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Taking no chances with ITER coils | 24/12/2013
The hopes for fusion power ride on the success of the next big international experiment, ITER. So when it comes to keeping ITER running safely, nothing is being left to chance. CCFE is using the latest computer modelling techniques to predict ‘worst case scenarios' for the magnets that will control the plasma of fusion fuel inside the machine; meaning the project team at ITER can ensure their protection systems are ready for anything.
ITER will use a series of coils around its vacuum vessel to form a magnetic ‘cage' to confine and shape the hot plasma required to produce energy. The magnets will work in much the same way as in present-day tokamaks such as JET and MAST at Culham, but with some crucial differences. The coils on ITER will be much larger and will create a stronger magnetic field. They will also be superconducting – cooled by a cryostat – requiring less power to run and enabling the machine to operate for long periods of time.
Increased specifications bring bigger risks if any faults in the magnets were to occur. Although a major failure is extremely unlikely, it is one of the main potential hazards once the giant ITER tokamak is up and running. The coils' proximity to the steel vacuum vessel means that a quench in the superconducting material – a breakdown causing resistance to enter the magnets – could cause arcing which damages the vessel as well as the coils themselves. With up to 41 gigajoules of energy in the main set of magnets (bearing in mind that one gigajoule can melt a tonne of stainless steel) the scale of the issue becomes clear.
Built-in quench detectors will give ITER operators early warning of any problems. But back-up systems need their own back-up; the plant must be prepared for all eventualities. The kind of accident sometimes seen in fission power stations is impossible in fusion due to the inherent safety features of tokamaks. However, ITER will still be a nuclear facility containing radioactive material, and it is essential to have a full picture of all possible scenarios and to design mechanisms to prevent them from happening.
Enter CCFE, which has recently won a contract to update ITER's magnet failure predictions. Shangliang Zheng is leading the project:
“My role is to look at the ‘what ifs',” she explains. “If a magnet fails, what effect will there be on the current and temperature? What will the consequences of thermal damage be? Recent advances in computing mean we're able to run more detailed scenario models so ITER can build on the comprehensive plans they already have in place.”
In addition to the obvious need to guard against accidents, there is another, more immediate reason for carrying out the work. If French nuclear regulators are not satisfied that all safety questions continue to be fully addressed, they have the power to step in and halt design and construction of the magnets, which would be a setback for the whole ITER project.
CCFE has been selected for the contract based on its experience in operating tokamaks and all-round capability in fusion technology, but will be calling on the knowledge of superconducting magnet experts from industry (such as world leaders Oxford Instruments), academia (Durham University) and the fusion community (Karlsruhe Institute of Technology) during the studies.
“It makes sense to get as much information as we can about the risks, however minimal,” says Shangliang. “With something as important as this, prevention is far better than cure.”