Separation of isotopes by laser excitation
Separation of isotopes by laser excitation (SILEX) is a process for isotope separation that is used to produce enriched uranium using lasers. It was developed in the 1990s, based on earlier technologies.
The SILEX process was developed in Australia by Dr. Michael Goldsworthy and Dr. Horst Struve, working at Silex Systems Limited, a company founded in 1988. Their process was based on earlier methods of laser enrichment developed starting in the early 1970s, such as AVLIS (atomic vapor laser isotope separation) and MLIS (molecular laser isotope separation).
In 1993, the foundation of a set of principles for the separation of isotopes by laser excitation to enrich uranium were established by Dr. Goldsworthy and Dr. Struve at SILEX headquarters in Sydney.
Silex Systems concluded the second stage of testing in 2005 and began its Test Loop Program. In 2007, Silex Systems signed an exclusive commercialization and licensing agreement with General Electric Corporation. The Test Loop Program was transferred to GE's facility in Wilmington, North Carolina. Also in 2007, GE Hitachi Nuclear Energy (GEH) signed letters of intent for uranium enrichment services with Exelon and Entergy - the two largest nuclear power utilities in the USA.
In 2008, GEH spun off Global Laser Enrichment (GLE) to commercialise the SILEX Technology and announced the first potential commercial uranium enrichment facility using the Silex process. The U.S. Nuclear Regulatory Commission (NRC) approved a license amendment allowing GLE to operate the Test Loop. Also in 2008, Cameco Corporation, 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. Compared to current enrichment technologies, the SILEX process requires as little as 25% of the space and consumes considerably less energy. It is reportedly almost undetectable from orbit, potentially allowing rogue governments' activities to go undetected by the international community.
In August 2011, GLE applied to the NRC for a permit to build a commercial plant at Wilmington, which would enrich uranium to a maximum of 8% 235U. On September 19, 2012, the NRC made its initial decision on GLE's application, and granted the requested permit. Silex has completed its phase I test loop program at GE-Hitachi Global Laser Enrichment’s (GLE) facility in North Carolina. The commercial plant's target enrichment level will be 8 percent, which puts it on the upper end of low-enriched uranium.
In 2016, the United States Department of Energy agreed to sell about 300,000 tonnes of depleted uranium hexafluoride to GLE for re-enrichment using the SILEX process over 40 years at a proposed Paducah Laser Enrichment Facility. 
According to Laser Focus World, the SILEX process exposes a cold stream of a mixture of uranium hexafluoride (UF6) molecules and a carrier gas to energy from a pulsed laser. The laser used is a CO2 laser operating at a wavelength of 10.8 μm (micrometres) and optically amplified to 16 μm, which is in the infrared spectrum. The amplification is achieved in a Raman conversion cell, a large vessel filled with high-pressure para-hydrogen.
The 16 μm wavelength laser preferentially excites the 235UF6, creating a difference in the isotope ratios in a product stream, which is enriched in 235U, and a tailings stream, which has an increased fraction of the more common 238U. The Sydney Morning Herald reports that "The lasers electrically charge the atoms, which become trapped in an electromagnetic field and drawn to a metal plate for collection."
According to John L. Lyman, the Silex Systems Ltd. (SSL) research facility in Australia uses a laser pulsed at a frequency of 50 Hz, a rate that results in great inefficiency. At 50 Hz, only 1% of the UF6 feedstock is processed. This results in a high fraction of feedstock entering the product stream and a low observed enrichment rates. Consequently, a working enrichment plant would have to substantially increase the laser duty cycle. In addition, the preparation time needed is prohibitively long for full-scale production. The SSL research facility requires ten hours of prep time for a one-hour enrichment test run, significantly restricting output.
Further details of the technology, such as how it differs from the older molecular laser isotope separation (MLIS) and atomic vapor laser isotope separation (AVLIS) processes are not known publicly. The technique can be used for the isotopic enrichment of chlorine, molybdenum and uranium, and similar technologies can be used with carbon and silicon.
Nuclear security concerns
A physicist at Princeton University, Ryan Snyder, noted that the SILEX process could create an easy path towards a nuclear weapon due to the ability to reach a high level of uranium enrichment, that is difficult to detect. 
SILEX is the only privately held information that is classified by the U.S. government. In June 2001, the U.S. Department of Energy classified "certain privately generated information concerning an innovative isotope separation process for enriching uranium." Under the Atomic Energy Act, all information not specifically declassified is classified as Restricted Data, whether it is privately or publicly held. This is in marked distinction to the national security classification executive order, which states that classification can only be assigned to information "owned by, produced by or for, or is under the control of the United States Government." This is the only known case of the Atomic Energy Act being used in such a manner.
The 2014 Australian Broadcasting Corporation drama The Code uses "Laser Uranium Enrichment" as a core plot device. The female protagonist Sophie Walsh states that the technology will be smaller, less energy-intensive, and more difficult to control once it is a viable alternative to current methods of enrichment. Ms. Walsh also states that the development of the technology has been protracted, and that there are significant governmental interests in maintaining the secrecy and classified status of the technology.
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