Disposal of Depleted Uranium
A discussion of risks associated with disposal of depleted uranium.
Uranium Forms Considered
The PEIS evaluated the potential environmental impacts, including potential health risks, associated with the disposal of depleted uranium oxide at a generic low level waste disposal facility. Two forms of uranium oxide were considered, triuranium octaoxide (U308) and uranium dioxide (U02). After completion of the PEIS, as documented in the Oak Ridge National Laboratory study, Assessment of Preferred Depleted Uranium Disposal Forms (ORNL/TM- 2000/161, June 2000), four potential conversion forms (U308, U02, uranium tetrafluoride (UF4), and uranium metal) were evaluated and found to be acceptable for near-surface disposal at low level radioactive waste disposal sites such as those at DOE's Nevada Test Site and Envirocare of Utah, Inc.
The actual risk from disposal of depleted uranium would depend on the form of the uranium as well as the characteristics of the disposal facility. During the licensing or authorization process for such a disposal facility, which includes an opportunity for public review, analyses are conducted to evaluate risks and demonstrate that the facility meets established performance criteria. The generic results for the PEIS are described below.
Oxide Form Disposal
Analysis Once depleted uranium has been converted from UF6 to the oxide form, the risk associated with handling at a disposal facility is greatly decreased because the corrosive fluorine component has been removed, and because the oxide form is not reactive. Under normal operating conditions at a disposal facility, there would be a small increase in cancer risk for workers due to exposure to external radiation from the uranium oxide; however, good work practices would minimize the exposure and the risk.
The PEIS evaluated a range of hypothetical accidents at a disposal facility, including container drops, earthquakes, tornadoes, and floods. Even under extreme accident conditions, such as if a receiving building were damaged in an earthquake, the risk of immediate chemical injury to the general public and to workers from exposure to released uranium oxide was found to be very small. Because uranium oxide is insoluble, amounts inhaled would reside in the lungs for a long period of time, so increased cancer risk from radiation would be the predominant risk from oxide exposure.
The most serious accident for a disposal facility evaluated in the PEIS was an earthquake damaging a receiving building. The probability of earthquakes depends on the location of the facility, and the probability of damage depends on the structural characteristics of the buildings. In the PEIS, the estimated frequency of this type of accident at a disposal facility was between once in 100 years and once in 10,000 years. However, if such an accident did occur, it was estimated that 370 pounds of uranium oxide could be released, resulting in an increased cancer risk of about 0.05 (1 chance in 20) for the most exposed worker. The increased cancer risk for the most exposed member of the general public was estimated to be about 0.0005 (1 chance in 2000). No immediate chemical health effects would be expected for the general public from accidental exposures, but irreversible or fatal effects among workers very near the accident scene would be possible.
The PEIS also considered the potential long-term impacts to groundwater and human health from a disposal facility. Although design criteria are such that disposal facilities would not be expected to fail (i.e., release material to the environment) until several hundred years after closure, for purposes of analysis, it was assumed that a disposal facility would fail 100 years after closure. Based on the PEIS analysis, if the disposal facility was located in a dry environment (typical of the western United States), no measurable groundwater contamination would occur even by 1,000 years after facility failure, because of the small amount of rainfall and large distance to the groundwater table typical of a dry environment. If the disposal facility was located in a wet environment (typical of the eastern United States), the estimated dose from the use of groundwater at 1,000 years after the assumed failure of the facility was estimated to be about 100 mrem/yr, which would exceed the regulatory dose limit of 25 mrem/yr specified for disposal facilities. In addition, the groundwater concentrations would be great enough to cause potential adverse kidney effects from chemical exposures. Based on these results, a dry environment would be preferred for disposal of depleted uranium. (For more details on risks from disposal, see also Appendix I of the PEIS.)
Uranium Tetrafluoride (UF4) Form Disposal Analysis
As mentioned above, following the completion of the PEIS, it was suggested that the depleted UF6 could also be converted to UF4 and disposed. Because of differences in the chemical properties of uranium oxide and UF4, it was unclear if the impacts estimated for the disposal of uranium oxide in the PEIS would be representative of the impacts associated with the disposal of depleted UF4. Therefore, the potential impacts of UF4 disposal were evaluated in a separate report, Environmental Impacts of Options for Disposal of Uranium Tetrafluoride (UF4) (ANL/EAD/TM-104). Because of issues identified in the PEIS associated with disposal in a wet environment, the UF4 disposal analyses were limited to the evaluation of disposal in a dry environment.
In order to facilitate comparisons and future decision making, the depleted UF4 disposal analyses were performed and the results were presented at the same level of detail as contained in the PEIS for disposal as an oxide. For the same reasons, the assumptions made and the methodologies used to evaluate environmental impacts were similar to those used in the PEIS. The disposal report found that the potential environmental impacts from disposal of UF4 were, in general, similar to those for disposal as uranium oxide – adverse impacts would be unlikely to occur within 1,000 years of the assumed failure of the disposal facility.