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ITER NEWSLINE 262
Take in the view and enjoy it while you can. In a couple months, the spectacular pattern formed by the 493 columns in the Tokamak Complex Seismic Pit—the emblematic image of ITER construction—will have vanished from view.
The lone scaffolding that was erected two weeks ago at the centre of the pit has already been joined by dozens of others. "Everything will be covered," says ITER Nuclear Buildings Section Leader Laurent Patisson. Once the structures are in place, a steel rebar skeleton will be installed on top of them and pouring of the 1.5-metre-thick B2 slab will begin plot by plot (or "pour by pour") —a process which should take about nine months.
"The propping and formwork structures will support the weight of the rebar and concrete until the hardening of the concrete makes it possible to transfer the effort onto the seismic pads," adds Laurent. "A total of 15,000 m³ of concrete will be poured, which—added to the weight of the rebar (~ 4,000 tonnes)—will amount to a load of some 37,500 tonnes."
Creating a reinforced slab over formwork structures that are supported by braced scaffoldings is a common technique. The very same process was implemented, two years ago, on the nuclear research reactor Jules-Horowitz (RJH) that is being constructed at CEA-Cadarache.
Every installation has its own geometry, however, which is reflected in the complex pattern of the steel reinforcement bars. "We have to demonstrate constructability prior to pouring the actual slab," explains Laurent. "We also have to qualify the concrete and test the efficiency of the vibration techniques."
To that effect, a 150 m² mockup has been created on the platform to the west of the Seismic Pit. "Although 3D models of the rebar arrangements have been produced, we need a hands-on experience of the difficulties we may encounter." Four different areas of rebar, presenting specific challenges (density, complexity), will be reproduced at 1:1 scale in the mockup. Work on the mockup began last week.
The mockup will also allow the practice installation and testing of the anchor plates that will be embedded into the concrete. These thick steel plates of various sizes, all dotted with long spikes (see picture), will reach deep into the concrete.
Once embedded into the concrete mass, the plates form an exceptionally solid base onto which equipment such as magnet feeders, drain tanks or cubicles can be welded. The Tokamak Complex will contain some 60,000 such plates.
"What is at stake beyond the B2 slab," summarizes Laurent, "is the robustness of the whole Tokamak support, from the cryostat bearing down to the ground."
Work progressed steadily last week on propping operations inside the Tokamak Pit and on its smaller mockup sibling nearby.
The meeting took place from 18-19 March at the headquarters of the European Domestic Agency in Barcelona in the attendance of high-level representatives of the ITER Organization and seven ITER Members.
Since the last special MAC meeting held in August 2012, the ITER Organization has worked closely with Domestic Agencies to complete the integration of Detailed Work Schedules (DWS)—detailed schedules that exist for every component or system. The ITER Organization and Domestic Agencies completed the integration of the remaining DWS, namely main vacuum vessel, ion cyclotron antenna, poloidal field coils and toroidal field structure, which will allow for monitoring of the schedule.
MAC requested that the Unique ITER Team continue to make significant efforts to take action focusing on super-critical milestones and to take all possible measures to keep to the Baseline schedule. The ITER Organization and Domestic Agencies are committed to doing their best to implement this request.
In pre-ITER times, the world production of niobium-tin (Nb3Sn) strands did not exceed 15 tonnes per year. Discovered in 1954, this intermetallic compound that exhibits a critical temperature of ~18 K and is able to withstand intense magnetic fields was used mainly in high field coils and nuclear magnetic resonance equipment.
To match the needs of ITER's 19 toroidal field coils (18 plus one spare), the world production capacity of Nb3Sn strand had to be ramped up by one order of magnitude. As of today, 400 tonnes of Nb3Sn have been produced by the industry of the six ITER Domestic Agencies involved in conductor procurement, representing 85 percent of toroidal field coil needs.
Nb3Sn conductors will also form the core of the central solenoid, the backbone of the ITER magnet system. Strand production has been launched in Japan for the lower module (CS3L) and conductor lengths will be shipped at a later time to the US where the central solenoid will be manufactured.
For ITER's third major magnet system—the poloidal field coils—because the magnetic field they produce is less intense, they can be manufactured from the metallic alloy niobium-titanium (NbTi), which is cheaper and easier to produce than the brittle Nb3Sn.
The Russian-European collaboration that procures NbTi strands for ITER has already produced 80 tonnes of Strand 1, destined for poloidal field 1 and 6. China, responsible for the procurement of conductors for poloidal field coils 2 to 5, has registered nearly 50 tonnes of of NbTi Strand 2 into the Conductor Database (this essential tool monitors the strand, cable, jacket and conductor production of each Domestic Agency). China will send its first poloidal field conductor shipment to the ITER site within the next two months.
Altogether, conductors for the magnet systems account for 13 percent of the total ITER Project credits.
These figures and other information relating to correction coils, feeder conductors, manufacturing issues, quality control, test results and technical issues were presented and discussed at last week's meeting on conductor production status at the Château de Cadarache.
The Conductor Meeting, which has been held twice a year since 2008 (at ITER in the spring and in one of the "Conductor Domestic Agencies" in the fall), gathers representatives of the ITER Organization, the Domestic Agencies and their suppliers.
"It is an opportunity to share and benefit from each other's experience," says ITER Superconductor Systems and Auxiliaries Section Leader Arnaud Devred who traditionally chairs the meeting. "As all conductors are now in the production phase, the feeling in our community is definitely optimistic. Everything is moving ahead and the collaborative spirit, not only between the ITER Organization and the Domestic Agencies but between the Domestic Agencies themselves, is truly excellent."
Scientists at the US Department of Energy's Princeton Plasma Physics Laboratory (PPPL) and the National Institute for Fusion Science (NIFS) in Japan have developed a rapid method for meeting a key challenge for fusion science. The challenge has been to simulate the diagnostic measurement of plasmas produced by twisting, or 3D, magnetic fields in fusion facilities. While such fields characterize facilities called stellarators, otherwise symmetric, or 2D, facilities such as tokamaks also can benefit from 3D fields.
Researchers led by PPPL physicist Sam Lazerson have now created a computer code that simulates the required diagnostics, and have validated the code on the Large Helical Device stellarator in Japan. Called "Diagno v2.0," the new program utilizes information from previous codes that simulate 3D plasmas without the diagnostic measurements. The addition of this new capability could, with further refinement, enable physicists to predict the outcome of 3D plasma experiments with a high degree of accuracy.
Read more here.