Uranium occurs naturally and is approximately 40 times more abundant than silver. It consists of three isotopes: uranium-234, uranium-235 and uranium-238 which are present in amounts of 0.005%, 0.7%, and 99.3%, by weight, respectively.
While Canadian designed reactors (CANDU) use natural uranium and a heavy water moderator, reactors in most other countries are cooled and moderated by ordinary (light) water and require uranium enriched in the 235-isotope. The proportion of uranium-235 is typically increased from 0.7% to between 3% and 5%. The enrichment process also increases the percent of uranium-234.
Depleted uranium is a byproduct of the enrichment process. The amounts of uranium-234 and uranium-235 remaining in depleted uranium metal are about 0.002% and 0.2%, respectively. The effect of removing these isotopes is that depleted uranium is about 40% less radioactive than natural uranium. For comparison, it is 10 million times less radioactive gram for gram than Americium-241 which is found in domestic smoke detectors.
Due to its high density (1.7 times the density of lead) and relative availability, depleted uranium is well suited for use as counterweights in aeroplanes, yachts and professional race cars, and as radiation shields for medical radiotherapy units and transportation of radioactive isotopes. Military applications include heavy tank armour, anti-tank missiles and projectiles. Depleted uranium weapons are regarded as conventional weapons and as such were widely used in the 1990 Gulf War (290 tonnes), 1994/95 in Bosnia and 1999 in Kosovo (10 tonnes). The Canadian Forces have recently replaced depleted uranium rounds in its Naval Close-In-Weapons System (CIWS) with tungsten rounds.
When a depleted uranium projectile strikes a target, the uranium usually burns. Combustion will also occur if any projectile strikes depleted uranium shielding or if an aircraft carrying depleted uranium crashes. The burning of depleted uranium produces dust and oxide aerosols which can be inhaled directly or ingested in contaminated food, drinking water or soil. For depleted uranium intake, inhalation is the dominant route of short term or acute exposure in occupational settings. Ingestion is the major source of long term or chronic intake to the general public.
Because uranium is ubiquitous throughout the natural environment, approximately 90 :g of uranium, on average, exists in the human body from normal daily intakes of air, water and food. The biological distribution is approximately 66% in the skeleton, 16% in liver, 8% in kidneys and 10% in other tissues. Due to the presence of natural uranium, a suspected intake of depleted uranium cannot be confirmed by techniques which measure total uranium or uranium-238 only. Depleted uranium is detected by measuring relative amounts of the isotopes on samples such as urine or hair, or on a person in a whole-body counter. Thermal Ionisation Mass Spectrometry (TIMS) or Inductively Coupled Plasma Mass Spectrometry (ICP/MS) measurements on urine or hair are approximately a million times more sensitive than whole body counting techniques.
Depleted uranium presents both a radiological and chemical risk to health. The level of risk depends on the route of exposure and the solubility.
As noted above, depleted uranium is 40% less radioactive than natural uranium. The most hazardous route of exposure from radiological point of view is inhalation, followed by ingestion and external exposure. The radiological risk can be understood by estimating the amount of depleted uranium that would deliver a dose equal to 1 millisievert in one year, the public dose limit for releases from regulated facilities in Canada. For comparison, the dose received from natural background radiation in Canada is about 2 millisievert per year. (A millisievert is the unit for effective dose of ionizing radiation. This dose is considered to be directly related to health risk.)
The chemical toxicity of depleted uranium is the same as that of natural uranium. Uranium is a heavy metal with similar health effects to lead, cadmium, nickel, cobalt, and tungsten. The kidney is the organ primarily affected and soluble uranium presents the greatest hazard. Studies of animals have shown observable changes in kidney tissue at daily intakes by ingestion equivalent to 4.2 milligrams per day for a person.
For comparison, at background concentrations of uranium dust in air, an average person would inhale about 0.004 milligrams of uranium in one year. The intake of natural uranium in water and food averages about 0.002 milligrams/day or 0.7 milligrams/year.
Chemical toxicity of depleted uranium presents a greater risk than the radiological toxicity. As an example, observable changes in kidney function could result from consuming uranium at 0.05 milligrams/day, or about 20 milligrams/year, in drinking water. The radiation dose from the ingestion of this amount of uranium is less than 1% of that from natural background radiation in Canada. One would not expect to observe any radiological effects from the ingestion of this much uranium. For the same chemical burden to the kidney, the equivalent uranium amount of acute inhalation is 0.6 milligrams of moderately soluble depleted uranium. This would produce a radiation dose of 0.015 millisievert, which is again insignificant compared to a possible chemical effect. For a given amount of uranium intake, whether through either ingestion or inhalation, chemical toxicity of depleted uranium is more significant than its radiological toxicity.