Gaseous diffusion cascade

Gaseous diffusion cascades for uranium enrichment have also been built in the United Kingdom, France, the former USSR, China, and, more recendy, in Argentina. 

The separation factor in ail of these processes is so close to unity that production of separated isotopes requires repeated partial separations in a multistage cascade generally similar to the gaseous diffusion cascade of Fig. 12.2. The remainder of this chapter develops theoretical principles of isotope separation in such cascades. 

Schematic Diagram of Flow of Process Gas in Gaseous Diffusion Cascade. Reprinted from Richard G. Hewlett and Oscar E. Anderson, Jr., The New World, 1939-1946, Volume I of A History of the United States Atomic Energy Commission (University Park Pennsylvania State University Press, 1962).

Rationale for Sprinkler Operation in a High Assay Gaseous Diffusion Cascade, AT. R. Yates (ONWI) R. S. Denning. R. G. Jung (BCD... 

For operational efficiency a number of gaseous diffusion stages are operated together in units referred to as cells and buildings. Cells and buildings. Cells and buildings can be removed from operation for routine maintenance and bypassed without disturbing the diffusion cascade. 

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 second type of apparatus based on the pressure diffusion effect is the separation nozzle. Pressure gradients in a curved expanding jet produce an isotopic separation similar to that in a centrifuge. The separation effect obtained with a single jet is relatively small, and separation nozzle stages, similar to gaseous diffusion stages, must be used in a cascade to realize most of the desired separations. 

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... 

Our brief discussion of cascade principles serves to demonstrate the critical dependence of the size and operating costs of isotope separation plants on the elementary separation factor c. The size and initial cost are proportional to c 2. The operating cost is less sensitive to c, but varies at least as c The economic importance of these factors is readily seen in context with the separation of In 1960 the USAEC had three gaseous diffusion plants in operation. The cost of each plant was approximately 1 billion dollars the power consumption in each plant was 1,800,000 kw. If the plants were to be built with processes or equipment giving separation factors one half the one used, the additional construction cost to the U.S. taxpayers would be nine billion dollars. The increase in the annual operating costs of the plants can be conservatively estimated from the increase in the reflux ratio or power consumption to be 100,000,000/yr. This is a realistic demonstration of the economic benefits and importance of fundamental research and development to society. 

Examples of separations of isotopes are in Table 20.3(b). The concentration of U-235 listed there was accomplished in a cascade of 2100 columns, each with an effective height of 14.6m, inner tube 5 cm dia, gap 0.25 mm, hot surface 87-143°C, and cold surface 63°C, just above the condensation temperature at the operating pressure of 6.7 MPa. Although the process was a technical success, it was abandoned in favor of separation by gaseous diffusion which had only 0.7% of the energy consumption. 

Table 12.5 shows expected isotopic uranium contents which might be expected as product from cascade type (gaseous diffusion or gas centrifuge) enrichment facilities. While the highly enriched U is actually the target of these efforts one should also be able to verify the production of low enriched uranium. Columns 5 and 6 of Table 12.5 show the isotopic enrichments which would result from a 1000-1 mixture of namral U and low or high enriched U. Table 12.6 shows the ratios of the isotopes U and U relative to the in each mixture. The 0.6% isotopic shift in the ratio in the case of LEU-MIX should be detectable by today s technology. The 0.13% in ratio... 

A cascade tiiat has the same number of units (i.e., the same capacity) in all stages of a group is known as a squared-off cascade. A cascade in which the number of units, or the capacity, in each stage decreases as the produce and waste ends of the cascade are approached is called a tapered cascade. A single multiplate distillation column is an example of a squared-off cascade a gaseous diffusion plant for uranium separation is an example of a tapered cascade. 

To iUustrate the shape of a typical ideal cascade, we shall woric out the variation of interstage flow virith stage number for an ideal cascade to separate natural uranium (zp = Oj0072) into enriched uranium with yp = 090 and depleted uranium tails with Xf r = 0.003 by gaseous diffusion, with a = 1.00429. 

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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