What's a SWU?

A separative work unit (SWU) is a unit of measurement used in the nuclear power industry, just as unit measurements such as calorie, watt, decibel, ampere, volt, etc., are used in other industries.

A SWU pertains to the process of enriching uranium so it can be used as fuel for nuclear power plants.

Most commercial nuclear reactors cannot use uranium in its natural form. More than 99% of natural uranium is composed of uranium-238 (U²³⁸) atoms. The majority of the balance—less than 1%—consists of slightly lighter U²³⁵ atoms. Most reactors need uranium fuel with a U²³⁵ content of between 4% and 5%. Uranium enrichment is the process that increases that U²³⁵ content.

A SWU is a unit of measurement of the effort needed to separate the U²³⁵ and U²³⁸ atoms in natural uranium in order to create a final product that is richer in U²³⁵ atoms. Material enriched to between 4% and 5% U²³⁵ is called low-enriched uranium (LEU). SWUs are measured using a standard formula derived from the physics of uranium enrichment.

What does a SWU do? Natural uranium, in the form of uranium hexafluoride ("natural UF₆"), is fed into an enrichment process. If, for example, you begin with 100 kilograms (220 pounds) of natural uranium, it takes about 60 SWU to produce 10 kilograms (22 pounds) of uranium enriched in U²³⁵ content to 4.5%.

It takes on the order of 100,000 SWU of enriched uranium to fuel a typical 1,000 megawatt (MW) commercial nuclear reactor for a year. A 1,000 MW plant can supply the electricity needs for a city of about 600,000. That’s approximately the size of Memphis, Tennessee or El Paso, Texas.

Customers order LEU by specifying the quantity of uranium required and the level of enrichment desired. Two uranium enrichment methods are currently in commercial use: gaseous diffusion and gas centrifuge. The finished product, LEU, is in the chemical form of UF₆ and is called "enriched UF₆." This is a solid at room temperature and a gas when heated. It is stored and shipped in sturdy metal cylinders. When shipped, these cylinders are protected by transportation overpacks.