Separation of isotopes by laser excitation (SILEX) is a process under development to enrich uranium on an industrial scale for nuclear reactors. It is strongly suspected that it utilizes laser condensation repression to excite the uranium-235 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 the heavier, unexcited 238UF6.[1] This differs greatly from previous methods of laser enrichment explored for their commercial prospects: 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 out as a solid.[1]
While the Australian company Silex Systems Limited is the most prominent developer of this technology (as part of the Global Laser Enrichment consortium), the acronym SILEX really only refers to a physical separation concept utilizing condensation repression that is well known and under development or being used for multiple applications around the world.[2] Slight variations in operating parameters, equipment arrangements, lasers and their capabilities, may exist from one SILEX-type process to the next (and be called by different names), but the physical separation concept remains the same if condensation repression is utilized, especially when compared to that used by AVLIS or MLIS.
Princeton physicist Ryan Snyder has suggested that this process may lead to the further proliferation of nuclear weapons due to its increasingly accessible technological pathway[2][3] and undetectable signatures (small area footprint and high energy efficiency).[1]
History
Development of various MLIS variants began already in the 1970s. In most of them, an infrared laser vibrationally excited one of the isotopes such as 235UF6 of gaseous uranium hexafluoride. This requires a wavelength near 16µm. The excited molecules were then further excited up to dissociation, either again at 16µm or by a UV laser.
After initial euphoria, LIS of uranium was mostly abandoned during the 1990s. Urenco published in 1992 the reasons for their decision.[4] One reason was that LIS processes seemed to require a number of further developments of uncertain outcome, whereas centrifuges had reached technical maturity around this time. But in Australia the SILEX variant was not discontinued.
In 1999, the United States signed the Agreement for Cooperation between the Government of Australia and the Government of the United States of America concerning Technology for the Separation of Isotopes of Uranium by Laser Excitation [SILEX Agreement], which allowed cooperative research and development between the two countries on the SILEX process.[6]
Silex Systems concluded the second stage of testing in 2005 and began its Test Loop Program. Already before, in 2003, USEC backed out from the project. In 2007, Silex Systems signed an exclusive commercialization and licensing agreement with General Electric Corporation (GE). 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.[7]
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, Canada, the world's largest uranium producer, joined GE and Hitachi as a part owner of GLE.[8]
In 2010, concerns were raised that the SILEX process poses a threat to global nuclear security (see Proliferation concerns).[9]
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.[10] On September 19, 2012, the NRC made its initial decision on GLE's application, and granted the requested permit.[11] Silex completed its phase 1 test loop program at GE-Hitachi Global Laser Enrichment's (GLE) facility in Wilmington. The commercial plant's target enrichment level is 8percent, which puts it on the upper end of low-enriched uranium.[12]
In 2014, both GLE and Silex Systems restructured, with Silex halving its workforce.[13] In 2016 GEH withdrew from GLE, writing-off their investment.[13][14]
In 2018, Silex Systems abandoned its plans for GLE, intending to repatriate the SILEX technology to Australia.[16]
In 2021, Silex Systems took majority ownership (51%) of GLE, with Cameco (49%) as minority owner. The path to market for the venture is underpinned by an agreement between GLE and the US Department of Energy under which DOE uranium tailings will be made available for the proposed Paducah Laser Enrichment project. Silex's technology will be used to produce natural grade uranium from the tailings.[17] GLE intends now (from 2022) to focus on re-enrichment in Paducah.[18] It is on the site, where the last diffusion plant for uranium enrichment worked until 2013. It left behind several hundred thousand tons of depleted UF6.
Process
Infrared absorption spectra of the two UF6 isotopes at 300 and 80 K.Schematic of a stage of an isotope separation plant for uranium enrichment with laser. An infrared laser with a wavelength of approx. 16 µm radiates at a high repetition rate onto a UF6 carrier gas mixture, which flows supersonically out of a laval nozzle. The excited component moves away from the axis of the molecular beam faster than the unexcited tailings stream which is separated at a skimmer.
The shortest-wavelength fundamental vibration of gaseous UF6 is around 16µm. At room temperature its width (around 20cm−1) is much larger than the isotopic shift (0.6cm−1). The broadening is due to thermally populated excited vibrational and rotational states. To allow for selective excitation, the UF6, diluted about 100 fold by a carrier gas (which can be argon or nitrogen), is cooled to about 80 K by adiabatic expansion through a nozzle into vacuum. Initially there are still collisions (which are necessary for cooling). But after traveling about 10 nozzle diameters, due to the expansion, they are so rare that condensation can no longer take place. Avoiding collisions is also necessary to suppress any collisional transfer of energy between the isotopes. Such a molecular beam method is used in all cases, where spectral narrowing is needed for selective excitation.
With SILEX, the pressure and nozzle diameter are chosen large enough to provide a sufficient number of collisions immediately after the nozzle, to allow for formation of clusters (UF6•G) with the carrier gas G. (UF6•UF6 clusters are practically not formed due to the much lower density of UF6 compared to G.) If 235UF6 is selectively excited at 628.3cm−1, then this molecule does not aggregate with G, whereas the nonexcited heavier 238UF6 does. Due to their higher thermal velocity, the free molecules leave the axis of the molecular beam faster than the clusters. The latter are therefore enriched in the part transmitted by a skimmer nozzle downstream, whereas the non-transmitted fraction is enriched in the 235UF6. The enrichment factor is the better, the larger the transmitted fraction (i.e. the smaller the depletion and the smaller the cut). That is, SILEX uses a separation nozzle, modified by a laser and profiting from selective repression of cluster formation ("condensation").
For that, the CO2 laser needs at least 20 MW. With a Raman shift of 354.3cm−1 and a CO2 laser wavenumber of 982.1cm−1 (10R30 line), one receives 627.8cm−1. This is only close to the Q-branch of 235UF6 (center at 628.3cm−1, width 0.01cm−1[19]) and is even closer to the Q-branch of 238UF6. GLE does not inform, how they do the necessary fine tuning. High-pressure CO2 lasers would cause additional problems with the pulse repetition rate. With common (atmospheric-pressure) CO2 lasers and with the stimulated Raman shifter the state of technology is 2–4kHz.[20] In order not to leave large parts of the molecular beam unirradiated, one needs at least 20kHz (according to Urenco several tens of kHz[21]), unless pulsed nozzles are used. The nozzles themselves must have slit form, in order to provide enough absorption length.
GLE informs that they reach separation factors of 2–20, the higher values probably coupled to a poorer depletion (which is not given). This is sufficient for enrichment from natural uranium (0,72% 235U) to reactor grade (> 3% 235U). The pioneer works of the van den Bergh group obtained only much smaller enrichments with SF6.[22]
Using other lasers with suitable wavelengths, SILEX can also be used for the isotopic enrichment of other elements such as chlorine, molybdenum, carbon and silicon.
Proliferation concerns
Compared to current enrichment technologies, SILEX obtains a higher enrichment. Hence fewer stages are necessary to reach bomb grade uranium (> 90% 235U). According to GLE, each stage requires as little as 25% of the space of the conventional methods. Hence it would facilitate to rogue governments to hide a production facility for bomb uranium.[9] The attractiveness is even enhanced by the claims of GLE that a SILEX plant is faster and cheaper to build, and consumes considerably less energy. Scientists therefore expressed their concerns repeatedly that SILEX could create an easy path towards a nuclear weapon (see e.g.[23]). The model calculations of Ryan Snyder substantiate these warnings.[1]
Security classification
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.[24][25]
Popular culture
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.
Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei. The fission process often produces gamma photons, and releases a very large amount of energy even by the energetic standards of radioactive decay.
Uranium is a chemical element with symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium radioactively decays by emitting an alpha particle. The half-life of this decay varies between 159,200 and 4.5 billion years for different isotopes, making them useful for dating the age of the Earth. The most common isotopes in natural uranium are uranium-238 and uranium-235. Uranium has the highest atomic weight of the primordially occurring elements. Its density is about 70% higher than that of lead, and slightly lower than that of gold or tungsten. It occurs naturally in low concentrations of a few parts per million in soil, rock and water, and is commercially extracted from uranium-bearing minerals such as uraninite.
Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes. The use of the nuclides produced is varied. The largest variety is used in research. By tonnage, separating natural uranium into enriched uranium and depleted uranium is the largest application. In the following text, mainly uranium enrichment is considered. This process is crucial in the manufacture of uranium fuel for nuclear power plants, and is also required for the creation of uranium-based nuclear weapons. Plutonium-based weapons use plutonium produced in a nuclear reactor, which must be operated in such a way as to produce plutonium already of suitable isotopic mix or grade.
Enriched uranium is a type of uranium in which the percent composition of uranium-235 has been increased through the process of isotope separation. Naturally occurring uranium is composed of three major isotopes: uranium-238, uranium-235, and uranium-234. 235U is the only nuclide existing in nature that is fissile with thermal neutrons.
Uranium hexafluoride, sometimes called hex, is an inorganic compound with the formula UF6. Uranium hexafluoride is a volatile white solid that reacts with water, releasing corrosive hydrofluoric acid. The compound reacts mildly with aluminium, forming a thin surface layer of AlF3 that resists any further reaction from the compound. UF6 is used in the process of enriching uranium, which produces fuel for nuclear reactors and nuclear weapons.
A gas centrifuge is a device that performs isotope separation of gases. A centrifuge relies on the principles of centrifugal force accelerating molecules so that particles of different masses are physically separated in a gradient along the radius of a rotating container. A prominent use of gas centrifuges is for the separation of uranium-235 (235U) from uranium-238 (238U). The gas centrifuge was developed to replace the gaseous diffusion method of uranium-235 extraction. High degrees of separation of these isotopes relies on using many individual centrifuges arranged in series, that achieve successively higher concentrations. This process yields higher concentrations of uranium-235 while using significantly less energy compared to the gaseous diffusion process.
Gaseous diffusion is a technology that was used to produce enriched uranium by forcing gaseous uranium hexafluoride (UF6) through microporous membranes. This produces a slight separation (enrichment factor 1.0043) between the molecules containing uranium-235 (235U) and uranium-238 (238U). By use of a large cascade of many stages, high separations can be achieved. It was the first process to be developed that was capable of producing enriched uranium in industrially useful quantities, but is nowadays considered obsolete, having been superseded by the more-efficient gas centrifuge process (enrichment factor 1.05 to 1.2).
The carbon-dioxide laser (CO2 laser) was one of the earliest gas lasers to be developed. It was invented by Kumar Patel of Bell Labs in 1964 and is still one of the most useful types of laser. Carbon-dioxide lasers are the highest-power continuous-wave lasers that are currently available. They are also quite efficient: the ratio of output power to pump power can be as large as 20%. The CO2 laser produces a beam of infrared light with the principal wavelength bands centering on 9.6 and 10.6 micrometers (μm).
The Australian Atomic Energy Commission (AAEC) was a statutory body of the Australian government.
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).
Cameco Corporation is the world's largest publicly traded uranium company, based in Saskatoon, Saskatchewan, Canada. In 2015, it was the world's second largest uranium producer, accounting for 18% of world production.
Uranium-234 is an isotope of uranium. In natural uranium and in uranium ore, 234U occurs as an indirect decay product of uranium-238, but it makes up only 0.0055% of the raw uranium because its half-life of just 245,500 years is only about 1/18,000 as long as that of 238U. Thus the ratio of 234 U to 238 U in a natural sample is equivalent to the ratio of their half-lives. The primary path of production of 234U via nuclear decay is as follows: uranium-238 nuclei emit an alpha particle to become thorium-234. Next, with a short half-life, 234Th nuclei emit a beta particle to become protactinium-234 (234Pa), or more likely a nuclear isomer denoted 234mPa. Finally, 234Pa or 234mPa nuclei emit another beta particle to become 234U nuclei.
Molecular laser isotope separation (MLIS) is a method of isotope separation, where specially tuned lasers are used to separate isotopes of uranium using selective ionization of hyperfine transitions of uranium hexafluoride molecules. It is similar to AVLIS. Its main advantage over AVLIS is low energy consumption and use of uranium hexafluoride instead of vaporized uranium.
Laser isotope separation, or laser enrichment, is a technology of isotope separation using selective ionization of atoms or molecules by the means of precisely tuned lasers.
The National Enrichment Facility (NEF) is a nuclear facility for the enrichment of uranium associated with the Los Alamos National Laboratory. The plant uses a gas centrifuge technology known as Zippe-type centrifuges. It is located 5 miles (8.0 km) east of Eunice, New Mexico. The NEF is operated by Louisiana Energy Services (LES), which is in turn owned by the Urenco Group. As of 2011, LES operates as URENCO USA.
The Zippe-type centrifuge is a gas centrifuge designed to enrich the rare fissile isotope uranium-235 (235U) from the mixture of isotopes found in naturally occurring uranium compounds. The isotopic separation is based on the slight difference in mass of the isotopes. The Zippe design was originally developed in the Soviet Union by a team led by 60 Austrian and German scientists and engineers captured after World War II, working in detention. In the West the type is known by the name of the man who recreated the technology after his return to the West in 1956, based on his recollection of his work in the Soviet program, Gernot Zippe. To the extent that it might be referred to in Soviet/Russian usage by any one person's name, it was known as a Kamenev centrifuge.
Project-706, also known as Project-786 was the codename of a research and development program to develop Pakistan's first nuclear weapons. The program was initiated by Prime Minister Zulfiqar Ali Bhutto in 1974 in response to the Indian nuclear tests conducted in May 1974. During the course of this program, Pakistani nuclear scientists and engineers developed the requisite nuclear infrastructure and gained expertise in the extraction, refining, processing and handling of fissile material with the ultimate goal of designing a nuclear device. These objectives were achieved by the early 1980s with the first successful cold test of a Pakistani nuclear device in 1983. The two institutions responsible for the execution of the program were the Pakistan Atomic Energy Commission and the Kahuta Research Laboratories, led by Munir Ahmed Khan and Abdul Qadeer Khan respectively. In 1976 an organization called Special Development Works (SDW) was created within the Pakistan Army, directly under the Chief of the Army Staff (Pakistan) (COAS). This organization worked closely with PAEC and KRL to secretly prepare the nuclear test sites in Baluchistan and other required civil infrastructure.
The Pakistan Atomic Research Reactor or (PARR) are two nuclear research reactors and two other experimental neutron sources located in the PINSTECH Laboratory, Nilore, Islamabad, Pakistan.
ConverDyn is a general partnership between American multinational firms General Atomics and Honeywell that provides uranium hexafluoride (UF6) conversion and related services to utilities operating nuclear power plants in North America, Europe, and Asia. The company is the sole marketing agent of UF6 produced at the Honeywell Uranium Hexafluoride Processing Facility in Metropolis, Illinois.
Depleted uranium hexafluoride (DUHF; also referred to as depleted uranium tails, depleted uranium tailings or DUF6) is a byproduct of the processing of uranium hexafluoride into enriched uranium. It is one of the chemical forms of depleted uranium (up to 73-75%), along with depleted triuranium octoxide (up to 25%) and depleted uranium metal (up to 2%). DUHF is 1.7 times less radioactive than uranium hexafluoride and natural uranium.
↑ W. Eberhardt (DESY), W. Fuss (MPQ), F. Lehner (DESY), and R. Snyder (IFSH) (2019-11-04). "FELs and Laser Isotope Separation". Deutsches Elektronen-Synchrotron (DESY) and European XFEL.{{cite web}}: CS1 maint: multiple names: authors list (link)
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