Predict first: if you want to stop a nuclear chain reaction, what would you insert into the reactor to absorb neutrons? The answer is control rods — typically made of boron — which absorb neutrons and prevent them triggering further fissions, slowing or stopping the chain reaction without removing the fuel itself.
What is nuclear fission?
Nuclear fission is the splitting of a large, unstable atomic nucleus into two smaller nuclei (called fission fragments), releasing a large amount of energy and typically two or three free neutrons.
The most commonly used fuel nuclei at GCSE are:
- Uranium-235 (³⁵U) — the primary fuel in most nuclear reactors.
- Plutonium-239 (²³⁹Pu) — used in some military and experimental reactors.
The fission reaction for uranium-235: ¹n + ²³⁵U → [²³⁶U] → fission fragments + 2–3 ¹n + energy*
A single fission event releases approximately 200 MeV of energy — about 50 million times more energy per atom than burning a coal atom in oxygen. This immense energy density is why a few kilograms of enriched uranium can produce the same energy as thousands of tonnes of coal.
What is a chain reaction and why must it be controlled?
The 2–3 neutrons released by each fission can each trigger another fission of a uranium-235 nucleus — which releases more neutrons — which trigger more fissions. This is a chain reaction.
If uncontrolled, the number of fissions doubles with each generation, leading to an exponential release of energy — this is the basis of nuclear weapons.
In a nuclear power station, the chain reaction must be kept at a steady, controlled rate — exactly one neutron from each fission goes on to cause exactly one more fission. This is called a critical chain reaction. If the rate is below this (sub-critical), the reaction dies away; above this (supercritical), it accelerates dangerously.
What are the key components of a nuclear reactor?
| Component | Material | Function |
|---|---|---|
| Fuel rods | Enriched uranium-235 (or Pu-239) | Source of fissile material; contain the fission reactions |
| Moderator | Graphite or water | Slows fast neutrons to speeds suitable for triggering U-235 fission |
| Control rods | Boron (or cadmium) | Absorb neutrons; inserted deeper → slower reaction; withdrawn → faster reaction |
| Coolant | Water or CO₂ gas | Carries heat from the reactor core to the steam generator |
| Containment vessel | Thick concrete and steel | Shields workers and environment from radiation; contains accidents |
Why does the moderator slow neutrons? Fast neutrons (released directly by fission) are not easily captured by U-235 nuclei — they tend to pass straight through. When slowed by multiple collisions with the moderator material, they become thermal neutrons, which are far more likely to be captured and trigger fission.
How does a nuclear power station generate electricity?
The energy chain from nucleus to national grid:
- Fission: uranium-235 nuclei in the fuel rods undergo fission, releasing energy mostly as kinetic energy of the fission fragments, which heats the fuel rods.
- Coolant: water or CO₂ gas flows past the fuel rods, absorbing the heat and becoming very hot (or turning to steam directly).
- Steam generator (heat exchanger): hot coolant transfers heat to a separate water circuit, producing high-pressure steam — this keeps the radioactive coolant isolated from the turbines.
- Turbine: high-pressure steam spins turbine blades.
- Generator: the turbine shaft drives a generator (an electromagnet rotating inside coils), producing alternating current (AC) electricity.
- Transformer: voltage is stepped up for efficient transmission on the National Grid.
- Cooling tower: waste heat is removed from the steam, condensing it back to water for re-use; the visible "smoke" from cooling towers is actually water vapour.
What are the advantages and disadvantages of nuclear power?
| Advantages | Disadvantages |
|---|---|
| Very low CO₂ emissions during operation | Radioactive waste remains hazardous for thousands of years |
| Very high energy density — small mass of fuel, large energy output | High capital cost and long construction time (10–20 years) |
| Reliable baseload power (unlike wind/solar, not weather-dependent) | Risk of nuclear accident (e.g. Chernobyl 1986, Fukushima 2011) |
| Long operational lifetime (40–60 years) | Uranium mining and enrichment have environmental impacts |
| No air pollution during operation | Public concern and political difficulty siting new plants |
The UK currently generates about 15% of its electricity from nuclear power (as of 2024). New reactors (e.g. Hinkley Point C in Somerset) are under construction.
Frequently asked questions
Why do nuclear power stations use enriched uranium rather than natural uranium?
Natural uranium is about 99.3% uranium-238 and only 0.7% uranium-235. Only U-235 undergoes fission readily when struck by thermal neutrons — U-238 mostly just absorbs them without fissioning. For a sustained chain reaction in a conventional reactor, the proportion of U-235 must be increased (enriched) to about 3–5%. This is done by a process of gaseous diffusion or centrifugation of uranium hexafluoride gas. Weapons-grade uranium is enriched to over 90% U-235, which is why civilian enrichment facilities are monitored by international inspectors.
What happens to nuclear waste after a reactor is used?
Spent fuel rods are still highly radioactive and very hot. They are initially stored in large pools of water at the reactor site for several years, where the water absorbs heat and radiation. After cooling, they are transferred to dry storage containers and eventually to long-term geological disposal — storing them deep underground in stable rock formations where they remain isolated from the biosphere for the thousands of years needed for radioactivity to decay to safe levels. The UK currently has no permanent geological disposal facility operational — long-term waste storage is one of the most challenging unresolved issues in nuclear power.
What is the difference between nuclear fission and nuclear fusion?
Nuclear fission splits large nuclei (uranium, plutonium) into smaller ones, releasing energy. It is the basis of current nuclear power stations and nuclear bombs. Nuclear fusion joins small nuclei (hydrogen isotopes — deuterium and tritium) into larger ones, releasing even more energy per unit mass. Fusion is the process powering the Sun. Fusion is theoretically preferable — the fuel (hydrogen from sea water) is almost unlimited, and it produces far less radioactive waste — but the conditions needed (temperatures of ~100 million °C) are extraordinarily difficult to achieve and sustain. Commercial fusion power has been "30 years away" for several decades; the ITER international project in France is the most advanced current attempt.
Why can a nuclear power station never explode like a nuclear bomb?
A nuclear weapon is designed to achieve an extremely rapid, supercritical chain reaction that releases all its energy in a tiny fraction of a second — this requires precisely engineered, weapons-grade enriched uranium or plutonium assembled in a specific geometry. A nuclear reactor uses only 3–5% enriched uranium spread across fuel rods — far too dilute for a weapons-type explosion. The worst-case accident is a loss of cooling (meltdown), where fuel rods overheat and melt, potentially releasing radioactive material — serious, but not a nuclear explosion.
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