Nuclear Fission and Fusion

Nuclear fission — a large nucleus splits into two smaller nuclei, releasing energy and neutrons.

The sequence:

1. A slow-moving neutron is absorbed by a uranium-235 nucleus.
2. The nucleus becomes unstable and splits into two roughly equal daughter nuclei.
3. Two or three neutrons are released, along with a large amount of energy.
4. Those neutrons can be absorbed by other U-235 nuclei, causing further fissions — a chain reaction.

Chain reaction — each fission produces neutrons that cause more fissions. Uncontrolled, this escalates enormously and is the principle of a nuclear weapon. In a reactor it is held steady, so that on average exactly one neutron from each fission goes on to cause another.

Controlling a reactor — three components and what each does:

Moderator (graphite or water) — slows the neutrons down. Fast neutrons are poorly absorbed by U-235; slow ones are absorbed readily. Without a moderator the chain reaction would not sustain itself.

Control rods (boron or cadmium) — absorb neutrons. Lowering them further into the core absorbs more neutrons and slows the reaction; raising them speeds it up. This is how output is controlled and how the reactor is shut down.

Shielding (thick concrete and steel) — absorbs the radiation so it does not escape and harm workers.

The moderator and the control rods are routinely confused. One slows neutrons so the reaction can happen; the other removes neutrons so it does not run away. Getting them the right way round is worth a mark almost every year.

Nuclear fusion — two light nuclei join to form a heavier nucleus, releasing energy. In the Sun, hydrogen nuclei fuse to form helium.

Why fusion needs extreme conditions. Nuclei are positively charged and repel one another strongly. To fuse, they must approach closely enough for the short-range attractive nuclear force to take over, which requires very high temperature (so they move fast enough) and very high pressure or density (so collisions are frequent enough). In the Sun, gravity supplies those conditions; on Earth they are extremely difficult and expensive to reproduce, which is why commercial fusion power does not yet exist.

The two compared:

Example: a reactor is shut down by lowering the control rods fully into the core. They absorb so many neutrons that fewer than one per fission survives to cause another, so the chain reaction dies away.