Depleted uranium
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"Depleted" Uranium (DU), is a by-product of uranium enrichment, the process used to create fuel for nuclear reactors or material for atomic bombs. The metal, DU, is chemically toxic, very dense, and hard. Like other heavy metals (such as lead and cadmium) it can cause chemical poisoning. Like other types of uranium it is radiologically hazardous. DU has been used to armour tanks and to create shells that penetrate artillery. It has been used in the Bosnia in 1995, in the Balkans in 1999 and in Iraq. It is indiscriminate, affecting the health of civilians and military personnel on both sides.
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Understanding depleted uranium (DU)
Natural uranium consists of two isotopes: U235 and U238. An atom of U238 is slightly heavier, containing 3 more neutrons (nuclear particles) than U235.
U238 is ~99.3% of natural uranium, and U235 is ~0.7%. Most nuclear reactors need uranium fuel in which the percentage of U235 has been raised to about 4%. Uranium-powered atomic bombs need U235 to be about 90% or more of the material.
Consequently there is an industry dedicated to enriching natural uranium, that is, raising the percentage of U235 as required.
The by-product is uranium in which the percentage of U235 has been depleted, usually to ~0.2% as it is unproductive to extract it all. Such uranium is known as DU or Depleted Uranium.
Metallic DU has several properties of interest:
- It is extremely dense, about 70% denser than lead
- It is very hard, and when very hot, burns in air in an unusual way
- Despite being slightly radioactive itself, it is a good screen against fierce radioactivity
This leads to DU being used in various ways such as:
- Keels of yachts (because of its density)
- Tank armour, and shell tips: it starts to burn as it hits the tank, and self-sharpens as a result
- Screening against intense gamma radiation
However, the vast majority of DU (probably over a million tons by now) is simply stored.
Technically, DU could eventually be converted to plutonium (Pu239) by irradiation in nuclear reactors. That is what happens to a fraction of the U238 which is included with the U235 fuel. Well known techniques exist to separate the Pu239 from the other wastes, and then to use the Pu239 as another nuclear fuel or bomb material. Special nuclear reactors can be used in which there are fast-moving neutrons to convert U238 into Pu239; they are known as fast breeder reactors. In practice, this technology has not yet proved commercial, and is certainly a safety concern.
The problems of using DU
Since 99.8% of DU is U238, which is only very mildly radioactive, peaceful uses of DU such as yacht keels and radioactive screening are not particularly controversial provided that the DU is suitably sealed from direct contact with the environment.
On the other hand, military use of DU in vehicles such as tanks which may be destroyed, and in shell tips which most certainly will be destroyed, is very dangerous to human health virtually for ever. This is because the uranium is converted to fine powdered oxide dust, which can blow everywhere and easily be inhaled, and will be radioactive for millions of years. U238 decays so that half of it is left after ~4,500,000,000 years (the "half life"). Battle ground clean up is almost impossible.
The half life of the tiny percentage of U235 in DU is ~700,000,000 years.
