Barrier, gaseous diffusion

Successful operation of the gaseous diffusion process requires a special, fine-pored diffusion barrier, mechanically rehable and chemically resistant to corrosive attack by the process gas. For an effective separating barrier, the diameter of the pores must approach the range of the mean free path of the gas molecules, and in order to keep the total barrier area required as small as possible, the number of pores per unit area must be large. Seals are needed on the compressors to prevent both the escape of process gas and the inflow of harm fill impurities. Some of the problems of cascade operation are discussed in Reference 16. 

A Back-Pressure Efficiency Factor. Because a gaseous diffusion stage operates with a low-side pressure p which is not negligible with respect to there is also some tendency for the lighter component to effuse preferentiahy back through the barrier. To a first approximation the back-pressure efficiency factor is equal to (1 — r), where ris the pressure ratiopjpj. 

If the power requirement of the gaseous diffusion process were no greater than the power required to recompress the stage upflow from the pressure on the low-pressure side of the barrier to that on the high-pressure side, then the power requirement of the stage would be Z RTLq (1 /r) for the case where the compression is performed isotherm ally. The power requirement per unit of separative capacity would then be given simply by the ratio... 

The optimum pressure level for gaseous diffusion operation is also determined by comparison at some pressure level the decrease ia equipment size and volume to be expected from increasing the pressure and density is outweighed by the losses that occur ia the barrier efficiency. Nevertheless, because it is weU known that the cost of power constitutes a large part of the total cost of operation of gaseous diffusion plants, it can perhaps be assumed that a practical value of r does not differ gready from the above optimum. Inclusion of this value ia the preceding equations yields... 

From equation 60 one can obtain a theoretical power requirement of about 900 kWh/SWU for uranium isotope separation assuming a reasonable operating temperature. A comparison of this number with the specific power requirements of the United States (2433 kWh/SWU) or Eurodif plants (2538 kWh/SWU) indicates that real gaseous diffusion plants have an efficiency of about 37%. This represents not only the barrier efficiency, the value of which has not been reported, but also electrical distribution losses, motor and compressor efficiencies, and frictional losses in the process gas flow. 

For ideal gaseous diffusion of UF6 through a porous yet restrictive barrier (see Fig. 8.2), Graham s law yields... 

Miszenti, G. S. and C. A. Mannctti. 1971. Process for preparing porous composite membranes or barriers for gaseous diffusion systems. Italian Patent 27802A/71. 

PORE. I A minute cavity in epidermal tissue as in skin, leaves, or leather, having a capillary channel to the surface that permits transport of water vapor from within outward but not the reverse. 2. A void of interstice between particles of a solid such as sand minerals or powdered metals, that permits passage of liquids or gases through the material in either direction. I11 some structures, such as gaseous diffusion barriers and molecular sieves, the pores ate of molecular dimensions, i.e 4-10 A units. Such microporous structures are useful for filtration and molecular separation purposes in various industrial operations. 3. A cell in a spongy structure made by gas formation (foamed plastic) that absorbs water on immersion but releases it when stressed. 

VOIDS. Empty spaces of molecular dimensions occurring between closely packed solid particles, as in powder metallurgy. Their presence permits barriers made by powder metallurgy techniques to act as diffusion membranes for separation of uranium isotopes in the gaseous diffusion process. 

Actual measurements of reduced gaseous diffusion across certain compressed surfactant monolayers, at an air/water interface, have been reported in detail by numerous investigators in the past (ref. 115-120). For such measurements to be completely trustworthy, it is first necessary to eliminate convection in the bulk (aqueous) phase, since the monolayer can reduce the rate of gas absorption by reducing convection at the surface, and this has a greater effect than a diffusion barrier (ref. 116). The problem of... 

Gaseous Diffusion. In the gaseous diffusion process, the UFfi flows through a porous nickel membrane called the barrier. The heavier U-238F6 flows more slowly than the U-235F6, and the theoretical separation factor for an equilibrium stage is ... 

Although the process is called gaseous diffusion, because the chambers are separated by barriers that effectively allow only individual UF6 molecules to pass through, it behaves like an effusion process. Thus we can find the actual ratio of the two types of UFg in chamber 2 from Graham s law ... 

A schematic diagram for the enrichment of by gaseous diffusion of UFe through an effusion barrier is shown in Figure 5, which also illustrates the counter-current flow and cascade principles. The limiting separation factor a is given by the kinetic theory of gases... 

Much of the impetus for the awakened interest and utilization of inorganic membranes recently came hom a history of about forty or fifty years of some large scale successes of porous ceramic membranes for gaseous diffusion to enrich uranium in the military weapons and nuclear power reactor applications. In the gaseous diffusion literature, the porous membranes are referred to as the porous barriers. For nuclear power generation, uranium enrichment can account for approximately 10% of the operating costs (Charpin and Rigny, 1989]. 

The volatile UF (sublimation at 65 °C) was, and remains, the key material for separation of 0.6% from the major uranium isotope (Figure 4.2). The reactivity of UFj is comparable to that of elemental fluorine, and it oxidizes most metals immediately and reacts violently with conventional organic materials. When the first large-scale separation of the uranium isotopes was achieved in a gaseous diffusion plant in 1943, PTFE seals in combination with compressed nickel powder diffusion barriers played a crucial role in this success. After the explosion of the first atomic bomb based on over Hiroshima on August 6, 1945, the atomic weapons programs in the West and the Eastern Bloc became one of the dominant forces driving the development of industrial fluorine chemistry. 

Reactions between solid substances can be very slow, because the reactants meet directly only at the interface between solid particles, and the bulk reaction requires the diffusion of atoms through the solids. Even when one reactant is gaseous or liquid the barrier to diffusion may prevent bulk reaction. For example the formation of inert oxide films on some reactive metals such as aluminum and titanium is important for their applications. Reactions confined to surface layers are exploited in the manufacture of electronic devices such as integrated circuits made from silicon. 

The importance of the separative capacity in isotope separation lies in the fact that it is a good measure of the magnitude of an isotope separation job. Many of the characteristics of the plant that make important contributions to its cost are proportional to the separative capacity. For example, in a gaseous diffusion plant built as an ideal cascade of stages operated at the same conditions, the total flow rate, the total ptimp capacity, the total power demand, and the total barrier area are all proportional to the separative capacity. In a distillation plant, the total column volume and total rate of loss of availability are proportional to the separative capacity. 

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