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. |
Powered by Discuz! X3.2 © 2001-2013 Comsenz Inc.