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Nuclear Fuel Cycle Conversion-Enrichment-Diffusion. SYE 4503 Mahmoud R. Ghavi , Ph.D. 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.

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Nuclear fuel cycle conversion enrichment diffusion

Nuclear Fuel CycleConversion-Enrichment-Diffusion

SYE 4503

Mahmoud R. Ghavi, Ph.D.

Center for Nuclear Studies


Uranium enrichment
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


Purification of u 3 0 8
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


Properties of uf 6
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


Conversion of u 3 0 8 to uf 6
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


Dry hydrofluor process
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


Wet solvent extraction process
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


Enrichment methods
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


Uranium enrichment in the u s
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


The oak ridge gaseous diffusion site
The Oak Ridge Gaseous Diffusion Site

Center for Nuclear Studies


Current status in the u s
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


The gaseous diffusion method
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


Gaseous diffusion process
Gaseous diffusion process

  • Typical converter

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Gaseous diffusion
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

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Gaseous diffusion operational concerns
Gaseous Diffusion Operational Concerns

  • Criticality

  • UF6 leaks

  • Air leaks

  • Diffusion barrier obstruction

Center for Nuclear Studies



Converter
Converter

Center for Nuclear Studies


Cascades in a gaseous diffusion plant
Cascades in a gaseous diffusion plant

  • The repetitive arrangement of the cascades In a gaseous diffusion plant

Center for Nuclear Studies


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