Atomic vapor laser isotope separation, or AVLIS, is a method by which specially tuned lasers are used to separate isotopes of uranium using selective ionization of hyperfine transitions. A similar technology, using molecules instead of atoms, is molecular laser isotope separation (MLIS). Natural uranium consists of a large mass of 238U and a much smaller mass of fissile 235U. Traditionally, the 235U is separated from the mass by dissolving it in acid to produce uranium hexafluoride and then using gas centrifuges to separate the isotopes. Each trip through the centrifuge "enriches" the amount of 235U and leaves behind depleted uranium. In contrast, AVLIS produces much higher enrichment in a single step without the need to mix it with acid. The technology could, in principle, also be used for isotope separation of other elements, which is uneconomic outside specialist applications with current non-laser-based technologies for most elements. As the process does not require the feedstock to be chemically processed before enrichment, it is also suitable for use with used nuclear fuel from light water reactors and other nuclear waste. At present, extracting from those sources is only economical up to a degree, leaving tons of still contained in waste products. AVLIS may offer an economic way to reprocess even the fuel that has undergone one cycle of reprocessing using existing methods. Due to the possibility of achieving much higher enrichment with much lower energy needs than conventional centrifuge based methods of uranium enrichment, AVLIS is a concern for nuclear proliferation. To date, no commercial-scale AVLIS production line is known to be in use. Principle The basic concept behind the AVLIS system is to selectively ionize the desired atoms in a vaporized source material. As the energy levels of the electrons are affected by the nuclear structure, causing the hyperfine structure, different isotopes have different energy levels. The designers pick a particular electron energy where the difference between isotopes is maximized and the energy level can be practically produced with a laser. The laser light causes the chosen electron to be photoexcited and thus ionize the atom, leaving it electrically charged. The ion can then be manipulated with electrostatic or magnetic fields. Other isotopes, which subtly different energy levels, will not be ionized and remain in the original mix. The choice of target electron has changed during the development of AVLIS as newer laser technologies have been developed. Early work generally focused