Nuclear Fuel Cycle Conversion-Enrichment-Diffusion

1. Nuclear Fuel CycleConversion-Enrichment-Diffusion SYE 4503 Mahmoud R. Ghavi, Ph.D. Center for Nuclear Studies

2. Uranium Enrichment • Natural Uranium found in ore deposits: • U-234 Trace amounts <0.006% • U-235 0.71% • U-238 99.28% Need 2-5% for LWR 20% for research & naval realtors 90+% for nuclear weapons Center for Nuclear Studies

3. Purification of U308 • Prior to conversion to UF6 impurities such as B, Cd, Cl, etc. in natural uranium must be removed • Use solvent extraction methods to separate out impurities • Once highly purified, it is ready for conversion from U308 to UF6 Center for Nuclear Studies

4. Properties of UF6 • White, dense crystalline material resembling rock salt, liquid and gaseous forms are colorless • UF6 is an ideal uranium compound for gaseous diffusion enrichment method due to its properties • UF6 can be solid, liquid or gas depending on its temperature and pressure (phase diagram shown) • It is solid at room temperature and at slightly elevated temperatures • The triple point is at 147 F and 22 psia • Direct Solid to vapor below triple point • Low atomic weight ideal due to non-interference with diffusion of uranium Center for Nuclear Studies

5. Conversion of U308 to UF6 • Conversion of U308 to UF6by one of the following methods: • Dry hydrofluor process • Wet solvent extraction process Center for Nuclear Studies

6. Dry hydrofluor process • U308 is ground to fine powder • At T=1000-1200 oF, reduced by hydrogen resulting in UO2 • UO2 is exposed to hydrogen fluoride at 900 to 1000 oF producing UF4 (green salt) • UF4 is treated with fluorine gas to form UF6 • To remove volatile impurities, the resultant UF6 undergoes a final solvent extraction process Center for Nuclear Studies

7. Wet solvent extraction process • Similar to the dry process, uses reduction, hydro-fluorination, and fluorination steps • The above steps are preceded by the wet solvent extraction process • Due to the upstream processing, theUF6 produced is pure and does not require a final solvent extraction process Center for Nuclear Studies

8. Enrichment methods • Uranium Enrichment Methods: • Gaseous diffusion method • Gaseous separation by centrifuge method • Separation nozzle method • Atomic Vapor Laser Isotope Separation (AVLIS) • The only commercial method currently used in the U.S. is the gaseous diffusion method Center for Nuclear Studies

9. Uranium enrichment in the U.S • The U.S. Uranium Enrichment program was created in the 1940’s for military purposes. • The three gaseous diffusion plants (GDP): • Oak Ridge, Tennessee (up to 90% enrichment capacity) • Paducah, Kentucky (up to 2% enrichment capacity) • Portsmouth, Ohio (97%+ enrichment capacity) • Feed material (UF6) for these plants is produced at other facilities and are then delivered to these sites. • The enriched UF6 product from these facilities sent to other facilities for fabrication • Depleted UF6tails is collected and stored on-site. Center for Nuclear Studies

10. The Oak Ridge Gaseous Diffusion Site Center for Nuclear Studies

11. Current status in the U.S. • Oakridge plant & high enrichment sections of the Portsmouth plant are closed • Enrichment capacity of Paducah plant is increased to 2.75% • United States Enrichment Corporation (USEC) was established (1993) as a government corporation to operate as a business entity. • Ownership of the corporation was transferred to private investors in 1998 (NYSE:USU) • Enrichment operation (U.S.) • USEC, Paducah & Portsmouth (KY) • Uranco (NM) • Areva (ID) – Not Operational yet • Global Laser Enrichment – GE/HITACHI (planning stages) Center for Nuclear Studies

12. The Gaseous diffusion method • The technique takes advantage of the slight mass differences between 235U & 238U • UF6 gas flows through a barrier tube with porous walls • Part of the gas (about 50%) diffuses through the tube walls • The 235UF6 molecules with lower molecular weight have a higher molecular velocity and diffuse more readily through the barrier pores. • = α(where α is the separation factor) • Gas that passes is more enriched in 235U isotope and gas that does not pass through is slightly depleted in 235U. • The higher the value of α, the easier it is to separate isotopes & preferentially enrich one isotope. • α = = 1.004289 Center for Nuclear Studies

13. Gaseous diffusion process • Typical converter Center for Nuclear Studies

14. Gaseous diffusion • Need to repeat this process thousands of times by cascading a large number of diffuser units in series • The large converter vessels in the U.S. GDP are 13 ft in diameter by 24 ft long for the low enrichment sections. • The converters in the high enrichment sections are smaller Center for Nuclear Studies

15. Gaseous Diffusion Operational Concerns • Criticality • UF6 leaks • Air leaks • Diffusion barrier obstruction Center for Nuclear Studies

16. Center for Nuclear Studies

17. Converter Center for Nuclear Studies

18. Cascades in a gaseous diffusion plant • The repetitive arrangement of the cascades In a gaseous diffusion plant Center for Nuclear Studies