Transcript Document

Nuclear Fuel Cycle
Conversion-Enrichment-Diffusion
SYE 4503
Mahmoud R. Ghavi, Ph.D.
7/17/2015
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Uranium Enrichment
• Natural Uranium found in ore deposits:
– U-234 Trace amounts <0.006%
– U-235 0.71%
– U-238 99.28%
Need
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2-5% for LWR
20% for research & naval realtors
90+% for nuclear weapons
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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
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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
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Conversion of U308 to UF6
• Conversion of U308 to UF6 by one of the
following methods:
– Dry hydrofluor process
– Wet solvent extraction process
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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
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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,
the UF6 produced is pure and does
not require a final solvent
extraction process
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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
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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 UF6 tails is collected and stored on-site.
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The Oak Ridge Gaseous Diffusion Site
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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.)
–
–
–
–
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USEC, Paducah & Portsmouth (KY)
Uranco (NM)
Areva (ID) – Not Operational yet
Global Laser Enrichment – GE/HITACHI (planning stages)
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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.
‒
1/2
𝑉𝑙𝑖𝑔ℎ𝑡
𝑀
ℎ𝑒𝑎𝑣𝑦
=
𝑉ℎ𝑒𝑎𝑣𝑦
𝑀𝑙𝑖𝑔ℎ𝑡
= 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=
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𝑀ℎ𝑒𝑎𝑣𝑦
𝑀𝑙𝑖𝑔ℎ𝑡
1/2
==
238+6𝑥19 1/2
235+6𝑥19
= 1.004289
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Gaseous diffusion process
• Typical converter
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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
•
•
•
•
Criticality
UF6 leaks
Air leaks
Diffusion barrier obstruction
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Converter
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Cascades in a gaseous diffusion plant
• The repetitive arrangement of the cascades In a gaseous diffusion plant
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