Lasers isotope separation

However, the greatest interest in isotope separation comes from the nuclear power industry. Production of heavy water for heavy-water reactors and separation of highly active components from the bumt-out uranium fiiel are two applications. The most important aspect by far is, however, isotope separation of Natural uraniimi contains only 0.7% 35  

Isotope shifts in a uranium emission line. A luanium sample enriched in U was used in the light source [10.156] 

Uranium isotope separation using a molecular approach is based on selective multi-photon dissocation of UPg. The relevant vibrational isotope shift is 0.6 cm in the primary vibrational transition at 628 cm (16 pm). In the development of the technique, experiments on SFg have been very important. The conditions are inucli more favourable for SFe than for UFg- The isotope shift is 17cm between and in the IR active vibrational mode that involves asymmetric stretdiing of two S-F bonds. The spectrum has a typical P, Q and R branch structure and the whole region of absorption for the rotational level popnlation distribution that is obtained at room temperature is 15 cm . Thus, the isotopic molecules are spectroscopically totally separated. Furthermore, the vibrational transition in SFg well matches the emission of a free-ruiniing pulsed CO2 laser. 

The multi-photon dissociation process is illustrated in Fig, 10.43. Because of the anharmonicity of the vibrational potential a photon energy that is resonant in the first vibrational step will successively pull out of resonance higher up in the vibrational energy level ladder. However, the molecule can be excited in a multi-photon process (Sect. 9.1.3) until it is dissociated. SPe then disintegrates into SF5 and F. By making the first step resonant for one isotopic molecule the probability of subsequent dissociation for this molecular species is significantly increased. 

To carry out the corresponding process for UPe the availability of eflS.cient lasers in the 16 pm region is a necessity. This has resulted in considerable effort being put into producing such lasers. An efficient way of generating laser radiation at 16 pm is to Raman shift a pulsed CO2 laser at 10.6 pm. A 16 pm beam can be produced at a 50% efficiency by stimulated Raman scattering in 

Invention of the laser provided the intense, monochromatic, tunable light source needed to make (diotochemical isotope separation apfdicabie to all elements, at least on a laboratory scale. The promise of this method was recognized as early as 1965 by Robieux and Audair [Rl], who were issued the first patent on it. Since the pioneering experiments of Tiffany et al. [Tl] on bromine isotopes in 1966, an enormous amount of work has been done with lasers, with small-scale separation reported for most elements. 

This text can describe only briefly the incomplete information publicly available on laser separation of uranium isotopes. For a more detailed discussion of the history and principles of laser isotope separation, reference may be made to the review articles of Letokhov and Moore [LI] and Aldridge et al. [A2], and to Farrar and Smith s report on uranium [FI]. 

Two general methods have been proposed for separating uranium isotopes. In the photoionization method to be discussed in Sec. 9.2, in uranium metal vapor is ionized selectively and then separated from unionized by deflection in electric or magnetic fields. In the photochemical method, to be described in Sec. 9.3, UFj in UFj vapor is excited selectively and caused to react chemically to produce a solid lower fluoride, which is then separated from unreacted UF5 vapor. 

2 Laser Isotope Separation of Uranium Metal Vapor 

Absorption spectrum of uranium metal vapor. The absorption spectrum of uranium metal vapor is very complex, with over 300,000 lines at visible wavelengths. However, many of these absorption lines are very sharp, with sufficient displacement between a absorption line and the absorption line for the corresponding transition, and without overlap of the line with the U line for a different transition, to permit selective excitation of the atoms. However, choice of the wavelength most suitable for a practical process is made difficult by the large number of possibilities. Janes et al. [J2] discuss some of the alternatives. 

The classical methods of isotope separation on a large, technical scale, such as thermal diffusion, or gas centrifuge techniques are expensive because they demand costly equipment or consume much energy [14.11], New techniques based on a combination of laser spectroscopy with photochemistry may considerably reduce the costs. Up to now several methods have been proposed and some of them already proved their feasibility in laboratory experiments. The extension to an industrial scale, however, demands still more efforts and many improvements.  

If a selectively excited isotope is irradiated by a second photon during the lifetime of the excited state, photoionization or photodissociation may take place. The ions can be collected and separated from the neutral species by electric fields. The photodissociation fragments may be separated by adding scavenger reactants S which preferably react v/ith the fragments A or B but not with the parent molecules AB. 

Another approach to laser isotope separation is offered by predissociation of laser-excited molecular isotopes into stable fragments. If the potential curve of the excited state of AB is intersected by a repulsive potential (Fig.14.3), the molecule may dissociate without absorbing a second photon. 

The recent discovery of multiphoton dissociation of polyatomic molecules, where molecules, such as SFg, can be dissociated by multiple absorption of infrared laser photons, has stimulated many theoretical [14.13] and experimental [14.14] investigations about the mechanism of this process. Since the first steps, namely the excitation of lower vibrational levels with moderate level density may be isotope selective, the multiphoton dissociation may turn out to become a cheap and efficient way of laser isotope separation. Infrared lasers, such as the CO2 laser, have a high conversion efficiency which makes CO2 laser photons inexpensive. For more detailed discussions of the various aspects of laser isotope separation see [14.15-17]. 

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B2.5.5.4 LASER ISOTOPE SEPARATION AND MODE-SELECTIVE REACTIONS... 

Although this limit is not always reaehed. The same is true for the eoherenee of the radiation. Eaeh of these properties ean be exploited for partieular ehemieal applieations. The monoeliromatieity ean be used to initiate a ehemieal reaetion of partieular moleeules in a mixture. The laser isotope separation of and in nafriral abimdanee exploits the isotope shift of moleeular vibrational frequeneies. At 10-50 em, the eorresponding shift of IR absorption wavenumbers is large eompared to the speetral width of the CO2 laser... 

Early laser Isotope separation after IR multiphoton excitation high selectivity at room temperature... 

Atomic vapor laser isotope separation (AVLIS)... 

Laser isotope separation techniques have been demonstrated for many elements, including hydrogen, boron, carbon, nitrogen, oxygen, sHicon, sulfur, chlorine, titanium, selenium, bromine, molybdenum, barium, osmium, mercury, and some of the rare-earth elements. The most significant separation involves uranium, separating uranium-235 [15117-96-1], from uranium-238 [7440-61-1], (see Uranium and uranium compounds). The... 

Atomic- Vapor Laser Isotope-Separation. Although the technology has been around since the 1970s, laser isotope separation has only recently matured to the point of industrialization. In particular, laser isotope separation for the production of fuel and moderators for nuclear power generation is on the threshold of pilot-plant demonstrations in several countries. In the atomic vapor laser isotope-separation (AVLIS) process, vibrationaHy cooled U metal atoms are selectively ionized by means of a high power (1—2 kW) tunable copper vapor or dye laser operated at high (kHz) repetition rates (51,59,60). 

A promising alternative is provided by Laser isotope separation . Because the ionization energies of and differ slightly, it is possible to ionize the former selectively by irradiating U vapour with laser beams precisely tuned to the appropriate wavelength. The ions can then be collected at a negative electrode. 

LLNL AVLIS Laser. The first WFS measurements using a Na LGS were performed at LLNL (Max et al., 1994 Avicola et al., 1994). These experiments utilized an 1100 W dye laser, developed for atomic vapor laser isotope separation (AVLIS). The wavefront was better than 0.03 wave rms. The dye laser was pumped by 1500 W copper vapor lasers. They are not well suited as a pump for LGSs because of their 26 kHz pulse rate and 32 ns pulse length. The peak intensity at the Na layer, with an atmospheric transmission of 0.6 and a spot diameter of 2.0 m, is 25 W/cm, 4x the saturation. The laser linewidth and shape were tailored to match the D2 line. The power was varied from 7 to 1100 W on Na layer to study saturation. The spot size was measured to be 7 arcsec FWHM at 1100 W. It reduced to 4.6 arcsec after accounting for satura-... 

Atomic systems, in lasers, 74 666-669 Atomic Vapor Laser Isotope Separation (AVLIS) process, 25 416 Atomic weight, 75 748 Atomization, 77 774-775 in spray coating, 7 69-74 technology of, 23 175 Atomizer operation, concerns related to, 23 195... 

Laser-induced plasma spectroscopy (LIPS), archaeological materials, 5 743 Laser isotope separation, 25 416 417 Laser light, 14 655-656 Laser light sources, in photochemical technology, 19 107-108... 

Molecular hydrogen, 23 759 Molecular imprinting, 6 397 Molecular interactions, 25 103 Molecular interaction theories, 24 38 Molecular Laser Isotope Separation (MLIS) process, 25 416 417 Molecular level machine, 2 7 58 Molecularly imprinted plastics (MIPs) smart, 22 717)... 

Uranium enrichment using LIS has been exhaustively studied and the conceptual outlines of two different methods can be found in the open literature. These methods are multi-photon dissociation of UF6 (SILEX, or Separation of Isotopes by Laser Excitation) and laser excitation of monatomic uranium vapor (Atomic Vapor Laser Isotope Separation, or AVLIS). Following an enormous investment, AVLIS was used by the United States DOE in the 1980s and early 1990s, but due to the present oversupply of separated uranium, the plant has been shut down. 

The dramatic growth occurring over the past few years in laser chemistry and laser isotope separation has refocused interests upon dissociative processes in molecules. Collectively, these interests are traceable to the pragmatic goals of producing appreciable populations of selected atomic or molecular states having useful reactive properties or isotopic content. From this perspective, it is natural that photodissociation of some parent molecule would appear to be the ideal means for obtaining a desired product. 

Laser isotope separation (LIS) utilizes small differences in the spectroscopic properties of isotopic substances. Each isotope-bearing substance absorbs a radiation of a particular wavelength. Separation of the excited species can be achieved by multiple-photon absorption or photopredissociation of molecules or chemical scavenging. 

A basic flow sheet containing common features of an LIS plant is schematically shown in Fig. 5. The two major components in a deuterium LIS plant are the laser isotopic separator and chemical exchange reactor. 

Vanderleeden, J.C. Generalized concepts in large-scale laser isotope separation, with application to deuterium. J. Appl. Phys. 1980, 51, 1273. 

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