Fusion Power - Fuel Cycle

Fuel Cycle

The basic concept behind any fusion reaction is to bring two or more nuclei close enough together so that the residual strong force (nuclear force) in their nuclei will pull them together into one larger nucleus. If two light nuclei fuse, they will generally form a single nucleus with a slightly smaller mass than the sum of their original masses (though this is not always the case). The difference in mass is released as energy according to Albert Einstein's mass-energy equivalence formula E = mc2. If the input nuclei are sufficiently massive, the resulting fusion product will be heavier than the sum of the reactants' original masses, in which case the reaction requires an external source of energy. The dividing line between "light" and "heavy" is iron-56. Above this atomic mass, energy will generally be released by nuclear fission reactions; below it, by fusion.

Fusion between the nuclei is opposed by the repulsive positive electrical charge common to all nuclei due to protons in the nucleus. To overcome this electrostatic force, or "Coulomb barrier", the kinetic energy of the atoms must be increased. The easiest way to do this is to heat the atoms, which has the side effect of stripping the electrons from the atoms and leaving them as bare nuclei.

In most experiments the nuclei and electrons are left in a fluid known as a plasma. The temperatures required to provide the nuclei with enough energy to overcome their repulsion is a function of the total charge, so hydrogen, which has the smallest nuclear charge therefore fuses at the lowest temperature. Helium has an extremely low mass per nucleon and therefore is energetically favoured as a fusion product. As a consequence, most fusion reactions combine isotopes of hydrogen ("protium", deuterium, or tritium) to form isotopes of helium (3He or 4He) as the fusion end product.

The reaction cross section, denoted σ, is a measure of the probability of a fusion reaction as a function of the relative velocity of the two reactant nuclei. If the reactants have a distribution of velocities, as is the case in a thermal distribution within a plasma, then it is useful to perform an average over the distributions of the product of cross section and velocity. The reaction rate (fusions per volume per time) is <σv> times the product of the reactant number densities:

ƒ = (½n)2 <σv> (for one reactant)

ƒ = n₁n₂ <σv> (for two reactants)

<σv> increases from virtually zero at room temperatures up to meaningful magnitudes at temperatures of 10–100 keV (2.2–22 fJ). The significance of <σv> as a function of temperature in a device with a particular energy confinement time is found by considering the Lawson criterion.

Perhaps the three most widely considered fuel cycles are based on the D-T, D-D, and p-11B reactions. Other fuel cycles (D-3He and 3He-3He) would require a supply of 3He, either from other nuclear reactions or from extraterrestrial sources, such as the surface of the moon or the atmospheres of the gas giant planets. The details of the calculations comparing these reactions can be found here.