Source: Appendix A of the PEIS (DOE/EIS-0269)
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.
Figure A.1 is the phase diagram covering the range of conditions usually encountered in working with UF6. It shows the correlation of pressure and temperature with the physical state of UF6. The triple point 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.
Figure A.1: Phase diagram covering the range of conditions usually encountered in working with UF6.
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.
Uranium hexafluoride (UF6) combines with water to form the soluble reaction products UO2F2 and HF. 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.