Uranium hexafluoride
Contents
Introduction A vial containing solid crystals of uranium hexafluoride. (Source: Argonne National Laboratory (Uranium hexafluoride) )
Uranium hexafluoride (UF6) is a chemical compound consisting of one atom of uranium combined with six atoms of fluorine. It is the chemical form of uranium that is used during the uranium enrichment process. Within a reasonable range of temperature and pressure, it can be a solid, liquid, or gas. Solid UF6 is a white, dense, crystalline material that resembles rock salt.
UF6 and Uranium Processing
The gaseous diffusion process used to enrich uranium requires uranium in the form of uranium hexafluoride (UF6). In the first step of UF6 production, uranium ore is mined and sent to a mill where uranium oxide (often called "yellowcake") is produced. The uranium oxide is then sent to a UF6 production facility. At the production facility, the uranium oxide is combined with anhydrous hydrogen fluoride (HF) and fluorine gas in a series of chemical reactions to form the chemical compound UF6. The product UF6 is placed into steel cylinders and shipped as a solid to a gaseous diffusion plant for enrichment.
Uranium hexafluoride is used in uranium processing because its unique properties make it very convenient. It can conveniently be used as a gas for processing, as a liquid for filling or emptying containers or equipment, and as a solid for storage—all at [[temperature]s] and [[pressure]s] commonly used in industrial processes.
Physical Properties
Uranium hexafluoride (UF6) at ambient conditions is a volatile, white, crystalline solid. Solid UF6 is readily transformed into the gaseous or liquid states by the application of heat. All three phases — solid, liquid, and gas — coexist at 147°F (64°C) (the triple point). Only the gaseous phase exists above 446°F (230°C), the critical temperature, at which the critical pressure is 45.5 atm (4.61 mPa). The vapor pressure above the solid reaches 1 atm (0.1 mPa) at 133°F (56°C), the sublimation temperature.
The triple point of UF6 occurs at 22 pounds per square inch, absolute (psia) and 147°F (64°C). These are the only conditions at which all three states — liquid, solid, and gas — can exist in equilibrium. If the temperature or pressure is greater than at the triple point, there will only be gas or liquid.
Density of solid UF6 at 68°F (20°C) is 317.8 lb/ft3 (5.1 g/cm3). A large decrease in UF6 density occurs when UF6 changes from the solid to the liquid state, which results in a large increase in volume. The thermal expansion of the liquid with increasing temperature is also high. Therefore, it is important to maintain control of the total mass and physical state of UF6 throughout an operational cycle. To avoid hydraulic rupture, when items with restricted volumes, such as traps and containers, are filled with UF6, full allowance must be made for the volume changes that will arise over the working temperature range to which the vessels will be subjected.
For UF6 to be handled as a liquid, the pressure must be in excess of 0.15 mPa (1.5 atm) and the temperature above 147°F (64°C) because the sublimation temperature lies below the triple point. Thus, any process using liquid UF6 is above atmospheric pressure and is subject to a potential leakage of UF6 to the environment, with vapor loss and cooling occurring simultaneously. Solidification occurs exothermically when the pressure falls below 1.5 atm (0.15 mPa). Thus, if a cylinder heated above the triple point is breached, a rapid outflow of the UF6 occurs until the pressure drops sufficiently to start the solidification process. The rate of outflow then decreases but continues until the contents cool to about 133°F (56°C), which is the atmospheric sublimation temperature. Some release of material may continue, depending on the type and location of the breach.
UF6 is hygroscopic (i.e., moisture-retaining) and, in contact with water (H2O), will decompose immediately to uranyl fluoride (UO2F2). When heated to decomposition, UF6 emits toxic fluoride fumes.
Chemical Properties
Uranium hexafluoride (UF6) does not react with oxygen, nitrogen, carbon dioxide, or dry air. However, UF6 combines with water to form the soluble reaction products uranyl fluoride (UO2F2) and hydrogen fluoride (HF). For this reason, UF6 is always handled in leak-tight containers and processing equipment. When UF6 comes into contact with water, such as water vapor in the air, the UF6 and water react, forming corrosive HF and a uranium-fluoride compound called uranyl fluoride (UO2F2). UF6 is essentially inert to clean aluminum, steel, Monel, nickel, aluminum, bronze, copper, and Teflon. Teflon is commonly used in the packing and cap gasket for cylinders storing depleted UF6.
When released to the atmosphere, gaseous UF6 combines with humidity to form a cloud of particulate UO2F2 and HF fumes. The reaction is very fast and is dependent on the availability of water vapor. Following a large-scale release of UF6 in an open area, the dispersion is governed by meteorological conditions, and the plume could still contain unhydrolyzed material even after traveling a distance of several hundred meters. After hydrolysis, UO2F2 can be deposited as a finely divided solid, while HF remains as part of the gas plume.
In enclosed situations, the reaction products form a dense fog, reducing visibility for occupants of the area and hindering evacuation and emergency response. Fog can occur in unconfined areas if the humidity is high.
In a fire, the reaction of UF6 with water is accelerated because of the increased UF6 vapor pressure and the large quantities of water formed in combustion of organic materials or hydrocarbons. Reaction of liquid UF6 with hydrocarbon vapors is extremely vigorous in flames, with formation of UF4 and low-molecular weight fluorinated compounds. More heat is generally released in these hydrocarbon interactions with UF6 than in the corresponding reactions of hydrocarbons with oxygen.
Health Effects
Uranium hexafluoride (UF6) and related compounds have radiological and chemical characteristics that pose potential health risks. Depleted uranium hexafluoride also has potential health and environmental effects.
Radiation Effects
The characteristics of UF6 pose potential health risks, and the material is handled accordingly. Uranium is radioactive and decays into a series of other radioactive elements. Therefore, UF6 in storage emits low levels of radiation. The radiation levels measured on the outside surface of filled depleted UF6 storage cylinders are typically about 2 to 3 millirem per hour (mrem/h), decreasing to about 1 mrem/h at a distance of 1 ft (0.3 m).
Chemical Effects
In addition, if UF6 is released to the atmosphere, the uranium compounds and hydrogen fluoride (HF) that are formed by reaction with moisture in the air can be chemically toxic. Uranium is a heavy metal that, in addition to being radioactive, can have toxic chemical effects (primarily on the kidneys) if it enters the bloodstream by means of ingestion or inhalation. HF is an extremely corrosive gas that can damage the lungs and cause death if inhaled at high enough concentrations.
Further Reading
Uranium Hexafluoride, Depleted UF6 Management Program Information Network, Argonne National Laboratory
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