Gaseous diffusion process principle

The operating principles of multi-stage Isotope separating processes, such as the gaseous diffusion process, were first developed by Karl Cohen (19), and further exposed in a form more directly useful to chemical engineers by Benedict and Plgford ( ). Blgelelsen has presented a concise summary of the theory In his review ( ). ... 

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. 

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. 

Of all the proportioning principles already known from the production of calibration gas from permanent gas, only the permeation method can be used because the diffusion process is not bound to the gaseous state and even for the dosage of permanent gases the permeation tubes contain two-phase mixtures. This method, as well as the mixed gas cylinders, are limited to the trace domain because the tubes only contain insignificant amounts. 

In natural uranium ores, the fraction of the atoms of the fissile isotope is about 0.72%. For many commercial applications, like production of fuel for light water reactors or several types of research reactors and other nuclear functions, its fraction must be increased, that is, isotope enrichment is carried ont. The main isotope separation methods, or isotope enrichment processes, ntilize the small differences in between the mass of U-235 and U-238. The two major commercial methods that have supplied most of the enriched uranium to date, gaseous diffusion and gas centrifuges, use the only gaseous compound of nraninm, nranium hexafluoride (UFg), as the feed material. Both methods utilize the difference between the mass of UFg (349 Da) and UFg (352 Da) where the mass ratio difference that is 0.86%. The product and tails of the enrichment process are also with the same chemical form, but the isotope composition of the material is altered in the enrichment process. Schematic diagrams of the principle of operation of these methods can be found on the web and in many textbooks, so will not be shown here. 

Thermal-Gradient Infiltration. The principle of thermal-gradient infiltration is illustrated in Fig. 5.15b. The porous structure is heated on one side only. The gaseous reactants diffuse from the cold side and deposition occurs only in the hot zone. Infiltration then proceeds from the hot surface toward the cold surface. There is no need to machine any skin and densification can be almost complete. Although the process is slow since diffusion is the controlling factor, it has been used extensively for the fabrication of carbon-carbon composites, including large reentry nose cones. 

The type of process where a solid is converted to a product with a smaller volume (a) by the action of a gas (eq. (6.14)) lends itself best to a general analysis. There is the well known "shrinking core" model, that describes the conversion of a massive solid reactant into a porous product by the action of a gaseous reactant. (Yagi and Kunii, 1952, Levenspiel, 1980). As the reaction process, the massive unreacted core will shrink and a layer of porous reaction product ("ash") will cover the core. Reactants have to dif fuse through the porous "ash" to reach the surface of the massive core, where the reaction takes place. In principle, the rate of the process is determined by diffusion through the porous layer, that becomes thicker on conversion, and reaction at the core si ace, that becomes smaller. 

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