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

• 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)

Enrichment

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 – The stream that is slightly enriched in U235 is withdrawn and fed into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.

Possible AVLIS-type system schematic