Nuclear reactions release millions of times more energy per atom than chemical reactions by altering the nucleus itself. Fission splits a heavy nucleus and powers all current nuclear reactors; fusion joins light nuclei together and powers the Sun, but achieving it sustainably on Earth remains an unsolved engineering challenge.
What is nuclear fission?
Nuclear fission is the splitting of a large, heavy, unstable nucleus into two smaller nuclei (called daughter nuclei or fission fragments), along with the release of two or three neutrons and a large amount of energy (as heat and gamma radiation).
The most commonly used fission fuel is uranium-235 (U-235):
¹n₀ + ²³⁵U₉₂ → ²³⁶U₉₂ → ⁹²Kr₃₆ + ¹⁴¹Ba₅₆ + 3¹n₀ + energy
(One neutron is absorbed by U-235, making unstable U-236, which immediately splits into krypton-92, barium-141, three neutrons, and releases energy.)
The key features of fission:
- A neutron must be absorbed by the heavy nucleus to trigger splitting.
- The mass of the products is slightly less than the mass of the original nucleus — this mass defect is converted to energy according to E = mc².
- The released neutrons can trigger further fission reactions.
What is a chain reaction?
The neutrons released by one fission event can be absorbed by neighbouring U-235 nuclei, causing them to undergo fission and release more neutrons — triggering further fissions. This is a chain reaction.
- Uncontrolled chain reaction — each fission produces more than one neutron that triggers a new fission. The number of fissions approximately doubles each generation. This is the basis of a nuclear weapon.
- Controlled chain reaction — the reaction is kept at a steady rate (one fission triggering exactly one further fission on average), producing a steady supply of heat. This is the basis of a nuclear reactor.
What are the components of a nuclear reactor?
| Component | Material | Function |
|---|---|---|
| Fuel rods | Uranium-235 (or plutonium-239) | Source of fission fuel |
| Control rods | Boron or hafnium | Absorb neutrons to control reaction rate; inserted further → slower; withdrawn → faster |
| Moderator | Graphite or water | Slows fast neutrons to thermal (slow) speeds so they can be absorbed by U-235 |
| Coolant | Water or gas (CO₂ in some UK designs) | Carries heat from reactor to steam generator |
| Pressure vessel | Thick steel | Contains the reactor core; prevents radiation escape |
| Containment structure | Reinforced concrete | Final barrier against radioactive release in accident |
The heat from the reactor turns water into steam, which drives a turbine connected to a generator — the same principle as a coal or gas power station, but with nuclear fission as the heat source.
What is nuclear fusion?
Nuclear fusion is the joining together of two light nuclei to form a heavier nucleus, releasing a very large amount of energy in the process. It is the reaction that powers the Sun and all other stars.
A common fusion reaction uses isotopes of hydrogen:
²H₁ (deuterium) + ³H₁ (tritium) → ⁴He₂ (helium-4) + ¹n₀ + energy
Fusion releases far more energy per kilogram of fuel than fission. It also produces no long-lived radioactive waste (helium-4 is stable) and is fuelled by hydrogen isotopes that are far more abundant than uranium. Deuterium can be extracted from seawater; tritium can be bred from lithium.
Why is fusion so difficult to achieve on Earth?
For two nuclei to fuse, they must overcome the electrostatic repulsion between their positive charges and get close enough for the strong nuclear force to take over (within about 10⁻¹⁵ m). This requires extremely high temperatures — around 100 million °C — to give nuclei enough kinetic energy to overcome repulsion. At these temperatures matter exists as a plasma (fully ionised gas).
Two main approaches are under development:
- Tokamak reactor (e.g. JET in the UK, ITER being built in France) — a doughnut-shaped chamber using powerful magnetic fields to confine and heat the plasma.
- Inertial confinement — lasers compress and heat a tiny fuel pellet from all sides simultaneously.
No fusion reactor has yet produced more energy than it consumed (achieved ignition on a sustained basis), though the US National Ignition Facility achieved a net energy gain briefly in 2022, a landmark result. Commercial fusion power remains a major engineering challenge.
How do fission and fusion compare?
| Feature | Fission | Fusion |
|---|---|---|
| Fuel | Uranium-235, plutonium-239 | Deuterium (from seawater), tritium |
| Energy per kg of fuel | Large | Much larger (roughly 4× fission) |
| Radioactive waste | Yes — long-lived (thousands of years) | Minimal; short-lived (decades) |
| Greenhouse gases | None during operation | None during operation |
| Risk of meltdown | Yes (loss of coolant → overheating) | No — plasma cools instantly if confined |
| Current status | Commercial reactors worldwide | Still under development |
Frequently asked questions
What is a mass defect and why does it release energy?
When a nucleus undergoes fission or fusion, the total mass of the products is slightly less than the total mass of the starting materials — this difference is the mass defect. The missing mass has been converted to energy according to Einstein's equation E = mc², where c is the speed of light (3 × 10⁸ m s⁻¹). Because c² is a huge number, even a tiny mass defect corresponds to an enormous release of energy. This is why nuclear reactions release millions of times more energy per reaction than chemical reactions, which involve only electron rearrangements and no change in nuclear mass.
Why does a nuclear reactor need a moderator?
Neutrons produced by fission are fast-moving (high energy). Fast neutrons are less likely to be absorbed by U-235 nuclei than slow (thermal) neutrons. The moderator (water or graphite) slows fast neutrons down to thermal speeds by elastic collisions. Slower neutrons are much more likely to cause further fission events, sustaining the chain reaction at a lower, controllable fuel enrichment level. Without a moderator, the reaction would need very highly enriched uranium (bomb-grade) to sustain itself.
What happens when control rods are fully inserted into a reactor?
When boron control rods are fully inserted, they absorb neutrons before those neutrons can reach uranium nuclei. The chain reaction stops because no fission events are triggered — the reactor shuts down (SCRAM). Control rods are routinely used to adjust power output: partially inserting them absorbs some neutrons and slows the reaction; withdrawing them allows more neutrons to reach the fuel and increases the reaction rate. In an emergency, rods fall fully in by gravity to halt the reaction immediately.
What are the advantages of fusion over fission if it can be made to work?
Nuclear fusion has several major advantages over fission. Its fuel (deuterium from seawater and lithium for tritium breeding) is effectively inexhaustible, whereas high-grade uranium is a finite resource. Fusion produces no long-lived radioactive waste — helium-4 is stable and the neutron-activated reactor materials have relatively short half-lives. There is no risk of a runaway chain reaction or meltdown, because the plasma is so difficult to sustain that any loss of containment immediately cools it and stops the reaction. If achieved commercially, fusion could provide abundant, low-carbon electricity for thousands of years.
For Socratic GCSE physics with Professor Newton — predicting nuclear behaviour from first principles before the equations — visit aitutors.me.