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Fusion Basics
Fusion is the process where two light nuclei collide and fuse to form a
heavier nucleus. A large amount of energy is released in the process and the most
suitable reaction occurs between the nuclei of two heavy forms (isotopes) of
hydrogen; deuterium and tritium. After fusing, helium and a neutron are
produced.
Water is made of hydrogen and oxygen. There is one atom of deuterium for
every 6000 atoms of hydrogen. So deuterium fuel is plentiful. The energy
released by fusing the deuterium in 1 litre of seawater is equivalent to that
released by burning 30 litres of gasoline.
Tritium is unstable and decays into helium with a half-life of 11 years. So
there is no natural tritium. It can be produced within the fusion reactor by
installing a blanket of lithium around the fusing plasma. When hit by the
neutron generated in a fusion reaction, lithium breaks into tritium and helium.
The tritium is rapidly extracted from the blanket and sent into the plasma to be
fused. So overall, the lithium is consumed and turned into helium. Lithium is an
abundant, inexpensive metal.
The fuel for fusion is therefore deuterium and lithium, both abundant and
inexpensive. The final by-product is helium, a safe, stable and environmentally
friendly gas.
However, there is no such thing as a free lunch. Creating the conditions for
the fusion reactions to happen is extremely difficult.
The difficulty is that both nuclei are positively charged and therefore repel
each other. In order to bring the nuclei close enough to fuse, they must be
hurled at each other at high velocity. Also, the nucleus is very small, so the
probability of a good head-on collision is low. The hotter the gas, the faster
the atoms travel. The velocity needed to overcome the repulsion corresponds to a
temperature of 150 million degrees C. At that temperature, the collisions
between the atoms are so violent that the electrons are knocked off their
nuclei. You obtain a soup of free electrons and free nuclei called a plasma. It
takes a considerable amount of energy to heat the deuterium-tritium mixture to
that temperature. You therefore need to make enough fusion reactions happen to
produce more energy than initially invested to heat the gas, which is a condition called
break-even. The denser the plasma, the more often head-on collisions will
happen. The plasma must also be kept hot long enough for the fusion reactions to
produce the desired energy.
Keeping together a dense, 150 million degree C plasma for any period of time
is the big problem.
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