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

Q

I actually have two questions. What does depleted mean in depleted uranium (DU)? Secondly, is the DU in DU rounds (ammunition) considered naturally occurring radioactive material?

A

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% by weight of the 238
isotope and 0.7% by weight of the 235 isotope. There is also a tiny bit
by weight of ²³⁴U, only about 0.01%. 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%) 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% 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