Laser Isotope Enrichment

The idea of laser isotope enrichment grew from the laser’s ability to concentrate its output power in a narrow range of wavelengths. Different isotopes of the same element are very hard or impossible to separate chemically, but the difference in their masses leads to differences in their spectra, which in principle can be used to selectively excite one isotope and isolate it by some photo-induced process. The first proposal came from the Atomic Energy Commission’s (AEC’s) Mound Laboratories in Miamisburg, Ohio, which in 1961 began a classified investigation of using lasers to enrich the concentration of fissionable uranium-235. Others independently proposed laser uranium enrichment. A company called Radioptics proposed it to the AEC in 1963 and later unsuccessfully sued the AEC for violating their trade secrets. A French group received a patent in France in 1965, and by the time a U.S. version of the patent issued in 1969 the idea was looking attractive. The impetus came from the development of the tunable dye laser and the growth of nuclear power. The U.S. depended on the gaseous diffusion process developed during World War II to enrich U-235 concentration to the levels needed for atomic bombs. Gaseous diffusion is energyintensive, expensive, and raises U-235 concentration only a small amount on each pass. Laser enrichment offered to reduce cost, improve efficiency, and increase recovery of U-235. At the Avco-Everett Research Laboratory, Richard Levy and G. Sargent Janes developed a two-step process to enrich U-235. First a dye laser would selectively excite U-235 atoms in uranium vapor, then an ultraviolet laser would ionize the excited U-235 atoms, so they could be collected [1]. (Figure 1 shows the process.) Avco lacked money to develop the technology, so they formed a joint venture with Exxon Nuclear, hoping to build a private uranium enrichment business. Avco-Everett founder Arthur Kantrowitz initially worried that laser enrichment might open the door to nuclear proliferation. “At first glimpse it seems like it’s a garage operation. A garage operation for separating uranium isotopes is a frightening thing,” he recalled in a 1985 interview. He imposed special security restrictions but eventually realized “this is not an easy way to make a bomb. It might be an easy way to make 1000 bombs, but it is not a terrorist operation” because of its technical complexity [2]. In 1972 the AEC launched competing laser uranium enrichment projects at its Los Alamos and Livermore laboratories. John Emmett, director of Livermore’s laser program, chose to try selective excitation of U-235 atoms in uranium vapor with the relatively well-developed tunable dye laser. That paralleled the Avco approach but was based on earlier work by Ray Kidder of Livermore. They proposed a two-step process, starting with using visible output of a narrow-band dye laser tuned to excite U-235, then ionizing the excited uranium atoms. In early 1973 Livermore hired three developers of the first continuous-wave dye laser from Eastman Kodak, Ben Snavely, Otis Peterson, and Sam Tuccio, to start and manage the program. “It seemed like an exciting thing to do at the time,” Snavely recalled many years later, an opinion echoed by the other two. At Los Alamos, Reed Jensen and John Lyman chose to try selective enrichment in UF6, the compound used in gaseous diffusion, which sublimes at about 55 deg Celsius and is easier to handle than uranium vapor. They found a large isotope shift in a 16-μm absorption band of UF6 and discovered that ultraviolet photons could photodissociatiate excited UF6 molecules, precipitating solid UF5 from the gas phase reaction and releasing free fluorine into the gas. Developing 1960–1974