How Nuclear Fission and Fusion Work

Nuclear fission and nuclear fusion both release large amounts of energy from changes in atomic nuclei, but the processes are very different.

Nuclear fission

Fission is the splitting of a large, heavy nucleus into two smaller nuclei, releasing energy and usually two or three neutrons.

Chain reaction: the neutrons released by one fission event can cause further fission in nearby nuclei, creating a self-sustaining chain reaction.

In a nuclear power station:

1. Uranium-235 nuclei absorb neutrons and become unstable.
2. Each nucleus splits into two smaller nuclei (fission products) and releases 2-3 neutrons.
3. These neutrons cause more fission (chain reaction).
4. The energy released heats water to steam, which drives turbines and generators.

Control:

• Control rods (boron or cadmium) absorb excess neutrons to control the rate of fission.
• Moderator (water or graphite) slows neutrons down so they are more likely to be absorbed by uranium nuclei.

Nuclear fusion

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

Example: hydrogen nuclei fuse to form helium in the Sun:

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

Why fusion is difficult on Earth:

• Nuclei are positively charged, so they repel each other.
• Extremely high temperatures (millions of degrees) and pressures are needed to overcome this repulsion and bring nuclei close enough to fuse.
• Containing the plasma at such temperatures is a major engineering challenge.

Comparison

Feature	Fission	Fusion
Process	Splitting heavy nuclei	Joining light nuclei
Fuel	Uranium, plutonium	Hydrogen isotopes
Conditions	Needs neutron bombardment	Needs extremely high temperature and pressure
Waste	Radioactive fission products	Minimal radioactive waste
Current use	Nuclear power stations	Stars; research reactors (not yet commercial)

Common errors and how to correct them

Confusing fission and fusion. Fission = splitting (think “fission = division”). Fusion = joining (think “fusion = fusing together”).

Saying fusion is used in nuclear power stations. Current nuclear power stations use fission. Fusion power is not yet commercially viable.

How to apply this in an exam

Define both processes clearly. For fission, describe the chain reaction and the role of control rods and moderator. For fusion, explain why high temperature is needed (to overcome electrostatic repulsion between positive nuclei).