ENRICHMENT METHOD

Enrichment methods

Isotope separation is difficult because two isotopes of the same elements have very nearly identical chemical properties, and can only be separated gradually using small mass differences. (235U is only 1.26% lighter than 238U.) This problem is compounded by the fact that 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.

highly enriched uranium

yellow cake

There are currently two generic commercial methods employed internationally for enrichment: gaseous diffusion (referred to as first generation) and gas centrifuge (second generation) which consumes only 6% as much energy as gaseous diffusion. Later generation methods will become established because they will be more efficient in terms of the energy input for the same degree of enrichment and the next method of enrichment to be commercialized will be referred to as third generation. 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.


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 continues to account for about 33% of enriched production but is now an obsolete technology that is steadily being replaced by the later generations of technology as the diffusion plants reach their ends-of-life.



Thermal diffusion utilizes the transfer of heat across a thin liquid or gas to accomplish isotope separation. The process exploits the fact that the lighter 235U gas molecules will diffuse toward a hot surface, and the heavier 238U gas molecules will diffuse toward a cold surface. The S-50 plant at Oak Ridge, Tennessee was used during World War II to prepare feed material for the EMIS process. It was abandoned in favor of gaseous diffusion.

A cascade of gas centrifuges at a U.S. enrichment plantThe gas centrifuge process uses a large number of rotating cylinders in series and parallel formations. Each cylinder's rotation creates a strong centrifugal force so that the heavier gas molecules containing 238U move 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,[8] which translates to about one-fiftieth of the energy requirements. Gas centrifuge techniques produce about 54% of the world's enriched uranium.

Diagram of the principles of a Zippe-type gas centrifuge with U-238 represented in dark blue and U-235 represented in light blueThe Zippe 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 their nuclear weapons program.



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.


None of the laser processes below are yet ready for commercial use, though SILEX is well advanced and expected to begin commercial production in 2012.(see here: 30 April 2008) and May 2010 Investor Presentation

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 which are 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 (see here). GEH has since begun construction of a demonstration test loop and announced plans to build an initial commercial facility. (see here: 30 April 2008). Details of the process are restricted by intergovernmental agreements between USA and Australia and the commercial entities. SILEX has been indicated to be an order of magnitude more efficient than existing production techniques but again, the exact figure is classified.

Schematic diagram of an aerodynamic nozzle. Many thousands of these small foils would be combined in an enrichment unit.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. 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 Helikon vortex separation process based on the vortex tube and a demonstration plant was built in Brazil by NUCLEI, a consortium led by Industrias Nucleares do Brasil that used the separation nozzle process. However both methods have high energy consumption and substantial requirements for removal of waste heat; neither is currently in use.


Electromagnetic isotope separation

Schematic diagram of uranium isotope separation in a calutron 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. The French CHEMEX process exploited a very slight difference in the two isotopes' propensity to change valency in oxidation/reduction, utilising immiscible aqueous and organic phases.


An ion-exchange process was developed by the Asahi Chemical Company in Japan which 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. The French developed their own version of PSP, which they 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.