In the Name of God

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In the Name of God

• Isfahan University of Technology
• Department of Chemistry

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Nuclear Fuel Cycle

• By Habib Soleimani
• Supervisor Dr. Ghaziaskar

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Contents

• Definitions
• Uranium and its compounds
• Uranium Mining and Milling
• Uranium Conversion
• Uranium enrichment
• Fuel fabrication
• Spent fuel
• Reprocessing

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Definitions

• Radioactivity ( radioactive decay )
• 1 Becquerel 1 decays/S
• 1 Curie 37 billion decays/S
• Half-life

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Definitions

• Radiation
• particles neutrons, alpha particles, and
  beta particles
• energy waves of pure energy, such as
  gamma and X-rays.

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Fission
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Element Sym-bol Atomic number Half-life Decay mode
Actinium Ac 89 22 y a,b-
Astatine At 85 8.3 h a
Francium Fr 87 22 min a,b-
Plutonium Pu 94 3.8 105 y a
Polonium Po 84 138.4 d a
Thorium Th 90 1.4 1010 y a
Uranium U 92 4.5 109 y a
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Uranium

• Uranium is present in the Earths crust at an
  average concentration of 2 ppm. Its natural
  abundance is equal that of Sn.
• Acidic rocks with high silicate, such as granite,
  have higher than average concentrations of
  uranium.
• sedimentary and basic rocks have lower than
  average concentrations .
• Isotopes U-233, U-234, U-235, U-236, U-237,
  U-238 and U-239
• Specific activity 24.9 103 Bq/g
• All isotopes decay by emission of a-radiation
  with a radiation energy between 4.2 and 5.2 MeV.

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Uranium Compounds

• Uranium metal
• Uranium dioxide ( UO2 )
• Thriuranium octaoxide ( U3O8 )
• Uranium tetrafluoride ( UF4 )
• Uranium hexafluoride ( UF6 )

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Uranium metal

• Uranium metal is heavy, silvery white, malleable,
  ductile, and softer than steel .
• d 19 g/cm3 , 1.6 times more dense than lead.
• it is subject to surface oxidation.
• Water attacks uranium metal slowly at room
  temperature and rapidly at higher temperatures.
• Uranium metal powder or chips will ignite
  spontaneously in air at ambient temperature.

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Uranium metal
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Uranium dioxide ( UO2 )

• It is an basic oxide.
• Most commonly used as a nuclear reactor fuel.
• It is a stable ceramic that can be heated almost
  to its melting point, 5,212F (2,878C), without
  serious mechanical deterioration .
• It does not react with water to any significant
  level.
• At ambient temperatures, UO2 will gradually
  convert to U3O8.
• Particle density 10.96 g/cm3 ,
  
  bulk density 2.0 - 5.0 g/cm3
• Uranium dioxide (UO2) will ignite spontaneously
  in heated air and burn brilliantly .

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Uranium dioxide ( UO2 )
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Thriuranium octaoxide ( U3O8 )

• It is an amphoteric oxide.
• Triuranium octaoxide (U3O8) occurs naturally as
  the olive-green-colored mineral pitchblende.
• In the presence of oxygen (O2), uranium dioxide
  (UO2) and uranium trioxide (UO3) are oxidized to
  U3O8.
• It is generally considered for disposal purposes
  because, under normal environmental conditions,
  U3O8 is one of the most kinetically and
  thermodynamically stable forms of uranium.
• It is insoluble in water
• Particle density 8.3 g/cm3
  bulk density 1.5
  - 4.0 g/cm3

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Thriuranium octaoxide ( U3O8 )
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Uranium tetrafluoride ( UF4 )

• Uranium tetrafluoride (UF4) is a green
  crystalline solid.
• m.p. 1,760F (96C)
• It is formed by the reaction UF6 H2 in a
  vertical tube-type reactor or by the action
  HFUO2 .
• It is generally an intermediate in the conversion
  of UF6 to either uranium oxide (U3O8 or UO2) or
  uranium metal.
• Uranium tetrafluoride (UF4) reacts slowly with
  moisture at ambient temperature, forming UO2 and
  HF, which are very corrosive.
• Bulk density 2.0 - 4.5 g/cm3.

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Uranium tetrafluoride ( UF4 )
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Uranium hexafluoride ( UF6 )

• Uranium hexafluoride (UF6) is the chemical form
  of uranium that is used during the uranium
  enrichment process.
• Within a reasonable range of temperature and
  pressure, it can be a solid, liquid, or gas.
• Disadvantage
  UF62H2O(g/l)
  4HF(g)UO2F2
• UF6 is not considered a preferred form for
  long-term storage or disposal because of its
  relative instability.

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Uranium hexafluoride ( UF6 )

• UF6 is characterised by an unusually high vapour
  pressure for a solid.
• UF6 is not flammable and is inert in dry air.

Temperat-ure (oC) Vapor Pressure(mbar)
0 24
20 107
56 1013.5
64 1516.5
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UF6
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Mining

• Excavation Excavation may be underground and
  open pit mining .
• In situ leaching (ISL) oxygenated acidic or
  basic groundwater is circulated through a very
  porous orebody to dissolve the uranium and bring
  it to the surface.

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Milling

• The ore is first crushed and ground to liberate
  mineral particles.
• The amphoteric oxide is then leached with
  sulfuric acid ( Leaching)
• UO3(s) 2H(aq)    UO22(aq) H2O
• UO22(aq) 3SO42-(aq)   
  UO2(SO4)34-(aq)
• The basic oxide is converted by a similar process
  to that of a water soluble UO2(CO3)34-(aq) ion.

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Milling

• Two methods are used to concentrate and purify
  the uranium ion exchange and solvent extraction
  ( more common ).
• solvent extraction uses tertiary amines in an
  organic kerosene solvent in a continuous process
  
• 2 R3N(org) H2SO4(aq)  
  (R3NH)2SO4(org)
• 2(R3NH)2SO4(org) UO2(SO4)34-(aq)
• (R3NH)4UO2(SO4)3(org
  ) 2SO42-(aq)

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Milling

• The solvents are removed by evaporating in a
  vacuum .
• Ammonium diuranate, (NH4)2U2O7 , is
  precipitated by adding ammonia to neutralize the
  solution.
• Then
• (NH4)2U2O7 heat U3O8 (yellow
  cake)

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Refining and converting U3O8 toUO3

• U3O8HNO3
  UO2(NO3)2 6H2O
• Uranyl nitrate, UO2(NO3)2 6H2O, is fed into a
  continuous solvent extraction process. The
  uranium is extracted into an organic phase
  (kerosene) with tributyl phosphate (TBP), and the
  impurities remain again in the aqueous phase.
• Washing from kerosene with dilute nitric acid and
  concentrated by evaporation to pure UO2(NO3)2
  6H2O .
• Then
• UO2(NO3)2 6H2O heat
  UO3 (pure)

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Continuous solvent extraction
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Converting UO3 to UF6

• The UO3 is reduced with hydrogen in a kiln
• UO3(s) H2(g) UO2(s) H2O(g)
  
• then
• UO2(s) 4HF(g)   UF4(s)
  4H2O(g)
• The tetrafluoride is then fed into a fluidized
  bed reactor and reacted with gaseous fluorine to
  obtain the hexafluoride
• UF4(s) F2(g)    UF6(g)

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Production of uranium metal

• Uranium metal is produced by reducing the uranium
  tetrafluoride with either calcium or magnesium,
  both active group IIA metals that are excellent
  reducing agents.
• UF4(s) 2Ca(s)   U(s)
  2CaF2(s)

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Enrichment
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Enriched uranium grades

• Highly Enriched Uranium ( HEU ) gt 20 235U
• 20 weapon-usable, 85 weapon-grade
• Low Enriched Uranium ( LEU ) lt 20 235U
• 12 - 19.75 used in research reactors
• 3 - 5 used in Light Water Reactors
• Slightly enriched Uranium ( SEU ) 0.9 - 2
  235U
• used in Heavy Water Reactors instead of
  natural uranium
• Recovered Uranium ( R U )
• recovered from spent fuel of Light Water
  Reactors

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Enrichment Methods

• Thermal Diffusion
• Gaseous Diffusion
• The Gas Centrifuge
• Aerodynamic Process
• Electromagnetic Isotope Separation( EMIS )
• Laser Processes
• Chemical Methods
• Plasma Separation

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Basic Facts of Separation Physics
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Laser Processes

• Atomic Vapor Laser Isotope Separation (AVLIS)
• Molecular Laser Isotope Separation (MLIS)

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Diffusion Cell
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separation factor of asingle diffusion process
step isdetermined as follows
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Gaseous Diffusion Cascade
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Separation factor Separative power of
centrifuge

• M1, M2 Molecular weight of the molecules to be
  separated
• R gas constant
• D diffusion constant of the process gas
• ? density of the process gas
• T temperature in degrees Kelvin
• d diameter of the rotor
• L length of the rotor
• V circumferential velocity of the rotor

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P1-centrifuge
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Gas Centrifuge Cascade
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Design of enrichment plants

• Several centrifuges are therefore operated in
  parallel in the separation stages of a centrifuge
  cascade.
• Centrifuge plants, are built of several operating
  units, which themselves consist of several
  cascades working in parallel.
• Diffusion plants consist of a single large
  cascade with approximately 1,400 stages.

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Fuel fabrication

• Enriched UF6 is converted into uranium UO2 powder
  which is then processed into pellet form
• UF6H2 (g)
  UF4(S)2HF(g)
• UF4(S)H2O
  UO2(S)2HF(g)

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Fuel fabrication

• The pellets are then fired in a high temperature
  sintering furnace (with H2) to create hard,
  ceramic pellets of enriched uranium.
• Fuel rods corrosion resistant metal alloy (
  zirconium ).

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Fuel bundle fuel pellet
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Fuel assembly
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Chain reaction
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Nuclear reactor
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Spent fuel

• Used fuel
• About 95 U-238
• About 1 U-235 that has not fissioned
• About 1 plutonium
• 3 fission products, which are highly radioactive
• With other transuranic elements formed in the
  reactor.

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Spent fuel storage
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Reprocessing

• The PUREX process is a liquid-liquid extraction
  method used to reprocess spent nuclear fuel, in
  order to extract uranium and plutonium,
  independent of each other, from the fission
  products.
• PUREX is an acronym standing for Plutonium and
  Uranium Recovery by Extraction.

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Reprocessing

1. Dissolving of fuel into nitric acid
2. Remove the fine insoluble solids
3. Organic solvent 30 tributyl phosphate (TBP)
   in odorless kerosene (or hydrogenated propylene
   trimer)
4. The extraction of U(VI) and Pu(IV)
5. Reduction of Pu(IV) to Pu(III)
6. Back extraction (stripping) of U(VI) by a low
   nitric acid concentration

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References

• www.nrc.gov
• www.stpnoc.com
• www.urenco.com
• http//chemcases.com
• www.wikipedia.com
• www.jnfl.co.jp
• www.globalsecurity.com
• http//daneshnameh.roshd.ir
• www.isotopetrace.com

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Uranium hexafluoride ( UF6 )

• With most metals and alloys (for example, Fe, Co,
  Cr, Al, Mg, high grade steel, brass) UF6 reacts
  slowly at room temperature to form metal
  fluorides and reacts somewhat faster at higher
  temperatures( grey, brown or green deposits).
• Absolutely dry glass and dry quartz sand are not
  attacked by UF6.
• Metals such as Ni and Pt and most of their alloys
  are practically resistant, even at 100 C.
• Synthetic polymers, for example Teflon and some
  copolymers,
• demonstrate similar resistance towards UF6.
• ether, ester, ketone and saturated and
  unsaturated hydrocarbons react at room
  temperature by fluorinating with UF6.

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Refining and converting U3O8 to UF6
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Basic Facts of Separation Physics

• The ability of the separation element to separate
  235Uand 238U is described by its separation
  factor.
• If N concentration of 235U
• NF is its concentration in feed stream(F)
• NP is its concentration in product stream(P)
• NT is its concentration in Tails stream(T)

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Basic Facts of Separation Physics

• However, the separation factor alone does not
  describe fully the efficiency of a separation
  element.
• To determine the "work that must be applied for
  separation, P, NP, NF and NT must be given.
• Mass balance 0PT-F
• Isotope balance 0PNP TNT -FNF

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Separative Work and Power

• dUPV(NP)TV(NT)-FV(NF)
• The value function V(N) is determined using
  mathematical methods so that the calculated
  separative work is independent of the 235U
  concentrations in the separation element and
  depends only on the archieved change in
  concentration and throughput.
• Kg SW/S or SWU/S

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