Fire Hazards with Uranium Hexafluoride Cylinders
HAZARDOUS MATERIALS
All photos by Frank Hoffman, ORO Photographic Services.
FIRE DEPARTMENT and other emergency service personnel must be aware of the fire hazards associated with the transportation of uranium hexafluoride (UF6).
UF6 is a chemical produced, transported, and processed by the nuclear industry prior to its conversion into reactor fuels. It is a radioactive and corrosive material with significant nonradiological chemical hazards. Since the 1940s, millions of tons of UF6 have been transported around the world, reportedly without a serious transportation accident that has caused any injury or death from either radiological or chemical exposure.
Although past experience indicates that the probability of a serious transportation accident is relatively low, the consequences of fire exposure to a UF6 cylinder could be disastrous. Recent events and analytical models have verified the potential hazards from the overheating of UF6 cylinders.
In 1986, at a uranium-to-UF6 conversion plant in Oklahoma, a UF6 cylinder was improperly overfilled and steamheated, causing overpressurization and rupture of its steel shipping cylinder. A large quantity of UF6 was released into the plant and off-site areas. One employee died from hydrogen fluoride chemical exposure, but the radiological consequences were apparently minor. This accident demonstrated that a large quantity of toxic chemicals released from overheating UF6 presents a significant risk.
The only transportation accident involving fire and UF6 was a train derailment in March, 1977 at Rockingham, North Carolina. Of the 33 railroad cars that derailed, four carried 14-ton natural UF6 cylinders. One cylinder was adjacent to a fire involving ammonium nitrate. Luckily, there was no release of UF6, and damage to the cylinder was minor.
UF6 is transported by commercial carriers (primarily by truck and rail) between uranium-to-UF6 conversion plants; between uranium enrichment plants located in Paducah, Kentucky, Portsmouth, Ohio, and Oak Ridge, Tennessee; and between UF6-to-fuel fabrication plants located in several areas of the country. UF6 is transported to and from seaports around the world. Naturally occurring UF6 and depleted UF6 are transported in cylinders of up to 14-ton capacity and without protective overpacks. Products that are enriched to higher concentrations of the uranium-235 isotope are transported in smaller containers packaged within protective overpacks that are tested and certified for thermal, puncture, and water-immersion stresses. This article deals only with the risks associated with the transportation of cylinders without protective overpacks.
Uranium hexafluoride at normal ambient temperatures is a crystalline solid, similar in appearance to rock salt. It converts to liquid at 147°F and undergoes thermal expansion. At progressively higher temperatures, the liquid UF6 will completely fill its steel shipping cylinder. A 14-ton uranium hexafluoride cylinder has a volume of approximately 140 cubic feet, and when heated to 278°F it will be completely filled with liquid UF6. Any further heating will cause the confined liquid to hydraulically expand and rupture the cylinder, which is not required to be equipped with engineered pressure-relief devices. UF6 is shipped only after it has solidified and the vapor pressure of the cylinder is below atmospheric pressure.
Analytical models for evaluating the thermal behavior of UF6 cylinders involved in a fire are currently being developed. The time to failure of a UF6 cylinder depends upon the size of the cylinder, availability of heat, and extent of mechanical damage. Models have estimated the time to failure for a large 14-ton cylinder to be on the order of 30 minutes. Valve failure may occur sooner. Furthermore, the reaction of liquid UF6 with hydrocarbon fuels, such as diesel oil or gasoline, is highly exothermic (that is, the reaction quickly produces a lot of heat). During tests, small cylinders containing approximately 250 pounds of UF6 failed when tested within 10 minutes from exposure to a fuel oil fire and created a fireball estimated at 75 to 100 feet in diameter.
Transportation accidents involving lire exposure, overheating, and rupture of a UF6 cylinder could have extremely serious consequences. Such accidents could release instantaneously up to 14 tons that could not be controlled, spreading contamination over a large area and exposing nearby populations to this highly toxic chemical. Hazardous materials other than UF6, may be present at accident scenes. The presence of burning materials such as gasoline, fuel oil, flammable or combustible liquids, and flammable compressed gases may expose UF6 cylinders to high temperature conditions. Emergency service personnel must be alert to interactions of these hazardous materials with packages of radioactive or corrosive materials such as UF6.
The chemical hazard is the dominant risk in a transportation accident involving depleted or natural UF6. The UF6 escaping from a ruptured tank in a fire will react with moisture in the air to form uranyl fluoride (U02F2) and hydrogen fluoride (HF). Exposure to these chemicals can cause serious damage to the kidneys and to the respiratory system, and in some cases lead to fatalities. Protection of personnel from chemical exposure is paramount.
Radiologicatly, UF6 is an alpha emitter which has a short biological half-life, and any UO2F2 absorbed by the lungs into the body is eliminated rapidly through the kidneys. In a major release of UF6 in a fire, the UO2F2 will probably be dissipated over a large area and not present significant radiological hazards. Animal experiments have concluded that the radio-toxicity hazard of is insignificant compared with the chemical-toxicity hazard. Personnel should avoid inhalation and skin contamination to minimize exposure, and appropriate authorities should be consulted.
Emergency Precautions for a Uranium Hexafluoride Release
POTENTIAL HAZARDS
Health
- Material is of a relatively low order of hazard due to external radiation (as from X-ray).
- Vapor, dust, or mist is poisonous, can be fatal if breathed in high concentrations.
- Contact with material may cause severe burns to skin and eyes.
- The reaction product with air is readily visible as a white cloud, settling as a dust on surfaces.
IMMEDIATE-ACTION INFORMATION
General
- No unnecessary personnel permitted in area. Keep upwind. Identify and isolate hazard area. Wear selfcontained breathing apparatus and full protective clothing.
Fire
- On small fires, use dry chemical or carbon dioxide.
- On large fires, use water spray or fog.
- Move exposed containers from fire area, if without risk.
- Keep undamaged packages cool with large volumes of water.
- Delay cleanup until arrival of qualified radiation monitoring assistance.
Spill or Leak
- Attempt to plug releases from container openings using wooden plugs or “freeze” leakage by cooling with water stream at point of opening.
- Use water spray to reduce vapor.
- Pressurized CO2 may also be effective in “freezing” leakage.
- Dilute spill with large amounts of water, dike for later disposal.
- Delay cleanup until arrival of qualified radiation monitoring assistance.
First Aid
- Call physician. Use standard first-aid procedures.
- Assume radioactive contamination on persons or equipment close to damaged packages in spill areas.
- Wash all exposed parts of body with soap and water, and shower if possible.
- Advise rescue personnel and physicians of possibility of radioactive contamination, chemical burns from exposure to spilled material.
Reference Emergency Handling of Radiation Accident Cases, DOE/EY-0023. published by the United States Department of Energy.
Natural Uranium Hexafluoride Shipments
Natural Uranium Hexafluoride Shipments By Mode to Transportations
NOTES: Natural uranium UF6 is non-fissile and is enriched at no greater than 0.72% uranium-235. Material shipped nonpackaged.
SOURCE: SHIPMENT MOBILITY/ACCOUNTABILITY COLLECTION (SMAC) SYSTEM Science Applications International Corporation, Oak Ridge, TN
Shipments of UF6, must be identified by Department of Transportation hazardous-materials warning placards for both radioactive and corrosive materials. The operator of the vehicle must carry a copy of shipping papers that contains detailed information on the material. These papers provide valuable information to emergency personnel, such as the nature and quantity of material, its form, origin, destination, and emergency response to an accident. In the event that shipping papers are lost or not available, any markings on the vehicle, especially the license number, should be used to identify the shipment.
It is essential that emergency preplanning for UF6 cylinder fire involvement be understood. All personnel should be evacuated from all potential release areas and a control zone set up. Cooling cylinders with hose streams from a safe distance may be undertaken for cylinders containing natural or depleted uranium-235. (Cylinders of enriched uranium-235, packaged in protective overpacks for transportation, will require a nuclear safety evaluation prior to the application of water to prevent an inadvertent nuclear chain reaction or criticality condition.) Personnel entry into released UF6 clouds will require acid suits approved for HF gas exposure and self-contained breathing apparatus.
FIRE HAZARDS WITH UF6 CYLINDERS
Requests for emergency assistance can be obtained directly through the Department of Energy Emergency Operations Center in Washington, D.C. (202-586-8100) or through the Chemical Transportation Emergency Center (CHEMTREC). A radiological assistance team will provide support to state and local authorities. Assistance may also be obtained from the Nuclear Regulatory Commission (NRC), the Environmental Protection Agency (EPA), and the Federal Emergency Management Agency (FEMA).
Additional information about the DOE’s programs as well as literature on the subject of hazardous-materials transportation may be obtained from the Transportation Management Division, U.S. Department of Energy, Washington, D.C. 20545.
LITERATURE REFERENCED
Recommendations for Providing Protection During the Transport of Uranium Hexafluoride, International Atomic Energy Agency, IAEA-TECDOC-423, Vienna, Austria, 1987.
A.J. Mallet, ORGDP Container and Developmen t Program Fire Tests of UF6 Filled Cylinders, K-D-1894, Union Carbide Corporation, January 1966.
K.E. Rapp. The Pattern of Explosive Reactions Between Uranium Hexafluoride and Hydrocarbon Oils, K/GD1631, Union Carbide Corporation, March 1977.
Uranium Hexafluoride: Handling Procedures and Container Criteria, U.S. Department of Energy, ORO-651, Rev. 5, September 1987.
Emergency Handling of Radiation Accident Case, U.S. Department of Energy, DOE/EV-0023.
W. Reid Williams, Investigation of UF6 Behavior in a Fire, Martin Marietta Energy Systems, Inc., May 1988.
R.A. Just, Acute Toxicity of Uranium Hexafluoride, Uranyl Fluoride, and Hydrogen Fluoride, Oak Ridge National Laboratory, Oak Ridge, TN, May 1988.