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Old 2017-03-19, 14:38   #3
Dr Sardonicus
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Feb 2017

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Originally Posted by ewmayer View Post
Also, in the part where MH talks about the economics of atomic power, I think he means 'spent' rather than 'depleted' uranium, since the latter is leftover from U-enrichment and not particularly toxic compared to the post-reactor spent stuff. (In fact the military loves DU to make penetrating projectiles out of - when one of those fragments on impact and even burns it does great increase the toxicity, but still nowhere near that of spent fuel.)
Yes, I've seen this error in news accounts, and I'm glad someone is alert enough to point it out.

Spent reactor fuel is a witches' brew of radioisotopes, some of them fission fragments with relatively short half-lives. Spent fuel rods are often kept at the bottom of large pools of water in part to dissipate the heat they generate. There are also isotopes of uranium in spent fuel that "crap up" the reactor. In theory, spent fuel could be "reprocessed," but in practice this would (as I understand it) create its own set of very nasty problems.

Depleted uranium (DU) is so called because it is depleted of its natural allotment (0.7% if memory serves) of the fissile isotope U-235, the separated U-235 going to "enrich" the uranium used as reactor fuel. DU is desirable for "sabots" (large slugs that punch through armor) because it is dense (tungsten is also so used for this reason), but DU has the added attraction mentioned above of being highly reactive, so likely to burn fiercely after a supersonic collision with armor. DU is (or at least used to be) obtainable fairly cheaply, as there aren't as many other uses for it as there are for tungsten, which among other things is a component of some kinds of steel. I have a vague recollection of reading that DU was also used on commercial aircraft for "balancing weights," which could be moved to compensate for unbalanced loading of cargo or passengers on the plane. I sometimes wondered if this was the case for the planes that were crashed on 9/11.

There is, of course, a distinction between radioactivity and chemical toxicity. DU (U-238) is chemically toxic if it gets into soluble compounds that you ingest. Its radioactivity is fairly low (half-life is about 4.5 billion years, or approximately the present age of the Earth according to current science). As an alpha emitter, it presents very little hazard if it is outside the body (your skin will stop alpha particles), but if it gets inside you (say, by breathing in uranium-oxide dust, or ingesting uranium compounds), it does present a radioactive hazard as well as a possible toxic risk.

One proven use for U-238 is as a source material for creating the well-known fissile isotope of plutonium (Pu-239). If a U-238 nucleus absorbs a neutron, it becomes unstable and kicks out two anti-electrons (positrons), thereby becoming a Pu-239 nucleus. (A much "hotter" but non-fissile isotope, Pu-238, is used as a heat source to generate power on space probes.)

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