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Nuclear fusion: What of the future? 209

5.3.3 Breeder blanket

The energy generated by the fusion plasma must be removed to generate electricity,
and new tritium fuel must be created. This is the role of the breeder blanket, a structure
that sits between the plasma and the rest of the machine to absorb neutrons and turns
them into something useful. The blanket contains a neutron multiplier such as beryl-
lium or lead, which allows a single fusion-produced neutron to create more than one
new tritium atom, and a lithium-6-enriched breeder material, which produces that new
tritium. The blanket also results in some energy multiplication due to the exothermic
nature of the neutron multiplication and tritium-breeding reactions.
This is also a replaceable component. However, the lifetime is generally limited by
neutron damage to the structural materials, not consumption of the neutron multiplier
or breeder materials, which must be separated and recycled.

5.3.4 Superconducting magnets

The stability and confinement of a magnetized plasma is inherently limited by the
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magnetic field applied. Tokamaks use powerful magnets generating fields 10 times
greater than Earth’s natural magnetic field. This requires large magnetic coils, which
carry high currents. In a standard conductor such as copper, these currents cause
heating and, for a power reactor, this energy loss would be economically prohibitive.
ITER, JT-60SA, and subsequent designs therefore use superconducting magnets,
which can carry such currents without deleterious losses. However, designing these
very large structures, which may be >20m across and have to bear high (and cyclical)
stresses at cryogenic temperatures, is a significant engineering challenge. Depending
on the design and superconductor used, it may be necessary to sinter the entire coil,
which must also be constructed on site as it will be too large to transport to a separate
location. During operation, the coils must be maintained at near absolute-zero temper-
atures and shielded from neutron radiation, which would disrupt the superconductiv-
ity. High-temperature superconductors are starting to become available but cannot yet
be produced in the quantity required for a large tokamak. These new superconductors
may permit much higher fields, although as the forces on a magnetic coil scale with the
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magnetic field pressure (∝B ) this also requires stronger structural materials.
A large fraction of the estimated cost of a fusion power plant is the magnets and
related systems (see Fig. 5.6).

5.3.5 Remote handling

Following a fusion burn, the interior of a fusion reactor will be a very hostile place
awash with ionizing radiation. Robotic systems must be used to remove and replace
components, which must then be detritiated and stored until safe to recycle. As
these components must be moved around the plant into storage with the risk of tritium
migration during operations, most of the space around and above the reactor and the
storage are also required to be fully robotic in normal operations.