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The design, construction and operation of future advanced fusion reactors require the development of a number of technologies for both the core and auxiliary systems.
Overview of technology research areas
The aim of the fusion technology research programme is to meet the technology needs of the next generation of experimental tokamaks, like ITER, and the future conceptual fusion power plants (DEMO).
The scope of work in the area of Next Step fusion technology within the EFDA framework will includes development and validation of key technologies, such as superconducting magnets, vacuum vessel, the breeder blanket system and shielding, heating and current drive systems, fuel cycle, and diagnostics.
Major examples of advanced technology for fusion applications are presented here.
Information on heating and current drive systems and diagnostics can be found on the Plasma Engineering page .
Superconducting magnet technology
Liquid helium at 4 K is continuously passed around the magnet strands to keep it at low temperature and ensure it remains in a super-conducting state. Once energised, these magnets can operate continuously with very high efficiency and therefore are perfect for a steady state fusion reactor. As these magnets run at liquid helium temperature it is necessary to operate them in vacuum to prevent the heat in the atmosphere from boiling off the helium.
Super-conducting magnets are made from kilometers of brittle ceramic strands that are packed together and encased in a steel jacket. These jacketed strands are then wound into the final shape of the magnet. One of the major engineering challenges for fusion has been to be able to manufacture very large superconducting magnets that operate reliably and safely. The superconducting magnets are the most expensive components of a fusion reactor.
Breeder blanket technology
The ‘breeder blanket’ technology is vital for heat transfer and fuel generation in future fusion power plants. The energetic neutrons released from fusion reactions do not interact with the plasma. The role of the blanket, which will surround a commercial reactor, is to slow the neutrons, recovering their energy and using them to transform lithium into tritium. The tritium can then be extracted, processed and added to deuterium for refuelling the reactor. The blanket also acts as effective shielding by capturing the neutrons.
A number of different concepts are being explored for breeder blankets. This technology will have to work at sufficiently high temperatures in a commercial reactor to provide efficient heat exchange to generate steam for the electricity generating process, whilst continuing to breed at least one tritium atom for every fusion reaction in the plasma. Research in this area is concentrating on the use of liquid-cooled lithium-lead and helium-cooled solid ceramic breeder pebbles.
Advanced Materials for a fusion reactor
The neutrons produced by the fusion reaction carry a high energy so, as well as breeding tritium in the lithium blanket, they can also interact with the walls of the plasma reactor changing the characteristics and radioactive properties of the wall materials. This process is called activation.
The total amount of radioactive waste a future fusion power plant could generate depends on the materials used in its construction. If conventional building materials were used this could result in very long decay times for the radioactivity because of the alloy components and impurities found in materials such as conventional steel.
Fusion reactors will require advanced low activation materials to ensure fusion wastes will not be a long-term burden to future generations. These are being developed within the long term fusion R&D programme. These materials must be resistant not only to neutrons but also to high surface heat loads and thermal cycling. To assess these materials it will be necessary to construct a test facility that can provide a similar neutron environment to that of a future fusion reactor. A proposal for such a facility is currently being considered under an international collaboration called IFMIF. To obtain more information on materials development visit the EFDA website, EFDA report : ”European Material Assessment Meeting”.
Fabrication of reactor components
The internal structure of a fusion reactor will become radioactive during operation due to neutron radiation and the use of Tritium. It is necessary, therefore, to be able to replace components inside the machine remotely.
On JET, engineers have mastered remote handling technology and can now replace all the necessary experimental components in the machine using a mascot master-slave technique.
In future reactors, such as ITER, robust and reliable remote handling equipment must be designed. This equipment must be capable of manipulating components weighing up to 50,000 kg. To start the design process, virtual prototyping is used. This uses a computer to model in great detail all the movements and mechanical behaviour of the robot so that the engineers can be certain that the equipment will perform first time.
This essential technology for ITER and future reactors is being developed in Europe. The conception of the remote handling techniques and the successful demonstration of the basic principles on full-scale mock-ups has been achieved.
In particular, a large R&D project has demonstrated the basic feasibility of the remote maintenance scenario for the ITER divertor which includes the in-vessel cassette removal/replacement and hot cell refurbishment. For this purpose, two full-scale remote handling facilities at the ENEA research centre at Brasimone, Italy have been constructed and tested.
Cryogenics and Vacuum systems
Large scale vacuum systems are required to ensure an ultra high vacuum in the large reactor vessels that will be used by commercial fusion power stations and to maintain the vacuum surrounding the superconducting magnets.
Developing all these technologies has already provided ‘spin-off’ benefits to numerous parts of European industry .
To know more about technology R&D go to the EFDA website.