Controlled Isotope Separation

As mentioned in the previous section a CO2 laser can be tuned to selectively excite the isotopically different species BCl3 or BCl3. This phenomenon has been exploited to separate the two isotopes of boron. 

As a matter of practical interest the molecule that one wants to separate isotopically is UFe- However, its infrared active vibrational frequencies do not fall in the 9-11 [xm range and are thus not accessible to excitation by a CO2 laser. Thus much work has been done on the structurally similar molecule SFg which has the infrared active fundamental at 10.6 [xm it has been possible to separate SFg from SFg. Here radiation is used to dissociate SFg, probably to SF4 and F2 the SF4 can be hydrolyzed with water and the SOFg separated. By using hundreds of laser pulses it is possible to increase the naturally occurring ratio by more than 

The enrichment ratio is sensitive to a range of variables laser frequency, pulse energy, pulse duration, pulse number, and gas pressure. Naturally, increase of the pressure is rapidly counterproductive above 10 atm V-T transfer is sufficiently rapid so that SFg cannot readily be driven into a highly excited state and the enrichment ratio falls. 

Efforts to maximize the enrichment ratio led to a variety of experiments, some superficially unlikely to succeed. One of the latter had most unexpected results. The SFe was irradiated using two lasers. One, at lower power (50kWcm to 2 MW cm 2), was tuned to the excitation frequency the ultrahigh-power laser ( 20 MW cm ) was not tuned. This mode of operation led to a substantial increase in the enrichment ratio. 

The impetus for laser isotope separation studies is given by UFg. As mentioned in the previous section the presently known infrared lasers emit in the vicinity of 2.7 [xm (HF), 5 [xm (CO), and 10 [xm (CO2). Thus to employ the laser approach in uranium isotope separation requires the development of a new high-power, low-frequency, laser source since the V3 excitation mode used in the SFg experiments appears at 16.0 [xm in 

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Due to the very high intensity of the laser beams and their coherent nature they may be used in a variety of ways where controlled energy is required. Lasers are used commercially for excitation with a specific energy, e.g. in Raman spectroscopy or isotope separation. 

Until the advent of modem physical methods for surface studies and computer control of experiments, our knowledge of electrode processes was derived mostly from electrochemical measurements (Chapter 12). By clever use of these measurements, together with electrocapillary studies, it was possible to derive considerable information on processes in the inner Helmholtz plane. Other important tools were the use of radioactive isotopes to study adsorption processes and the derivation of mechanisms for hydrogen evolution from isotope separation factors. Early on, extensive use was made of optical microscopy and X-ray diffraction (XRD) in the study of electrocrystallization of metals. In the past 30 years enormous progress has been made in the development and application of new physical methods for study of electrode processes at the molecular and atomic level. 

B is a powerful neutron absorber and has been employed in reactor control rods, neutron detectors, and other applications. Cascades based on exchange distillation of boron-ether complexes have usefully large a s and were used for 10B/UB isotope separation by the US DOE. Exchange distillation takes advantage of the fact that condensed phase/vapor phase separation factors can be enhanced (as compared to liquid/vapor a s) by association/dissociation equilibria in one or the other phase. At the normal boiling point (173 K) the VPIE for... 

The optimum pH for separating cations is pK + 0.30 K. K.-C. Yeung and C. A. Lucy, Isotopic Separation of [14N]- and fI5N] Aniline by Capillary Electrophoresis Using Surfactant-Controlled Reversed Electroosmotic Flow, Anal. Chem 1998, 70. 3286. 

Other uses of lasers include eye surgery on detached retinas, spot welding, holography, isotope separation, accurate determination of the moon s orbit by reflection of laser light off a reflector placed on the moon s surface, and laser-guided bombs and missiles. Possible future uses include terrestrial and extraterrestrial communication, applications to computers, and production of the high temperatures needed for controlled nuclear-fusion reactions. 

Fig. 2 Layout of isotope separator in beam line C. The numbers indicate (1) He-jet recoil chamber, (2) beam-stdpper, (3) ion-source, (4) analysing magnet, (3) Einzel lens for transport of mass-separated beam, (6) tape-transport and counting station, (7) power/control rack of tape-transport, (8) power/control of mass-separa tor.

Selective excitation of wavepackets with ultrashort broadband laser pulses is of fundamental importance for a variety of processes, such as the coherent control of photochemical reactions [36-39] or isotope separation [40--42]. It can also be used to actively control the molecular dynamics in a dissipative environment if the excitation process is much faster than relaxation. For practical applications it is desirable to establish an efficient method that allows one to increase the target product yield by using short laser pulses of moderate intensity before relaxation occurs [38]. 

Fig. 22. Schematic diagram of the automatically controlled multi-stage isotopic separation system ...

This chapter gives a brief account of the nuclear fission reaction and the most important fissile fuels. It continues with a short description of a typical nuclear power plant and outlines the characteristics of the principal reactor types proposed for nuclear power generation. It sketches the principal fuel cycles for nuclear power plants and points out the chemical engineering processes needed to make these fuel cycles feasible and economical. The chapter concludes with an outline of another process that may some day become of practical importance for the production of power the controlled fusion of light elements. The fusion process makes use of rare isotopes of hydrogen and lithium, which may be produced by isotop>e separation methods analogous to those used for materials for fission reactors. As isotope separation processes are of such importance in nuclear chemical engineering, they are discussed briefly in this chapter and in some detail in the last three chapters of this book. 

For the mass diffusion screen, Maier used a variety of materials, such as plates perforated with 0.4-mm holes, fine-mesh wire screen, or alundum filter plates. Very fine holes, such as is needed in gaseous diffusion, are not required, although holes with diameter under 10 are preferred because control of mass flow through the screen is easier. In the uranium isotope separation design example to be given in Sec. 7.4, electroformed nickel screen with holes 6.76 tm in diameter and 30 percent free area was specified. 

By proper control of flow rates of separating agent and light- and heavy-stream feed rates, it is possible to make the molar velocity of light component inward just equal to the molar velocity of heavy component outward, a preferred condition for isotope separation in this equipment. 

Nuclear control for reprocessing plants [163] with spectrophotometry [34, 206, 210-212] and fluorescence [166, 175, 207] for gaseous isotopic separations [213, 214] or in the environmental sciences (see Chapter 18). The results in pilot plants [34, 206, 210] indicate a signiHcant future for these new techniques. 

The flrst U.K. uranium isotope separation plant, built in the early 1950 s, was based on gaseous difiusion. This phenomenon depends on the observation that the rate of passage of molecules through a membrane is inversely proportional to the square root of the molecular weight. The application of this principle in a cascade stage requires a compressor, membrane, control valve, and cooler. 

Experiments with Pu confirmed theoretical predictions that it would exhibit high fissibility with both thermal and fast neutrons. This meant that Pu in sufficient quantity would also experience an instantaneous nuclear explosion like If controlled nuclear fission could be accomplished in a nuclear reactor, it would be possible to produce large amounts of plutonium by neutron bombardment of U. The Pu could be isolated by chemical methods which were expected to be simpler than the isotopic separation required to obtain pure As a consequence, the production of Pu became a major project of the atomic bomb program of the United States during World War II see further Ch. 19. 

Review. A review of cryptates (82 references) not only discusses known effects of inclusion complexes of this type but notes possible future uses. Thus a cryptate has been prepared that forms a complex with the highly toxic Cd but not with Zn and Ca +. Such cryptates should be useful in treatment of metal poisoning and in control of pollution. Some cryptates are very efficient phase-transfer catalysts (liquid to liquid). Isotope separation may be possible with cryptates. 

Based on established isotope uses, on the projected increase in the pollution problem, and on the apparent social and economic pressure for pollution abatement, a significant demand for enriched isotopes appears to be developing for the assessment and control of air, water, and soil pollutants. Isotopic techniques will be used in combination with conventional methods of detection and measurement, such as gas chromatography, x-ray fluorescence, and atomic absorption. Recent advances in economical isotope separation methods, instrumentation, and methodology promise to place isotopic technology within the reach of most research and industrial institutions. Increased application of isotope techniques appears most likely to occur in areas where data are needed to characterize the movement, behavior, and fate of pollutants in the environment. 

Boron trifluoride is also employed in nuclear technology by uti1i2ing several nuclear characteristics of the boron atom. Of the two isotopes, B and B, only B has a significant absorption cross section for thermal neutrons. It is used in " BF as a neutron-absorbing medium in proportional neutron counters and for controlling nuclear reactors (qv). Some of the complexes of trifluoroborane have been used for the separation of the boron isotopes and the enrichment of B as (84). 

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