What enrichment actually does
Natural uranium is only about 0.7% uranium-235 — the isotope that sustains a chain reaction — with the rest being uranium-238. Most power reactors need that figure raised to between 3% and 5%. Enrichment is the industrial process that does this, and it's one of the most demanding steps in the entire nuclear fuel cycle.
The process splits a stream of uranium into two: the product (enriched, sent to fuel fabrication) and the tails (depleted, with less U-235 than natural uranium). How much you can squeeze out depends on the tails assay — leave more U-235 in the tails and you need less effort but more raw feed; strip the tails harder and you save uranium but spend more enrichment work.
SWU: measuring the effort
The work of separating isotopes is measured in separative work units (SWU). A SWU isn't a mass or an energy — it's a measure of effort, and it's how enrichment services are priced worldwide. The calculation uses the "value function" applied to the product, feed and tails streams:
As a benchmark, making 1 kg of 5%-enriched fuel with 0.25% tails takes roughly 10 kg of natural uranium and about 8 SWU. Multiply up for a reactor reload and the numbers become industrial in scale — which is why enrichment capacity is a strategic national asset.
How it connects to power output
That enriched fuel is what ultimately drives a reactor. Once you've sized the fuel here, the reactor output calculator shows the electricity it produces, while the radioactive decay calculator covers the half-lives of the isotopes involved across the fuel's life.
Frequently asked questions
What is uranium enrichment?
Raising the proportion of fissile U-235 in uranium. Natural uranium is ~0.7% U-235; power reactors need 3–5% to sustain a chain reaction.
What is a SWU?
A separative work unit — a measure of the effort to separate uranium isotopes, and the main unit in which enrichment is priced. It's not a mass or an energy.
How much natural uranium is needed for reactor fuel?
About 10 kg of natural uranium and ~8 SWU per kg of 5% fuel at 0.25% tails. Lower enrichment or higher tails change the figures.