Since the end of World War II, large quantities of uranium were processed by gaseous diffusion to produce uranium having a higher concentration of uranium-235 (235U) than the 0.72% that occurs naturally (called “enriched” uranium) for use in U.S. national defense and civilian applications. The uranium remaining after removal of the enriched fraction contains about 99.8% uranium-238, 0.2% uranium-235 and 0.001% uranium-234 by mass; this is referred to as depleted uranium. The main difference between depleted uranium and natural uranium is that the former contains at least three times less 235U than the latter.
Uses of Depleted Uranium
Depleted uranium has peaceful applications, such as counterweights in aircraft, missiles and racing sailboat keels and as a material used in hospitals for shielding x-rays or gamma radiation from equipment used for radiation therapy. Depleted uranium is used in armor-piercing ammunition because it has a high density (1.7 times that of lead), and is also used for military armor to reduce the effect of other conventional munitions.
Due to the pyrophoric nature of uranium metal and the extreme temperatures generated on impact of depleted uranium ammunition on a hard target, it ignites and produces an aerosol of fine particles of uranium oxides if the temperature exceeds 600°C. The main oxidation product is triuranium octaoxide (U3O8), but uranium dioxide (UO2) and uranium trioxide (UO3) will also be formed. It is assumed that a large fraction (50 to 96%) of the aerosol consists of respirable particles that could enter the lower respiratory tract and cannot be expelled.
Potential Health Risks
Depleted uranium is not a significant health hazard unless it is taken into the body. External exposure to radiation from depleted uranium is generally not a major concern because the alpha particles emitted by its isotopes travel only a few centimeters in air or can be stopped by a sheet of paper. Also, the uranium-235 that remains in depleted uranium emits only a small amount of low-energy gamma radiation. According to the World Health Organization, a radiation dose from it would be about 60% of that from purified natural uranium with the same mass.
Under most circumstances, use of depleted uranium will make a negligible contribution to the overall natural background levels of uranium in the environment. However, if allowed to enter the body, depleted uranium, like natural uranium, has the potential for both chemical and radiological toxicity, with the two important target organs being the kidneys and the lungs. The most likely pathways by which uranium could enter the body are ingestion and inhalation. The relative contribution of each pathway to the total uptake into the body depends on the physical and chemical nature of the uranium, as well as the level and duration of exposure.
In general, potential health and environmental impacts from a depleted uranium handling or processing facility could occur (1) during construction of the facility, (2) during operations of the facility under both normal conditions and during postulated accidents, and (3) during any transportation of cylinders, uranium, and hydrogen fluoride (HF) products that may be required to support the facility. The potential impacts associated with facility construction would result from typical land-clearing and construction activities. Potential impacts during operations would occur primarily to workers during handling operations and to the public as a result of routine releases of small amounts of contaminants through exhaust stacks and treated liquid effluent discharges. Potential impacts to workers and the public from processing or storage also might occur as a result of accidents that release hazardous materials, during both facility operations and transportation.
Probably the greatest potential for depleted uranium exposure will follow conflict where depleted uranium munitions are used. Possible pathways for exposure of the general population to depleted uranium from an attack site include:
- Inhalation of uranium oxides in smoke and dust;
- Depleted uranium fragments entering the body in wounds;
- External contact with depleted uranium penetrators;
- Uptake of uranium by crops for human consumption;
- Uptake of uranium in grass or soil by cattle grazing; and
- Uranium accumulation in drinking water.
A United Nations Environment Programme (UNEP) report giving field measurements taken around selected impact sites in Kosovo (Federal Republic of Yugoslavia) indicates that contamination by depleted uranium in the environment was localized to a few tens of meters around impact sites. Contamination by depleted uranium dusts of local vegetation and water supplies was found to be extremely low. Thus, the probability of significant exposure to local populations was considered to be very low. A NATO study reached similar conclusions.
Other groups claim that the health effects of depleted uranium are significant. For example, the U.S. used more than 300 tons of depleted uranium during the Gulf War of 1991. Much of that was converted at high temperature into an aerosol, that is, minute insoluble particles of uranium oxide, UO2 or UO3, in a mist or fog. Some attribute exposure to depleted uranium in Iraq as a cause of Gulf War Syndrome, a widely used term to describe the unexplained illnesses occurring in Gulf War veterans, civilian defense workers, and military families. Former U.S. Attorney General Ramsey Clark has issued an International Appeal to Ban the Use of Depleted Uranium Weapons due in part their perceived health effects. Some health officials call for a more rigourous testing of Gulf War veterans for exposure to depleted uranium.
Transportation of Depleted Uranium Materials
In the U.S., the Department of Transportation (DOT) has regulatory responsibility for safety in transportation of all hazardous materials, including radioactive material. DOT developed a single a set of safety standards that assured that properly prepared shipments of hazardous materials would be acceptable for transport by all modes (rail, highway, air, and water).
Under the [Atomic Energy Act, United States|Atomic Energy Act of 1954] as amended, the U.S. Nuclear Regulatory Commission (NRC) also has responsibility for safety in the transport of radioactive materials. Due to the overlap in statutory authorities of the NRC and DOT, the two agencies have a Memorandum of Understanding (MOU) with regard to regulation of the transport of radioactive material. Consistent with the MOU, the NRC has promulgated, in 10 CFR Part 71, shipping requirements for radioactive materials.
The primary regulatory approach used by DOT and NRC for ensuring safety during transportation of radioactive materials is by specifying standards for the proper packaging of such materials. Packaging for transporting radioactive materials must be designed, constructed, and maintained to ensure that they will contain and shield their contents during normal transportation. The type of packaging used is determined by the radioactive hazard associated with the packaged material. The hazard is determined by the characteristics of the specific radioactive material and its physical form (e.g., solid, liquid, or gas). The regulations also specify many requirements for labeling, marking, training, and administrative controls.
The shipment of radioactive materials may take place by truck, rail, or barge. Federal regulations do not place route restrictions on the movement of depleted uranium hexafluoride (UF6) cylinders or depleted uranium on United States highways or railroads.
It should be noted that the nuclear properties of depleted uranium are such that the occurrence of a nuclear criticality (i.e., a nuclear chain reaction) is not a concern, regardless of the amount of depleted uranium present. However, criticality is a concern for the handling, packaging, and shipping of enriched uranium. For enriched uranium, criticality control is accomplished by employing, individually or collectively, specific limits on uranium-235 enrichment, mass, volume, geometry, moderation, and spacing for each type of package. The amount of uranium that may be contained in an individual package and the total number of packages that may be transported together are determined by the nuclear properties of the enriched uranium.
Shipment of Depleted Uranium Hexafluoride (UF6) Cylinders
Specific requirements exist for the shipment of uranium hexafluoride (UF6) cylinders. Among other things, UF6 cylinders must be designed, fabricated, inspected, tested, and marked in accordance with the version of American National Standard N14.1, "Uranium Hexafluoride - Packaging for Transport", that was in effect at the time the cylinder was manufactured. Although a detailed discussion of depleted UF6 transportation regulations is not included here, three requirements are particularly important relative to depleted UF6 cylinder shipments: (1) cylinders must be filled to less than 62% of the certified volumetric capacity (the fill-limit was reduced to 62% from 64% around 1987); (2) the pressure within cylinders must be less than 14.8 psia (absolute pressure); and (3) cylinders must be free of cracks, excessive distortion, bent or broken valves or plugs, and broken or torn stiffening rings or skirts, and must not have shell thicknesses that have decreased below a specified minimum value. Cylinders not meeting these requirements are often referred to as substandard or noncompliant.
Although the exact number is not yet known, preliminary reports suggest that many of the cylinders at East Tennessee Technology Park (ETTP) will not meet the DOT transportation requirements. Three options exist for shipping these noncompliant cylinders:
- The UF6 contents could be transferred from noncompliant cylinders into new or compliant cylinders.
- An exemption could be obtained from the DOT, allowing the UF6 cylinder to be transported either "as is" or following repairs. The primary finding that DOT must make to justify granting an application for an exemption is that the proposed alternative will achieve a level of safety that either: (1) is at least equal to the level of safety required by the otherwise applicable regulation; or, (2) if the otherwise applicable regulations do not establish a required level of safety, is consistent with the public interest and will adequately protect against the risks to life and property inherent in the transportation of hazardous materials in commerce.
- Noncompliant cylinders could be shipped in an "overpack." In this case, the shipper would have to obtain an exemption from DOT allowing the existing cylinder, regardless of its condition, to be transported if it is placed into a metal overpack. The metal overpack would have to be specially designed. Furthermore, DOT would have to determine that, if the overpack is fabricated, inspected, and marked according to its design, the resulting packaging (including the cylinder and the overpack) would have a level of safety at least equal to the level of safety required for a new UF6 cylinder.
The depleted uranium conversion product will be shipped as low specific activity, group I, (LSA-I) material. All LSA materials have a characteristic of presenting limited radiation hazard, because of their relatively low concentration of radioactivity.
Impacts from Storage of Uranium Hexafluoride (UF6) Cylinders
The U.S. Department of Energy's inventory of depleted uranium hexafluoride (UF6) consists of approximately 700,000 metric tons of depleted UF6, containing about 470,000 metric tons of uranium, currently stored at the Paducah Site in Kentucky, the Portsmouth Site in Ohio, and the East Tennessee Technology Park (ETTP) in Tennessee (formerly known as the K-25 Site). This inventory of depleted UF6 is stored in about 57,000 steel cylinders. Continued storage of the UF6 cylinders would require extending the use of a total of about 100 acres of land currently used to store the cylinders.
The day to day management of the depleted UF6 cylinders includes actions designed to cost-effectively improve their storage conditions, such as:
- Performing regular inspections and general maintenance of cylinders and storage yards;
- Restacking and respacing the cylinders to improve drainage and to allow for more thorough inspections;
- Repainting ends of skirted cylinders and repainting cylinder bodies as needed to arrest corrosion; and
- Constructing new concrete cylinder storage yards and reconditioning existing yards from gravel to concrete to improve storage conditions.
If continued cylinder maintenance and painting are effective in controlling corrosion, impacts to the environment, such as air or groundwater contamination, would be kept within regulatory standards at all storage sites.
However, the potential for adverse impacts from continued cylinder storage was identified for air and groundwater quality, if worst-case conditions were assumed (i.e., that cylinder maintenance and painting activities would not reduce cylinder corrosion rates). Under these conditions, it is possible that cylinder breaches could result in hydrogen fluoride (HF) air concentrations greater than the regulatory standard level at the ETTP storage site around the year 2020; HF concentrations at the Paducah and Portsmouth sites were estimated to remain within applicable standards or guidelines. Additionally, uranium concentrations in groundwater could exceed the 20 µg/L guideline level at all three sites at some time in the future (earliest about the year 2100 at the Paducah site). However, if continued cylinder maintenance and painting are effective in controlling corrosion, as expected, air and groundwater concentrations of pollutants related to continued storage would be kept within regulatory standards at all storage sites.
Impacts from Storage of Depleted Uranium Oxide
Storage as depleted uranium oxide could potentially result in adverse impacts to air, water, and soil quality as a result of construction activities—similar to the impacts expected for construction of any facility of a similar size. Potential air quality impacts would be from particulate matter generated during construction; such impacts could be controlled by good construction practices. Also, construction activities have the potential to result in surface water, groundwater, or soil contamination through spills of construction chemicals. However, by following good engineering practices, concentrations in soil and wastewater (and therefore surface water and groundwater) could be kept well within applicable standards or guidelines.
Long-term storage as depleted uranium oxide could require excavation of large quantities of soil and rock, if subsurface storage were selected as the preferred option. Impacts from the excavated materials could be mitigated by contouring and reseeding, or by trucking the excavated material off-site. Additionally, long-term storage as an oxide could require a relatively large land area, ranging from 80 acres for storage as uranium dioxide (UO2) in a mine, and up to 260 acres for storage as triuranium octaoxide (U3O8) in a vault.
Impacts from Uranium Hexafluoride (UF6) Conversion
Conversion of the depleted uranium inventory could result in adverse impacts to air, water, and soil quality as a result of the construction of conversion facilities. Such impacts would be typical of construction of any facility of similar size. A conversion facility was estimated to require about 40 acres of land area or less. Potential air quality impacts would be from particulate matter generated during construction; such impacts could be controlled by good construction practices. Also, construction activities have the potential to result in surface water, groundwater, or soil contamination through spills of construction chemicals. However, by following good engineering practices, concentrations in soil and wastewater (and therefore surface water and groundwater) could be kept well within applicable standards or guidelines.
Removal of Fluorine
Conversion of depleted uranium would likely include the removal of the fluorine component from the uranium hexafluoride (UF6). It is possible that hydrogen fluoride (HF) would be produced and sold for use, thus avoiding a waste management problem. However, if an option involving calcium fluoride (CaF2) or magnesium fluoride (MgF2) production is selected, it is currently unknown whether the product generated could be sold, disposed of as nonhazardous solid waste, or whether disposal as low-level waste (LLW) would be required. The low level of uranium contamination expected for CaF2 (i.e., less than 1 ppm) suggests that sale or disposal as nonhazardous solid waste would be most likely. It is more likely that MgF2 would require disposal as LLW. If the CaF2 and MgF2 were both considered to be LLW, the largest generation volumes (about 550,000 m3) would represent about a 13% addition to the projected U.S. Department of Energy (DOE) complex-wide LLW disposal volume, and could result in a moderate adverse impact on DOE's waste management system as a whole.
Cylinder Treatment and Disposal
Any conversion options would involve emptying the cylinders currently containing the depleted uranium hexafluoride (UF6). It is was assumed that the empty cylinders would be treated to remove the residual materials and crushed. It is also assumed that the treated, crushed cylinders would become part of scrap metal inventory, which would be disposed of as low-level waste (LLW).
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