separative work unit

A unit used to express the magnitude of the effort needed to separate two isotopes of an element, so that the proportion of one isotope is increased in one of the resulting products. Symbol, SWU. Almost always, SWU's refer to treating feedstock to increase the percentage of uranium-235 (see below).  The term is used to describe the capacity of separation plants (SWUs per annum), and the fuel requirements of nuclear power plants. It is the unit by which separative work is sold. A 1300-megawatt power plant requires in its fuel about 25 tonnes of uranium that is 3.75% U-235.  That amount of fuel can be prepared from 210 tonnes of natural uranium, using 120 tonne SWUs.  In the United States, utilities purchase separative work units from the Department of Energy.

When it is not qualified, an SWU is a kilogram separative work unit. (In Germany, the term kg UTA is used.) There is also, however, a tonne separative work unit (the tonne is the metric ton, 1000 kilograms). The dimension of the SWU is mass.

Although the SWU is often described as a unit of energy, it is not, as the table below shows.

Process Energy used per SWU
Gaseous diffusion 2400 kilowatt-hours (8600 megajoules)
Modern gas centrifuge 50 kWh (180 MJ)

Why U-235?

Uranium as it is mined is a mixture of isotopes, principally U-238. However, the main uranium isotope from which we get energy is U-235. Unlike U-238, U-235 fissions when it is struck by a slow neutron--its nucleus splits, releasing energy. Uranium as it comes from a mine is only 0.711% U-235. Although some nuclear power reactors run on natural uranium (CANDU reactors in Canada and Magnox reactors in Britain), most require that the percentage of U-235 in the fuel be at least 4%. For nuclear weapons, the percentage must be at least 90%.

Calculating SWU's

Precise calculation of SWUs involves differential equations. In ordinary practice, however, the equation below can be used. (For a more complete explanation, see the book cited in Resources.)




K. Cohen.
The Theory of Isotope Separation as Applied to the Large-scale Production of U235.
New York: McGraw-Hill, 1951.

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