In the Name of God
Isfahan University of Technology
Department of Chemistry
Nuclear Fuel Cycle
By: Habib Soleimani
Supervisor: Dr. Ghaziaskar
Contents:
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Definitions
Uranium and its compounds
Uranium Mining and Milling
Uranium Conversion
Uranium enrichment
Fuel fabrication
Spent fuel
Reprocessing
Definitions
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Radioactivity ( radioactive decay ):
1 Becquerel= 1 decays/S
1 Curie= 37 billion decays/S
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Half-life
Definitions
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Radiation:
particles: neutrons,
alpha particles, and beta
particles
energy : waves of
pure energy, such as
gamma and X-rays.
Fission
Element
Actinium
Sym- Atomic
bol number
Ac
89
Half-life
Decay
mode
22 y
α,b-
Astatine
At
85
8.3 h
α
Francium
Fr
87
22 min
α,b-
Plutonium
Pu
94
3.8 ×105 y
α
Polonium
Po
84
138.4 d
α
Thorium
Th
90
1.4 *1010 y
α
Uranium
U
92
4.5 *109 y
α
Uranium
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Uranium is present in the Earth’s 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 α-radiation with a radiation
energy between 4.2 and 5.2 MeV.
Uranium Compounds
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Uranium metal
Uranium dioxide ( UO2 )
Thriuranium octaoxide ( U3O8 )
Uranium tetrafluoride ( UF4 )
Uranium hexafluoride ( UF6 )
Uranium metal
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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.
Uranium metal
Uranium dioxide ( UO2 )
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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, 5,212°F (2,878°C), 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 .
Uranium dioxide ( UO2 )
Thriuranium octaoxide ( U3O8 )
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It is an amphoteric oxide.
Triuranium octaoxide (U3O8) occurs naturally as the olive-greencolored 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
Thriuranium octaoxide ( U3O8 )
Uranium tetrafluoride ( UF4 )
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Uranium tetrafluoride (UF4) is a green crystalline solid.
m.p.= 1,760°F (96°C)
It is formed by the reaction UF6 + H2 in a vertical
tube-type reactor or by the action HF+UO2 .
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.
Uranium tetrafluoride ( UF4 )
Uranium hexafluoride ( UF6 )
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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:
UF6+2H2O(g/l)
4HF(g)+UO2F2
UF6 is not considered a preferred form for long-term
storage or disposal because of its relative instability.
Uranium hexafluoride ( UF6 )
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UF6 is characterised by
an unusually high vapour
pressure for a solid.
UF6 is not flammable
and is inert in dry air.
Temperat- Vapor
ure (oC)
Pressure(mbar)
0
24
20
107
56
1013.5
64
1516.5
UF6
Mining
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Excavation : Excavation may be underground
and open pit mining .
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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.
Milling
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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)
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UO2(SO4)34-(aq)
The basic oxide is converted by a similar process to
that of a water soluble UO2(CO3)34-(aq) ion.
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)
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2(R3NH)2SO4(org) + UO2(SO4)34-(aq)
(R3NH)4UO2(SO4)3(org) + 2SO42-(aq)
Milling
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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)
Refining and converting U3O8 toUO3
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U3O8+HNO3
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)
Continuous solvent extraction
Converting UO3 to UF6
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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)
Production of uranium metal
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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)
Enrichment
Enriched uranium grades
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Highly Enriched Uranium ( HEU ): > 20%
20% weapon-usable, 85% weapon-grade
235U
Low Enriched Uranium ( LEU ): < 20% 235U
12% - 19.75% used in research reactors
3% - 5% used in Light Water Reactors
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Slightly enriched Uranium ( SEU ): 0.9% - 2% 235U
used in Heavy Water Reactors instead of natural uranium
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Recovered Uranium ( R U ):
recovered from spent fuel of Light Water Reactors
Enrichment Methods
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Thermal Diffusion
Gaseous Diffusion
The Gas Centrifuge
Aerodynamic Process
Electromagnetic Isotope Separation( EMIS )
Laser Processes
Chemical Methods
Plasma Separation
Basic Facts of Separation Physics
Laser Processes
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Atomic Vapor Laser
Isotope Separation
(AVLIS)
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Molecular Laser Isotope
Separation (MLIS)
Diffusion Cell
separation factor of a
single diffusion process step is
determined as follows:

 b 
 1.00429
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Gaseous Diffusion Cascade
Separation factor & Separative
power of centrifuge
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M1, M2 ; Molecular weight of the molecules to be separated
R ; gas constant
D ; diffusion constant of the process gas
ρ ; 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
Vmax
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R
P1-centrifuge
Gas Centrifuge Cascade
Design of enrichment plants
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Several centrifuges are therefore operated in parallel in
the separation stages of a centrifuge cascade.
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Centrifuge plants, are built of several operating units,
which themselves consist of several cascades working
in parallel.
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Diffusion plants consist of a single large cascade with
approximately 1,400 stages.
Fuel fabrication
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Enriched UF6 is converted into uranium UO2
powder which is then processed into pellet
form:
UF6+H2 (g)
UF4(S)+2HF(g)
UF4(S)+H2O
UO2(S)+2HF(g)
Fuel fabrication
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The pellets are then fired in a high temperature
sintering furnace (with H2) to create hard, ceramic
pellets of enriched uranium.
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Fuel rods : corrosion resistant metal alloy ( zirconium ).
Fuel bundle & fuel pellet
Fuel assembly
Chain reaction
Nuclear reactor
Spent fuel
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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.
Spent fuel storage
Reprocessing
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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.
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PUREX is an acronym standing for Plutonium
and Uranium Recovery by Extraction.
Reprocessing
1.
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5.
6.
Dissolving of fuel into nitric acid
Remove the fine insoluble solids
Organic solvent : 30% tributyl phosphate
(TBP) in odorless kerosene (or hydrogenated
propylene trimer)
The extraction of U(VI) and Pu(IV)
Reduction of Pu(IV) to Pu(III)
Back extraction (stripping) of U(VI) by a low
nitric acid concentration
References:
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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
Uranium hexafluoride ( UF6 )
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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.
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Metals such as Ni and Pt and most of their alloys are practically resistant,
even at 100 °C.
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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
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
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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 : 0=P+T-F
Isotope balance: 0=PNP +TNT -FNF
N P - NT
F
P
N F - NT
Separative Work and Power
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δU=PV(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