Natural uranium isn't reactor-ready
Uranium as it comes out of the ground is almost entirely U-238, an isotope that doesn't sustain a chain reaction on its own. Only about 0.7% is U-235, the fissile isotope that does. Most power reactors need that concentration raised to somewhere between 3% and 5% before it will work as fuel — and that's the entire job of enrichment.
Enrichment splits a stream of uranium hexafluoride gas into two: the product, enriched and sent on to fuel fabrication, and the tails, depleted uranium left over with less U-235 than the natural uranium it started as.
Work out SWU and feed for your own fuel order
Enter enrichment level and tails assay to see the separative work and natural uranium required.
SWU: how the effort is measured and priced
The effort involved in separating isotopes is measured in separative work units (SWU) — not a mass, not an energy, but a measure of the work the enrichment plant does. SWU is the unit enrichment services are priced in worldwide, calculated from a "value function" applied to the product, feed and tails streams:
As a benchmark, producing 1 kg of fuel enriched to 5% with a 0.25% tails assay takes roughly 10 kg of natural uranium feed and about 8 SWU of separative work. Scale that up to a full reactor reload and both numbers become industrial — which is why enrichment capacity is treated as a strategic national asset in several countries.
The tails assay trade-off
How much U-235 gets left behind in the tails is a genuine economic choice. A lower tails assay strips more U-235 out of the feed, needing less natural uranium but more separative work. A higher tails assay is the opposite — less enrichment effort, but more natural uranium feed required and more U-235 wasted in the depleted stream. Enrichers balance this against the relative cost of raw uranium versus enrichment capacity at the time.
From here to power on the grid
Enriched fuel is what a reactor actually runs on — see the MWth vs MWe guide for how that fuel becomes electricity, and the energy density guide for just how much energy that small mass of enriched fuel unlocks compared with fossil fuel. The fuel's radioactive life cycle, from fresh to spent, follows the same half-life maths covered in the half-life guide.
Frequently asked questions
Why does natural uranium need to be enriched?
Natural uranium is only ~0.7% U-235, the fissile isotope. Power reactors need that raised to 3–5% to sustain a chain reaction.
What is a tails assay?
The U-235 concentration left in depleted uranium after enrichment. Lower tails extracts more U-235 but needs more separative work; higher tails needs less work but wastes more U-235.
Is enriched reactor fuel the same as weapons-grade uranium?
No. Reactor fuel is enriched to roughly 3–5% U-235. Weapons-grade requires above ~90%, a far more intensive process using the same underlying technology pushed much further.