Nuclear fission, fusion and fusion requirements

Nuclear fission is the splitting of a heavy nucleus (U-235, Pu-239) after absorbing a neutron into two medium nuclei, with 2–3 neutrons and about 200 MeV of energy per fission. Nuclear fusion is the joining of two light nuclei (isotopes of hydrogen) into a heavier nucleus with release of energy, as in the sun.

Main reaction: $^2_1D + {}^3_1T \rightarrow {}^4_2He + {}^1_0n + 17.6\ \text{MeV}$

Requirements for fusion

Nuclei are positively charged and repel each other (Coulomb barrier). To fuse they must have very high kinetic energy, which gives these requirements:

1. High temperature: about 100–150 million K for D-T (10–15 keV) so the fuel is a fully ionised plasma.
2. Sufficient density $n$ of the plasma, so that collisions are frequent.
3. Confinement time $\tau$: the plasma must be held away from walls long enough.
4. Lawson criterion: to produce more energy than is spent in heating, the product $n\tau \ge 10^{20}\ \text{s/m}^3$ for D-T at about 10 keV (triple product $nT\tau \approx 3\times10^{21}\ \text{keV s/m}^3$).
5. Fuel supply of deuterium (from seawater) and tritium (bred from lithium in the blanket), and purity of plasma.

Confinement approaches

• Magnetic confinement: the plasma is held in a magnetic field in a doughnut-shaped chamber (tokamak, stellarator). Low density, seconds-long confinement.
• Inertial confinement: a small fuel pellet is compressed and heated by lasers or ion beams to extremely high density for a very short time (about 10⁻¹¹ s), so the fusion burns before the pellet blows apart.