Enrichment Processes. Nuclear Fuel Cycle Diagram. * Figure from The Nuclear Fuel of Pressurized Water Reactors and Fast Reactors , ed. H. Bailly, D. Menessier, and C. Prunnier, (Lasovier Publishing, 1999) p. 14. . Enrichment Review.
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Nuclear Fuel Cycle Diagram * Figure from The Nuclear Fuel of Pressurized Water Reactors and Fast Reactors, ed. H. Bailly, D. Menessier, and C. Prunnier, (Lasovier Publishing, 1999) p. 14.
Enrichment Review • Isotope separation enabled by minor differences in the molecular weight of UF6 or the atomic weight of U-metal. • Three most prominent methods • Gaseous diffusion (proven technology – VERY high power requirements) • Gaseous centrifuge (under development) • Laser isotope separation (maybe someday . . . but why)
Gaseous Diffusion • Based on the separation effect arising from molecular effusion. • Effusion = the flow of gas through small holes. • The GD vessel contains a mixture of two gases • Gas molecules with lower molecular weight (e.g., UF6 with U-235) travel faster and strike the vessel walls more frequently, relative to their concentration, than molecules with higher molecular weight (e.g., UF6 with U-238). • If walls of the vessel are semi-permeable, more of the lighter molecules flow through the wall than the heavier molecules. • Therefore, gas passing through the vessel wall is slightly enriched in the lighter isotope.
Gaseous Diffusion • The United States currently uses the gaseous diffusion process to enrich uranium. • Piketon, Ohio (no longer operating) • Paducah, Kentucky • Both operated by the United States Enrichment Corporation (USEC). • USEC which was created as a government corporation under the Energy Act of 1992 and privatized by legislation in 1996.
Gas Centrifuge • Separation using rotation • The process uses a large number of rotating cylinders interconnected to form cascades. • Uranium hexafluoride (UF6) gas enters a cylinder and is rotated at a high speed. • The strong “centrifugal” force draws more of the heavier gas molecules (UF6 with U-238) toward the cylinder wall. • The lighter gas molecules (UF6 with U-235) tend to collect closer to the center. • The stream that is slightly enriched in U235 is withdrawn and fed into the next higher stage, while the slightly depleted stream is recycled back into the next lower stage. • Significantly more U-235 enrichment can be obtained from a single gas centrifuge machine than from a single gaseous diffusion stage.
Who uses Gas Centrifuges • It has been widely used in Europe for about 30 years for the commercial nuclear power market. • In February 2004, the NRC issued a license authorizing USEC to construct and operate a demonstration and test facility known as the Lead Cascade. • To be located at the Piketon, Ohio gaseous diffusion plant site. • In August 2004, USEC submitted an application for a commercial facility to be located in Piketon. • The staff review of the USEC application is scheduled to be completed by February 2007. • In June 2006, the NRC issued a license to Louisiana Energy Services (LES) to construct and operate a commercial gas centrifuge enrichment facility in Lea County, New Mexico.
Laser Isotope Separation • Atomic Vapor Laser Isotope Separation (AVLIS) • 235 U atoms and 238 U atoms absorb light of slightly different frequencies (or colors). • Dye lasers 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.
AVLIS • 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. • Conceptually simple . . . but • The actual implementation of the process is likely to be difficult and expensive, • especially for countries with limited technical resources. • No country has yet deployed an AVLIS process, although several have demonstrated the capability to enrich uranium with the process.