SWU (separative work unit)
A separative work unit (SWU) measures the effort needed to enrich uranium and is the unit enrichment is priced in. How SWU works and how much a reactor uses.
A separative work unit (SWU) is the standard measure of the effort required to enrich uranium, and the unit in which enrichment services are priced and sold. It quantifies the work an enrichment facility performs to raise the concentration of the fissile isotope uranium-235, separately from the quantity of uranium involved. The cost of enrichment is a number of SWU multiplied by a price per SWU.
What determines how much SWU is needed
The SWU a batch of fuel requires depends on three variables.
- Feed. The quantity of natural uranium fed into the enrichment plant.
- Product assay. The uranium-235 concentration of the enriched product.
- Tails assay. The uranium-235 concentration left in the depleted uranium, or tails.
As an example, producing one kilogram of uranium enriched to 4 percent, with a tails assay of 0.25 percent, takes about 8 kilograms of natural uranium feed and about 5.8 SWU.
The feed and SWU tradeoff
A buyer can use more natural uranium and fewer SWU by enriching to a higher tails assay, or less uranium and more SWU by enriching to a lower one. Enrichers adjust the tails assay depending on the relative prices of uranium and enrichment. This is why uranium feed and SWU are tracked and priced as separate markets.
How much SWU a reactor uses
A typical 1,000-megawatt light-water reactor requires roughly 100,000 to 120,000 SWU each year to enrich its fuel. Enrichment is bought mostly under long-term contracts, with SWU price indicators published separately from the uranium price.
SWU is one of the separately priced components of reactor fuel, alongside the uranium itself, priced per pound of U₃O₈, and conversion. See enriched uranium and the nuclear fuel cycle.
FAQ
SWU stands for separative work unit. It is the standard measure of the work required to separate uranium into an enriched product and depleted tails, and the unit in which uranium enrichment services are priced and contracted.
SWU is calculated from the quantities and uranium-235 concentrations of the feed, the enriched product, and the tails, using a standard value function for each stream. In practice, producing one kilogram of 4 percent enriched uranium with 0.25 percent tails requires about 5.8 SWU and about 8 kilograms of natural uranium feed.
Uranium and SWU are priced separately because they are partly interchangeable. Enriching to a lower tails assay uses less uranium feed but more SWU, and a higher tails assay does the reverse. Utilities buy uranium, conversion, and enrichment as separate inputs and balance them according to their relative prices.