Melting tungsten for a good cause



Over the past two years ITER physicists and engineers, along with many scientific colleagues within the fusion research community, have been working to establish the design and physics basis for a modified divertor—the component located at the bottom of the huge ITER vacuum vessel responsible for exhausting most of the heat and all of the particles which will continuously flow out of ITER’s fusion plasmas. 

Our current Baseline begins plasma operations with divertor targets armoured with carbon fibre composite (CFC) material in the regions that will be subject to the highest heat flux densities. After the initial years of ITER exploitation, in which only hydrogen or helium will be used as plasma fuel producing no nuclear activation, this divertor is to be replaced. The replacement—a variant of the first component but fully armoured with tungsten—would be the heat and particle flux exhaust workhorse once the nuclear phase, using deuterium and then deuterium/tritium fuel, begins.

In 2011 the ITER Organization proposed to eliminate the first divertor and instead go for the full-tungsten („full-W”) version right from the start. This makes more operational sense and has the potential for substantial cost savings. By June 2013, the design was at a sufficiently advanced stage and we were confident that the necessary tungsten high heat flux handling technology was mature enough to invite external experts to examine our progress during the full-W divertor Final Design Review

But making a choice to begin operations with tungsten in the most severely loaded regions of the divertor is not just a question of having a design ready to build. 

Tungsten, a refractory metal with high melting temperature (3400 Celsius), is a much more difficult material than carbon when it comes to handling very high heat loads and running the plasmas which ITER will require to reach good fusion performance. Why? For two principal reasons: as a metal, tungsten will melt if the heat flux placed on it is high enough; also, as an element with high atomic number it can only be tolerated in minute concentrations in the burning plasma core.

Carbon, on the other hand, does not melt but sublimes (passing directly from solid to vapour) and is low atomic number, so can be tolerated in much higher quantities in the core plasma. Unfortunately, carbon is a difficult option for ITER nuclear phase operations as a result of its great capacity for swallowing up precious tritium fuel and efficiently trapping it inside the vacuum vessel. Tungsten retains fusion fuel only at comparatively low levels.

Why is melting such a problem? Because a melted metal surface is no longer the flat, pristine surface which is installed when the component is new. One of the ways the ITER divertor is able to handle the enormous power flux densities which will be carried along the magnetic field lines connecting to the target surfaces is to make the target intersect the field lines at very glancing angles, so that the power is spread over a wider surface. But a small angle means that any non-flat feature on the surface will receive a higher-than-average heat flux and can be further melted, producing a cascade effect.

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.

We intend to stop this happening as much as possible by applying ELM control techniques, but occasional larger events cannot always be excluded. So one of the big physics questions we have tried to answer over the past two years is: what exactly happens when a burst of energy, sufficient to melt tungsten, strikes our divertor targets?

Until recently we had only rather complex computer simulations with which to establish the physics design specifications. One of the main worries was not just that energy bursts could roughen up and damage divertor component surfaces, but that the very rapid melting induced by the burst could lead to the expulsion, or spraying, of micro-droplets of tungsten back into the plasma leading to intolerable contamination and a decrease in performance.

The computer simulations say this shouldn’t happen, but the process of melt ejection is so complex that experiment is the only sure test. But how to test the behaviour under conditions which only ITER can create? Well, as far as tokamaks are concerned, the only place where this was even conceivable was at JET, in which natural ELM energy bursts can be generated at levels similar to those expected for controlled ELMs in ITER. The problem is that these comparatively benign transients will not melt a tungsten surface!

In an experiment proposed and planned jointly between JET and the ITER Organization over the past two years, a small region of one of the full-W modules in the JET divertor was carefully modified to create a situation which every divertor designer would do anything to avoid—a deliberately misaligned edge.

The JET divertor modules are made up of about 9,000 small tungsten plates („W lamellas”), bound together by a complex spring loading system. The lamellas are only 5 mm wide and about 60 mm long with 1 mm gaps between neighbouring elements. For the experiment, a few lamellas were machined to make a single element stand up out of the crowd, presenting an edge of about 1.5 mm on average to the plasma in one of the hottest zones of the divertor.
The result: reassuringly unsurprising! Although there was some evidence suggesting the occasional ejection of very small droplets from the melted area, there was very little impact on the confined plasma. As the ELM plasma bursts repetitively melted the edge of the misaligned lamella, the molten material continuously migrated away from the heat deposition zone, accumulating harmlessly into a small mass of re-solidified tungsten (see video at left, courtesy of EFDA-JET). The JET plasmas with 3 MA of plasma current were able to produce ELM plasma pulses very similar to the lowest amplitude events we need to guarantee for 15 MA operation in ITER—a fact which makes the experiments very relevant from a plasma physics point of view.

Much more analysis is required to see how the results can be matched quantitatively by simulation, but the observations are clearly in qualitative agreement with theory. That’s the most reassuring part: that physics codes used to assist in component design for ITER tomorrow can be validated on experiments performed today. We will have to wait another year now for the damaged lamella to be retrieved from JET before the full picture of these important experiments can be completed, but this is already extremely valuable physics input for the important decisions coming up later this year with regard to our divertor strategy.

First design review within Test Blanket Module program



Last week the ITER project—and the worldwide fusion community—celebrated yet another premiere: the first conceptual design review within the Test Blanket Module (TBM) program, a key technology development paving the way to fusion power. It was not yet the turn of the tritium-breeding test modules to be assessed, but that of the components required for hosting them.
During its operational phase, ITER will draw upon the global (civil) inventory of tritium, currently estimated at 20 kilos.

But future fusion power stations would have to create their own supply of tritium. Part of ITER’s mission is to test different tritium breeding concepts proposed and developed by the Members … concepts that will enable future fusion reactors to produce their fuel within the machine (tritium self-sufficiency) and at the same time extract the heat produced by the fusion reaction and convert it into electricity.

While six different tritium breeding concepts—the Test Blanket Modules—are currently in their pre-conceptual design phase, a group of experts lead by ITER Senior Engineer Guenter Janeschitz last week concluded the first design check of the modules' frames and housings, as well as the dummy modules that will be needed to substitute for the actual TBM sets in order to close and seal the port plugs in the case of delayed delivery or in case replacement is required. Mario Merola, in charge of ITER’s in-vessel components, called the design review „a significant step forward toward the goal of testing tritium breeding technology.”

The current strategy foresees that the dummy TBM sets and the frames shall be made of water-cooled 316-L(N) steel (ITER grade), a special metal that guarantees reduced activation when exposed to neutrons, no ferromagnetic effects and adequate mechanical properties. To reduce maintenance time, the replacement of a TBM will be performed „off-line,” meaning that the entire port plug (with its TBM sets, plus frame) will be removed, stored in the Hot Cell, and replaced by a new plug with a new set of equipment. Delivery and installation of the six Test Blanket Systems is planned during the machine’s first shutdown period following First Plasma.

„We looked at the design concept from all possible different angles and the requirements have been clearly identified,” the Chairman Guenter Janeschitz stated in the panel’s close-out session, praising the high level of preparation of the review. „A significant effort was made in the presentations to cover, in a quite comprehensive manner, systems requirements, design analysis, interface requirements and manufacturing aspects—therefore, the objectives of the design review were achieved. However, a few issues such as the potential contamination of the port flange, the still-insufficient shielding performance, the attachment of the TBM sets or their dummies to the frame structure, and the expected thermal stresses these components could be exposed will have to be further considered during the post-conceptual design phase.”

In dealing with the press, openness is key

On 22 and 23 April, the ITER Organization welcomed 19 science journalists from the European Union’s Science Journalist Association (EUSJA). This was the result of an initiative taken jointly by the Russian journalist Viola Egikova, vice-president of EUSJA, and ITER Communication to present ITER and the project’s underlying fusion science and technology to a group of selected science journalists.

The two-day program included a visit of the worksite and presentations by several ITER scientists and engineers on status of the project, plasma physics, the chemistry of tritium, etc. Interviews were also organized at the requests of the journalists.

As Head of Communications, I believe it is essential to work with the press and to handle their requests as swiftly as possible, as there is still a huge information gap and major communication needs relative to ITER and fusion. In my opinion, the aim is not so much the information that you deliver but the openness and the dialogue that you establish (or make visible) … and  the respect for journalistic work.

„Indeed, I was pleased to see the openness of the ITER Communication team,” said Amanda Verdonck, a free-lance Dutch journalist who participated in the EUSJA visit. „But I was really impressed by the scale of the project and the sophisticated scientific knowledge that has gone into the machine. And I will be further impressed to see all this functioning! Like your videoconference system — quite impressive to me!”

Green light for ITER’s blanket design



After three days and 29 presentations, a comprehensive design review with probably the largest participation in the history of the ITER project was completed last week. More than 80 experts from the ITER Organization, Domestic Agencies and industry attended the Final Design Review of the ITER blanket system.

„The development and validation of the final design of the blanket system is a major achievement on our way to deuterium-tritium operation—the main goal of the ITER project,” Blanket Integrated Product Team Leader (BIPT) and Section Leader Rene Raffray concluded at the end of the meeting, obviously relieved at the success of this tremendous endeavour. „We are looking at a first-of-a-kind fusion blanket which will operate in a first-of-a-kind fusion experimental reactor.”

The ITER blanket system provides the physical boundary for the plasma and contributes to the thermal and nuclear shielding of the vacuum vessel and the external machine components such as the superconducting magnets operating in the range of 4 Kelvin (-269°C). Directly facing the ultra-hot plasma and having to cope with large electromagnetic forces, while interacting with major systems and other components, the blanket is arguably the most critical and technically challenging component in ITER.

The blanket consists of 440 individual modules covering a surface of 600 m2, with more than 180 design variants depending on the segments' position inside the vacuum vessel and their functionality. Each module consists of a shield block and first wall, together measuring 1 x 1.5 metres and weighing up to 4.5 tons—dimensions  that not only demand sophisticated remote handling in view of maintenance requirements during deuterium-tritium operation, but also an approach to attaching the modules which is far from trivial when considering the enormous electromagnetic forces. 

The first wall is made out of shaped „fingers.” These fingers are individually attached to a poloidal beam, the structural backbone of each first wall panel through which the cooling water will be distributed. Depending on their position inside the vacuum vessel, these panels are subject to different heat fluxes. Two different kinds of panels have been developed: a normal heat flux panel designed for heat fluxes of up to 2 MW/m2 and an enhanced heat flux panel designed for heat fluxes of up to 4.7 MW/m2.

The enhanced heat flux panels are located in areas of the vacuum vessel with greater plasma-wall interaction and they make use of the hyper-vapotron technology which is similar to that used for the divertor dome elements. All panels are designed for up to 15,000 full power cycles and are planned to be replaced at least once during ITER’s lifetime. A sophisticated R&D program is currently under way in Japan for the development of remote handling tools to dismantle and precisely re-position the panels.  

Due to the high heat deposition expected during plasma operation—the blanket is designed to take a maximum thermal load of 736 MW—ITER will be the first fusion device with an actively cooled blanket. The cooling water is fed to and from the shield blocks through manifolds and branch pipes. Furthermore, the modules have to provide passage for the multiple plasma diagnostic technologies, for the viewing systems, and for the plasma heating systems.

Because of its low plasma-contamination properties, beryllium has been chosen as the element to cover the first wall. Other materials used for the blanket system are CuCrZr for the heat sink, ITER-grade steel 316L(N)-IG for the  steel structure, Inconel 718 for the bolts and cartridges, an aluminium-bronze alloy for the pads that will buffer the electromechanical loads acting on the segments, and alumina for the insulating layer. 

The procurement of the 440 shield blocks is equally shared between China and Korea. The first wall panels will be manufactured by Europe (50%), Russia (40%) and China (10%). Russia will, in addition, provide the flexible supports, the key pads and the electrical straps. The assembly of the blanket is scheduled for the second assembly phase of the ITER machine starting in May 2021 and lasting until August 2022. The work will be performed with the help of two in-vessel transporters working in parallel.

In assessing the work presented at the Final Design Review, Andre Grosman, deputy head of Magnetic Fusion Research Institute at CEA and chair of the review panel, enthusiastically commended the BIPT for its achievements since the Preliminary Design Review in December 2011 which were „beyond the expectation of the panel.” He added: „We have singled out the continuity and benefit of the work done by the ITER Organization and the Domestic Agencies within the BIPT framework with a sharing of risk and information among all stakeholders.”

The panel nevertheless pointed out some remaining issues, including a few challenging issues that need to be addressed at the project level. But thanks to the excellent quality of work performed by the BIPT, the ITER blanket design can today be called „approved.” The BIPT can now turn its focus to addressing the feedback received at the Final Design Review, applying the final touches to the design, and preparing for the Procurement Arrangements, where fabrication is handed over to the Domestic Agencies, starting at the end of 2013.

Fusion draws on Japanese traditions

The Japanese people have a long history of creating ceramics of great beauty and elegance. Now they are putting their skills towards the search for materials for future fusion plants — in this case not crafting elegant forms, but elegant solutions: ceramics are nearly impervious to tritium.

In a colloquium delivered at JET last week, Assistant Professor Takumi Chikada from the University of Tokyo outlined promising progress in research into the ceramic coating, erbium oxide, which may prove to be a vital coating for use in tritium-carrying pipework. „Without solving this problem it will be impossible to operate a fusion reactor,” he stated.

Because of its very small size, tritium tends to permeate through materials readily — an undesirable characteristic in a tritium processing plant, where tritium would be exposed to a large surface area as it passes through cooling, ducting and processing pipework.
Assistant Professor Chikada’s results showed that a layer of erbium oxide only tens of microns thick on a steel surface could reduce permeation of tritium by 100 000 times.

Erbium oxide was originally chosen as an insulation coating because it has a high thermodynamic stability and is resistant to liquid lithium-lead — a proposed blanket material for fusion plants, which is corrosive to many materials.

Read more on the EFDA website.

DivSOL wagon rolls EAST



With the EAST tokamak in the middle of an extended maintenance period—during which the ASIPP team in Hefei, China will take the audacious step of installing an ITER-like, full tungsten divertor in the upper part of the vacuum vessel by the end of this year—what better place to hold the latest in the series of regular meetings of the International Tokamak Physics Activity (ITPA) Topical Group on Divertor and Scrape-Off Layer physics. Known in ITPA circles as the DivSOL TG, this group focuses on issues of importance to ITER in the area of heat and particle exhaust from the tokamak plasma and the unavoidable plasma-surface interactions which occur at the plasma-materials boundary. 

Plasma and materials physicists work together within DivSOL to address a host of questions, from movement of material by the plasma and tritium trapping in surfaces, to turbulent transport of heat in the plasma boundary and plasma-facing component lifetime under intense heat fluxes. In common with all ITPA groups, DivSOL is reactive to urgent ITER physics R&D issues and works to find answers to specific requests.

One such example is the flurry of activity stimulated by the ITER Organization proposal in autumn 2011 to eliminate one of the two divertors planned for the first years of ITER operation, up to achievement of burning plasmas. The idea is to go the whole way with a single unit in which tungsten (chemical symbol W) would be the only material intercepting the majority of the tokamak heat exhaust. A single divertor would be a major cost saving to the project, but it is a calculated risk: W is a harder material to work with from the plasma point of view than the carbon fibre composite in originally planned first divertor.

Finding out just how much of a risk, and making sure that a workable design with qualified technology can be ready in time for procurement which must begin next year, was the task set by the ITER Council to the ITER Organization, with a reporting deadline near the end of 2013. All the ITPA groups are lending a helping hand by trying to assess the physics risks of „beginning full-W.” DivSOL has a major role to play given that most, but by no means all, of the issues concern the plasma-materials interface.

Not surprisingly, living with tungsten was a major theme in the 18th DivSOL meeting, hosted by ASIPP from 19-22 March. It was also a record breaking meeting that reunited over 90 representatives from the ITER Members, including about 50 Chinese participants representing universities and technology institutes from all over China. Such high attendance reflects the importance of plasma-materials interaction not just to ITER, but to the long-term future of fusion as a viable energy source. The meeting was also a good example of the less visible, but essential, role which ITPA fulfills in addition to supporting ITER as a vehicle through which newcomers can take part in lively discussions and presentations, in a workshop atmosphere, with experts from across the ITER Members.

The success of any workshop or conference depends to a large part on organization. Our Chinese hosts led by Houyang Guo of ASIPP (and ITPA DivSOL co-chair), provided a seamless environment for the first DivSOL meeting ever to be held at the Institute. The next DivSol TG will be held in Japan in January 2014.

ITPA DivSOL wagon rolls EAST



With the EAST tokamak in the middle of an extended maintenance period—during which the ASIPP team in Hefei, China will take the audacious step of installing an ITER-like, full tungsten divertor in the upper part of the vacuum vessel by the end of this year—what better place to hold the latest in the series of regular meetings of the International Tokamak Physics Activity (ITPA) Topical Group on Divertor and Scrape-Off Layer physics. Known in ITPA circles as the DivSOL TG, this group focuses on issues of importance to ITER in the area of heat and particle exhaust from the tokamak plasma and the unavoidable plasma-surface interactions which occur at the plasma-materials boundary. 

Plasma and materials physicists work together within DivSOL to address a host of questions, from movement of material by the plasma and tritium trapping in surfaces, to turbulent transport of heat in the plasma boundary and plasma-facing component lifetime under intense heat fluxes. In common with all ITPA groups, DivSOL is reactive to urgent ITER physics R&D issues and works to find answers to specific requests.

One such example is the flurry of activity stimulated by the ITER Organization proposal in autumn 2011 to eliminate one of the two divertors planned for the first years of ITER operation, up to achievement of burning plasmas. The idea is to go the whole way with a single unit in which tungsten (chemical symbol W) would be the only material intercepting the majority of the tokamak heat exhaust. A single divertor would be a major cost saving to the project, but it is a calculated risk: W is a harder material to work with from the plasma point of view than the carbon fibre composite in originally planned first divertor.

Finding out just how much of a risk, and making sure that a workable design with qualified technology can be ready in time for procurement which must begin next year, was the task set by the ITER Council to the ITER Organization, with a reporting deadline near the end of 2013. All the ITPA groups are lending a helping hand by trying to assess the physics risks of „beginning full-W.” DivSOL has a major role to play given that most, but by no means all, of the issues concern the plasma-materials interface.

Not surprisingly, living with tungsten was a major theme in the 18th DivSOL meeting, hosted by ASIPP from 19-22 March. It was also a record breaking meeting that reunited over 90 representatives from the ITER Members, including about 50 Chinese participants representing universities and technology institutes from all over China. Such high attendance reflects the importance of plasma-materials interaction not just to ITER, but to the long-term future of fusion as a viable energy source. The meeting was also a good example of the less visible, but essential, role which ITPA fulfills in addition to supporting ITER as a vehicle through which newcomers can take part in lively discussions and presentations, in a workshop atmosphere, with experts from across the ITER Members.

The success of any workshop or conference depends to a large part on organization. Our Chinese hosts led by Houyang Guo of ASIPP (and ITPA DivSOL co-chair), provided a seamless environment for the first DivSOL meeting ever to be held at the Institute. The next DivSol TG will be held in Japan in January 2014.

The Sun never sets on the CODAC empire


Every year in February, when almond trees begin to bloom in Provence, the ITER CODAC team releases a new version of the CODAC Core System.

The 2013 edition (CODAC Core System v 4.0) is more robust, comes with a better operator interface, offers more features, and supports plant systems that need „fast control,” for example plasma control systems that have to react within a strictly defined period of time. „Version 3.0 did it okay,” says ITER Control System Division Head Anders Wallander. „Version 4.0 does it better.”

CODAC (Control, Data Access and Communication) can be described as a software conductor that orchestrates the dialogue between the hundred-odd ITER plant systems …”the system of systems that makes one entity of everything” … the lingua franca that allows the magnets, blanket, tritium plant, cryostat and diagnostics to exchange signals and share information.

Working for the ITER project here and abroad, 55 organizations (Domestic Agencies, fusion labs, contractors) are presently using the CODAC Core System. An infrastructure has been set up to distribute the software to these and future organizations and to keep track of versions used. Training and user support is also provided.

The software package has recently demonstrated its efficiency on the Korean tokamak KSTAR and celebrated its „First Plasma,” so to speak, last June at the Frascati Tokamak Upgrade (FTU) project in Italy.  „The ITER CODAC system is truly becoming a world language,” says Anders.

CODAC is already implemented and deployed to monitor the power consumption on the ITER site, providing the „power people” with a global view and data with which to charge the different contractors operating on site. „With these pilot applications, we’re demonstrating that the system meets our expectations,” says CODAC System Engineer Franck Di Maio. „We’re demonstrating the system’s credibility.”

_To_44_Tx_CODAC users throughout the world are no different from any personal software user: switching to an upgrade is both exciting and challenging. „Although we provide support for older versions, we want to convince companies to upgrade. And the way to do it is to provide new features and make the upgrade easy.” In Franck Di Maio’s v 4.0 User Manual, the list of changes, fixed bugs, and enhancements of all kinds occupy no less than six pages …

Optimizing, upgrading and adapting ITER CODAC is „a process that will never end,” says Anders. „There will always be new requirements—this is the main difference between an experimental facility and a power plant.”

Romanelli sees JET as "main risk mitigation" for ITER

On the afternoon last year when the European tokamak JET attempted first plasma after an 18-month shutdown, Associate Leader Francesco Romanelli remained in his first-floor office. „I wasn’t expecting the machine to perform so faultlessly on its first attempt,” he later explained. „Besides, things had a way of going wrong when I entered the room, so maybe it was better after all.”

That anecdote and others were related by Romanelli at last week’s Inside ITER seminar, during which he gave a first-hand overview of the ITER-like wall campaign that has been running at JET since that first (very successful) day back in August 2011. Three thousand installable items and 16,000 tiles had been replaced in the machine (non-metal carbon tiles were replaced by the metals beryllium and tungsten) to equip JET with the same materials mix chosen for ITER.

Romanelli reported in detail on the experimental results so far: demonstration of low fuel retention, tungsten divertor successfully tested, observations related to the dynamics of disruptions …

„Overall, the operation of the ITER-like wall has been easier than expected, giving us the confidence that the fusion community is making the right choice for ITER. We see JET as the main risk mitigation measure in support of ITER.”

The European Fusion Development Agreement is already looking ahead to other roles for JET—developing plasma scenarios in ITER-relevant configurations and testing the compatibility of the wall with the use of tritium. „JET can provide unique input in a number of technical and operational areas.”

David Campbell, director of ITER’s Plasma Operation Directorate, agrees: „The crucial ITER-like wall experiment will give us insight—ahead of ITER operation—as to how fusion plasmas will behave in the presence of the plasma-facing mixture that we’re planning to use in ITER.”
For more on JET’s ITER-like wall campaign, visit the EFDA-JET website.

First four levels of Tokamak Complex now defined

The heart of the ITER facility will be the Tokamak Complex, comprising the Tokamak Building, the Diagnostic Building, and the Tritium Plant.

The seven-storey Complex measuring 118 by 80 metres and towering 57 metres above the platform will contain more than 30 different plant systems including cooling systems and electrical power supplies, all having physical as well as functional interfaces. As you can tell from the configuration drawing there won’t be much extra room for manoeuvring. The house is pretty busy!

In order to make sure that all the necessary pipes, ducts, structures, cable trays and penetrations are correctly defined before the pouring of the concrete, a Building Integration Task Force was created in April last year to go through the building floor by floor. All the required documentation has now been delivered for the basement level (B2), the lower level (B1) and the equatorial level (L1) according to the agreed schedule with the European Domestic Agency Fusion for Energy and the Architect Engineer ENGAGE. The upper level (L2) has also been reviewed and the data files will be handed over by the end of this month.
The configuration for each level was reviewed in compliance with the safety files and the installation and assembly feasibility of the systems and components. The design also respects the requirements on the civil works such as radiation shielding, fire protection and sectorization, and confinement leak tightness.

For the Level B2 slab, the detailed design of the rebar arrangement will be completed by F4E’s designer Engage by December followed by the review of all the embedded steel plates that will be cast into the concrete to support the heavy loads.

About 55,000 such plates have been identified and tagged in the floors, walls and ceilings of the Tokamak Complex. The review will focus on the exact position of each plate with respect to the concrete rebar grid. Furthermore, for each plate the plate type and anchoring system need to be confirmed. Following the finalization of the design in March 2013, pouring will begin on the B2 slab. This process will continue for the remaining levels of the Tokamak Complex.

Tore Supra ready to go WEST

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.


One more step towards the final green light

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.