The levels of performance required by ITER in the field of cryomagnetism are extremely challenging. The conductor, made of Nb2Sn and cooled by liquid helium,must sustain a current of 40 kA in a magnetic field of 12 T. The cable consists of about a thousand strands, each with a diameter of 0.7 mm, placed together in a stainless steel matrix surrounding a channel through which the helium coolant flows. Samples 4 m long have already been successfully tested in the SULTAN facility (Euratom-Switzerland).
A model coil for the ITER toroidal magnet will be manufactured in Europe and tested in the TOSKA facility (Euratom-FZK, Karlsruhe, Germany). Here, the coil structures can be subjected to forces of up to 100 MN.
The components which line tokamak's vacuum chamber internal walls undergo high thermal flows (up to 15 MW/m2 in the case of the divertor plates). Development work on materials with good thermomechanical properties, which would be brazed onto the metal supports where the coolant circulates, is being concentrated on elements with a low atomic number, such as Si, C, Be or B. Testing facilities have been constructed, in particular on the Framatome site at Creusot, France, to simulate disruption conditions (1000 MW/m2 for 3s) as well as continuous operation with a thermal flow of 60 MW/m2. These conditions are simulated on large models (up to 2 m long).
The total tritium inventory in a fusion reactor would be about 1 kg whilst the amount discharged into the environment during normal operation should be less than 2 g per year, so that the dose received by the general public would still be less than 1% of the dose due to natural radioactivity. It will not be easy to construct and maintain tritium circuits to meet this high standard of reliability. Specialized tritium-handling laboratories are working to develop methods (such as cryodistillation and gas chromatography) for purifying the gases which leave the torus, ways of storing them on uranium beds, high-capacity pumping systems, etc. Valuable lessons have been learned from the tritium storage, distribution and reprocessing system designed and already applied at JET. In particular, the amount of tritium remaining in the vacuum chamber after the November 1991 experiments was reduced to a very low level after several purgings (discharges in deuterium). In Europe, a good deal of the research into tritium technology is undertaken at the Joint Research Centre at Ispra (Italy) and at the FZK facility in Karlsruhe (Germany).
The purpose of safety studies is to describe the consequences of the major referred accidents (loss of coolant, electric power failure, consequences of an accident in the tritium system, etc.). In the event of an accidental failure, the plasma would be extinguished within a very short time (┤ 5 s) and no melting of critical components (such as the divertor) would occur. Detailed studies on these topics have been carried out by the NET (Next European Torus) research team.
Some materials will become radioactive during the lifetime of a reactor, and will have to be processed as radioactive waste. Although the volume of activated material is comparable with that of the waste from a fission reactor, since fusion waste contains no actinides and is shorter-lived, the biological hazards presented by fusion waste are, after 10 years, one thousand times smaller than those associated with fission waste.
One of the long term aims of the materials development programme is to use components which can be recycled after 50 or 100 years at the most.
Robots specifically designed for changing the modules of the tritium breeding and coolant blankets in a tokamak-type fusion reactor are being developed at the Joint Research Centre, Ispra, and at the FZK facility in Karlsruhe.
For carrying out operations inside the vacuum chamber, large robot arms have been built at JET, capable of lifting 1 tonne at 9 m and 400 kg at 14 m. Maintenance and repair work around tokamaks requires very powerful telemanipulators. (At JET, a telescopic robot arm 10 m long, transported by a 40 tonne crane, can lift objects weighing 400 kg within a useful volume of 68 000 m3.)
Although the extrapolation of technology for heating at the ion cyclotron frequency is a relatively straightforward process, it is much more difficult to design equipment for generating waves at the electron cyclotron frequency. That is why Europe supports the industrial development of millimetre-length wave sources (gyrotrons) having a power rating of at least 1 MW for several seconds.
Similarly, careful attention is being paid to neutral beam injectors based on negative ion beams, the neutralization of which at very high energy (500 to 1000 kV) is more effective than in the case of positive ions. A development programme for such injectors is underway.
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