separation of stable isotopes
DESCRIPTION
Different methods for enrichment of stable isotopes are decribed in this powerpoint presentation. This was given to College faculty. Some of the figures are taken from internet and other open sourcesTRANSCRIPT
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Karanam L. Ramakumar 1
Methods for the separation of stable isotopes
Karanam L. RamakumarIndia
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Karanam L. Ramakumar 2
Isotopes of an element have very similar chemical properties
e.g. 235U3O8 and 238U3O8 Chemical reactivity is nearly identical
They behave as completely different substances in nuclear reactions
e.g. 235U is a fissile isotope while 238U is not
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Karanam L. Ramakumar 3
Many of the stable isotopes find wide spread applications in chemical, industrial, agricultural and clinical research
�Elucidate and understand reaction pathways
�Mechanisms and kinetics
�Effect of trace elements on physico-chemical properties
�Up-take and plant metabolism studies
�Behaviour of trace elements from toxicological and human metabolism point of view
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Karanam L. Ramakumar 4
Mass differences result in
Thermodynamic isotopic effects
Shift in equilibrium in reactions
Kinetic isotopic effects
Shift in rate of reactions
Isotopic effects are quite pronounced in light elements
Negligible in heavy elements
“The reasonable man adapts himself to the world, the unreasonable man persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man”
George Bernard Shaw ‘The Revolutionist’s Handbook’
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Karanam L. Ramakumar 5
Separation Factor in a Typical Enrichment Process
Two types of separation factor
(i) separation factor (αααα) and
(ii) enrichment factor (ε)
y/(1-y)z/(1-z)
α=
y/(1-y)x/(1-x)
ε=
z is the atom % abundance of the desired isotope in the feed. (1-z) is the corresponding quantity of the other isotope in the feed
y and (1-y) refer to the corresponding quantities in the product and
x and (1-x) are defined for tails
Feed
z, (1-z)
Product
y, (1-y)
Tails
x, (1-x)
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For material balance:Total U = F = P + W (F = Feed, P = Product, W = Waste, all in Kgs)U-235 = F.xf = P.xp + W.xw (x is atom fraction of U-235)
w pp f
w wf f
x -xx -xF=P W=P
x -x x -x
The ratio of products
flow rate to feed flow rate is called “cut” θθθθ
wF
wP
x -x=
x -x
θ
Cut (θθθθ ) for a given enrichment cascade is optimised
Product flow rate PFFeed flow rate
θ= =
Fraction of desired
component in products
stream is called recovery
“r”y x(1 )r 1z z
θ −θ= = −1 - θθθθ = W/F
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Karanam L. Ramakumar 7
Separating Unit, Stage and Cascade
Separating Unit: The smallest element of an isotope separation plant that effects some separation of the process material
Examples of a single separating unit are one gas centrifuge, or one electrolytic cell etc.
No single separating unit can enrich any material to desired value. Throughput is also very very small
To multiply the effects of the enrichment of one unit and to achieve adequate throughput large numbers of units are interconnected in parallel
Stage: A group of parallel-connected separating units, all fed with material of same composition and producing partially separated product streams of the same composition
Cascade: A series-connected group or stages
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Karanam L. Ramakumar 8
CascadeStage
UnitZ1
Feed
x1
x1
x1
y1
y1
y1
x2
x2
x3
y2
y2
y3
Concept of Unit, Stage and cascade
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FeedWaste
Product
A square cascade has the same flow rates in all stages and therefore the same number of machines per stage. Rarely used because they are not very efficient.Constant flow rate results in constant cut and mixing of concentrations and therefore loss ofseparative work.
Square Cascade
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FeedWaste
Product
Simple cascade
Waste
Waste
No attempt is made to reprocess the partially depleted waste streams leaving each stage.
it is impossible to obtain high recovery of desired component because of losses in the waste streams leaving every stage, the recovery falls rapidly as the over all enrichment factor desired is increasing.
A simple cascade has only enrichment section.
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FeedWaste
Product
Feed for each stage consists of heads from the next lower stage and wastes from the next higher stage
The most commonly employed cascade
Two sections: the enriching section, consisting of the stages above the point at which the feed enters the cascade and produces material of increased concentration. The stripping section is below the feed point and increases the recovery of the material
In a symmetric counter-current cascade, the waste stream is recycled back to the immediately preceding stage. In an asymmetric counter-current cascade, the waste is recycled more than one stage back.
Counter current or recycle cascade
As (α-l) < < < 1, in most of
the cases, these are also known as close-separation cascades.
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Separation of heavy isotopes
e.g. 235U from 238U
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Concept of Separative Power, Separative capacity andSeparative Work Unit
In conventional industries, where the level of separation is almost 100%, throughput parameter is sufficient to indicate the capacity of the separating plant
e.g. a heavy water production plant, where the grade is fixed for reactor use.Petroleum refineries
In the case of uranium enrichment, two parameters namely extent of enrichment and total quantity of enriched isotope decide the plant’s capacity
e.g. 3% for LWRs to 90% & above for weapon grade
To compare the capacities of two different plants, only throughput may not be sufficient to gauge the size of an enrichment plant, particularly when enrichment levels at which the plants are operating are different
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Separative power or Separative capacity
A combined function of quality & quantity of separation performed by a separating element or a plantIt is independent of the level of concentration of feed material
Separative power: A change in the Value effected by a separating element, i.e. the increase in the value of output over the value of input. A quantity called value function is defined as a function of the concentration, x, of the desired isotope by the relation:
The work WSWU (separative work per unit time) necessary to separate a mass F of feed of assay xf into a mass P of product assay xp, and tails of mass T and assay xt:
xV(x)=(2x-1)ln1-x
p tSWU fW P.V(x ) T.V(x )-F.V(x )= +
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For material balance:Total U = F = P + W (F = Feed, P = Product, W = Waste all in Kgs)U-235 = F.xf = P.xp + W.xw (x is atom fraction of U-235)
w pp f
w wf f
x -xx -xF=P W=P
x -x x -x
p w fwpSWU f
wp f
xx xW =P(2x -1)ln +W(2x -1)ln -F(2x -1)ln
1-x 1-x 1-x
pSWU w fwp f
wp f
W xx xW F=(2x -1)ln + (2x -1)ln - (2x -1)ln1-x P 1-x 1-x
P P
wF
wP
x -x=
x -x
θCut P F
wP
x -xx -x
(1-θ) =
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Let us calculate the amount of feed (F in kg) required to produce 1 kg of product and the number of SWUs needed for this operation in two cases:
193218Case 2: Xf = 0.00711,
Xp = 0.9, Xw = 0.003
229176Case 1: Xf = 0.00711,
Xp = 0.9, Xw = 0.002
SWUF(kg)Case #
Feed and SWUs operate in opposite direction. If the availability of feed is no problem, one can save on energy consumption by allowing larger fraction of desired isotope in the waste streams.
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1/22 1
[M /M]α=
The gaseous diffusion process makes use of the phenomenon of molecular effusion to effect separation. If a gas is allowed to pass through a porous membrane with pore sizes equal to the at molecular dimensions, the relative frequency with which molecules of different species pass through the pores is inversely proportional to the square root of their molecular weights. For a mixture of two masses M1 and M2 (MI < M2), this ratio, called separation factor, αααα, is given by
Gaseous Diffusion Process
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Gaseous diffusion process
Feed Product
z, M1, p1 y
1-z, M2, p2 (1-y)
Porous membrane
Rate of diffusion (D) αDensity
1
No. of molecules crossing the barrier α pressure x diffusion rate
J α P.D (p α z)
y = J1 α . z. (1-y) =J2 α (1-z)
Mass1
(D) α
/M
1 21
1/
M1 22
1
yy
MzMz
2
11 1
=− −
M
M2
1
α =
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Separation of U-235 from U-238 by gaseous diffusion
Feed : UF6 M1 = 235UF6 = 349 M2 = 238UF6 = 352
Separation factor M
My/( y)z/( z)
2
1
11
− α−
.352 1 00429349
=
Separation factor is very close to 1!!
Back-diffusion brings it down further.
For useful degree of enrichment, many stages in series
(Cascade) are employed.
Lower elements have better separation factor
20Ne-22Ne = 1.0488 36Ar-40Ar = 1.0541 D-H2 = 1.414
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Natural uranium U-235 : 0.00711%
Product U-235 : 0.03%
Tails U-235 : 0.002%
No. of stages required (n): 1275
UF6 is highly reactive, powerful fluorinating reagent
B.Pt. = 56.40C Vapour pressure at 250C = 111.9 mm (Hg)
Gaseous diffusion of UF6 : a technological challenge
Materials compatible with UF6
Lubricants
Seals and gaskets
Diffusion cells
Diffusion membranes
Compressor materials
Complete elimination of air
leakage inside the process
system!!
wp
w p
x ( x )n ln
x ( x )
−=
α− −
121 1
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Materials used in diffusion plants
Fluorocarbons and chlorocarbons as lubricants and gasket valves
Alumina or Nickel vessel protected by chemisorbed Nickel fluoride layer
Alumina or Aluminium protected by alumina for construction of plant
Diffusion membrane : Chemically resistant, even sized and shaped pores of radius ≤ 10 nm
Large porosity : 109 / cm2
Small thickness and sufficient mechanical strength
Diffusion membrane is Key to the process
Method of manufacture and performance characteristics remain classified
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Diffusion membrane materials
Metals : Au, Ag, Ni, Al, Cu
Oxides : Al2O3
Fluorides : CaF2
Fluorocarbons : Teflon
Film type membranes : Pores are bored through an initially non-porous membrane
Alloy of Ag(66) + Zn(34) HCl leaching of Zn
Au(40) + Ag(60) HNO3 leaching of Ag
Al sheet anodically oxidised by 5% H2SO4
Aggregate type membranes : Pores are the voids left when fine particles are agglomerated under pressure or sintered at convenient temperature
Sintered Al or Ni powders
Teflon granules pored into a grid
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~~
~~
~~
~~~
~
~~
TailsWt.Fr. U-235 = 0.002
Natural U feed
Wt.Fr. U-235 =0.00711
ProductWt. Fr. U-235 = 0.03
Stage 594
Cooler
CompressorStage 1
Controlvalve
0.007095
0.007125
Stage 1275
Ideal Gas Diffusion Cascade
A
A
A
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Centrifugal Methods
M1 and M2 are the masses of the lighter and heavier isotopesω is the peripheral velocity of the moleculesa is the radius of the centrifuge r is the radius at any given location in the centrifuge R is the molar gas constant T is the absolute temperature
αααα increases with the length of the rotor, the peripheral speed and also with the radius.αααα depends on the difference between the massesBetter separation possible, of the 235U and 238U isotopes of uranium than of the isotopes of hydrogen with masses 1 and 2 Since the difference in the atomic masses is always same for a given element, the efficiency is independent of the molecular weight of the compound whose vapour is being centrifuged
2 2 2122
(M -M )ω a rα=exp (1- )a2RT
Separation factor α
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2 2 2122
(M -M )ω a r=exp (1- )a2RT
α
When a gas or vapour flows into a rapidly rotating centrifuge, the force acting on the molecules will produce an increased concentration of the heavier isotope at the walls, while the lighter isotope tends to collect nearer the axis of rotation. If the centrifuge is vertical, a current of vapour can be made to flow down near the axis and up near the wall. It should then be possible to draw off a product richer in the lighter isotope at the bottom of the apparatus, near the centre, whereas the heavier species would be removed at the top near the periphery. The separation factor α for centrifugal method along the radius is given by
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Pressure gradient of the gas
Ph = P0 exp (-Mgh/RT)
For two masses M1 and M2 (M1 < M2)
h o
P Pexp[ (M M )gh/RT]
P P1 1
2 2
1 2
= − −
Pressure gradient between the axis and the wall
Pa <<<<< Pw
Lighter isotope accumulates near the axis
Heavier isotope accumulates near the wall
Separation factor
depends on mass
difference
Separation factor
same for same
mass difference
(light and heavy
elements!!)
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For a given mass difference between the isotopes, the stage separation factor is more than in gaseous diffusion plant.
To get 3 % enriched uranium, 13 stages are needed in centrifuge as compared to about 1300 stages required in the gaseous diffusion plant.
This advantage is partly off set by a lower yield per stage compared to the process of gaseous diffusion.
Large number of centrifuges need to he operated in parallel to multiply the net yield
With current technology, a single gas centrifuge is capable of about 5 separative work units [SWU] annually, while advanced gas centrifuge machines can operate at a level of up to perhaps 40 SWUs annually. About 120,000 SWU is required to enrich the annual fuel loading for a typical 1000 MWe light water reactor.
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Separation factor does depend on the angular velocity (peripheral
speed of the rotor)
3.3 x 10141.329700
4.5 x 10101.233600
2.5 x 1071.156500
5.5 x 1041.0975400
Pressure ratio between axis
and wall
Separation factorPeripheral speed
m/s
Maximum velocity is limited by tensile strength of the rotor T.S. >
ρω2r2
Aluminium alloys
Titanium alloys
High tensile steels
Polyamides
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Advantages of gas centrifuge over gas diffusion processes of enrichment
Higher separation factor hence requires less number of stages.
Absence of inter-stage gas compressors in centrifuge plant allows it to be squared off more towards ideality. Whereas in case of gas diffusion plant use of compressors makes it necessary to go for bigger squaring off (more off from ideality) in order to avoid use of large number of compressors of different capacities. This makes the centrifuge cascades more efficient.
Gas centrifuge being modular in construction, capacity addition can be done more easily. The plant can initially be constructed for lower capacity and can subsequently be expanded without much penalty.
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Gas diffusion plants must be of large capacity to be economical due to requirement of large number of supporting systems like captive power plants etc. Whereas the gas centrifuge plants can be economical in smaller capacities.
Higher material inventory in gas diffusion plant makes it more difficult to switch over from one level of enrichment to another in an operating plant without a sufficient lead-time. This reduces flexibility of the plant in catering to different users requiring different enrichment levels in short delivery periods. In G.C. plants this problem does not arise due to much lower material inventories.
It has low equilibrium time, which reduces time between start up of the plant and start of withdrawal of product. Gas centrifuge process is considered superior above nozzle process also because of low separation factor (compared to gas centrifuge) and very high-energy consumption of nozzle process.
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Feed
Product
Tails
Aerodynamic methods Nozzle separation process
S = 0.03 mm
A = 0.1 mm
95% H2 + 5% UF6 feed
α = 1.01 to 1.05
Processes in which isotopic composition changes are produced when a flowing gas mixture experiences large linear or centrifugal acceleration are termed aerodynamic processes.
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Mixture of about 95% H2 and 5% UF6 at a pressure of about 1 atm. is allowed to expand through a narrow nozzle (0.01 mm wide) in to a curved (0.1 mm radius) wall.
The high speed gas experiences forces 160 million times the gravitational force in the curved nozzle.
The gas stream coming out of the nozzle is divided into lighter and heavier fractions by a very sharp skimmer knife.
Separation factor depends on the configuration of the nozzle, type of diluent gas and its abundance in relation to UF6, the inlet's absolute pressure and expansion ratio of the heavy fraction.
About 740 stages are required to produce 3% enriched uranium.
Jet nozzle process
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Dilution of UF6 with hydrogen has two beneficial effects:
It helps to increase the speed of flow
It delays the establishment of a hypsometric distribution of Uf6 density. This delay reduces the re-mingling by diffusion of the isotopes of uranium already separated by the centrifugal forces.
Aerodynamic methods Nozzle separation process
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Thermal diffusion methods
When heat flows through a mixture initially of uniform composition, small diffusion currents are set up, with one component transported in the direction of heat flow, and the other in opposite direction. This is known as thermal diffusion effect. The effect is generally small. For example when a mixture of 50% hydrogen and 50 % nitrogen is held in temperature gradient between 260 and 10°C the difference in composition at steady state is only 5%. In isotopic mixtures the effect is even smaller.
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Thermal diffusion methods
Separation of molecules of different masses by radial diffusion in cylindrical columns due to temperature gradient across cylinder walls
Vertical separation due to temperature-induced convection currents
One of the methods first adopted in Manhattan project
Uranium was enriched to about 1% which was taken to electromagnetic separation for further enrichment
Solutions can also be enriched
Separation factor ~2
High energy incentive!!
SS
Cu
Ni
15 meters heightInner tube (Ni) at 2860COuter tube (SS) at 640CGap between Ni and Cu tubes ~ 1mmMaterial passes through this gapEquilibration time ~ weeks
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Electromagnetic separation methods
Mass spectrometric principle
Mono-energetic ion beams are deflected by magnetic fields to
different m/e charge ratios
M H re V
2 2
2=
Requirements
Ion source
Acceleration field
Magnetic field analyser
Suitable collectors
Efficient pumping system
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Very large separation factors possible
Production of large ion currents (space charge effects)
Strong stable magnetic fields
Suitable material for collectors (proper cooling)
Suitable for producing small amounts of isotopes
60 stable isotopes have been enriched
One of the first methods employed in Manhattan project in conjunction with thermal diffusion method
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Laser separation methods
Electronic levels of atoms and vibrational levels molecules differ marginally depending on the isotopic mass
e.g. Hydrogen spectrumR
n n2 21 2
1 1
υ = −
Rydberg constant R ech
= π µ2 43
2 µ = reduced mass =MM
M M+1 2
1 2
HH
H
mM
M mµ =
+D
DD
mM
M mµ =
+
For a given transition n1→→→→n2
H
D
H H
DD
R
R
µ= = µ
υυ For D, λλλλD - λλλλH = 0.1785 nm
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In the case of molecules, the fundamental frequency of a
diatomic moleculekυ= µπ
12
For different isotopes µ is different.
∆µ for lighter isotopes is large and for heavier isotopes small
By selecting a suitable wavelength it is possible to selectively
excite and ionise isotopic atoms
Uranium enrichment by lasers
Still at development stage
AVLIS: Atomic Vapour Laser Isotope Separation
MLIS: Molecular Laser Isotope Separation
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Karanam L. Ramakumar 40
AVLIS Process
Reservoir of uranium atoms by heating U metal
U atoms vapour pressure: a few torr
First U-235 atoms are selectively excited and then ionised by
another laser.
Ions are collected by electric or magnetic fields
235U*
Ground state
235U+
591.94 nm
210 - 310 nm
Xenon laser
Copper laser
Nitrogen laser
Dye laser
Nd-Yag laser
MLIS Process
235UF6 molecules are
selectively excited
with IR-laser.
Excited species are
irradiated with UV-
laser.
235UF6 →→→→ 235UF5 + F235UF5 is solid and is condensed
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Karanam L. Ramakumar 41
151133MLIS
170110AVLIS
350014001.012Nozzle
210671.25Gas centrifuge
210039201.00429Gas diffusion
Energy kwh/SWUStagesSeparation factorProcess
Performance of different processes for uranium enrichment
Feed: U-235 = 0.00711
Product U-235 = 0.9
Tails: U-235 = 0.002
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Karanam L. Ramakumar 42
Ion exchange enrichment of uranium isotopes
238UO2+2 + 235U(IV) � 235UO2
+2 + 238U(IV)
K = 1.0015
Ion exchange resin
Uranium loaded on column in H2SO4 medium
Repetitive oxidation, reduction carried out on the column
U(VI) is strongly absorbed
Many process conditions are classified
20 days of continuous operation yielded ~ 3% U-235
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Karanam L. Ramakumar 43
Separation of light isotopes
e.g. Deuterium from Hydrogen
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Karanam L. Ramakumar 44
Mass differences result in small but significant differences
in physico-chemical properties
1.106 g/cc0.991 g/cc------Density at 200C
201842Molecular weight
276.8K273K18.65K13.95KFreezing point
374.4K373K23.67K20.39KBoiling point
D2OH2OD2H2Property
Differences in the behaviour of isotopes due to mass
difference:
Diffusion
Evaporation
Mobility
Reactivity
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Karanam L. Ramakumar 45
Separation factors from vapour pressure ratios at boiling point
1.038-245.920Ne/22Ne
1.0046100H216O/H2
18O
1.004-195.8
1.029100
1.026100
1.036-33.6
1.81-252.9Ortho-H2/HD
αααα At boiling pointBoiling point (0C)Compounds
NH /ND33 3
H O/D O2 2
H O/T O2 2
N / N14 152 2
Conversion from ortho to para form should be minimised (large
power consumption!!!!)
No paramagnetic or ferromagnetic materials for construction!!
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Karanam L. Ramakumar 46
Distillation methods
Small differences in vapour pressure (boiling point) between the species
containing different isotopes
Separation factor x/( x)y/( y)
−α =−
11
x = atom fraction of desired isotope in liquid phase
y = atom fraction of desired isotope in vapour phase
AAB
B
πα = π
H2O + D2O ⇌⇌⇌⇌ 2HDO K = 4
H2O, D2O, HDO species in liquid phase
HDO D O H O HDO*
H O HDO HDO D O
x x y y
x x y y
+ +α =
+ +2 2
2 2
2 2
2 2x
HDO H O D Oy x x .x
Pπ
= =2 2
H O H
DD O
π π
π π2 2
22
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Karanam L. Ramakumar 47
P
Hydrogen rich gas,
depleted in D to ammonia
plant
Hydrogen
from
ammonia
plant
Recycle
compresso
r
First
coolin
g
refrigeratio
n
Secon
d
coolin
g
Feed
compresso
r
refrigeratio
n
First
cooling
and
water
removal
Second
cooling
and
nitrogen
removal
Joule-Thomson
coolingNormally
closed
Depleted liquid hydrogen
flux
Primary distillation tower
Generalised flow sheet for hydrogen distillation heavy water plants
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Karanam L. Ramakumar 48
⊗⊗⊗
HD-Free
hydrogen
low pressure
Secondary
towers
Exchange reactor
2HD ⇄ H2 + D2
Pure D2
Heat exchanger
HD
HD + H2
H2 + HD + D2
Pure D2HD-free hydrogen
high pressure
Primary
tower
Cold natural
hydrogen
0.028%HD
5.14%HD
Final concentration of deuterium by distillation of liquid hydrogen
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Karanam L. Ramakumar 49
≈≈≈≈
Electrolysis
Once-through process
≈≈≈≈≈≈≈≈
≈≈≈≈≈≈≈≈
Feed water
Partially enriched water
Counter-current process
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Karanam L. Ramakumar 50
F
E1 E2 E3
T1
600CT2 T3
BB
T
P
C1C2
C3C4
C5 C6
C7 C8
C9 C10
Three-stage cascade of electrolytic cells and exchange towers
T1, T2, T3 Exchange
towers
E1, E2, E3 Electrolytic
cells
F Feed water 10000
moles, 0.0148% D
T 9999 moles of
depleted water 0.005# D
B Burners
P 0.982 moles of Product
99.8% D
C1 0.0598% D, C2 0.0501% D, C3 2.013% D, C4 1.818% D, C5 98.89% D, C6 98.81%
D C7 491.92 moles of 0.101% D, C8 13.818 moles of 3.618% D, C9 492.9 moles of
0.300% D, C10 14.80 moles of 10.0% D
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Karanam L. Ramakumar 51
Chemical exchange methodsHD + H2O(l) � H2 + HDO(l) Catalyst Pt or Ni
K = 3.78 at 250C (Separation factor)
HDS(g) + H2O(l) � H2S(g) + HDO(l)
K = 2.32 at 320C
HD(g) + NH3(l) � H2(g) + NH2D(l) Catalyst KNH2 in NH3
K = 3.60 at 250C (ammonia plant needed!!!)
Water-hydrogen exchange reaction needs catalyst. Finely
divided Pt or Ni
Wetting of catalyst inhibits catalytic exchange
Water has to be in vapour form
Alternatively hydrophobic catalysts may be used.
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Karanam L. Ramakumar 52
Depleted water Waste
SulphurAir
Sulphur
recovery
unit
H2S + ½ O2 →H2O + S
H2S
320C
α = 2.32
Water
Heavy water
product
D2S
generator
Al2S3
producer
2Al + 3S → Al2S3
3D2O + 2Al2S3 → 3D2S + Al2O3
Al2O3 Al
Mono-thermal water – H2S
exchange
G.S. Process
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Karanam L. Ramakumar 53
Feed:Natural water
Cold towerT = 320Cα = 2.32
Hot towerT = 1280Cα = 1.80
D2S flow
D2O flowProduct
Recycle D2S
Depleted water
Blower
Dual temperature Water – H2S ExchangeGirdler – Spevack Process (GS Process)
Heat exchangers
Dual temperature exchange or bi-thermal exchange processAvoids reconverting the products into initial reactants to achieve the multiplication effect in the separation factor. Basis: Temperature dependent property of the equilibrium constant for the exchange reaction.
H2O(l) + HD(g) < = = > HDO(I) + H2(g)Keq = 3.78 at 25
0C and 2.60 at 800C
H2O(l) + HDS(g) < = = > HDO(l) + H2S(g), Keq = 2.32 at 32°C and 1.30 at 138°C.
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Karanam L. Ramakumar 54
Ion Migration
Slight differences in velocities of isotopic ions in solution under an electric field
These small differences are due to the different sizes and masses of the ions.Contributions due to differences in the degree of dissociation and in complex formation also to be considered.Ion migration can occur not only in aqueous media where the ions are invariably hydrated but also in fused salt media where the ions are relatively more free from solvation effects.
Advantage of the fused salt medium: Absence of ion solvation resulting in larger mass effects in the
migration of isotopic ions.
Separation factor αααα ∆∆∆∆v/v ∆∆∆∆v is difference in velocities between the isotopes and v is the mean velocity
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Karanam L. Ramakumar 55
v/v-m/m
∆µ =∆
The extent of separation effect between the two isotopes can also be expressed in terms of relative mass effect given as
where m is the mass of the ion. Thus the actual enrichment factor is somewhat less than expected when only velocities are considered. More over, whileelectromigration builds up a concentration gradient along the field direction, the reverse flow of the electrolyte due to diffusion tends to neutralise the effect partially.
In a typical example, 39K and 41K were separated by theelectromigration of potassium chloride solution in a U-shaped tube using platinum gauge electrodes,
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Karanam L. Ramakumar 56
1. K. Cohen, The theory of isotope separation, Mc Graw Hill, New York (1951)
2. M. Benedict and T.H. Pigford, Nuclear chemical engineering, Mc Graw Hill, New York (1965)
3. H. London, Separation of isotopes, George Newnes, London (1961)
4. S. Villani, Isotope separation, Amer. Nucl. Soc., Hinsdale, (1976)
5. H.J. Arnikar, Isotopes in atomic age, Wiley Eastern, New Delhi, (1989)
6. J. Koch(Ed.), Electromagnetic isotope separators and applications of magnetically enriched isotopes, Interscience, New York (1958)
7. G.M. Murphy(Ed.), Production of heavy water, Mc GrawHill, New York (1955)