The isotopes with which we are concerned are those of uranium; namely U²³⁵ and U²³⁸. in order that the isotopes of uranium may be separated by the gaseous diffusion process, it is necessary that the uranium be in a gaseous state. This is accomplished by combining the uranium with fluorine to form uranium hexafluoride, which is a gas at workable temperatures and pressures. Another advantageous quality of uranium hexafluoride is that fluorine has no isotopes to further complicate the separation process. In general, the following discussion is concerned with the separation of U²³⁵F₆ and U²³⁸F₆

Separation Factor

The ability of a barrier to separate isotopes is expressed by a value called the separation factor. It is denoted by the Greek letter a, and is defined as the mole ratio of light component to heavy component in the gas passing through the barrier at a given point divided by a similar mole ratio on the high pressure surface of the barrier at the same point. The separation factor can be expressed by the following equation.

In this equation, y and x are the mol fraction of light component on the low and high pressure sides of the barrier respectively. It should be emphasized that a, as shown in Figure A-1, is a point value, and that x and y are concentrations at a given point on the two surfaces of the barrier. In practice, point values are usually not determined. The average a's of the surface of samples are calculated from experimental separation data.

As previously stated, the separation of isotopes is dependent upon the difference in the mass of the particles. Under ideal conditions, the separation factor would be equal to the ratio of the square roots of the molecular weights. For a mixture of U²³⁵F₆ and U²³⁸F₆, this value is 1.0043 and is commonly referred to as the ideal separation factor. In actual practice, however, these ideal conditions cannot be duplicated. For example, to approach the ideal separation factor, only a small fraction of the total gas involved could be diffused through the barrier. For such a process to be productive, the plant size and its power consumption would be too large to be economically feasible. Limitations of plant size and power consumption have, necessitated a reduction in the separation factor to a lower value. This lower value is called the actual separation factor.