Tungsten, actually
[link|http://www.defensetech.org/archives/002434.html|Anonymous comment], but it sounds reasonable.
The shrapnel used in the cancer tests were "tungsten cobalt" and "tungsten cobalt nickel" alloys. Pure tungsten metal powder has been used in medicine for ages, biomedical implants, and even nutritional suplements. It is an inert element, and has never been linked to cancer.
It's interesting to note that the Gov't is stuck with Hundreds of tons of DU, Tungsten costs about 200X that of DU, and the cancer tests conspicuously avoided using pure tungsten powder (cobalt has been known to cause cancer for years, and nickel is poisonous as well).
If I mixed sugar & tobacco, had rats smoke it, then published on the resulting cancer, I could easily say "Sugar product linked with cancer." Although the effects of pure sugar smoking was never tested, I could easily say "There is much more to learn about sugar, and it's possible effects on human beings."
The spin works for everyone. The military gets to use more DU, the government spends less money, the scientist gets to do more testing because "There is much more to learn." Tungsten carbide cobalt is dangerous and there are procedures to handle the material safely.