Isotope Separation Process

All these experimentally demonstrated phenomena may influence the chromatographic separation processes. Isotope effects on the chromatographic behavior of compoimds labeled with isotopes of heavier elements (carbon, nitrogen, oxygen, etc.) are so small that they can only be detected for simple compounds of low relative molecular mass. The following discussion is hmited to deuterated and tritiated compounds and only a few examples are given of separation of heavier isotopes. 

Another impetus to expansion of this field was the advent of World War 11 and the development of the atomic bomb. The desired isotope of uranium, in the form of UF was prepared by a gaseous diffusion separation process of the mixed isotopes (see Fluorine). UF is extremely reactive and required contact with inert organic materials as process seals and greases. The wartime Manhattan Project successfully developed a family of stable materials for UF service. These early materials later evolved into the current fluorochemical and fluoropolymer materials industry. A detailed description of the fluorine research performed on the Manhattan Project has been pubUshed (2). 

The electromagnetic separation plant built during World War 11 at Oak Ridge, involved two types of calutrons, alpha and beta. The larger alpha calutrons were used for the enrichment of natural uranium, and the beta calutrons were used for the final separation of U from the pre-enriched alpha product. For the electromagnetic separation process, UO was converted into UCl [10026-10-5] with CCl. The UCl was fed into the calutron for separation. The calutron technique has been used to separate pure samples of and stable isotopes of many other elements. The Y-12 calutron... 

Irreversible processes are mainly appHed for the separation of heavy stable isotopes, where the separation factors of the more reversible methods, eg, distillation, absorption, or chemical exchange, are so low that the diffusion separation methods become economically more attractive. Although appHcation of these processes is presented in terms of isotope separation, the results are equally vaUd for the description of separation processes for any ideal mixture of very similar constituents such as close-cut petroleum fractions, members of a homologous series of organic compounds, isomeric chemical compounds, or biological materials. 

Cascade Design. The efficiency of a Zippe-type centrifuge, separating uranium isotopes when UF is the process gas, operating at a peripheral speed of 350 m/s and at a temperature of 320 K A = 2.85), would be expected to be... 

G. F. Mailing and E. Von H.a]le,Merocfnamic Isotope Separation Processes for Cranium Enrichment Process Requirement, paper presented at the Symposium on New Advances ia Isotope Separation, Div. of Nuclear Chemistry and Technology, American Chemical Society, San Francisco, Calif., Aug. 1976 CCC-ND Report K/OM-2872, Oak Ridge Gaseous Diffusion Plant, Oak Ridge, Term., Oct. 7, 1976. 

Our discussion concentrates on the uranium-235 isotope. It makes up only about 0.7% of naturally occurring uranium. The more abundant isotope, uranium-238, does not undergo fission. The first process used to separate these isotopes, and until recently the only one available, was that of gaseous effusion (Chapter 5). The volatile compound uranium hexafluoride, UF6, which sublimes at 56°C, is used for this purpose. 

Other reasons for investigating plutonium photochemistry in the mid-seventies included the widely known uranyl photochemistry and the similarities of the actinyl species, the exciting possibilities of isotope separation or enrichment, the potential for chemical separation or interference in separation processes for nuclear fuel reprocessing, the possible photoredox effects on plutonium in the environment, and the desire to expand the fundamental knowledge of plutonium chemistry. 

Enrichment, Isotopic—An isotopic separation process by which the relative abundances of the isotopes of a given element are altered, thus producing a form of the element that has been enriched in one or more isotopes and depleted in others. In uranium enrichment, the percentage of uranium-235 in natural uranium can be increased from 0.7% to >90% in a gaseous diffusion process based on the different thermal velocities of the constituents of natural uranium (234U, 235U, 238U) in the molecular form UF6. 

G-S [Girdler sulphide] A process for separating hydrogen isotopes, using the equilibrium between water and hydrogen sulfide ... 

Hypersorption A continuous chromatographic separation process using a moving bed. Invented in 1919 by F. D. Soddy (famed for his work on isotopes) at Oxford and developed commercially for petroleum refinery separations by the Union Oil Company of California in 1946. Six plants were built in the late 1940s, using activated carbon as the adsorbent. The process was abandoned because attrition of the bed particles proved uneconomic. 

Separation-nozzle method, 25 417 Separation of Isotopes by Laser Excitation (SILEX) technology, 25 416-417 Separation processes enhanced, 27 670-673 foams in, 72 19, 21-22 for supercritical fluids, 24 13-14 sustainable development and, 24 175-176... 

Mulliken, R.S. and Harkins, W.D. (1922). The separation of isotopes. Theory of resolution of isotopic mixtures by diffusion and similar processes. Experimental separation of mercury by evaporation in a vacuum. Journal of the American Chemical Society 44 37-65. 

The separation factor, r, in an isotope separation process is the ratio of the relative concentration of desired isotope in the product, p, to its relative concentration in the feed, f. Denoting the feed to the i th stage, the i th section of an overall plant, or the ith overall plant, as mol/s of isotope with an isotope fraction Zj, yielding product at a rate of Yj mol/s of analysis yi, and waste at a rate of Xj mol/s and isotope analysis Xj, we have for a two isotope feed... 

The selection of an isotope separation process including the design of the separative units and the cascade depends principally on engineering and economic considerations. One must consider the amount of product which is desired, choose the starting material, consider energy demand, etc. It comes as no surprise, then, that many different methods have been used for isotope separation. Some of these have been listed in Table 8.1 and a few are discussed in more detail in the material which follows. 

The interest in ceramic membranes grew, together with the interest in membrane separation processes, due to their specific properties. They are chemically stable, can withstand high temperatures and are noncompressible. These characteristics made them the only materials available, which could withstand the harsh environment in the isotope separation. On the other hand, the brittleness of most materials is a problem and so is the selectivity. 

A primary goal of chemical separation processes in the nuclear industry is to recover actinide isotopes contained in mixtures of fission products. To separate the actinide cations, advantage can be taken of their general chemical properties [18]. The different oxidation states of the actinide ions lead to ions of charges from +1 (e.g., NpOj) to +4 (e.g., Pu" " ) (see Fig. 12.1), which allows the design of processes based on oxidation reduction reactions. In the Purex process, for example, uranium is separated from plutonium by reducing extractable Pu(IV) to nonextractable Pu(III). Under these conditions, U(VI) (as U02 ) and also U(IV) (as if present, remain in the... 

Of special interest in stable isotope geochemistry are evaporation-condensation processes, because differences in the vapour pressures of isotopic compounds lead to significant isotope fractionations. For example, from the vapour pressure data for water given in Table 1.2, it is evident that the lighter molecnlar species are preferentially enriched in the vaponr phase, the extent depending upon the temperature. Such an isotopic separation process can be treated theoretically in terms of fractional distillation or condensation under equilibrium conditions as is expressed by the Rayleigh (1896) equation. For a condensation process, this equation is... 

Fig. 5.5. Decomposition of Solar System abundances into r and s processes. Once an isotopic abundance table has been established for the Solar System, the nuclei are then very carefully separated into two groups those produced by the r process and those produced by the s process. Isotope by isotope, the nuclei are sorted into their respective categories. In order to determine the relative contributions of the two processes to solar abundances, the s component is first extracted, being the more easily identified. Indeed, the product of the neutron capture cross-section with the abundance is approximately constant for aU the elements in this class. The figure shows that europium, iridium and thorium come essentially from the r process, unlike strontium, zirconium, lanthanum and cerium, which originate mainly from the s process. Other elements have more mixed origins. (From Sneden 2001.)... 

The compound is used in the gaseous diffusion process to separate uranium isotopes... 

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