Molecular laser isotope separation

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Molecular laser isotope separation (MLIS) is a method of isotope separation, where specially tuned lasers are used to separate isotopes of uranium using selective ionization of hyperfine transitions of uranium hexafluoride molecules. It is similar to AVLIS. Its main advantage is low energy consumption and use of uranium hexafluoride instead of vaporized uranium.

MLIS was conceived in 1961 at the Los Alamos National Laboratory.

MLIS operates in cascade setup, like the gaseous diffusion process. Instead of vaporized uranium of AVLIS, the working medium of the MLIS is uranium hexafluoride, which requires much lower temperature to vaporize. In every stage, the stream of UF₆ is irradiated with an infrared laser operating at the wavelength of 16 µm. The mix is then irradiated with another laser, infrared or ultraviolet, are selectively absorbed by the excited ²³⁵UF₆, causing its photolysis to UF₅ and fluorine. The pentafluoride forms a solid, which is then separated from the gas by filtration or a cyclone separator. The precipitated UF₅ is relatively enriched with uranium-235 and, after conversion back to hexafluoride, it is fed to the next stage of the cascade to be further enriched. The UF5 gas is mixed with a suitable carrier gas (hydrogen or some noble gas), which allows the molecules to stay in gaseous phase even after being cooled, and a scavenger gas (eg. methane), whose role is to bind and remove the released fluorine, which would otherwise tend to react with the pentafluoride.

The laser for the excitation is usually a carbon dioxide laser with output wavelength shifted from 10.6 µm to 16 µm, the photolysis laser may be a Xe Cl excimer laser operating at 308 nm, however infrared lasers are mostly used in existing implementations.

The process is considerably complex, with many technical difficulties. United States, France, United Kingdom, Germany and South Africa reported termination of their MLIS programs, however Japan has a small scale program in operation.