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Source: Uranium Hexafluoride: A Manual of Good Handling Practices (USEC-651, Revision 7)

January 1995

United States Enrichment Corporation

4.1 General

Uranium hexafluoride is a compound of hexavalent uranium and fluorine. It is the process gas used by the gaseous diffusion plants to increase the concentration of the fissionable isotope uranium-235 in the mixture of uranium-238, uranium-235, and uranium-234 found in naturally occurring uranium ore. UF6 is used for two reasons. First, it can conveniently be used as a gas for processing, as a liquid for feeding and withdrawing, and as a solid for storage. Each of these states is achievable at relatively low temperatures and pressures. Second, because fluorine has only one natural isotope, all the isotopic separative capacity of the diffusion plant is used to enrich the concentration of the lighter uranium isotopes.

4.2 Chemical Characteristics

In the solid state, UF6 is a nearly white, dense crystalline solid. The appearance of the solid is a function of whether it was formed by freezing from the liquid phase or desubliming from the vapor phase. In the first case, the solid particles will be irregularly shaped grains somewhat like rock salt, and in the second case, the solid will be a formless mass. The liquid is colorless and, even though it is very heavy, has a low viscosity so it flows freely and completely wets the surface of its container. The liquid phase is not stable at atmospheric pressure. The gas is colorless.

Uranium hexafluoride does not react with oxygen, nitrogen, carbon dioxide, or dry air; however, each of these gases is soluble in the UF6 liquid phase, and very much less soluble in the solid phase. Gaseous UF6 does react rapidly with water vapor as does the exposed surface of solid UF6. Because of this, UF6 is always handled in leak-tight containers and processing equipment to prevent it from reacting with water vapor in the air. The reaction of gaseous UF6 with water vapor at elevated temperatures is shown in Equation 1.

Equation 1:

At room temperature, depending upon the relative humidity of the air, the products of this reaction are UO2F2 hydrates and HF-H2O fog, which are seen as a white cloud. A typical reaction with excess water is given in Equation 2.

Equation 2:

If, because of extremely low humidity, the HF-H2O fog is not formed, the finely divided UO2F2 causes only a faint haze. The UO2F2 is a water soluble, yellow solid whose exact coloring depends on the degree of hydration, as well as the particle size.

The heat release for this reaction, as written in Equation 1, is 124 BTUs per pound of UF6 gas reacted. The heat release is much larger if the UO2F2 is hydrated and HF-H2O fog is formed. Thus, the heat release for Equation 2 is 1057 BTUs per pound of UF6 vapor, due mostly to the condensation of water vapor as it is incorporated into the UF2F2 hydrate and the HF-H2O fog, and to the interaction of the HF and the water. For the reaction of the solid UF6 and liquid water to form a solution as shown in Equation 3, the heat release is 258 BTU's per pound of solid UF6 reacted.

Equation 3:

Most of the additional heat released by the reaction in Equation 3, over that of reaction in Equation 1, is from the interaction of HF and water; however, the rate of heat release is usually considerably slower because the complex uranium oxyfluoride layer formed on the surface of the solid UF6 constitutes a diffusion barrier that limits the access of water to the UF6 surface. This also explains the slow hydrolysis rate of solid UF6 by water vapor, while the reaction in the gas phase is almost instantaneous.

When there is a release of UF6 as a gas to the atmosphere, reactions similar to that shown in Equation 2 normally occur, and the visible white cloud rises rapidly because of the heat generated by the reaction.

Uranium hexafluoride reacts with most metals to form a fluoride of the metal and a poorly volatile or non-volatile lower-valence uranium fluoride. Nickel and Nickel-plated steel, Monel, copper, and some aluminum alloys are generally used for processing equipment. Mild steel is corroded by UF6 and the resulting film greatly reduces, but does not prevent, further attack. Steel is used for shipping and storage cylinders, since the small amount of corrosion that occurs does not warrant the cost of more expensive metals.

Uranium hexafluoride reacts rapidly with hydrocarbons. If the UF6 is in the gas phase, the reaction forms a black residue of uranium-carbon compounds. In the liquid phase, the reaction proceeds at an accelerated rate and has been known to cause explosions in cylinders. Great care must be taken to avoid introducing hydrocarbon oil into processing equipment or cylinders.

Uranium hexafluoride is a chemically stable compound. However, in a field of intense alpha radiation, it slowly decomposes to solid UF5 and fluorine gas.

4.3 Physical Properties
Phase Diagram

Safe handling of UF6 requires a detailed knowledge of its physical characteristics. Because UF6 is always processed in leak-tight piping, equipment and containers, it is not visible to the operator. The operator must follow its presence by observing changes in pressures or weights. Such changes are conveniently illustrated by means of a phase diagram that shows the physical state (i.e., solid, liquid, or gas), of UF6 as a function of its pressure and temperature. It should be noted that these data are for UF6 alone, as a single-component system. If air, nitrogen, HF, or other gases are present, the total pressure condition for a given temperature will be higher (i.e., the sum of the partial pressures of the system components).

Figure 3 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 the UF6. The triple point occurs at 22 psia and 147.3ºF. These are the only conditions at which all three states -- solid, liquid, and gas -- can exist together in equilibrium. If the temperature or pressure is greater than at the triple point, there will be only gas or liquid. If the temperature or pressure is lower, there will be only solid or gas. For instance, at atmospheric pressure, 14.7 psia, there can only be gas or solid regardless of the temperature. At 100ºF, for example, the pressure of gas over solid is 5 psia. This is a typical condition in a UF6 cylinder in storage.

Graph 1, Phase Diagram

Table 4. Comparison of Phase Changes for UF6 and Water
Description UF6 Water
Heat of Sublimation 58.2 BTU/lb (135,373 J/kg) 1116 BTU/lb (2,595,816 J/kg)
Heat of Fusion 23.5 BTU/lb (54,661 J/kg) 143 BTU/lb (332,618 J/kg)
Heat of Vaporization 35.1 BTU/lb (81,643 J/kg) 973 BTU/lb (2,263,198 J/kg)
Specific Heat of Solid 0.114 BTU/lb/°F (477 J/kg/K) 0.5 BTU/lb/°F (2,093 J/kg/K)
Specific Heat of Liquid 0.130 BTU/lb/°F (544 J/kg/K) 1.0 BTU/lb/°F (4,186 J/kg/K)

Table 5. Physical Properties of UF4
Sublimation Point 14.7 psia (760 mm Hg) (101 kPa)
133.8°F (56.6°C)
Triple Point 22 psia (1140 mm Hg) (152 KPa)
147.3°F (64.1°C)
Density, Solid 68°F (20°C)
Liquid, 147.3°F (64.1°C)
Liquid, 200°F, (93°C)
Liquid, 235°F, (113°C)
Liquid, 250°F, (121°C)
317.8 lb/ft3 (5.1 g/cc)
227.7 lb/ft3 (3.6 g/cc)
215.6 lb/ft3 (3.5 g/cc)
207.1 lb/ft3 (3.3 g/cc)
203.3 lb/ft3 (3.3 g/cc)
Heat of Sublimation, 147.3°F (64.1°C) 58.2 BTU/lb (135,373 J/kg)
Heat of Fusion, 147.3°F (64.1°C) 23.5 BTU/lb (54,661 J/kg)
Heat of Vaporization, 147.3°F (64.1°C) 35.1 BTU/lb (81,643 J/kg)
Critical Pressure 668.8 psia (34,577 mm Hg) (4610 kPa)
Critical Temperature 446.4°F (230.2°C)
Specific Heat, Solid, 81°F (27°C) 0.114 BTU/lb/°F (477 J/kg/K)
Specific Heat, Liquid, 162°F (72°C) 0.130 BTU/lb/°F (544 J/kg/K)

The curve in Figure 3 also shows the terms used to describe the changes of phase that occur as temperature and/or pressure are altered. Below the triple point, solid UF6 sublimes to gas, and gaseous UF6 desublimes to solid. Above the triple point, liquid UF6 vaporizes to gas, and gaseous UF6 condenses to liquid. At 147.3ºF and pressures above 22 psia, liquid UF6 and solid UF6 exist in equilibrium with no driving force for phase alteration.

As heat is added to solid UF6 in a closed system, the solid mass absorbs heat and some of it sublimes to gas. Sublimation continues until the pressure of 22 psia and the temperature of 147.3ºF are reached, at which point solid, gas and liquid coexist. Additional heat incrementally melts the remaining solid, and when all of the solid UF6 has melted, further addition of heat increases the temperature of the liquid and causes a portion of the liquid to vaporize to gas. These are the same kinds of phase changes that take place when ice melts to water and when water boils, but the amount of heat required for UF6 to change states is smaller than for water. Table 4 compares these values in BTUs. Table 5 summarizes the physical properties of UF6.

4.3.2 Density

Most materials, unlike water, undergo a volume expansion, and thus a decrease in density upon transformation from solid to liquid. This volume expansion for UF6 is among the largest known, with the density decreasing as much at 40% when heated to a maximum temperature (250ºF). The large increase in volume places certain restrictions on handling systems and procedures.

Figure 4 shows the density change of the solid as it is heated from 70ºF to the triple point of 147.3ºF. Figure 5 shows the density of the liquid from 147.3ºF to 220ºF. Comparing the two curves shows that at the melting point, the density changes from 303 lb/ft3 for the solid to 227 lg/ft3 for the liquid. To put this in more familiar terms, a gallon of solid UF6, when melted, would fill a 1.33 gallon container. Figure 5 also shows that the liquid continues to expand as it is heated. These facts are important for determining the weight of UF6 that can be safely contained in a cylinder. If a cylinder were filled with solid UF6 as, for instance, by desubliming in a cold trapping facility, it is possible to put more solid UF6 in the cylinder than the cylinder can hold as liquid UF6. If such a cylinder were heated, the melting of the solid and the expansion of the liquid UF6 would completely fill it. Continued heating would cause the cylinder to rupture, resulting in a release of the UF6.

Figure 4

Figure 5

The same rupture potential exists if UF6 freezes and plugs a process line. Application of external heat to the middle portion of the plug can melt the solid and develop large hydraulic forces on the pipe and the ends of the plug, creating the potential for a UF6 release due to pipe rupture. To remove a pipe plug, the best practice is to evacuate the pipe to a low enough pressure that the UF6 will be removed by sublimation without entering the liquid phase.

4.3.3 Pressure Units

Depending on the temperature, the pressure in a UF6 cylinder is either positive or negative with respect to atmospheric pressure. Several scales are used to express the measurement of the pressure in a container. Two pressure scales based on English units (pounds per square inch or psi) are in common use in American industrial practice. One of these is an absolute scale (psia), while the other measures incremental pressure above atmospheric pressure (psig, also known as gage pressure). For pressure below atmospheric, units are given in inches of mercury (in. Hg) with an absolute vacuum being indicated as -30 in. Hg and atmospheric pressure as 0 in. Hg. The inches-of-Mercury scale is commonly used on compound gages to give a scale that is continuous with gage pressure units.

The pressure scale, commonly used in laboratory work, is based on the barometric scale where atmospheric pressure will support a mercury column 30 in. high. This scale is used extensively in measurement of subatmospheric pressures. Subatmospheric pressure can be expressed in inches of mercury (in. Hg) which is an inversion of the absolute pressure scale. Barometric pressure is also measured in millimeters of Mercury (mm Hg), where atmospheric pressure will support a mercury column 760 mm high.

In the International System of Units, pressure is measured in pascals (Pa), with 101,330 Pa or 101.33 kilopascals (101.33kPa) equal to atmospheric pressure.

These pressure and vacuum scales are compared in Figure 6. The scales can be used to convert any of the pressure or vacuum values in this document to other units that may be more useful or more familiar to the reader.

Figure 6

Conversion factors, or stoichiometric equivalents, for seven chemical forms of uranium are listed in Table 6. To compute the quantity of the compound named at the top of the column, a given quantity of the compound named in the left column is multiplied by the factor in the column and line indicated. Example: 1.4790 kg of UF6 contains 1 kg of uranium; the uranium in 1.3037 kg of UF6 will produce 1 kg of UO2.

Table 6. Conversion Factors