFMIF Accelerator Prototype ( LIPAc) which will accelerate a current of 125 mA Deuterons at 9 MeV. The current status of the LIPAc control, safety and machine protection systems were presented and discussed.
The LIPAc, like ITER, is also based on in-kind procurements. The collaborating organizations in Japan and Europe are in charge of building and installing the different plant systems of the accelerator’s prototype including their local controls, safety and machine protection systems. The international team in Rokkasho is in charge of the development of the central control systems and the entire integration and commissioning.
Not surprisingly, they are facing many of the issues and challenges related to the integration of the I&C systems that the CODAC team at ITER has been solving during the last years by the development of tools such as the Plant Control Design Handbook (a new version will be released at the beginning of 2013) and the CODAC Core System software.
One of the main meeting conclusions was that the similarities between the two project control systems – the fact that both are based on EPICS and the equivalent procurement strategy – makes a more detailed analysis of the potential collaboration between ITER and IFMIF very desirable. The potential to share the developed tools, and procedures and apply knowledge and lessons learnt from the ITER controls and interlocks teams to the design and implementation of the LIPAc control systems could result in an efficient and cost-effective collaborative approach.