Nuclear fusion

What fusion is

Nuclear fusion is the joining of two light nuclei to form a single heavier nucleus, releasing energy.

• Fusion is the process that powers the Sun and every other star.
• A typical fusion reaction joins the two heavy isotopes of hydrogen — deuterium ($^{2}_{1}\text{H}$) and tritium ($^{3}_{1}\text{H}$):

$$^{2}_{1}\text{H} + {}^{3}_{1}\text{H} \rightarrow {}^{4}_{2}\text{He} + {}^{1}_{0}\text{n} + \text{energy}$$

• Check the conservation rules as always: mass numbers $2 + 3 = 5$ and $4 + 1 = 5$ ✓; atomic numbers $1 + 1 = 2$ and $2 + 0 = 2$ ✓.

Why fusion is hard to achieve

• Both nuclei are positively charged, so they repel each other electrically. The closer they get, the stronger the repulsion.
• To fuse, they must get close enough for the strong nuclear force to take over — and the strong force only acts over an extremely short range.
• Getting that close requires the nuclei to be moving extremely fast, which means an extremely high temperature (millions of degrees).
• Extremely high pressure or density is also needed, so that fast-moving nuclei actually meet each other often enough.
  • In the Sun, gravity supplies the pressure and the core temperature is around 15 million K.
  • On Earth, containing a gas that hot is the central engineering problem — no solid container can touch it.

Fission and fusion compared

	Fission	Fusion
What happens	one heavy nucleus splits	two light nuclei join
Typical fuel	uranium-235, plutonium-239	deuterium, tritium
Triggered by	absorbing a neutron	very high temperature and pressure
Waste	radioactive daughter nuclei, dangerous for a long time	helium, which is not radioactive
Used commercially?	yes — nuclear power stations	not yet — still experimental
Energy released per reaction	large	large (and larger per kilogram of fuel)

• Both release energy because in both cases the products have less total mass than the reactants — the missing mass appears as energy (next page).

Why fusion would be worth the effort

• The fuel is abundant. Deuterium can be extracted from ordinary seawater, and tritium can be bred from lithium. Uranium, by contrast, must be mined and is limited.

• The waste is not the problem it is for fission. The main product is helium, which is not radioactive. Fission produces daughter nuclei that stay dangerously radioactive for thousands of years.

• It cannot run away. A fusion reaction requires extreme conditions to be actively maintained; if containment fails the reaction simply stops. A fission chain reaction, by contrast, must be actively controlled to prevent it accelerating.

• The energy per kilogram of fuel is larger than for fission.

• The obstacle is entirely one of engineering, not physics: sustaining a plasma at over a hundred million kelvin, held away from any wall, for long enough to release more energy than was put in.