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Nuclear Safeguards Education Portal


Natural uranium only contains about 0.7% U-235. The other 99.3% is almost all U-238. U-235 is a fissile isotope, which means that it has a high probability of fission when interacting with neutrons of any energy. This property makes it very useful as a nuclear fuel. U-238 is not fissile. Thus, neutrons interacting with natural uranium, which is mostly U-238, have a low probability of causing fission reactions. Therefore, natural uranium is generally not a very good nuclear fuel. To make it more useful as a fuel for nuclear reactors, natural uranium is usually artificially enriched in the isotope U-235. Most fuels for nuclear reactors are enriched to about 3-5% U-235 (compared to the 0.7% U-235 in natural uranium).

While U-235 and U-238 behave very differently in nuclear reactions, they are chemically almost identical. Thus, the separation of these isotopes cannot be accomplished using techniques of the kind normally used to purify substances. Large-scale isotope separation has required the development of special processes and devices to make efficient use of the small chemical and physical differences among isotopes.

The device used to enrich the uranium is called a separator. The material fed into the separator is called the feed (which is fed into the separator at a rate of F kgU per unit time). The feed is then separated into a product (which is produced by the separator at a rate of P kgU per unit time) and a waste (which is generated by the separator at a rate of W kgU per unit time). The product is enriched in the isotope U-235 compared to the feed, and the waste is depleted in the isotope U-235 compared to the feed. (Note: the waste is sometimes also referred to as the tails.)

The ratio of the mass of U-235 to total mass of U in a material is referred to as its assay (denoted by the symbol X). So the feed has an assay of XF, the product has an assay of XP, and the waste has an assay of XW. The separator is characterized by a separation factor (α) which is the ratio of XP to XW or


If α=1, then no separation has occurred and the feed, product, and waste materials all have the same assay. A value of α that is greater than 1 denotes that some enrichment of the product has occurred. The large the value of α, then the more enriched the product will be from a single pass through a separator

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