Technology R & D

The design, construction and operation of future advanced fusion reactors requires the development of a number of technologies, such as superconducting magnets, vacuum vessel, the breeder blanket system and shielding, heating and current drive systems, fuel cycle, and diagnostics. Many of the required technologies have already been developed and tested by the fusion research community and its industrial partners.

Superconducting magnet technology

Figure 1:Prototype of ITER
coils being tested at the
TOSKA facility in Karlsruhe
© Image: EFDA

Very strong magnetic fields are required to confine the plasma in the vacuum vessel and prevent it from touching the walls. If conventional (copper) electromagnets are used, too much energy is wasted in the form of heat. To limit the energy needed to produce the magnetic field, super-conducting magnets are used. These magnets have to operate at liquid helium temperature (4 K or -269 °C). At this low temperature the resistance of the magnet´s special alloy falls to zero and the energy required to energise the magnet is greatly reduced. 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. Even when cooled with liquid helium, these magnets would warm up to much if they would be operated in normal air - the warm air would heat the magnets. That is why the magnets need to be kept in vacuum.

Super-conducting magnets are made from kilometres 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 blanket is the structure surrounding the plasma. It´s function is twofold: first of all, the neutrons produced by the fusion reactions - which are not confined by the magnetic field as they have no charge - penetrate the blanket and react with lithium, producing the fusion fuel tritium. The tritium can then be extracted, processed and added to deuterium for refuelling the reactor. The second function of the blanket is to remove the heat deposited by the fast neutrons. The heat is removed by a cooling fluid, and in a commercial reactor can be used to produce electricity.

Figure 2:Breeder blankets will be complex technology © Image: FZK/ IMF III

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

The neutrons produced by the fusion reaction carry a high energy, which means that 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.

Figure 3:Irradiation test samples for
divertor materials © Image: ENEA

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 long decay times for the radioactivity because of the alloy components and impurities found in materials such as conventional steel.

Figure 4:Full scale vertical
divertor target used to extract
gases from the plasma
© Image EFDA

Fusion reactors will require advanced low activation materials to ensure fusion waste will not be a long-term burden to future generations. Advanced materials with low activation and good thermo-mechanical properties such as low activation steel and silicon carbide composites 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.

In addition, there is a large programme devoted to manufacturing techniques needed to fabricate reactor components including novel welding processes, such as high-pressure electron beam welding, hybrid metal inert gas and laser welding, which could improve quality and reduce manufacturing time and cost.

Fabrication of reactor components

Figure 5:High power laser welding
(11 kW) for vacuum vessel sectors.

The future construction of advanced fusion devices requires the development of a whole range of sophisticated processes and manufacturing techniques. Therefore, as well as research on the materials, there is a large programme devoted to manufacturing reactor components. Novel welding processes, such as high-pressure electron beam welding, hybrid metal inert gas and laser welding, are being investigated in order to improve quality and reduce manufacturing time and cost. Although these techniques are being developed within the fusion programme because of its specific needs, they have a very wide range of application. Improvements are also being made in the superconducting material and their surrounding structures that are used in the fabrication of superconducting magnetic coils which will increase their operating margins and reliability.

Remote handling

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.

Figure 6:ITER divertor remote
handling test platform.

On JET, engineers have mastered remote handling technology and can now replace all the necessary experimental components in the machine. In JET the whole of the inside of the machine is modelled on a computer. The operator types in the coordinates of the component he wants to work on and then with the press of a button sends the robot arm to the component. The operator then takes over control of the robot arm for the final delicate manoeuvres.

Figure 7:Remote handling device
in JET © Image: Jet

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 at the first time it is used.

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 removal, replacement and refurbishment of a divertor cassette. 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

Figure 8:Large vacuum pumps
are needed for ITER © Image: EFDA

In a fusion power plant, cryogenic techniques, which make use of extremely low temperatures, are used to remove the waste and impurities from the plasma, cool the super-conducting coils to allow them to operate, separate the waste gasses into their different individual components for disposal or recycling, provide the cooling for the Radio Frequency heating sources and control the gas pressure of neutral beam 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.