Separative work units

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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)

• 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. In the early 2020s the cost of 1 SWU was approximately $100.^[1] The unit was introduced by Paul Dirac in 1941.^[2]

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:^[3]

${\displaystyle W_{\mathrm {SWU} }=P\cdot V\left(x_{p}\right)+T\cdot V(x_{t})-F\cdot V(x_{f})}$

where ${\displaystyle V\left(x\right)}$ is the value function, defined as:^[2]

${\displaystyle V(x)=(2x-1)\ln \left({\frac {x}{1-x}}\right)}$

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

${\displaystyle F={\frac {x_{p}-x_{t}}{x_{f}-x_{t}}}\cdot P}$

${\displaystyle T={\frac {x_{p}-x_{f}}{x_{f}-x_{t}}}\cdot P}$

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%.

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.^[4]

Example:

A large nuclear power station with a net electrical capacity of 1300 MW requires about 25 tonnes per year (25 t/a) 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.