5.7 - Nuclear Fusion

What nuclear fusion is

• Nuclear fusion is when two light nuclei collide at high speed and join together to make a single larger, heavier nucleus.
• For example, hydrogen nuclei fuse to form a helium nucleus.
• Fusion is the process that powers stars, including the Sun, releasing energy by fusing hydrogen into helium in their cores.

The deuterium-tritium fusion reaction

In a fusion reactor, two isotopes of hydrogen - deuterium (hydrogen-2) and tritium (hydrogen-3) - fuse to form helium and a neutron:

$${}_1^2\text{H}+{}_1^3\text{H}\to {}_2^4\text{He}+{}_0^1\text{n}+\text{energy}$$

The nucleon numbers balance (2 + 3 = 4 + 1) and the proton numbers balance (1 + 1 = 2 + 0), so the equation conserves both mass number and charge.

Fusion is extremely hard to achieve

• Nuclei are positively charged, so they repel each other by electrostatic repulsion.
• To overcome this repulsion and fuse, nuclei must travel at very high speeds, which requires very high temperatures (around 10,000,000°C) and very high pressures.

The plasma problem

• At these temperatures, the fuel becomes a plasma (atoms stripped of their electrons).
• No container can withstand contact with plasma this hot, so strong magnetic fields are used to contain it away from the reactor walls.

Energy deficit

Creating and maintaining these extreme conditions currently takes more energy than the fusion releases, so no reactor generates usable electricity yet.

Fusion as a clean, plentiful energy source

If fusion reactors become practical, they offer major advantages:

• Plentiful fuel - Hydrogen isotopes (such as deuterium) can be extracted from seawater.
• No greenhouse gases - The main by-product is inert, non-toxic helium.
• No long-lived radioactive waste - Unlike nuclear fission.

Mass is converted to energy in fusion

• When two light nuclei fuse, the heavier nucleus formed has slightly less mass than the two separate nuclei started with.
• That lost mass is converted into energy, released as radiation and the kinetic energy of the particles.

Per kilogram of fuel, fusion releases:

• About four million times more energy than burning fossil fuels (such as coal, oil or gas).
• About four times as much energy as nuclear fission.