EDITOR’S CHOICE
STANDING THE HEAT
MATMATCH
MATERIALS SELECTION FOR FACING
NUCLEAR FUSION PLASMA
As of today, there is still no net electricity
contribution from nuclear fusion in our
power grids. Here, materials scientist
Benjamin Spilker, on behalf of materials
search engine Matmatch, explores the
world of nuclear fusion and the material
characteristics that make research possible.
Plasma physicists were surprised by the
behaviour of the plasma more than once
in the decades of fusion plasma research. A
highly prominent and important discovery
has been made with the high-confinement
mode (H-mode), in which the plasma
suddenly changes its characteristics to a
steep pressure gradient at the edge and an
overall better confinement.
Even though we have come quite far,
our understanding of the underlying
mechanisms of the H-mode transition is still
incomplete. Thorough modelling efforts are
ongoing to reliably predict the operational
scenarios and conditions in future fusion
devices.
Despite all of the measures that can be
taken to control the fusion plasma, the
surplus energy and the product of the fusion
reactions in the form of helium particles and
other impurities need to be removed from
the plasma in order to allow for a continuous
operation and the generation of electricity.
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So-called plasma-facing materials are
deployed to protect the vacuum vessel from
the hot plasma particles and to efficiently
remove the incident power fluxes. Nuclear
fusion is sought after as a clean and
sustainable energy source. Because of this, all
in-vessel components are carefully selected
in order to avoid the generation of any long-
term radioactive waste that would require
geological disposal.
The anticipated deuterium-tritium (DT) fusion
reaction generates a continuous flux of highly
energetic neutrons. These neutrons carry the
potential to knock atoms from their lattice
positions, causing material defects, but also
to transmute atoms (transform the atom into
a different chemical element) if a neutron is
captured.
This neutron irradiation environment requires
that the materials, which are in place to shield
the rest of the machine from these neutrons,
do not degrade to the point where they
could overheat and fail under operational
conditions.
Another important aspect for the
environmental impact of fusion is neutron
induced activation. When stable atomic
nuclei capture a neutron, they can enter an
excited state and become radioactive. These
newly formed radioactive isotopes decay
with a certain half-life time, which should
remain within specified limits.
Unfortunately, a major drawback of carbon
has been discovered in the course of said
research. Even when subjected to only
small doses of neutron irradiation of 0.2 dpa
(displacements per atom), which could be
accumulated in a couple of days in a running
fusion reactor, the thermal conductivity of
carbon decreases to less than 20% of its
original value. As a consequence, the material
quickly overheats because the incident heat
flux cannot be transferred to the cooling
channels efficiently enough.
From these considerations and many others,
it has been concluded that all-metal plasma-
facing materials, such as beryllium, are the
best way to go forwards.
For design engineers working on these
projects, Matmatch’s materials comparison
platform provides the ideal means of
researching and sourcing materials suitable
for handling nuclear fusion. By choosing the
right materials, engineers and scientists can
make a brighter future for fusion power.
www.matmatch.com