UK researchers have proposed a concept of a neutron facility to test materials for future fusion power plants.
The facility, basically a large neutron source, would be used to bombard materials considered for use in tokamaks by a powerful stream of neutrons, creating similarly harsh conditions as occur inside a tokamak.
“We've come up with a workable plan to build a neutron source quickly and at low technical risk, largely using what we know now,” said Michael Porton, from the Culham Centre for Fusion Energy, the lead author of the Facility for Fusion Neutron Irradiation Research (FAFNIR) project.
With the construction of ITER, the world's largest experimental tokamak nuclear fusion reactor, underway, the scientists are already thinking about future and the DEMOnstration Power Plant (DEMO), foreseen as the next step after ITER, which will bridge the gap between ITER and the first commercial fusion power plants.
If approved, the FAFNIR project would give the designers of DEMO crucial data on materials with which to build the machine. It would also serve as a bridge to the planned International Fusion Materials Irradiation Facility (IFMIF), expected to play a similar role for the first generation of commercial fusion reactors.
“It's generally accepted that there's a gap to be filled between today's devices and IFMIF,” Porton said. “Whatever shape the final facility takes, we hope our work is a positive step to get the idea off the ground.”
The extremely fast neutrons produced by fusion reactions in tokamaks carry an energy of 14 million electron volts (MeV) – about 70 times more than photons in hospital X-ray equipment – and pose a threat to the tokamak's structures.
The neutrons cause damage within the structure of the material which leads to swelling through the creation of voids. Subsequently, the material becomes brittle and hardens because of accumulation of hydrogen gases.
Scientists know that to make fusion energy viable, new and extremely sturdy materials need to be developed that are capable of lasting for the designed life of the tokamak.
The materials will also have to be able to get rid of the radioactivity generated during the fusion process to allow for safe decommissioning.
The proposed facility will be able to shoot a beam of 40 MeV deuterium ions at a graphite target, releasing a stream of neutrons to fire at samples of candidate materials and simulating the effect of deuterium-tritium fusion reactions on materials in the reactor.
Using accelerated ion beams to produce neutrons is well established, but at present there are no devices able to produce sufficient neutrons with the correct range of energies that fusion researchers need to work with. As the engineering design for DEMO is planned to be locked down by 2030 such a facility has been included in the European roadmap to fusion electricity, published in 2013.
The neutrons produced in FAFNIR would cause enough irradiation to observe material degradation relevant for DEMO – measured by displacements per atom, a phenomenon leading to material damage.
The FAFNIR proposal has been put together by CCFE, the Science and Technology Facilities Council, the University of Birmingham, the University of Manchester and the University of Oxford.
The consortium foresees building the facility could cost about €300m and the construction would take about seven years followed by a three-year cycle of operation to generate data.