What is nuclear fission?
Fission means splitting. A large, unstable nucleus splits into two smaller nuclei (called daughter nuclei) of roughly equal size.
The usual fuels are uranium-235 and plutonium-239. Fission rarely happens on its own (spontaneous fission). Usually the nucleus must first absorb a neutron. A slow (thermal) neutron works best.
When it splits, it releases:
- two smaller nuclei, such as barium and krypton,
- 2 or 3 neutrons,
- gamma rays,
- a large amount of energy, carried mostly as kinetic energy of the pieces.
Example: n + U-235 → Ba-141 + Kr-92 + 3n + energy. The mass numbers add up: 1 + 235 = 141 + 92 + 3.
Chain reactions
Each fission gives out 2 or 3 neutrons. If these split more nuclei, which split more nuclei, the number of fissions grows very fast: 1, 3, 9, 27… This is a chain reaction.
- Uncontrolled: the reaction grows faster and faster and releases huge energy at once. This happens in a nuclear weapon.
- Controlled: on average exactly one neutron from each fission causes another fission. The energy comes out at a steady rate. This happens in a nuclear reactor.
Inside a nuclear reactor
- Fuel rods hold uranium. Fission heats them.
- The moderator (often water or graphite) slows the fast neutrons down so they are more likely to be absorbed by U-235.
- Control rods (often boron) absorb neutrons. Lowering them in slows the reaction; raising them speeds it up. In an emergency they drop all the way in.
- A coolant carries the heat away to boil water. The steam turns a turbine and generator, just like other power stations.
- Thick concrete shielding stops radiation escaping.
Good points: no carbon dioxide while running, lots of energy from little fuel. Problems: radioactive waste that stays dangerous for thousands of years, risk of accidents, high cost to build and close down.
What is nuclear fusion?
Fusion means joining. Two light nuclei join to make one heavier nucleus. For example, two forms of hydrogen (deuterium and tritium) join to make helium and a neutron.
The mass of the new nucleus is a little less than the total mass of the two starting nuclei. This missing mass becomes energy (E = mc²). Fusion gives even more energy per kilogram of fuel than fission.
Why it is hard: both nuclei are positive, so they repel each other. They only join if they crash together very fast, which needs a temperature of millions of degrees and a very high pressure or density. The Sun's core has this naturally; on Earth, machines like tokamaks hold the hot gas (plasma) in a magnetic field, but we cannot yet get more energy out than we put in for a long time.
Fission vs fusion
- Process: fission splits a heavy nucleus; fusion joins light nuclei.
- Fuel: fission uses uranium or plutonium; fusion uses hydrogen (deuterium from seawater, tritium).
- Where: fission in power stations today; fusion in the Sun and stars, and in experiments.
- Conditions: fission needs a slow neutron; fusion needs huge temperature and pressure.
- Waste: fission makes long-lived radioactive waste; fusion makes much less.
- Both turn a little mass into a lot of energy.
Try it: a domino chain reaction
Stand dominoes (or folded paper cards) so that each one can knock down two others, like a branching tree. Push the first one and watch how many fall in each row: 1, 2, 4, 8… That is a growing chain reaction. Now remove some dominoes (your 'control rods') so each falling one knocks down only one. The chain becomes steady. Then try the control-rod slider in the 3D and compare.
Key formulas and definitions
- n + U-235 → 2 smaller nuclei + 2–3 n + energy
- Mass numbers and atomic numbers balance on both sides
- hydrogen + hydrogen → helium (+ neutron) + energy
- E = mc² (lost mass becomes energy)
- Steady chain reaction: 1 neutron passed on per fission
Worked examples
1. In the fission n + U-235 → Ba-141 + Kr-92 + x n, find x.
Mass numbers: 1 + 235 = 236. Right side: 141 + 92 = 233, so x = 3 neutrons.
2. What would happen in a reactor if all the control rods were pulled out?
Fewer neutrons would be absorbed, so each fission would cause more than one new fission. The chain reaction would grow, the core would overheat and could be damaged. That is why rods drop in automatically in an emergency.
3. Why can't fusion happen at room temperature?
Nuclei are positively charged and repel. At room temperature they move far too slowly to get close enough to join. Only at millions of degrees are they fast enough.
4. In fusion, ²H + ³H → ⁴He + ¹n. Check that the mass numbers balance.
Left: 2 + 3 = 5. Right: 4 + 1 = 5. They balance.
Common mistakes
- Mixing up the words: fission splits, fusion joins.
- Thinking any neutron causes fission easily. U-235 absorbs slow neutrons best, which is why a moderator is used.
- Saying control rods slow neutrons down. The moderator slows them; control rods absorb them.
- Thinking fusion power stations already supply electricity. Fusion is still at the experiment stage.