Separative work units


Separative work – the amount of separation done by a Uranium enrichment process – is a function of the concentrations of the feedstock, the enriched output, and the depleted tailings; and is expressed in units which are so calculated as to be proportional to the total input and to the mass processed.
The same amount of separative work will require different amounts of energy depending on the efficiency of the separation technology. Separative work is measured in Separative work units SWU, kg SW, or kg UTA
  • 1 SWU = 1 kg SW = 1 kg UTA
  • 1 kSWU = 1 tSW = 1 t UTA
  • 1 MSWU = 1 ktSW = 1 kt UTA
Separative work unit is not a unit of energy, but serves as a measure of the enrichment services. As of August 2025, spot prices per SWU were $188, though most SWU are bought in long-term contracts which averaged $97/SWU in 2024. The unit was introduced by Paul Dirac in 1941.

Definition

The work necessary to separate a mass of feed of assay into a mass of product assay, and tails of mass and assay is given by the expression:
where is the value function, defined as:
Given the desired amount of product, the necessary feed and resulting tails are:

Relation to energy required

The number of separative work units provided by an enrichment facility is directly related to the amount of energy that the facility consumes. Modern gaseous diffusion plants typically require 2,400 to 2,500 kilowatt-hours, or 8.6–9 gigajoules, of electricity per SWU while gas centrifuge plants require just 50 to 60 kW·h of electricity per SWU.

Examples

Simple NU to LEU example

For example, beginning with of natural uranium, it takes about 62 SWU to produce of Low-enriched uranium in 235U content to 4.5%, at a tails assay of 0.3%.

Fuel generation: example for power stations

A large nuclear power station with a net electrical capacity of 1300 MW requires about 25 tonnes per year of LEU with a 235U concentration of 3.75%. This quantity is produced from about 210 t of NU using about 120 kSWU. An enrichment plant with a capacity of 1000 kSWU/a is, therefore, able to enrich the uranium needed to fuel about eight large nuclear power stations.