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 explainers

Six pages that each answer one question properly, with the arithmetic shown and every figure traced to a source. The rest of this page is the overview they sit under.

Moderators and coolants The two choices that determine everything else about a reactor: what slows the neutrons, and what carries the heat away.
Decay heat Why a reactor that has been shut down for a week still needs cooling, and why that single fact is behind most of the accident record.
Enrichment levels Natural, LEU, HALEU, HEU and weapons-grade. What each is for, where the legal lines sit, and who is allowed to hold what.
What a SWU is The unit the entire enrichment industry is priced in, derived from first principles, with the arithmetic shown.
Gross, net and capacity factor Why the same reactor is quoted at three different capacities, and why a 1,000 MW plant does not produce 8,760 GWh a year.
The INES scale How nuclear events are rated from 0 to 7, what the scale measures, and what it conspicuously does not.

How fission works

A uranium-235 nucleus that absorbs a neutron becomes unstable and splits. The two fragments fly apart carrying most of the released energy as heat, and two or three spare neutrons come out.

A neutron absorbed by a uranium-235 nucleus makes it unstable enough to split. The fragments fly apart with enormous kinetic energy, which is the heat, and two or three spare neutrons come out. Arrange enough uranium so that exactly one of those causes another fission and the reaction is steady. That condition is criticality, and a reactor is a machine for holding it.

Why uranium and not something else

Fission releases roughly fifty million times more energy per atom than a chemical reaction. That single ratio explains almost every practical feature of nuclear power: the small fuel volumes, the long refuelling intervals, the tiny waste mass, and the fact that the entire world fuel chain moves tens of thousands of tonnes a year rather than billions.

Energy released per kilogram of fuel · logarithmic scale

Uranium, enriched 4.4% in a reactor 3.9 million

Natural uranium, fast reactor (theoretical) 80 million

Hydrogen 142

Natural gas 55

Diesel 46

Coal, bituminous 30

Wood 16

Lithium-ion battery 0.9

The scale is logarithmic, because on a linear one every chemical fuel would be an invisible sliver. Uranium in a conventional reactor releases roughly 130,000 times as much energy per kilogram as coal; in a fast reactor that could extract the U-238 as well, the theoretical figure is another twenty times higher again. This is why a reactor refuels once every 18 to 24 months while a coal station needs a train of fuel every day, and why the fuel chain handles thousands of tonnes a year rather than millions.

The three things a reactor has to do

Sustain the reaction

Keep exactly one neutron from each fission causing another, 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. Control rods absorb the surplus.

Carry the heat away

The coolant takes heat from the fuel 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. This choice sets efficiency and most of the safety case.

Keep cooling after shutdown

The one people miss, and the one that causes accidents. Fission products keep decaying after the chain reaction stops, producing several percent of full power as decay heat for days.

The problem that causes accidents

Scramming a reactor stops the chain reaction in about two seconds. It does not stop the heat.

Stopping the chain reaction is not stopping the reactor. Fission products keep decaying, and a 3,000 MW thermal core still produces about 200 MW of heat one second after shutdown and roughly 5 MW a day later, with no way to switch it off. Every reactor must therefore be cooled for days after it stops, and failing to do that is what actually happened at Three Mile Island and Fukushima. Both reactors scrammed correctly. The accidents came afterwards.

What the fuel physically is

Ceramic uranium dioxide pellets, stacked in sealed metal tubes, bundled into assemblies. It is unremarkable to look at, and the dimensions are where the fuel chain stops being a commodity.

Fabrication is where the fuel chain stops being a commodity. Rod diameter, lattice pitch, grid spacer design and the burnable poisons blended into the pellets are all specific to a reactor design. That is how a Westinghouse four-loop PWR cannot use VVER fuel even though both are pressurised water reactors, and why moving Eastern European reactors off Russian fuel after 2022 took years rather than months.

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	309
Pressurised heavy water reactor PHWR	Heavy water	Heavy water	Natural uranium, 0.71% U-235	46
Boiling water reactor BWR	Light water	Light water	3–5% U-235	45
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 concentrates.

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?
Interim storage, Where is spent nuclear fuel stored?
Reprocessing, What is nuclear fuel reprocessing?
Geological disposal, Is there a permanent repository for nuclear waste?
Depleted uranium, What happens to depleted uranium?

Also worth reading: the accidents that shaped how all of this is regulated, which new designs are actually moving through regulators, what the fuel costs, and the posts, which take single questions further than a reference page can.

Sources

How these sources are used, including the corrections applied on top of them and what is deliberately left blank · download the underlying data