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7. Fuels and resources
We have seen that nuclear fusion reactions involve the lightest nuclei, that is, hydrogen and its isotopes. Currently, the most interesting fusion reaction is the one that occurs between the two heavy nuclei of hydrogen – deuterium and tritium - (at temperatures reaching 100 million degrees):
deuterium + tritium = helium4 + neutron + 17.5 MeV of energy
This reaction is the easiest to bring about, as well as being the most efficient for producing energy. The reaction products are helium4 - the helium isotope – also called the alpha particle, which carries 1/5 (3.5 MeV) of the total fusion energy in the form of kinetic energy, and a neutron, which carries 4/5 (14.1 MeV).
IDeuterium is present in significant amounts in seawater (30 g /m3) .Tritium, which is a radioactive material with a decay time of 12.36 years, does not exist in nature in significant quantities, but it can be generated.
In the future fusion reactor the neutrons, which carry 80% of the energy produced, will get absorbed by a “blanket” placed around the core of the reactor. The blanket will contain lithium (Li), which transforms into tritium and helium according to the reaction:
Li17 + n = He4 + T + n* - 2.5 MeV
Li6 + n = He4 + T + 4.86 MeV
(where n* = slow neutron)
Natural lithium (composed of 92.5% Li7 and 7.5% Li6)
is abundant in rocks (30 parts to a million/unit weight) and is present,
in minor concentrations, in the oceans.
The lithium blanket, together with
other materials, helps to mitigate the effect of the neutrons.
Other
exothermic reactions
There are other exothermic reactions of interest to controlled thermonuclear fusion, that is, reactions involving deuterium and helium nuclei. As they require more extreme conditions for the plasma (e.g., temperatures much higher than 100 million degrees), they are difficult to bring about. However, they can be considered important for fusion reactors in the more distant future because they prevent or limit the strong neutron flux.
In fact, it is the neutrons that are responsible for inducing radioactivity in the reactor materials.
Reaction
probalility vs. temperature
In the figure, “reaction probability” means that its value (for each fusion reaction indicated) multiplied by the density of interacting nuclei gives the number of fusion reactions per unit time and volume.
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