1. Uranium Enrichment
Louis Croquette

2. Fundamental Question:
How does the uranium enrichment process work?

3. Contents U 238 vs 235 Grades of uranium Enrichment Methods
Separative Work Units

4. U238 vs U235 U238 Not fissile but very common (99.284%)
Half-life of billion years
After two beta decays becomes Plutonium-239 (fissile)
Cannot support chain reaction

5. U238 vs U235 (continued) U238 (continued) Uses
Source material in breeder reactors (Makes Pu-239)
Radiation shielding
Downblending
Tamper in nuclear weapons

6. U238 vs U235 (continued) U235 Fissile but not very common (.72%)
Can support a chain reaction
Half-life of million years
Most neutron absorptions result in fission
Used mostly in making reactor- and weapon-grade material

7. Grades of Uranium Slightly-enriched uranium Low-enriched uranium
Highly-enriched uranium

8. Slightly-enriched uranium
.9%-2%
Can be used to replace natural uranium in heavy water reactors
Increases safety and decreases cost
Increased efficiency

9. Low-enriched uranium Concentration <20%
3%-5% uranium is typically used in reactors
Research reactors use 12%-19.75%

10. Highly-enriched uranium
>20%
Weapons-grade is typically 85% or more
Enrichments of over 97% have been achieved
Critical mass needed for weapon decreases as purity increases

11. Enrichment Methods
Diffusion
Gaseous
Thermal
Centrifuge
Zippe
Laser

12. Enrichment Methods (continued)
Very difficult since isotopes have very similar chemical and physics properties
Can only be separated by small differences in mass
Mostly done by use of compound UF6

13. Uranium hexafluoride
Highly toxic, violent reaction with water, radioactive
Used often in uranium enrichment because it has a triple point (64.05C) and slightly higher than normal atmospheric pressure
Yellowcake (U3O8) dissolved in nitric acid, giving uranyl nitrate
Pure uranyl nitrate is extracted by solvent extraction
Treated with ammonia to produce ammonium diuranate (ADU, (NH4)2U2O7)
Reduction with hydrogen to get UO2
Converted with hydrofluoric acid to uranium tetrafluoride
Oxidized with fluorine to get UF6

14. Diffusion Gaseous Forcing hex through semi-permeable membranes
Based on Graham’s law (rate of effusion is inversely proportional to square root of molar mass)
Slight separation between 235 and 238
Large number of stages are required for decent grades to be produced (called a cascade)

15. Diffusion (continued)
Gaseous (continued)
Eventually U235 and U238 are separated by comparable amount
Currently being phased out in favor of centrifuge enrichment methods
Thermal
Uses heat across a thin liquid
Smaller U235 tend towards the hotter surface and heavier U238 tends towards the colder surface
Not very efficient, quickly abandoned after World War II

16. Centrifuge Uses centrifugal acceleration to separate molecules Gas
Heavier particles move towards wall and lighter particles stay close to the center
Allows hex to flow in and out constantly of the device
Allows for easy cascading
Casing contains vacuum to minimize friction

17. Centrifuge (continued)
Gas (continued)
Motor spins rotor to generate centripetal force
RPM typically above 50,000
Process is silent, sound indicates failure
Zippe
Gas centrifuge with thermal components
Enriches uranium more quickly but process is more complicated

18. Laser Laser lower energy inputs lower capital costs
significant economic advantages
Atomic Vapor Laser Isotope Separation and Molecular Laser Isotope Separation
Absorption lines of U235 and U238 differ very slightly
Only U235 gets excited
U235 is collected on an electromagnetic plate and unwanted U238 passes through

19. Separative Work Units
Function of amount of uranium used and degree of enrichment
Slightly complicated formula used to calculate it
Gas centrifuges are a lot more efficient than diffusion
Shown by comparing SWU’s

20. Separative Work Units (continued)
Equation
SWU = P·V(Np) + W·V(Nw) – F·V(Nf)
Where P is the product amount, Np is the product concentration, W is the waste amount, Nw is the waste concentration, F is the feed amount, Nf is the feed concentration, and V(x) is a value function that takes the form:
V(x)=(2x-1)ln(x/(1-x)) where x is a given concentration

21. Works Cited
Brown. Chemistry: The Central Science. Ninth ed. Place of Publication Not Identified: Prentice Hall, Print.
Schwarcz, Joseph A. The Genie in the Bottle: 64 All New Commentaries on the Fascinating Chemistry of Everyday Life. New York: W.H. Freeman, Print.