| 1) "the whole point of DU is that it's significantly less radioactive than naturally occurring Uranium" The D in Depleted (U-238) has nothing to do with it's radioactivity but that it's not fissile. Fissile U-235 is not too badly radioactive either (4 billion vs. 0.7 million year half life i.e. 77% more radioactive 0.5^(0.7/4) / 0.5). We don't make depleted uranium to have less radioactive uranium (naturally occurring uranium is < 1% U-235). We have DU because we made nuclear bombs. Interestingly the Ruskies have made DU arms far more expensive now with their breeder reactor (U-238 is fertile). 2) DU is an alpha emitter. I would bath in a bathtub made of alpha-emitters and sleep in a bed of alpha-emitters. However, Uranium shatters making dust. I would not want to breath, eat, drink an alpha-emitter. It's a heavy hitting He ion that will knock the socks off anything in it's short path. Not to mention U-238 becomes Th-234 with a half life of 24 days (very radioactive) with beta emission (ouchy!). There'll never be a lot of Th-234 (U-238's low radioactivity) but it'll all decay into you (Th-234 small nuclear half-life and large biological one). So U-238 effectively decays in your body as a an alpha and beta! What happens when you blow up DU? Gets scattered throughout the local environment. 3) ...Low biological half life That's only relevant for disaster tourists. For people whose environment has been seeded with the stuff, the biological half life is completely irrelevant as it is replenished as you eat, breath and drink. An equilibrium concentration is therefore established. Therefore, environmental half-life is far more important than a biological one. |
The fissibility of Uranium isotopes is critically important in weapon design and storage. With regard to biological danger, it's the radioactivity that is important (unless you happen to be near a prompt critical event, of course). DU is about half as radioactive as natural Uranium, so I think my "significantly" was justified.
Body burden and biological half-life are better indicators of a given radioactive substance's danger to living creatures, especially in the case of alpha emitters. That's why proactive measures like prophylactic iodine and showers are effective in contaminated areas. Of course accumulating more doses of an agent is always going to be more dangerous---what's important in comparisons is how dangerous one dose is.
Your Th-234 argument is misleading, and you know it. Yes it's nasty, but U-238 decays to Th-234 with a half-life of 4 billion years. That event would occur in a human body so rarely as to be completely immeasurable unless the human was contaminated with so much U-238 they'd die instantly from the toxic exposure and whatever horrific event deposited that much.
We ingest alpha-emitters with every meal, and while we should avoid ingesting more than we have to, at the small rates we do even in the presence of U-238 (or a diet rich in Bananas and Brazil Nuts, for that matter), there's no statistical impact on lifespan.