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Bridging the gap to IFMIF | 11/08/2014
CCFE, in a consortium with UK universities and Rutherford Appleton Laboratory, is developing a concept for a large neutron source to test materials for future fusion power plants including the proposed prototype, DEMO, that will follow the ITER project.
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), which is expected to play a similar role for the first generation of commercial fusion reactors.
Fusion scientists and engineers are increasingly focusing on materials research as attention turns to designs for reactors that will put power on the electricity grid. 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. Effects such as embrittlement and hardening of the metal caused by accumulation of helium and hydrogen gases produced by transmutation (transformation of one element into another) mean that special materials must be developed that can stay the course throughout the reactor's lifespan. As a result of transmutations caused by the neutrons, radioactive elements are produced within the tokamak components, so choosing materials that will shed their radioactivity quickly is another priority for safe decommissioning.
But how can we be sure that fusion materials will be up to the task? The surest way is to put them through rigorous tests in conditions similar to those they will face inside a tokamak. That would be the role of the Facility for Fusion Neutron Irradiation Research (FAFNIR) being put forward by the group of UK research partners led by CCFE*.
FAFNIR is designed to accelerate a powerful beam of 40 MeV deuterium ions at a graphite target, in turn releasing a stream of neutrons to fire at samples of candidate materials. The effect will simulate 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' (dpa). Although some DEMO materials will be subject to over 20 dpa, the phenomena leading to damage can be seen at the 10 dpa range, which FAFNIR would approach in three years of operation.
“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,” says Michael Porton of CCFE, who has been instrumental in the FAFNIR design.
He estimates FAFNIR's price tag at about €300 million; a comparatively low cost for a machine making a key contribution to fusion's development. Construction would take seven years, followed by a three year cycle of operation to generate data – well in time for the big design decisions on DEMO. At this stage, though, concepts are still being put forward and the UK's is just one of a number that F4E will consider before hopefully giving a European early neutron source the green light.
Michael Porton adds: “It's generally accepted that there's a gap to be filled between today's devices and IFMIF. Whatever shape the final facility takes, we hope our work is a positive step to get the idea off the ground.”
*The FAFNIR proposal has been produced by CCFE with the Science and Technology Facilities Council, the University of Birmingham, the University of Manchester and the University of Oxford.
1. Plan of the IFMIF device (click to enlarge) – courtesy www.ifmif.org
2. CCFE's Michael Porton