Depleted uranium is a by-product or waste product of uranium enrichment.
Its density makes it useful in a variety of roles, including radiation shielding and military projectiles. The health hazards associated with any uranium are much the same as those for lead.
Uranium was discovered by Martin Klaproth in 1789 in the mineral pitchblende, and was named after the planet Uranus. Uranium is thought to have been formed in super novae some 6.6 billion years ago, and is reasonably common in the earth's crust. In places it is sufficiently concentrated to be mined, yielding a mixed uranium oxide product, (U3O8).
Uranium is chiefly used as an energy source for electricity generation. Uranium-235 is the only naturally-occurring material which can sustain a fission chain reaction, releasing large amounts of energy.
Most nuclear reactors require natural uranium (having 0.7 percent U-235) to be enriched, so as to increase the proportion of the fissile isotope U-235 about five- or six-fold.
Depleted Uranium: Every tone of natural uranium produced and enriched for use in a nuclear reactor gives about 130 kg of enriched fuel (3.5 percent or more U-235). The balance is depleted uranium (0.25-0.30 percent U-235).
This major portion has been depleted in its fissile U-235 isotope by the enrichment process. It is commonly known as DU.
DU is stored either as UF6 or it is de-converted back to U3O8, which is more benign chemically and thus more suited for long-term storage. It is also less toxic.
Every year almost 30,000 tones of depleted uranium joins already substantial stockpiles in USA, Europe and Russia.
Some is drawn from these stockpiles to dilute high-enriched (>90 percent) uranium released from weapons programs, particularly in Russia, and destined for use in civil reactors.
This weapons-grade material is diluted about 25:1 with depleted uranium, or with depleted uranium that has been enriched slightly (to 1.5 percent U-235) to minimize levels of (natural) U-234 in the product.
Other uses are more mundane, and depend on the metal's very high density (1.7 times that of lead). Hence, where maximum mass must fit in minimum space, such as aircraft control surface counterweights, yacht keels, etc, it is often well suited.
In addition it is used for radiation shielding, being some five times more effective than lead in this role.
Also because of its density, it is used as solid slugs or penetrates in Armour-piercing projectiles, alloyed with abut 0.75 percent titanium.
DU is prophetic, so that upon impact about 30 percent of the projectile atomizes and burns to uranium oxide dust. It was widely used in the Kuwait war (300 tones) and less so in Kosovo (11 tones).
Health aspects: Depleted uranium is not classified as a dangerous substance radiologically, though it is a potential hazard in large quantities, beyond what could conceivably be breathed.
Its emissions are very low, since the half-life of U-238 is the same as the age of the earth (4.5 billion years). There are no reputable reports of cancer or other negative health effects from radiation exposure to ingested or inhaled natural or depleted uranium.
However, uranium does have a chemical toxicity about the same as that of lead, so inhaled fume or ingested oxide is considered a health hazard.
Most uranium actually absorbed into the body is excreted within days, the balance being laid down in bone. Its biological effect is principally kidney damage.
WHO has set a Tolerable Daily Intake level for U of 0.6 micrograms/kg body weight, orally. (This is about eight times our normal background intake from natural sources.) Standards for drinking water and concentrations in air are set accordingly.
Like most radionuclides, it is not known as a carcinogen, or to cause birth defects (from effects in utero) or to cause genetic mutations.
Radiation from DU munitions depends on how long the uranium has been separated chemically from its decay products.
If some thorium-234 and protactinium-234 has built up through decay of U-238, these will give rise to some beta emissions. On this basis, DU is "weakly radioactive" with an activity of 39 Bq/mg quoted.
Thus DU is clearly dangerous for people in vehicles which are military targets, but for anyone else Ð even in a war zone Ð there is little hazard.
Ingestion or inhalation of uranium oxide dust resulting from the impact of DU munitions on their targets is the main possible exposure route. See also WHO briefing on DU and WHO fact sheet on DU.
Uranium in the earth's core: The primary source of energy driving the convection of the earth's mantle today is uranium-238 decay.
This causes the movement in plate tectonics and provides an enormous source of heat. As decay proceeds, the final product, lead, increases in relative abundance.
Reprocessed Uranium: When spent nuclear fuel is reprocessed, both plutonium and uranium are recovered separately. Uranium comprises about 96 percent of that spent fuel.
The composition of reprocessed uranium depends on the time the fuel has been in the reactor, but it is mostly U-238. Typically it will have about 1 percent U-235 and small amounts of U-232 and U-236.
The former is a gamma-emitter, making the material difficult to handle, even with trace amounts.
The latter, comprising about 0.5 percent of the material, is a neutron absorber which means that if reprocessed uranium is used for fresh fuel it must be enriched slightly more than is required for natural uranium. In the future, laser enrichment techniques may be able to remove these isotopes.
Source:www.uic.com.au
© 2001 Mena Report (www.menareport.com)
