Separation of isotopes by laser excitation (SILEX) is an experimental process for producing enriched uranium. It is strongly suspected that SILEX utilizes laser condensation repression to excite a vibrational mode of the 235Uranium isotope in uranium hexafluoride (UF6), allowing this lighter molecule to move more rapidly to the outer rim of a gaseous jet and resist condensing compared to heavier, unexcited 238UF6. This differs from previous laser enrichment methods: one using atomic uranium (atomic vapor laser isotope separation (AVLIS)) and another molecular method that uses lasers to dissociate a fluorine atom from 235UF6 (molecular laser isotope separation (MLIS)), allowing the enriched product to precipitate. While Australian company Silex Systems Limited is the most prominent developer of this technology (as part of the Global Laser Enrichment consortium), the acronym SILEX refers to a physical separation concept utilizing condensation repression. Slight variations in operating parameters, equipment arrangements, lasers and their capabilities, may exist from one SILEX-type process to the next (under a different name), but the physical separation concept remains the same if condensation repression is utilized, especially when compared to that used by AVLIS or MLIS.
History Development of various molecular laser isotope separation (MLIS) variants began in the 1970s. The key technology is an infrared laser, which vibrationally excites only one isotope in gaseous uranium hexafluoride. This requires a wavelength near 16 μm. Traditional MLIS continued to excite the molecules unto dissociation, at which point they crystallized as uranium-235 pentafluoride. After initial euphoria, laser isotope separation research was mostly abandoned during the 1990s, mainly because it still required extensive and uncertain research and development work, while gas centrifuges had reached technological maturity. However, Australia continued SILEX research. In November 1996, Silex Systems Limited licensed its technology exclusively to United States Enrichment Corporation (USEC). In 1999, the United States and Australia signed an international treaty for SILEX R&D. However, in 2003 USEC left the project. Silex Systems concluded the second stage of testing in 2005 and began its Test Loop Program. In 2007, the company signed an exclusive commercialization and licensing agreement with General Electric Corporation (GE), transferring their test loop to GE's facility in Wilmington, North Carolina. That year, GE Hitachi Nuclear Energy (GEH) signed letters of intent for uranium enrichment services with Exelon and Entergy - the US' two largest nuclear utilities. In 2008, GEH spun off Global Laser Enrichment (GLE) and announced the first potential commercial SILEX facility. The Nuclear Regulatory Commission (NRC) approved a license amendment allowing GLE to operate the Test Loop. Also in 2008, Cameco Corporation, Canada, the world's largest uranium producer, joined GE and Hitachi as a part owner of GLE. In 2010, concerns were raised that the SILEX process poses a threat to global nuclear security. Between 2011 and 2012, GLE applied for and received a permit to build a commercial plant at Wilmington. The plant would enrich to 8% 235U, the upper end of low-enriched uranium. In 2014, both GLE and Silex Systems restructured, with Silex halving its workforce. In 2016 GEH withdrew from GLE, writing off their investment. In 2016, the United States Department of Energy (DOE) agreed to sell about 300,000 tonnes of depleted uranium hexafluoride to GLE for re-enrichment (from 0.35 to 0.7 % 235U) over 40 years at a proposed Paducah, Kentucky site. In 2018, Silex Systems abandoned plans for GLE, intending to repatriate the SILEX technology to Australia. In 2021, Silex Systems took majority ownership (51%) of GLE, with Cameco (49%) as minority owner. Under an agreement between GLE and DOE, GLE agreed to re-enrich to natural levels several hundred kilotons of depleted uranium tailings from the last enrichment plant to use gaseous diffusion. That plant operated until 2013.
Process
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