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 (energy / machine operation time) 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 (from the German Urantrennarbeit – literally uranium separation work)

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.

The work ${\displaystyle W_{\mathrm {SWU} }}$ necessary to separate a mass ${\displaystyle F}$ of feed of assay ${\displaystyle x_{f}}$ into a mass ${\displaystyle P}$ of product assay ${\displaystyle x_{p}}$, and tails of mass ${\displaystyle T}$ and assay ${\displaystyle x_{t}}$ is given by the expression:

where ${\displaystyle V\left(x\right)}$ is the value function, defined as:

Given the desired amount of product ${\displaystyle P}$, the necessary feed ${\displaystyle F}$ and resulting tails ${\displaystyle T}$ are:

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 (kW·h), or 8.6–9 gigajoules, (GJ) of electricity per SWU while gas centrifuge plants require just 50 to 60 kW·h (180–220 MJ) of electricity per SWU.

For example, beginning with 102 kilograms (225 lb) of natural uranium (NU), it takes about 62 SWU to produce 10 kilograms (22 lb) of Low-enriched uranium (LEU) in ²³⁵U content to 4.5%, at a tails assay of 0.3%.