Laser Isotope Separation Uranium Enrichment
Atomic and molecular laser isotope separation (LIS) techniques use lasers to selectively excite atoms or molecules containing one isotope of uranium so that they can be preferentially extracted. Although LIS appears promising, the technology has proven to be extremely difficult to master and may be beyond the reach of even technically advanced states.
In the early 1970’s, significant work began on the development of laser isotope separation technologies for uranium enrichment. Present systems for enrichment processes using lasers fall into two categories: those in which the process medium is atomic uranium vapor and those in which the process medium is the vapor of a uranium compound. Common nomenclature for such processes include “first category— atomic vapor laser isotope separation (AVLIS or SILVA)” and “second category— molecular laser isotope separation (MLIS or MOLIS).”
The systems, equipment, and components for laser-enrichment plants embrace (a) devices to feed uranium-metal vapor (for selective photoionization) or devices to feed the vapor of a uranium compound (for photo-dissociation or chemical activation); (b) devices to collect enriched and depleted uranium metal as product and tails in the first category and devices to collect dissociated or reacted compounds as product and unaffected material as tails in the second category; (c) process laser systems to selectively excite the 235 U species; and (d) feed preparation and product conversion equipment. The complexity of the spectroscopy of uranium atoms and compounds may require incorporation of any number of available laser technologies.
The atomic vapor laser isotope separation (AVLIS) process is based on the fact that 235 U atoms and 238 U atoms absorb light of different frequencies (or colors). Although the absorption frequencies of these two isotopes differ only by a very small amount (about one part in a million), the dye lasers used in AVLIS can be tuned so that only the 235 U atoms absorb the laser light. As the 235 U atom absorbs the laser light, its electrons are excited to a higher energy state. With the absorption of sufficient energy, a 235 U atom will eject an electron and become a positively charged ion. The 235 U ions may then be deflected by an electrostatic field to a product collector. The 238 U atoms remain neutral and pass through the product collector section and are deposited on a tails collector.
The AVLIS process consists of a laser system and a separation system. The separator system contains a vaporizer and a collector. In the vaporizer, metallic uranium is melted and vaporized to form an atomic vapor stream. The vapor stream flows through the collector, where it is illuminated by the precisely tuned laser light. The AVLIS laser system is a pumped laser system comprised of one laser used to optically pump a separate dye laser, which produces the light used in the separation process. Dye master oscillator lasers provide precise laser beam frequency, timing, and quality control. The laser light emerging from the dye master oscillator laser is increased in power by passage through a dye laser amplifier. A total of three colors are used to ionize the 235 U atoms.
Many countries are pursuing some level of AVLIS research and/or development, and major programs exist in the United States, France, Japan, and probably Russia. Principal advantages of the AVLIS process include a high separation factor, low energy consumption (approximately the same as the centrifuge process), and a small volume of generated waste. However, no country has yet deployed an AVLIS process, although several have demonstrated the capability to enrich uranium with the process.
Lawrence Livermore National Laboratory was responsible for the development of the Atomic Vapor Laser Isotope Separation process for enriching uranium and transfer of the technology to the U.S. Enrichment Corporation (USEC). The Atomic Vapor Laser Isotope Separation process was operated in 1985 at the Former K-25 Site at the East Tennessee Technology Park. DOE spent more than $1.7 billion developing the technology. USEC announced in June 1999 that it was suspending further In July 1994, the USEC Board of Directors authorized their management to begin taking necessary steps to commercialize AVLIS technology. In April 1995, USEC and DOE entered into an agreement that provided for the transfer of intellectual and physical property pertaining to AVLIS enrichment technology to USEC. USEC expected to operate AVLIS commercially in 2004. The U.S. Enrichment Corporation came to the conclusion that AVLIS would never be profitable. In June 1999, USEC announced that it was discontinuing its development of the AVLIS process. While USEC owns the AVLIS technology, the Department retains the right to utilize the intellectual property for government purposes. When USEC terminated development of the AVLIS technology, it argued that the rates of return were not sufficient to outweigh the risks and ongoing capital expenditures necessary to develop and construct an AVLIS production plant. USEC had spent about $100 million in development of the technology since the corporation was privatized in July 1998. The estimated cost to develop AVLIS is $659 million.
The suspension left USEC without a complete plan to replace its existing gaseous diffusion enrichment technology, which is nearly 50 years old and very costly when compared with its competitors’ centrifuge enrichment technology. USEC is evaluating both centrifuge technology and another laser-based technology called SILEX as a replacement for its gaseous diffusion technology. One advanced enrichment technology being evaluated is the laser-based technology developed by Silex Systems Ltd. of Australia. In 2001, the third-generation Silex /USEC Inc. project moved into the pilot engineering study phase, which includes the construction and testing of prototype equipment.
While conceptually simple, the actual implementation of the process is likely to be difficult and expensive, especially for countries with limited technical resources. The AVLIS process requires much sophisticated hardware constructed of specialized materials that must be capable of reliable operation for extended periods of time in a harsh environment.
The idea for the molecular laser isotope separation (MLIS) process was conceived by a group of scientists at the Los Alamos National Laboratory in 1971. There are two basic steps involved in the MLIS process. In the first step, UF 6 is irradiated by an infrared laser system operating near the 16 mm wavelength, which selectively excites the 235 UF 6 , leaving the 238 UF 6 relatively unexcited. In the second step, photons from a second laser system (infrared or ultraviolet) preferentially dissociate the excited 235 UF 6 to form 235 UF 5 and free fluorine atoms. The 235 UF 5 formed from the dissociation precipitates from the gas as a powder that can be filtered from the gas stream.
MLIS is a stagewise process, and each stage requires conversion of the enriched UF 5 product back to UF 6 for further enrichment. CO 2 lasers are suitable for exciting the 235 UF 6 during the first step. A XeCl excimer laser producing ultraviolet light may be suitable for the dissociation of 235 UF 6 during the second step. However, there is currently no known MLIS optical system which has been successfully designed to handle both infrared and ultraviolet. Consequently, most MLIS concepts use an all infrared optical system.
In terms of the gas flow for the MLIS process, gaseous UF 6 mixed with a carrier gas and a scavenger gas is expanded through a supersonic nozzle that cools the gas to low temperatures. Hydrogen or a noble gas are suitable as carriers. A scavenger gas (such as methane) is used to capture the fluorine atoms that are released as a result of the dissociation of 235 UF 6 molecules.
There are many complexities associated with the process, and the United States, UK, France, and Germany have stated that their MLIS programs have been terminated. Japan also has had a small MLIS program. South Africa has recently stated that their MLIS program is ready to be deployed for low-enriched uranium (LEU) production.
Principal advantages of the MLIS process are its low power consumption and its use of UF 6 as its process gas.