What Is Depleted Uranium ?

Depleted Uranium is nuclear waste, and that benign-sounding prefix of "depleted" is simply intended to dull our appreciation of what that really means.
It's uranium - radioactive and toxic - so let's just call it that.

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Uranium is the heaviest natural metal, and is a radioactive element found in trace quantities all over the earth. It is a ubiquitous mineral, forming on average 2-4 ppm (parts per million) of the earth's crust and about 3 millionths of a gramme per litre of sea water.
Its most concentrated sources, capable of being mined, are sometimes referred to as uranium ores although even they contain less than 2 kg of uranium per tonne of ore, or 1:500 by weight (but since uranium is so much denser than its ores, the ratio is even lower by volume). The uranium particles in these ore deposits are generally sealed inside the mineral surface, and only a minimal radioactivity can escape. To summarise, uranium does not occur in its pure metallic form, in nature.

Natural uranium comes in three isotopes:
- U238 or Uranium-238 (makes up 99.284% of natural uranium)
- U235 (0.711%)
- U234 (0.0055%).

Nuclear reactors and nuclear weapons require U235. To obtain this, uranium deposits are mined, and pure uranium metal is then extracted and refined from these diffuse natural uranium sources. This pure uranium then needs to be "enriched" prior to use, an industrial process which consists of extracting U235 to produce "enriched uranium". This has a much higher proportion of the fissile U235 isotope (about 3.5% for reactor fuel, and above 90% for nuclear bombs - although the bombs can also be made from Plutonium-239, which is industrially derived from U238).

This enrichment procedure produces waste byproducts, of which the so-called "depleted uranium" is the major part. Hence DU is the leftover uranium that has been depleted of its U235 and U234 content, but its non-fissile U238 quotient has been correspondingly increased. DU is effectively U238 (and for the rest of this report, we use the terms interchangeably), although the other uranium isotopes cannot be entirely removed. In fact, the U235 quotient is reduced by about 60%, and U234 by 80%. Up to 100 kg of depleted uranium can be produced, to obtain each kg of enriched uranium.

See IEER factsheet, for more details on the industrial processes.

After more than half a century producing nuclear energy and atomic weapons, the US Dept of Energy has accumulated over 700,000 tonnes of DU in its storage facilities (stored as Uranium Hexafluoride, UF6).
Unlike the more active U235 and U234 isotopes, U238 is classed as a low-level radioactive waste, but it is still radioactive to a hazardous degree, and has a half-life of 4.5 billion years (as opposed to "only" 704 million years for U235, and 245,000 years for U234). It therefore has to be stored safely for an indefinite period of time (basically, forever), an extremely costly procedure. In addition, it is chemically highly toxic, like all heavy metals.

Incidentally, the natural uranium sources mentioned above are also ultimately responsible for the radioactive radon gas found in some homes and localities, courtesy of radioactive decay from uranium to radium to radon.
This also accounts for the high incidence of cancer amongst uranium mining workers (USA, Canada, Germany, South Africa and France coming 1st, 2nd, 3rd, 4th and 7th respectively, in the world's uranium mining league - WISE)
In the US (and Canada), the burden of contamination from uranium mining falls mainly on Native American reservations.