Lecture 6(2) - Enrichmentx - International Atomic Energy Agency

Report
Sources of Radiation
Nuclear Fuel Cycle – Enrichment
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Day 4 – Lecture 6(2)
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Enrichment
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Gaseous Diffusion
 Two enrichment processes:
 Gaseous Diffusion
 Gas Centrifuge
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Basic Theory of
Gaseous Diffusion
•
Gaseous Diffusion uses molecular diffusion to
separate the isotopes of uranium
•
Three basic requirements are needed
• Combine Uranium with Fluorine to form Uranium
hexafluoride (UF6)
• Pass UF6 through a porous membrane
• Utilize the different molecular velocities of the
two isotopes to achieve separation
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Basic Theory of
Gaseous Diffusion
• Enrichment of 235U through one
porous membrane (or barrier) is very
minute
• Thousands of passes are required to
increase the enrichment of natural
uranium (0.711%) to a usable assay
of 4 or 5% for use in reactors
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Cylinder Filled with Solid UF6
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Gaseous Diffusion
Enrichment of 235U through one porous
membrane (or barrier) is very minute
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Tails Condensation and Withdrawal
• Tails are the depleted UF6 stream
• UF6 is compressed and condensed into a
liquid
• Withdrawn into 10- or 14-ton cylinders
• Cooled at ambient conditions until UF6 is
solid, taking at least 5 days
• Typical assay of tails is between 0.2% and
0.4%
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Potential Hazards
• Primary overall hazard is a major UF6
release
• Liquid cylinder drop is most credible
• When UF6 reacts with water, it forms
hydrofluoric acid
• Both corrosive and toxic
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Significant Hazards
•
•
•
•
•
UF6 – Uranium Hexafluoride
HF – Hydrogen Fluoride
Cl2 - Chlorine
NH3 - Ammonia
ClF3 – Chlorine Trifluoride
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Gaseous Diffusion
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Gas Centrifuge
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Why Gas Centrifuge?
 Large enrichment effect per stage
 > 1.05 vs 1.004 for GDP
 More compact design
 Reduced uranium inventories in cascades
 Better energy efficiencies
 < 5% of GDP energy typically stated
 More rapidly achieves equilibrium/steady state
 about a day instead of “weeks” for GDP
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Gas Centrifuge
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View of a Urenco cascade
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Safety Comparison: GC vs GDP
 Centrifuge
 Lower pressures, less inventory, more isolation
 Newer facility
 Liquid UF6 areas comparable
 Conclude GC risk probably lower
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What is Depleted Uranium?
 Definition:
 Depleted uranium (DU) is uranium that contains less
than the natural assay of uranium-235
 Natural assay = approximately 0.712% U-235
 “Normal DU” is around 0.2-0.4% U-235
 DU comes as a “byproduct” - some say “waste” from
enrichment
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DU Background
 Generated by every enrichment process
 DU generation cannot be avoided
 Perceived hazards
 old and rusting containers
 chemical - UF6, F2
 liability for cleanup, accidents
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A DUF6 Cylinder
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UF6 Tailings Cylinders
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Depleted uranium
Little U-235 remains in the tails (usually less
than 0.3%) so it is of no further use for energy.
Depleted uranium is used in metal form in
yacht keels, as counterweights, and as
radiation shielding, since it is 1.7 times
denser than lead.
Military uses include defensive armor plating
and bullets.
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Depleted uranium – hazards
External exposure to radiation from pure depleted uranium is
less of a concern because the alpha radiation emitted by its
isotopes travel only a few cms in air or can be stopped by a
sheet of paper. Further, the low concentration of uranium-235
that remains in depleted uranium emits only a small amount of
low energy gamma radiation.
The chemical toxicity of depleted uranium is about a
million times greater in vivo than its radiological
hazard.
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What are the potential
hazards with DUF6?
 DU more chemically toxic than radiotoxic
 heavy metal
 ingestion equivalent to about 1 rem dose can be
fatal
 DUF6 readily reacts with atmospheric water vapor to
form UO2F2 and Hydrogen Fluoride (both “bad”)
 DUF6 corrosive (+ HF effect)
 DUF6 reacts violently with most organic materials
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Reference
 International Atomic Energy Agency, Postgraduate
Educational Course in Radiation Protection and the Safety of
Radiation Sources (PGEC), Training Course Series 18,
IAEA, Vienna (2002)
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