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[[Category:Technology]]
Thermonuclear fusion involves combining the nuclei
of two or more light atoms to produce the nucleus of a
heavier atom. Fusion requires tremendous heat and pressure
to overcome nuclear forces, but liberates more energy
than was used to initiate the reaction. Hydrogen bombs
and stars demonstrate the power of fusion.

Several different fusion reactions exist, generally
involving the fusion of various isotopes of one or both of
the lightest elements, hydrogen and helium. Solar fusion
involves a series of reactions that combine hydrogen
nuclei (protons) to form helium nuclei, emitting neutrinos,
positrons, and electromagnetic radiation (including
heat and light) in the process. This type of “proton-proton”
fusion, although very efficient, is nearly impossible
to achieve anywhere but inside a star, which creates
the necessary pressures and temperatures in its core by
virtue of its immense size. Human technology uses other
means to produce a fusion reaction. In a hydrogen bomb,
these conditions are achieved by exploding a nuclear fission
bomb as a trigger, but the reaction is over in an
instant. Aself-sustaining fusion reaction that can power
a city or drive a spacecraft is trickier.

The most successful method has proven to be
magnetic confinement. Gas is ionized, forming a plasma,
which is then squeezed by magnetic fields until it is hot
and dense enough for fusion to take place. Fusion research
initially concentrated on the deuterium-tritium (D-T) reaction,
which required the lowest ignition temperature. This
fuses two isotopes of hydrogen into helium, liberating vast
quantities of energy in the process. The majority of its fuel
is an isotope of hydrogen called deuterium, which is ordinary
hydrogen plus an extra neutron. Deuterium is fairly
common: in the form of deuterium oxide (heavy water) it
forms one part in 5,000 of ordinary water, and can be distilled
at some expense using electrolysis. Tritium is a rare
radioactive isotope of hydrogen, but can be “bred” by surrounding
the fusion reactor core with a jacket of the element
lithium, which transforms into tritium under neutron bombardment.

However, D-T fusion has a disadvantage: much of the
energy liberated is in the form of energetic neutrons. Neutrons
are dangerous and cannot be directly converted into
electrical power. The neutrons must heat water, which produces
steam, which drives a turbine, all of which adds extra
bulk and cost. Moreover, the bombardment of neutrons irradiates
and degrades the structural material of the reactor
itself. Even with a careful choice of structural materials, this
still means a high maintenance and upkeep cost. Finally, tritium
is an essential component in hydrogen bombs, and as
such the global use of commercial reactors that require or
breed tritium does not help nuclear nonproliferation. As a
result, D-T fusion reactors failed to displace other types of
power plants on Earth. A few were built as experimental
systems, and some are still used in space, especially by the
Red Duncanites, but in general, they have been superseded
by D-He-3 reactors.

Second-generation fusion reactors fuse deuterium with
helium-3, a rare isotope of helium. The He-3 reaction
requires higher temperatures to ignite (and thus awaited the
development of more advanced magnetic confinement technology),
but its main products are charged particles instead
of neutrons.

A D-He-3 reactor is environmentally safer, and does
not require the same heavy shielding. (There is a tiny
amount of radiation produced by secondary reactions, so
some shielding is needed.) The charged particles are also
easier to convert into electricity. This means a D-He-3 reactor
can be lighter, more efficient, and more easily maintained.
The smallest present-day D-He-3 reactors mass several
tons and generate megawatts of energy. Building-sized reactors
generating a gigawatt of energy are common for cities,
producing power that costs a few pennies per kilowatt-hour.
D-He-3 reactors are also used in many spacecraft, space
habitats, and colonies, powering energy-intensive processes
such as agriculture, desalination, heavy industry, electrolysis,
and terraforming. Fusion torch drives are
variations on these reactors; pulse drives use
different technology.
===He-3 Mining===
The He-3 concentration on Luna is small,
only a few parts per billion. It requires 500,000
tons of raw material (an area of about 1,000
square yards to the depth of four inches) to produce
one pound of He-3. Lunar processing
plants use automated machinery: robot bulldozers
to scoop up the regolith, ovens to bake the
soil to 1,300°F, conveyors, and waste processing
plants. This is a huge amount of effort, only
justified by the worth of each pound of He-3,
which can generate staggering amounts of energy
when fused with deuterium. With all the
other costs of operating a lunar mining base, the
profits are not huge. However, a side effect of
the processing is that it also yields economically
useful quantities of elements such as oxygen
and hydrogen, which support other Luna
colony projects.

Extracting He-3 from Saturn (and potentially,
from other gas giants) is cheaper, as it can
be scooped directly out of the atmosphere. Specially
designed drone scoop craft dive into the
atmosphere and use high-thrust fission rockets
to lift gas out to orbiting refineries. The gas is
refined into He-3, then shipped via fusionpowered
tanker to Earth or elsewhere. A few
thousand tons are used annually (a tanker every
few months), but demand is expected to double
every 15-20 years. Even so, there’s enough
He-3 in Saturn alone to last centuries, and more
in the other gas giants.

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