Uranium is the heaviest naturally occurring element on earth. It has 92 protons and 92 electrons and has neutrons of varying number from 141 to 146 which corresponds to U-233 to U-238. In nature, uranium exists in oxidised form such as uranite (UO22+) and pitchblende (U3O82+).  The most abundant natural uranium isotope is U-238 (99.2742%) followed by U-235 (0.7204%) and U-234 (0.0054%).  Natural uranium has too low of U-235 content for fission reaction in nuclear power plants other than heavy water plants, therefore it is enriched to increase the U-235 content. The product of enrichment has more than 2% U-235 content and the byproduct, depleted uranium (DU) or DU tails (if still in fuel cycle), has U-235 content of approximately 0.2-0.3%.  Every ton of natural uranium produces approximately 130 kg of enriched uranium (>3.5% U-235) and the balance is depleted uranium.  The production rate of DU is over 50,000 tons/year and the current world stockpile is about 1.5 million tons.  Activity of DU is 14.80 Bq/mg which is less than the activity natural uranium of 25.28Bq/mg.  As a reference, human adult has activity of 100 Bq/kg and bananas 130 Bq/kg. [4,5] The human body contains approximately 56 μg of uranium and daily intake is estimated to be 1-2μg from food and 1.5μg from drinking water. 
DU is used to dilute weapons grade uranium (>90%) released from weapons program for use in nuclear power plants.  DU has density of 19.1 g/cm3 which is 68% denser than lead. This is exploited in civilian use for radiation shielding, counter weights and ballasts. DU is five times more effective in radiation shielding compared to lead. Other proposed uses of uranium include applications as catalyst, semiconductor and electrodes.  DU also has excellent properties for military use. Its high density and pyrophoricity as well as availability and low cost has led to its use as solid slugs and penetrator in armour piercing projectiles. DU can also be used as armour plating as it will not ignite on impact for temperatures below 600°C. 
Depleted Uranium is a possible carcinogen and potential health hazard.  All uranium isotopes decay alpha particles of various energy until reaching a non radioactive isotope of lead. Alpha particles has low penetrating power but deposit large amount of energy. Therefore exposure effect of DU is the mainly the result of its ingestion, inhalation and dermal contact.  Entry of uranium into the body thus results in a combined chemical/radiation exposure.
Ingestion pathways include ingestion of soil by children through hand contamination or through livestock. Uranium transfer from soil to pasture grass is negligible.  Uptake of uranium with drinking water is one of the major ways of ingestion pathway (UNSCEAR, 2000). About 2-5% of ingested soluble uranium (such as UF6) is absorbed into the body from the intestines and the rest is eliminated rapidly. For insoluble DU, 0.2% is absorbed into the blood and about 90% will be filtered by the kidneys within the first week.  The remaining DU will be distributed to organs and tissues where it will persist for a few weeks to 25 years.  Although gastrointestinal absorption is not a major pathway of uranium exposure, high intake of DU over a long period of time will impair or cause failure of organ functions such as kidney, liver, heart and brain leading to death. 
Inhalation of DU is significant in war zones where DU munitions are used. DU munitions create DU aerosol upon impact due to its low melting point and pyrophoricity. Inhaled particles smaller than 10 μm can reach deeper pulmonary regions and deposit for a significant amount of time, leading to longer exposure. DU aerosols are also created when DU are burnt in the case of NLI plant in Colonie, New York emitted many metric tons of uranium aerosols. DU enter the body systemically and form deposits in bones and other organs once uranium is deposited in the lungs.  Leakage of depleted UF6 from tanks is another factor of inhalation exposure. (D)UF6 is a soluble uranium compound and are absorbed within days while insoluble forms generally take months to years.  Toxic chemical effects are mostly associated with soluble forms of uranium while radiation effects are associated with the insoluble form.  Approximately 10% of the dissolved uranium reaching the blood compartment is deposited in bones, kidneys and other organs. About 6.4% of inhaled soluble DU and 0.3% of inhaled insoluble DU is ultimately transferred to the kidneys (CHPPM, 2000). Similar to ingestion pathway, inhalation of DU aerosols can also induce kidney failure.
Dermal contact is another routes that uranium can enter the body. The amount of uranium absorbed depends on factors such as solubility, length of time, exposed size and physical and physiological conditions.  Animal studies shows that uranium content is the highest in kidney and bone when uranium is embedded in them. This led to the conclusion that kidney and bone are the reservoir for uranium redistribution for embedded uranium. 
Studies of local population and soldiers of the gulf war showed increased immune system disorders, birth defect, neurocognitive disorders and cancer. DU is considered to be the possible cause as DU munitions are used widely in the gulf war. 
Animal and chronic exposure studies indicate that DU causes birth defects but data concerning DU's carcinogenic properties are conflicting. This is due to a variety of factors such as small sample size, insufficient time to assess long latency outcomes. It has been proven that DU are hazardous at high levels but conclusive studies are required to determine the health effects of DU at low to moderate levels.
© Eric Adijanto. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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