Isotope separation is difficult because two isotopes of the same element have nearly identical chemical properties, and can be separated only gradually, using small mass differences. (235U is only 1.26% lighter than 238U.) This problem is compounded because uranium is rarely separated in its atomic form, but instead as a compound (235UF6 is only 0.852% lighter than 238UF6). A
cascade of identical stages produces successively higher concentrations of 235U. Each stage passes a slightly more concentrated product to the next stage and returns a slightly less concentrated residue to the previous stage. There are currently two commercial methods employed internationally for enrichment:
gaseous diffusion (referred to as first generation) and
gas centrifuge (second generation), which consumes only 2% to 2.5% as much energy as gaseous diffusion. Some work is being done that would use
nuclear resonance; however, there is no reliable evidence that any nuclear resonance processes have been scaled up to production.
Diffusion techniques Gaseous diffusion Gaseous diffusion is a technology used to produce enriched uranium by forcing gaseous
uranium hexafluoride ('hex') through
semi-permeable membranes. This produces a slight separation between the molecules containing 235U and 238U. Throughout the
Cold War, gaseous diffusion played a major role as a uranium enrichment technique, and as of 2008 accounted for about 33% of enriched uranium production, but in 2011 was deemed an obsolete technology that is steadily being replaced by the later generations of technology as the diffusion plants reach their ends of life. In 2013, the
Paducah facility in the U.S. ceased operating; it was the last commercial 235U gaseous diffusion plant in the world.
Thermal diffusion Thermal diffusion uses the transfer of heat across a thin liquid or gas to accomplish isotope separation. The facility was used to prepare feed material for the Electromagnetic Isotope Separation (EMIS) process but was later abandoned in favor of gaseous diffusion.
Centrifuge techniques Gas centrifuge The gas centrifuge process uses a large number of rotating cylinders in series and parallel formations. Each cylinder's rotation creates a strong
centripetal force so that the heavier gas molecules containing 238U move tangentially toward the outside of the cylinder and the lighter gas molecules rich in 235U collect closer to the center. It requires much less energy to achieve the same separation than the older gaseous diffusion process, which it has largely replaced and so is the current method of choice and is termed second generation. It has a separation factor per stage of 1.3 relative to gaseous diffusion of 1.005,
Zippe centrifuge The Zippe-type centrifuge is an improvement on the standard gas centrifuge, the primary difference being the use of heat. The bottom of the rotating cylinder is heated, producing convection currents that move the 235U up the cylinder, where it can be collected by scoops. This improved centrifuge design is used commercially by
Urenco to produce nuclear fuel and was used by Pakistan in its nuclear weapons program.
Laser techniques Laser processes promise lower energy inputs, lower capital costs and lower tails assays, hence significant economic advantages. Several laser processes have been investigated or are under development.
Separation of isotopes by laser excitation (SILEX) is well developed and is licensed for commercial operation as of 2012. Separation of isotopes by laser excitation is a very effective and cheap method of uranium separation, able to be done in small facilities requiring much less energy and space than previous separation techniques. The cost of uranium enrichment using laser enrichment technologies is approximately $30 per SWU More than 20 countries worked with laser separation during the 1990s and 2000s, though all achieved very limited success.
Atomic vapor laser isotope separation (AVLIS) Atomic vapor laser isotope separation employs specially tuned lasers to separate isotopes of uranium using selective ionization of
hyperfine transitions. The technique uses
lasers tuned to frequencies that ionize 235U atoms and no others. The positively charged 235U ions are then attracted to a negatively charged plate and collected.
Molecular laser isotope separation (MLIS) Molecular laser isotope separation uses an infrared laser directed at
UF6, exciting molecules that contain a 235U atom. A second laser frees a
fluorine atom, leaving
uranium pentafluoride, which then precipitates out of the gas.
Separation of isotopes by laser excitation (SILEX) Separation of isotopes by laser excitation is an Australian development that also uses
UF6. After a protracted development process involving U.S. enrichment company
USEC acquiring and then relinquishing commercialization rights to the technology,
GE Hitachi Nuclear Energy (GEH) signed a commercialization agreement with Silex Systems in 2006. GEH has since built a demonstration test loop and announced plans to build an initial commercial facility. Details of the process are classified and restricted by intergovernmental agreements between United States, Australia, and the commercial entities. SILEX has been projected to be an order of magnitude more efficient than existing production techniques but again, the exact figure is classified. In September 2012, the NRC issued a license for GEH to build and operate a commercial SILEX enrichment plant, although the company had not yet decided whether the project would be profitable enough to begin construction, and despite concerns that the technology could contribute to
nuclear proliferation. The fear of nuclear proliferation arose in part due to laser separation technology requiring less than 25% of the space of typical separation techniques, as well as requiring only the energy that would power 12 typical houses, putting a laser separation plant that works by means of laser excitation well below the detection threshold of existing surveillance technologies. It includes a detailed analysis of how the technology works and the challenges it presents for detecting clandestine uranium enrichment facilities.
Other techniques Aerodynamic processes manufacturing process was originally developed at the Forschungszentrum Karlsruhe, Germany, to produce nozzles for isotope enrichment. Aerodynamic enrichment processes include the Becker jet nozzle techniques developed by E. W. Becker and associates using the
LIGA process and the
vortex tube separation process. These
aerodynamic separation processes depend upon diffusion driven by pressure gradients, as does the gas centrifuge. They in general have the disadvantage of requiring complex systems of cascading of individual separating elements to minimize energy consumption. In effect, aerodynamic processes can be considered as non-rotating centrifuges. Enhancement of the centrifugal forces is achieved by dilution of
UF6 with
hydrogen or
helium as a carrier gas achieving a much higher flow velocity for the gas than could be obtained using pure uranium hexafluoride. The
Uranium Enrichment Corporation of South Africa (UCOR) developed and deployed the continuous Helikon vortex separation cascade for high production rate low-enrichment and the substantially different semi-batch Pelsakon low production rate high enrichment cascade both using a particular vortex tube separator design, and both embodied in industrial plant. A demonstration plant was built in Brazil by NUCLEI, a consortium led by
Industrias Nucleares do Brasil that used the separation nozzle process. All methods have high energy consumption and substantial requirements for removal of waste heat; none is currently still in use.
Electromagnetic isotope separation shows how a strong magnetic field is used to redirect a stream of uranium ions to a target, resulting in a higher concentration of uranium-235 (represented here in dark blue) in the inner fringes of the stream. In the
electromagnetic isotope separation process (EMIS), metallic uranium is first vaporized, and then ionized to positively charged ions. The cations are then accelerated and subsequently deflected by magnetic fields onto their respective collection targets. A production-scale
mass spectrometer named the
calutron was developed during World War II that provided some of the 235U used for the
Little Boy nuclear bomb, which was dropped over
Hiroshima in 1945. Properly the term 'calutron' applies to a multistage device arranged in a large oval around a powerful electromagnet. Electromagnetic isotope separation has been largely abandoned in favour of more effective methods.
Chemical methods One chemical process has been demonstrated to pilot plant stage but not used for production. The French CHEMEX process exploited a very slight difference in the two isotopes' propensity to change
valency in
oxidation/reduction, using immiscible aqueous and organic phases. An ion-exchange process was developed by the
Asahi Chemical Company in Japan that applies similar chemistry but effects separation on a proprietary resin
ion-exchange column.
Plasma separation Plasma separation process (PSP) describes a technique that makes use of
superconducting magnets and
plasma physics. In this process, the principle of
ion cyclotron resonance is used to selectively energize the 235U isotope in a
plasma containing a mix of
ions. France developed its own version of PSP, which it called RCI. Funding for RCI was drastically reduced in 1986, and the program was suspended around 1990, although RCI is still used for stable isotope separation. ==Separative work unit==