A discussion of accidents involving depleted UF6 storage cylinders, including possible health effects, accident risk, and accident history.
Potential Health Effects from Cylinder Accidents
Accidents involving depleted UF6 storage cylinders are a concern because they could result in an uncontrolled release of UF6 to the environment, which could potentially affect the health of workers and members of the public living downwind of the accident site. Accidental release of UF6 from storage cylinders or during processing activities could result in injuries or fatalities. The most immediate hazard after a release would be from inhalation of hydrogen fluoride (HF), a highly corrosive gas formed when UF6 reacts with moisture in air. Exposure to HF could result in a range of health effects, from eye and respiratory irritation to death, depending on the exposure level.
Solid uranyl fluoride (UO2F2) is also formed when depleted UF6 reacts with moisture in the air after an accidental release. Uranyl fluoride is a particulate which can be dispersed in air and inhaled. Once inhaled, uranyl fluoride is easily absorbed into the bloodstream because it is soluble. If large quantities are inhaled, kidney toxicity can result.
The risks from an accident depend on the severity and characteristics of the accident. The consequences would depend on how much UF6 was released and how many people were near or downwind of the site when the accident occurred. In addition, the presence of water or a fire affects accident consequences because either can lead to more rapid release and dispersal of the UF6. All cylinder handling and storage operations are conducted in a manner that minimizes the chances of an accident occurring.
The PEIS evaluated a range of hypothetical accidents involving depleted UF6 in storage, including cylinder drops, valve sheers, earthquakes, and vehicle and airplane crashes into the cylinder yards. In the PEIS, the hypothetical storage accident estimated to have the largest potential consequences was a fire involving the rupture of three full cylinders. Such a fire could be caused by a vehicle accident in the storage yards, where the impact and fuel from the vehicle caused the large fire. In the PEIS, the frequency of this type of accident was estimated to be about once in 100,000 years. If such an extremely unlikely accident did occur, it was estimated that up to 1,900 members of the general public around the conversion facility might experience adverse effects from chemical exposures (mostly mild and temporary effects, such as respiratory irritation or temporary decrease in kidney function). However, of these only about 1 individual might experience irreversible adverse effects (such as lung damage or kidney damage), with no fatalities expected. In addition, irreversible or fatal effects among workers very near the accident scene would be possible. For more details on the risks from accidents, see Appendix D of the PEIS.
Uranium Hexafluoride Accident History
There have been several accidents involving uranium hexafluoride in the United States. In 1944, a research and development pilot plant for thermal diffusion was temporarily shut down for piping modifications. During reactivation of the plant, a weld ruptured on an 8-ft long cylinder containing gaseous natural UF6 that was being heated by steam. An estimated 400 lb of UF6 was released, which reacted with steam from the process and created HF and uranyl fluoride. This accident resulted in two deaths from HF inhalation and three individuals seriously injured from both HF inhalation and uranium toxicity. The injured individuals eventually recovered, and a follow-up many years later showed no evidence of lasting kidney damage from the uranium exposure.
In 1978 a cylinder containing liquid depleted UF6 was accidentally dropped and ruptured in a storage yard at the Portsmouth Gaseous Diffusion Plant. Cold weather limited the dispersion of the UF6. Cleanup efforts were conducted to collect as much of the released material as possible. No one was injured in this accident. (Note: Storage cylinders contain liquid UF6 only for a few days immediately after filling. Once the cylinder cools the UF6 is a solid and would be released much more slowly if an accident resulted in cylinder rupture).
Another UF6 accident involving a cylinder rupture occurred at a commercial uranium conversion facility (Sequoyah Fuels Corp., Gore OK) in 1986. The accident occurred when an over-loaded shipping cylinder was reheated to remove an excess of UF6. The cylinder ruptured, releasing a dense cloud of UF6 and its reaction products. This accident resulted in the death of one individual from HF inhalation. An additional 31 workers were exposed to the released cloud. Although some of the more highly exposed workers showed evidence of short-term kidney damage (e.g., protein in the urine), none of these workers had lasting kidney toxicity from the uranium exposure.