Answer to Question #1281 Submitted to "Ask the Experts"

And good questions they are. To respond to the first, depleted uranium refers to uranium that has been depleted in the ²³⁵U isotope. As it occurs in nature, uranium is 99.3 percent by weight of the 238 isotope and 0.7 percent by weight of the 235 isotope. There is also a tiny bit by weight of ²³⁴U, only about 0.01 percent. Anyway, to go on with the story, by various physical means, one can change the isotopic composition of uranium. It is the ²³⁵U that is fissile and hence of value as fuel in power reactors, so uranium used as fuel for light-water reactors (that is, reactors cooled and moderated with ordinary water) is enriched in the 235 isotope. A typical enrichment is on the order of a few percent (for example, 4 percent) of the 235 isotope. The ²³⁵U that provides the enrichment is drawn from natural uranium, depleting it of its 235; depleted uranium typically has only 0.2 percent by weight of the 235 isotope.

Now for the second question. Although uranium is a naturally occurring radioactive metal, the DU in munitions is not considered naturally occurring radioactive material as it has been altered from its natural state or isotopic composition, as well as being purified. When a naturally occurring radioactive material is concentrated through purification, it is sometimes known as "technologically enhanced," a term that really does not apply to depleted uranium as the specific activity (that is, radioactivity per unit mass) has actually been reduced.

Ronald L. Kathren, CHP
Professor Emeritus Washington State University