How fission works

Natural uranium is 0.7204% U-235, 99.2742% U-238 and 0.0054% U-234 by atom fraction (standard atomic weight 238.02891).

Industry and regulators describe natural uranium as approximately 99.3% U-238 and 0.7% U-235 (with a trace of U-234).

U-238 and Th-232 are the main naturally occurring fertile isotopes: U-238 captures a neutron to become U-239, which decays via neptunium-239 to fissile Pu-239 - the reason reactors breed and burn plutonium.

Nuclear fission occurs when a uranium-235 nucleus absorbs a neutron and splits into two lighter fragments, releasing about 200 MeV of energy and, on average, 2.45 further neutrons that can sustain a chain reaction.

Neutrons from fission are 'fast' (energies above about 1 MeV); in a thermal reactor a moderator slows them to around 0.02 eV, where U-235 is far more likely to fission.

The moderator slows fission neutrons so that they cause more fission; it is usually ordinary (light) water, heavy water or graphite.

Control rods or blades made of neutron-absorbing material such as cadmium, hafnium or boron are inserted into or withdrawn from the core to control or halt the chain reaction.

About 0.66% of the neutrons from U-235 fission are 'delayed' - emitted by fission products up to tens of seconds later (the longest-lived precursor group has a half-life of about 56 s) - and this delay is what makes a reactor controllable.

The coolant carries heat out of the core: ordinary water in PWRs and BWRs, heavy water in PHWRs, carbon dioxide in British AGRs, helium in high-temperature gas reactors and liquid sodium in fast reactors.

In a PWR the water in the core reaches about 325 C and is kept under about 150 times atmospheric pressure so that it does not boil; steam is raised in separate steam generators.

Commercial reactor fuel is enriched to 3 to 5% U-235 from the natural 0.7%.

Enrichment vocabulary: LEU up to 5% (current fuel), 'LEU+' up to 10%, HALEU 5-20% (DOE: 5% to less than 20%), weapons-grade at least 90% U-235; 20% is the boundary above which uranium counts as highly enriched (HEU).

Producing 1 kg of uranium enriched to 5% U-235 requires about 7.9 SWU when the tails assay is 0.25% U-235.

Fuel is ceramic UO2 pellets about 1 cm across stacked in zirconium-alloy tubes (fuel rods); a PWR assembly is a 17 x 17 array 4-5 m long, and a 1,100 MWe core holds 193 assemblies, more than 50,000 rods and about 18 million pellets.

Enriched UO2 powder is pressed into ceramic pellets just under 1 cm in diameter and a little over 1 cm long, which are sintered at about 1,750 C.

A typical PWR fuel assembly is a 17 x 17 square array of fuel rods and guide tubes, 4-5 m long and about 20 cm across, weighing about half a tonne.

Uranium is about 18.7 times denser than water, and a single 1,000 MWe reactor core contains about 51,000 fuel rods and 18 million pellets.

A 1,000 MWe reactor needs about 27 t of fresh enriched fuel a year, made from roughly 200 t of U3O8 concentrate (249 t UOC containing 211 tU for 4.5%-enriched fuel; 192 t UOC / 163 tU at 5%).

About one-third of the fuel is replaced at each refuelling every 12 to 18 months (some plants run 24-month cycles), so an assembly spends roughly three years (18-36 months of cycles) in the core.

Fuel burn-up - the energy extracted per tonne of uranium - has risen from about 40 GWd/tU to over 60 GWd/tU for the best fuel; about 50 GWd/t is typical in the USA.