What a SWU is
A separative work unit is a measure of the effort required to separate uranium isotopes, not a measure of uranium. One SWU is the work needed to produce one kilogram of separated material at the standard reference, and enrichment is sold in SWU because the enricher supplies only the separation: the customer usually supplies the uranium. Making one kilogram of ordinary 4.4% reactor fuel takes 7.11 SWU and 8.51 kilograms of natural uranium.
Why enrichment needs its own unit
An enrichment plant does not make uranium and it does not consume it. Uranium goes in at one assay and comes out as two streams: a small enriched product and a much larger depleted tails stream. Nothing is destroyed. What the plant sells is the separation itself.
That makes ordinary units useless for pricing it. Charging per kilogram of product would be wrong, because product at 2% and product at 19% represent wildly different amounts of work. So the industry uses a unit of effort, the separative work unit, and prices in dollars per SWU. A utility typically buys uranium from a miner, has it converted, and then pays an enricher for a number of SWU rather than for material at all.
The value function
Separative work is defined through a value function that assigns a number to any assay:
V(x) = (2x - 1) · ln( x / (1 - x) )
For a batch with product mass P at assay xp, tails mass T at xt and feed mass F at xf, the separative work is:
SWU = P·V(xp) + T·V(xt) − F·V(xf)
The function has no physical units. It is a mathematical device that happens to be additive in exactly the way separation work is, so the difference between the value of what comes out and the value of what went in is the work done. Feed follows from mass balance alone: F = P(xp − xt) / (xf − xt).
Worked, for one kilogram of ordinary fuel
Take natural uranium at 0.711% U-235, a product at 4.4%, and a tails assay of 0.22%:
| Quantity | Mass, per kg of product | V(x) |
|---|---|---|
| Product at 4.4% | 1.00kg | 2.808 |
| Feed at 0.711% | 8.51kg | 4.869 |
| Tails at 0.22% | 7.51kg | 6.090 |
| Separative work | - | 7.11 SWU |
So a kilogram of ordinary reactor fuel is 8.51 kilograms of mined uranium plus 7.11 SWU of work, and 7.51 kilograms of depleted uranium left behind. At the August 2026 spot indicator of US$ 200.00 per SWU, the enrichment alone is US$ 1,423 per kilogram. The fuel price page stacks that against the other three stages.
The trade-off nobody outside the industry sees
The tails assay is a choice, not a constant. Strip the tails harder and you extract more U-235 from each kilogram of feed, so you buy less uranium but spend more work. Leave the tails richer and the opposite. Compare the same product at two tails assays:
| Tails assay | Natural uranium per kg product | SWU per kg product |
|---|---|---|
| 0.22% | 8.51kgU | 7.11 |
| 0.35% | 11.22kgU | 5.53 |
Enrichers move this number continuously against the ratio between the uranium price and the SWU price. When uranium is expensive relative to separative work they strip harder; when separative work is expensive they leave more behind. It means world enrichment demand in SWU is not a fixed function of how many reactors are running, and it is one reason the two price series on the price page do not move together.
The scale of the industry, in SWU
World enrichment capacity is about 68,076 tonnes of SWU a year, which is 68,076,000 SWU. At 7.11 SWU per kilogram, a first core for a large reactor of roughly 80 tonnes of enriched uranium represents about 569 tonnes of SWU, or 0.84% of a full year of world capacity for a single core.
The 80-tonne core figure is an order-of-magnitude illustration for a large light water reactor, not a figure for any specific unit; core loadings are not consistently published per unit and none is asserted here. The world capacity figure is sourced.
Why this matters for the SMR pipeline
Because separative work rises so steeply with product assay, a design that needs HALEU is not asking for a slightly different fuel. It is asking for several times the separative work per kilogram, from a supply chain that barely exists outside Russia. That single fact does more to explain the state of the SMR pipeline than any regulatory question.
Sources
- Separative work definition and value function: World Nuclear Association, Uranium Enrichment
- World enrichment capacity: World Nuclear Association, Uranium Enrichment
- Cameco published market prices, and TradeTech and UxC month-end indicators · retrieved August 2026
- The worked figures on this page are computed from the equations shown, not quoted
How these sources are used, including the corrections applied on top of them and what is deliberately left blank · download the underlying data