Gaseous Diffusion - Technology

Technology

Scientific basis

Gaseous diffusion is based on Graham's law, which states that the rate of effusion of a gas is inversely proportional to the square root of its molecular mass. For example, in a box with a semi-permeable membrane containing a mixture of two gases, the lighter molecules will pass out of the container more rapidly than the heavier molecules. The gas leaving the container is somewhat enriched in the lighter molecules, while the residual gas is somewhat depleted. A single container wherein the enrichment process takes place through gaseous diffusion is called a diffuser.

Uranium hexafluoride

UF₆ is the only compound of uranium sufficiently volatile to be used in the gaseous diffusion process. Fortunately, fluorine consists of only a single isotope 19F, so that the 1% difference in molecular weights between 235UF₆ and 238UF₆ is due only to the difference in weights of the uranium isotopes. For these reasons, UF₆ is the only choice as a feedstock for the gaseous diffusion process. UF₆, a solid at room temperature, sublimes at 56.5 °C (133 °F) at 1 atmosphere. The triple point is at 64.05 °C and 1.5 bar. Applying Graham's Law to uranium hexafluoride:

where:

Rate₁ is the rate of effusion of 235UF₆.

Rate₂ is the rate of effusion of 238UF₆.

M₁ is the molar mass of 235UF₆ = 235.043930 + 6 × 18.998403  = 349.034348 g·mol−1

M₂ is the molar mass of 238UF₆ = 238.050788 + 6 × 18.998403  = 352.041206 g·mol−1

This explains the 0.4% difference in the average velocities of 235UF₆ molecules over that of 238UF₆ molecules.

UF₆ is a highly corrosive substance. It is an oxidant and a Lewis acid which is able to bind to fluoride, for instance the reaction of copper(II) fluoride with uranium hexafluoride in acetonitrile is reported to form copper(II) heptafluorouranate(VI), Cu(UF₇)₂. It reacts with water to form a solid compound, and is very difficult to handle on an industrial scale. As a consequence, internal gaseous pathways must be fabricated from austenitic stainless steel and other heat-stabilized metals. Non-reactive fluoropolymers such as Teflon must be applied as a coating to all valves and seals in the system.

Barrier materials

Gaseous diffusion plants typically use aggregate barriers (porous membranes) constructed of sintered nickel or aluminum, with a pore size of 10–25 nanometers (this is less than one-tenth the mean free path of the UF₆ molecule). They may also use film-type barriers, which are made by boring pores through an initially nonporous medium. One way this can be done is by removing one constituent in an alloy, for instance using hydrogen chloride to remove the zinc from silver-zinc (Ag-Zn).

Energy requirements

Because the molecular weights of 235UF₆ and 238UF₆ are nearly equal, very little separation of the 235U and 238U is effected by a single pass through a barrier, that is, in one diffuser. It is therefore necessary to connect a great many diffusers together in a sequence of stages, using the outputs of the preceding stage as the inputs for the next stage. Such a sequence of stages is called a cascade. In practice, diffusion cascades require thousands of stages, depending on the desired level of enrichment.

All components of a diffusion plant must be maintained at an appropriate temperature and pressure to assure that the UF₆ remains in the gaseous phase. The gas must be compressed at each stage to make up for a loss in pressure across the diffuser. This leads to compression heating of the gas, which then must be cooled before entering the diffuser. The requirements for pumping and cooling make diffusion plants enormous consumers of electric power. Because of this, gaseous diffusion is the most expensive method currently used for producing enriched uranium.