How nuclear reactors work
Every reactor does the same thing: split uranium atoms, use the heat to make steam, drive a turbine. Everything that separates one design from another comes down to two choices — what carries the heat away from the fuel, and what slows the neutrons down. Those two answers determine the enrichment the fuel needs, whether the plant can refuel while running, how hot it can get, and how it fails.
The three things a reactor has to do
Sustain the reaction
A uranium-235 nucleus that absorbs a neutron splits, releasing energy and two or three more neutrons. Keep exactly one of those causing another fission and the reaction is steady — "critical". Fast neutrons are poor at causing fission in U-235, so most reactors use a moderator — water, heavy water or graphite — to slow them down.
Carry the heat away
The coolant removes heat from the fuel and takes it to a turbine. Water is cheap and well understood but must be pressurised to stay liquid and boils at a low temperature. Gas and liquid metal run far hotter. The choice sets the plant's efficiency and most of its safety case.
Keep cooling after shutdown
Stopping the chain reaction is not enough. Fission products keep decaying, producing several percent of full power as decay heat for days. Nearly every serious accident — Three Mile Island, Fukushima — was a failure to remove decay heat from a reactor that had already shut down correctly.
The reactor families
419 reactors are operating worldwide. The table gives the essential trade-off for each family; each page has a schematic of how heat reaches the turbine, and a full list of advantages and drawbacks.
| Type | Coolant | Moderator | Enrichment | Operating |
|---|---|---|---|---|
| Pressurised water reactor PWR | Light water | Light water | 3–5% U-235 | 311 |
| Boiling water reactor BWR | Light water | Light water | 3–5% U-235 | 45 |
| Pressurised heavy water reactor PHWR | Heavy water | Heavy water | Natural uranium, 0.71% U-235 | 44 |
| Gas-cooled reactor GCR | Carbon dioxide | Graphite | Natural uranium to 2.5–3.5% U-235 | 9 |
| Light water graphite reactor LWGR | Light water | Graphite | 2–2.6% U-235 | 7 |
| Fast reactor FBR | Liquid sodium or lead | None | 15–20% fissile, or plutonium | 2 |
| High-temperature gas reactor HTGR | Helium | Graphite | 8–20% U-235, frequently HALEU | 1 |
| Molten salt reactor MSR | Molten fluoride or chloride salt | Graphite, or none in fast-spectrum designs | 5–20% U-235, often HALEU | — |
The trade-off in one line each
- PWR
- Two loops keep the turbine clean and the technology is mature — but it needs enriched fuel and a very thick, very high-pressure vessel.
- BWR
- One loop is simpler and more efficient — but the turbine hall becomes mildly radioactive.
- PHWR / CANDU
- Runs on natural uranium and refuels without shutting down — but needs a costly heavy water inventory and has a positive void coefficient.
- GCR
- Gas runs hotter, giving the best thermal efficiency of any fleet — but the cores are enormous and graphite ageing ends the plant's life.
- HTGR
- Hot enough for industrial process heat, with fuel that contains its own fission products — but it needs HALEU that barely exists.
- Fast reactor
- Breeds more fuel than it burns and can consume long-lived waste — but sodium burns in air and explodes on contact with water.
- MSR
- Fuel dissolved in coolant at near-atmospheric pressure removes the worst accident mechanisms — but nothing commercial has ever operated.
- LWGR / RBMK
- Large, cheap and refuellable on load — and the design whose positive void coefficient destroyed Chernobyl.
Then follow the fuel
A reactor's type decides what fuel it needs, and that decides who can supply it. The fuel chain is where the industry is genuinely concentrated.
- Mining and milling — Where is uranium mined?
- Conversion — What is uranium conversion and who does it?
- Enrichment — Who enriches uranium and how concentrated is the market?
- Fabrication — Which companies fabricate nuclear fuel, and for which reactors?
- HALEU — What is HALEU and who can actually produce it?
- Back end — What happens to spent nuclear fuel?
Also worth reading: the accidents that shaped how all of this is regulated, and which new designs are actually moving through regulators.