Depleted Uranium (DU)
Natural uranium is made up mostly of Uranium 238 (99.2745% by atom percent ) and Uranium 235 (0.72% atom percent). While both uranium isotopes have long half-lives, because U-235 has a shorter one, in one ton of natural uranium, there is 320 mCi of U-238 and 14 mCi of U-235. There is a small amount of U-234 in natural uranium too at 0.0055% atom percent.
Removing most of the usable U-235, to be used for weapons or nuclear reactor fuel, creates depleted uranium. DU is made into a metal, and due to the high density, very useful for shielding and weapons.
Due to the use of DU in weapons and shielding over the last 15 years, there has been some press on health hazards that are alleged to be due to the DU.
From the DOE "Guide of Good Practices for Occupational Radiological Protection in Uranium Facilities", is a good description of uranium processing:
The process of reducing uranium ore to metal begins with the discovery and mining of uranium in ore bodies. Most medium grade ore consists of oxides of uranium, of which carnotite is predominant. Although some ore is mined using in situ leach techniques, most is hard-rock mined with a small amount removed by open pit mining. Uranium ore is milled by crushing, leaching, extracting, and precipitating, usually to ammonium diuranate, commonly called yellow cake. The radioactivity of this product is low because the decay products have been stripped away and it is in an unenriched form. The yellow cake is purified and converted to UF4 and then further fluorinated to uranium hexafluoride (UF6). Gaseous diffusion enrichment changes the uranium isotopic, but not the chemical, composition of the gas. The UF6 is hydrolyzed to uranyl oxyfluoride, which is precipitated with an ammonia solution to ammonium diuranate. This precipitate is filtered or centrifuged, dried, and calcined. The uranium compound is reduced to UO2 powder, which is pelletized, sintered, and encapsulated in tubes for reactor usage.
Conversion of UF6 to uranium metal involves, first, the production of UF4 , commonly called green salt. Enriched uranium green salt is reacted with granular calcium to produce metal slag. This product is then reacted with magnesium or calcium to reduce the material to metal. Depleted uranium green salt is more commonly reacted with magnesium to produce DU metal as a derby. In both cases, most of the uranium decay products are concentrated in the calcium or magnesium slag, leaving the metal relatively pure and with a reduced level of radioactivity. Buildup of decay products to near-equilibrium levels takes about six months.
U-238 which is most of DU decays by:
238U --> 234Th ---> 234MPa ---> 234U
234U is long lived (2.4E5 years), so DU would not have much in the way of decay products beyond that.
The Health Physics Society members have provided a good resource on uranium and DU in their Ask the Expert section:
The World Health Organization has provided a good set of resources on DU ammunition
Uranium has some special chemical and biological characteristics: since natural uranium has a low specific activity, chemical damage to the kidneys is likely to be more limiting than radiation damage. If U-235 is concentrated for fuel or weapons, it is called enriched. Normally radiation damage to the lung and kidney are considered if exposure is to enriched uranium.
Specific information on uranium are:
Principle Organ: mineralized bone volume
Amount of Uranium in Body: 90 �g with 59 �g found in the skeleton
Daily intake in food and fluids of Uranium: 1.9 �g
Natural Levels: 0.5 to 4.7 ppm uranium in the common rock types
Biological half-life: two compartments for each of the following: mineralized bone - 20 and 5000 days, kidney and other tissue - 6 and 1500 days
Isotope: Uranium-235 (235U)
Physical Half-life: 7.038 x 108 years
Sources: Naturally occurring primordial nuclide, 0.720% of the atoms in natural Uranium in the Earth's crust. Enriched with respect to 238U in fissionable material for reactors and weapons.
Specific Activity for naturally occuring uranium: 15.8 nCi of U-235/g of Uranium (582 kBq/kg) or ~64 kg per mCi (37 MBq)
Principle Modes and Energies of Decay (MeV): alpha -> 4.22 (5.7%), 4.32 (4.4%), 4.40 (55%), 4.56 (4.2%), 4.37 (17%); gamma 0.196 (61%)
Isotope: Uranium-238 (238U)
Physical Half-life: 4.468 x 109 years
Sources: Naturally occurring primordial nuclide, 99.2745% the atoms in natural uranium.
Specific Activity: 330 nCi/g (12 kBq/g) or 3 grams = 1 �Ci (37 kBq)
Principle Modes and Energies of Decay (MeV): alpha -> 4.21 (77%), 4.15 (23%)
The Agency for Toxic Substances and Disease Registry (ATSDR), part of U.S. Department of Health and Human Services, Public Health Service statement on how can uranium affect health:
All uranium mixtures (natural, depleted, and enriched) have the same chemical effect on your body. Large amounts of uranium can react with the tissues in your body and damage your kidneys. The radiation damage from exposure to high levels of natural or depleted uranium are not known to cause cancer