A discussion of health risks associated with transport of depleted UF6.
Transport Regulations and Requirements
In the future, it is likely that depleted uranium hexafluoride cylinders will be transported to a conversion facility. For example, it is currently anticipated that the cylinders at the ETTP Site in Oak Ridge, TN, will be transported to the Portsmouth Site, OH, for conversion. Uranium hexafluoride has been shipped safely in the United States for over 40 years by both truck and rail.
Shipments of depleted UF6 would be made in accordance with all applicable transportation regulations. Shipment of depleted UF6 is regulated by the Department of Transportation (DOT) in 49 CFR part 173, subpart I, "Class 7 (Radioactive) Materials." Among other things, 49 CFR 173.420 requires that each UF6 cylinder be designed, fabricated, inspected, tested, and marked in accordance with the version of American National Standard N14.1, "Uranium Hexafluoride - Packaging for Transport" (ANSI N14.1) that was in effect at the time the cylinder was manufactured. Although a detailed discussion of depleted UF6 transportation requirements is not included here, three provisions in 49 CFR 173.420 and ANSI N14.1 are particularly important relative to depleted UF6 cylinder shipments: (1) a cylinder 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 a cylinder must be less than 14.8 psia; and (3) a cylinder 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 referred to as overfilled, overpressurized, and substandard, respectively. Any depleted UF6 currently stored in such a nonconforming cylinder cannot be transported without some type of prior preparation, such as placement of the nonconforming cylinder in an overpack, or transfer of the depleted UF6 to a conforming cylinder.
Risks from Transportation
The transportation of any material is associated with some potential health risk; namely, adverse health effects from inhalation of vehicle exhausts, and risk of injury or death associated with physical trauma from vehicle accidents. These risks are often referred to as "vehicle-related" risks because they depend on the distance traveled and not on the type of cargo being transported. In addition to vehicle-related risks, the shipment of depleted uranium products also will have some "cargo-related" risks because of the radioactive nature of the material. Risks from the transportation of radioactive material can occur during normal operating conditions from exposure to low levels of penetrating gamma radiation in the vicinity of a shipment, and from accidents that could potentially release the radioactive material to the environment. Moreover, accidents involving the shipment of depleted uranium hexafluoride could also potentially result in the release of hydrofluoric acid created if UF6 is released and reacts with moisture in the air.
The PEIS provided a detailed evaluation of transportation risks for the shipment of depleted UF6 and other depleted uranium compounds in Appendix J. Although there would be a low level gamma radiation field in the vicinity of depleted uranium shipments, the PEIS results indicate that the levels of exposure from such shipments would be well below levels expected to cause health effects among persons on or near the shipment routes.
Accidents involving the shipment of depleted uranium hexafluoride (UF6)
The PEIS also evaluated the impacts of hypothetical rail and truck accidents involving the shipment of depleted uranium hexafluoride. If a transportation accident involving release of UF6 from cylinders occurred, potential health risks would be associated with inhalation of HF and uranyl fluoride generated from the reaction of UF6 with moisture in the air. At high exposure levels, inhalation of the corrosive gas HF can cause death, and the uranyl fluoride could cause kidney damage. The most serious hypothetical transportation accident evaluated in the PEIS involved a rail shipment through an urban area with release of contents from four cylinders. The estimated frequency of such an accident was about once in 250,000 years. If such an accident did occur, it was estimated that exposures of workers and the general public would result in 4 cases of irreversible adverse chemical effects, but no immediate deaths. Also, over the long-term, approximately 60 latent cancer fatalities could occur from radiation exposures, in addition to those occurring from all other causes. (Statistically, in the population of 3 million assumed to be exposed, 700,000 would be expected to die of cancer from all causes.) Historically, no transportation accidents involving a release of UF6 have occurred.
Accidents involving the shipment of depleted uranium oxide and uranium metal
The PEIS also evaluated risks from accidents involving the shipment of depleted uranium oxide and uranium metal. For a hypothetical railcar accident involving powder U3O8 that was assumed to occur in a highly-populated urban area under stable (nighttime) weather conditions, it was estimated that up to 20 people might experience irreversible adverse effects from chemical toxicity, with no fatalities expected. Approximately 2 potential latent cancer fatalities from radiological hazards are estimated for an accident under the same conditions. The probability of such an accident occurring is very low. The consequences from a truck accident would be lower, because trucks have a smaller shipment capacity. The consequences of transportation accidents involving depleted uranium metal would be much smaller than those involving uranium oxide because uranium metal would be in the form of solid blocks and would not be easily dispersed in an accident.
(For more details on risks from transportation of uranium oxide or metal, see also Appendix J of the PEIS.)