AVLIS

Atomic vapor laser isotope separation (AVLIS)... 

The exact status of the development of the AVLIS process is subject to security classification. It is beHeved that the process is ready for transition to an industrial operator for commercial development. 

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). 

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-... 

Lick Observatory. The success of the LLNL/AVLIS demonstration led to the deployment of a pulsed dye laser / AO system on the Lick Observatory 3-m telescope (Friedman et al., 1995). LGS system (Fig. 14). The dye cells are pumped by 4 70 W, frequency-doubled, flashlamp-pumped, solid-state Nd YAG lasers. Each laser dissipates 8 kW, which is removed by watercooling. The YAG lasers, oscillator, dye pumps and control system are located in a room in the Observatory basement to isolate heat production and vibrations from the telescope. A grazing incidence dye master oscillator (DMO) provides a single frequency 589.2 nm pulse, 100-150 ns in length at an 11 kHz repetition rate. The pulse width is a compromise between the requirements for Na excitation and the need for efficient conversion in the dye, for which shorter pulses are optimum. The laser utilizes a custom designed laser dye, R-2 perchlorate, that lasts for 1-2 years of use before replacement is required. 

Experiments on the sky. Two experiments have been carried out at the sky, using two laser installations built for the American and French programmes for Uranium isotope separation, respectively AVLIS at the Lawrence Livermore Nat l Lab (California) in 1996 and SILVA at CEA/Pierrelatte (Southern France) in 1999. The average power was high pa 2 x 175 W, with a pulse repetition rate of 12.9 and 4.3 kHz, a pulse width of 40 ns and a spectral width of 1 and 3 GHz. Polarization was linear. The return flux was < 5 10 photons/m /s (Foy et al., 2000). Thus incoherent two-photon resonant absorption works, with a behavior consistent with models. But we do need lower powers at observatories ... 

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... 

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 spectrum of uranium vapor is complex with more than 3 x 10s lines in the visible region. Still, many of these are sharp and show sufficient isotope separation to permit selective excitation. The basic idea of AVLIS is to irradiate uranium vapor at a concentration around 1013 atoms cm-3 (higher concentrations are ineffective... 

The formazan reaction offers a means for clarifying the structure of the mixed A and mixed B osazones. The former was described by Voto6ek and Vondra6ek as acyclic n-glucose l-(iV -phenyl)-2-(Ar -methyl-A -phenyl)osazone, and the latter as acyclic n-glucose l-(A -methyl-i r -phenyl)-2-(V -phenyl)osazone. The Percivals assigned the same n-fruc-topyranose l-(Ar -phenyl)-2-(V -methyl-Ar -phenyl)osazone structure to the compounds, but differentiated them as probable syn and avli forms. 

However, if the molecules are cooled, the population of thermally excited vib-rot states falls drastically and the spectrum simplifies. Thus, at 77 K, 69% of UFs is in its lowest vibrational state, and this increases to 85% at 55 K. However, the vapor pressure is untenably low at such temperatures (7 x 10 Pa at 77 K), and equilibrium cooling is out of the question. Still by cooling in a nonequilibrium expansion nozzle, uranium vapor concentration can be kept at a useful level and LIS is possible. None of this changes the fact that molecular LIS was abandoned in favor of AVLIS. 

Developmenf work continued to further increase the output and reduce the cost of fhe machines while consfruction of the plant proceeded. However, in June 1985, the Department of Energy (DOE) made the decision (after an expenditure of nearly 3 billion) to shut down the centrifuge plant and concentrate on the development of the AVLIS process for uranium enrichment at LLNL. 

The Energy Research and Development Agency (ERDA), the forerunner to the DOE, through the late 1970s to 1981 supported the study of three new experimental processes for uranium enrichment. Two were based upon laser separation, and one on plasma separation. Jersey Nuclear-Avco Isotopes Incorporated (subsidiary of Exxon) and the LLNL worked on atomic uranium vapor. LLNL referred to it as AVUS. The LANL and a group at Exxon Research Laboratories (not connected with Jersey-Avco) worked on molecular UFg. TRW Incorporated pursued research work on a plasma separation process. Union Carbide Nuclear Division (UCC-ND) supported each in their efforts. In 1981, the AVLIS process at LLNL was selected as the process to be developed further and the other processes were subsequently phased out. 

The advanfages of AVLIS over gaseous diffusion and gas centrifuge were seen as ... 

DOE supported the development work of AVLIS at LLNL until 1994 when the USEC became an entirely private company. At that time, DOE turned the program over to USEC for continuation. In the first half of 1999, USEC gave up on AVLIS and started to search for a replacement. 

Upon the closure of AVLIS, the only remaining laser process on the world stage was (separation of isotopes by laser excitation [SILEX]), a molecular separation process developed by the Australian company Silex Systems Limited. The French had ceased work on their laser program, SILVA, in 2003. 

The gaseous diffusion and gas cenfrifuge techniques exploit the small mass differences between U and U in the gaseous form of UFg. AVLIS is based on an entirely different concept. U and LP isotopes have different electron energies, so that they absorb... 

The AVLIS process consists of a laser system and a separator system. The latter contains a vaporizer and a collector. The working medium is metallic uranium that is melted and vaporized to form an atomic vapor stream. The vapor stream flows through the collector where it is illuminated by precisely tuned laser light. The selected atoms become charged by photoionization and are removed from the vapor stream by an electronic field. 

MLIS uses UFg as its feedstock, thereby fitting more readily into the conventional fuel cycle than AVLIS. There are two steps involved in the MLIS process excitation with infrared lasers and then dissociation with an ultraviolet laser. Gaseous UF mixed with a carrier gas (argon) is expanded through a nozzle that cools the gas to low temperatures. The UF is irradiated by infrared lasers, which selectively excite the LP Fg, leaving the U F unexcited. Photons from an ultraviolet laser then preferentially disassociate the excited U F to form and free fluorine atoms. The U Fj formed in this manner precipitates from the gas as a solid powder which can be filtered from the gas stream. 

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