Separation Of Stable Isotopes

Methods for the separation of stable isotopes

Karanam L. Ramakumar India

Karanam L. Ramakumar

1

Isotopes of an element have very similar chemical properties 235U O 238U O e.g. and 3 8 3 8 reactivity is nearly identical

Chemical

They behave as completely substances in nuclear reactions

different

e.g. not

Karanam L. Ramakumar

235U

is a fissile isotope while

238U

is

2

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

Karanam L. Ramakumar

3

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’ Karanam L. Ramakumar

4

Separation Factor in a Typical Enrichment Process Two types of separation factor (i) separation factor (α α) and

α= y/(1-y) z/(1-z) (ii) enrichment factor (ε)

ε= y/(1-y) x/(1-x)

Feed

Product

z, (1-z)

y, (1-y)

Tails x, (1-x)

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 Karanam L. Ramakumar

5

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)   x -x  x -x  

The ratio of products flow rate to feed flow rate is called “cut” θ

θ= Product flow rate = P F Feed flow rate 1 - θ = W/F





W=P  x p-x f 





 

     



F=P  xp-x w 

xF-xw  θ= x -x  P w 

f

w

 

f

w

Fraction of desired component in products stream is called recovery “r” r = yzθ = 1 − x(1z−θ)

Cut (θ θ ) for a given enrichment cascade is optimised Karanam L. Ramakumar

6

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 Karanam L. Ramakumar

7

Cascade

Stage y1

y2

x1 Z1 Feed

Unit

y1

y3

x2 y2

x1 y1

x3

x2

x1

Concept of Unit, Stage and cascade

Karanam L. Ramakumar

8

Square Cascade Feed

Waste

Product

Karanam L. Ramakumar

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 of separative work. 9

Simple cascade Feed Waste

Waste

Waste

Product

Karanam L. Ramakumar

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. 10

Counter current or recycle cascade As (α-l) < < < 1, in most of the cases, these are also known as close-separation cascades. Feed

Feed for each stage consists of heads from the next lower stage and wastes from the next higher stage The most commonly employed cascade

Waste

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

Product

Karanam L. Ramakumar

In a symmetric counter-current cascade, the waste stream is recycled back to the immediately preceding stage. In an asymmetric countercurrent cascade, the waste is recycled more than one stage back. 11

Separation of heavy isotopes e.g.

Karanam L. Ramakumar

235U

from

238U

12

Concept of Separative Power, Separative capacity and Separative 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 Karanam L. Ramakumar

13

Separative power or Separative capacity A combined function of quality & quantity of separation performed by a separating element or a plant It 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:  x   V(x)=(2x-1)ln  1-x   

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:

W

SWU

Karanam L. Ramakumar

= P.V(xp) + T.V(xt)-F.V(x ) f

14

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)      

xp-xw  F=P x -x  w     

f



xF-xw  Cut θ= x -x  P w

     

xp-x  W=P x -x f  w f



    

xP-xF  (1-θ) = x -x  P w



     







   x xp   x  w f   -F(2x -1)ln WSWU =P(2xp-1)ln  +W(2xw-1)ln    f 1-xp  1-x 1-xw  





f



     

      WSWU x  xp   x   W(2x -1)ln w  - F(2x -1)ln f  + =(2xp-1)ln w f 1-x  1-x  P 1-xp  P P w   f 

Karanam L. Ramakumar











15

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: Case # Case 1: Xf = 0.00711, Xp = 0.9, Xw = 0.002 Case 2: Xf = 0.00711, Xp = 0.9, Xw = 0.003

F(kg)

SWU

176

229

218

193

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. Karanam L. Ramakumar

16

Gaseous Diffusion Process

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, α, is given by

α=[M /M ]1/2 2

Karanam L. Ramakumar

1

17

Gaseous diffusion process Feed z, M1, p1

Product y

1-z, M2, p2

(1-y) Porous membrane

Rate of diffusion (D) α

1 Density

1 (D) α Mass

No. of molecules crossing the barrier α pressure x diffusion rate J α P.D

(p α z)

y = J1 α . z.

1

1 /2

M1

(1-y) =J2 α (1-z) y = z M2 1−y 1−z M 1

Karanam L. Ramakumar

1

1 /2

M2

α=

M2 M1 18

Separation of U-235 from U-238 by gaseous diffusion Feed : UF6

M1 = 235UF6 = 349 M2 = 238UF6 = 352

Separation factor

   y /(1 − y)     z /(1 − z)   

M α 2 M1

    



352  = 1.00429  349 

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 Karanam L. Ramakumar

19

Natural uranium U-235 : 0.00711% Product U-235 : 0.03%    x (1 − x ) w  Tails U-235 : 0.002% n = 2 ln p α−1  xw(1 − xp) 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 Complete elimination of air leakage inside the process Seals and gaskets system!! Diffusion cells Diffusion membranes Compressor materials Karanam L. Ramakumar

20

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 Karanam L. Ramakumar

21

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 Karanam L. Ramakumar

22

Product Wt. Fr. U-235 = 0.03 Stage 1275

~ ~

~ ~

A

A

Natural U feed Wt.Fr. U-235 =0.00711

~ ~

Stage 594

~ ~

0.007125

A Cooler ~ ~ Compressor

Control valve

0.007095

~ ~

Stage 1

Tails Wt.Fr. U-235 = 0.002

Ideal Gas Diffusion Cascade Karanam L. Ramakumar

23

Centrifugal Methods Separation factor α

     

 (M2-M1)ω2a2  2 α=exp (1-r 2 ) a  2RT 

M1 and M2 are the masses of the lighter and heavier isotopes ω is the peripheral velocity of the molecules a 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 α increases with the length of the rotor, the peripheral speed and also with the radius. α depends on the difference between the masses Better 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 Karanam L. Ramakumar

24

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  (M2-M1)ω2a2 2  r α =exp (1- ) a2  2RT

Karanam L. Ramakumar

     



25

Pressure gradient of the gas Ph = P0 exp (-Mgh/RT) For two masses M1 and M2 (M1 < M2)      

P1   P1   =  exp[−(M1 − M2)gh/RT]   P2   P2  h



o

Pressure gradient between the axis and the wall Pa <<<<< Pw

Separation factor depends on mass difference Separation factor same for same mass difference (light and heavy elements!!)

Lighter isotope accumulates near the axis Heavier isotope accumulates near the wall Karanam L. Ramakumar

26

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. Karanam L. Ramakumar

27

Separation factor does depend on the angular velocity (peripheral speed of the rotor) Peripheral speed m/s

Separation factor Pressure ratio between axis and wall

400

1.0975

5.5 x 104

500

1.156

2.5 x 107

600

1.233

4.5 x 1010

700

1.329

3.3 x 1014

Maximum velocity is limited by tensile strength of the rotor ρω2r2 Aluminium alloys Titanium alloys High tensile steels Polyamides Karanam L. Ramakumar

T.S. >

28

Advantages of gas centrifuge processes of enrichment

over

gas

diffusion

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. Karanam L. Ramakumar

29

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. Karanam L. Ramakumar

30

Aerodynamic methods Nozzle separation process Processes in which isotopic composition changes are produced when a flowing gas mixture experiences large linear or centrifugal acceleration are termed aerodynamic processes. Product

Feed

Tails

S = 0.03 mm A = 0.1 mm 95% H2 + 5% UF6 feed α = 1.01 to 1.05 Karanam L. Ramakumar

31

Jet nozzle process 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. Karanam L. Ramakumar

32

Aerodynamic methods Nozzle separation process 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.

Karanam L. Ramakumar

33

Thermal diffusion methods

Karanam L. Ramakumar

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. 34

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!! Karanam L. Ramakumar

SS Cu Ni

15 meters height Inner tube (Ni) at 2860C Outer tube (SS) at 640C Gap between Ni and Cu tubes ~ 1mm Material passes through this gap Equilibration time ~ weeks 35

Electromagnetic separation methods Mass spectrometric principle Mono-energetic ion beams are deflected by magnetic fields to different m/e charge ratios

M = H2r2 e 2V Requirements Ion source Acceleration field Magnetic field analyser Suitable collectors Efficient pumping system Karanam L. Ramakumar

36

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

Karanam L. Ramakumar

37

Laser separation methods Electronic levels of atoms and vibrational levels molecules differ marginally depending on the   isotopic mass   e.g. Hydrogen spectrum 2 4 Rydberg constant R = 2π e µ ch3 mMH µH = MH + m

υ = R  1 − 1  2 n2  n 1 2 



M1M2 µ = reduced mass = M1 + M2 mMD µD = MD + m

For a given transition n1→ →n2

υH = RH = µH υD RD µD Karanam L. Ramakumar

For D, λD - λH = 0.1785 nm 38

In the case of molecules, the fundamental frequency of a diatomic molecule

υ= 1 k 2π µ

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 Karanam L. Ramakumar

39

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 Xenon laser 235U+ MLIS Process Copper laser 210 - 310 nm Nitrogen laser 235UF6 molecules are 235U* selectively excited Dye laser 591.94 nm Nd-Yag laser with IR-laser. Excited species are Ground state irradiated with UVlaser. 235UF 5 Karanam L. Ramakumar

is solid and is condensed

235UF 6

→ 235UF5 + F 40

Performance of different processes for uranium enrichment Process

Separation factor

Stages

Energy kwh/SWU

Gas diffusion

1.00429

3920

2100

Gas centrifuge

1.25

67

210

Nozzle

1.012

1400

3500

AVLIS

10

1

170

MLIS

33

1

151

Feed: U-235 = 0.00711 Product U-235 = 0.9 Tails: U-235 = 0.002

Karanam L. Ramakumar

41

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 Karanam L. Ramakumar

42

Separation of light isotopes e.g. Deuterium from Hydrogen

Karanam L. Ramakumar

43

Mass differences result in small but significant differences in physico-chemical properties Property

H2

D2

H2O

D2O

Boiling point

20.39K

23.67K

373K

374.4K

Freezing point

13.95K

18.65K

273K

276.8K

Molecular weight

2

4

18

20

Density at 200C

---

---

0.991 g/cc

1.106 g/cc

Differences in the behaviour of isotopes due to mass difference: Diffusion Evaporation Mobility Reactivity Karanam L. Ramakumar

44

Separation factors from vapour pressure ratios at boiling point Compounds

Boiling point (0C)

α At boiling point

Ortho-H2/HD

-252.9

1.81

-33.6

1.036

H2O /D2O

100

1.026

H2O / T2O

100

1.029

-245.9

1.038

-195.8

1.004

100

1.0046

3 NH /ND 3 3

20Ne/22Ne 14N

2

/ 15N2

H216O/H218O

Conversion from ortho to para form should be minimised (large power consumption!!!!) No paramagnetic or ferromagnetic materials for construction!! Karanam L. Ramakumar

45

Distillation methods Small differences in vapour pressure (boiling point) between the species containing different isotopes Separation factor

α = x /(1 − x) y /(1 − y)

x = atom fraction of desired isotope in liquid phase y = atom fraction of desired isotope in vapour phase

πA αAB = π B

πH O

πH

πD O

πD

2

H2O + D2O ⇌ 2HDO K = 4

2

2 2

H2O, D2O, HDO species in liquid phase α*

=

    xHDO + 2xD O   2yH O + yHDO  2  2    2x  y  2 + x + y H2O HDO   HDO D2O     

Karanam L. Ramakumar

y=

πx P

xHDO = xH O.xD O 2

2

46

Hydrogen rich gas, depleted in D to ammonia plant

Recycle compresso r Normally closed

Hydrogen from ammonia plant

Feed compresso r

Depleted liquid hydrogen flux

First refrigeratio Secon d coolin n coolin g g Joule-Thomson cooling

First refrigeratio Second cooling n cooling and and water nitrogen removal removal

P

Primary distillation tower

Generalised flow sheet for hydrogen distillation heavy water plants Karanam L. Ramakumar

47

HD-Free hydrogen low pressure

Secondary towers

⊗

Cold natural hydrogen 0.028%HD

Exchange reactor 2HD ⇄ H2 + D2

HD + H2

⊗

⊗

Pure D2

Heat exchanger

Primary tower HD

H2 + HD + D2 5.14%HD

HD-free hydrogen high pressure

Pure D2

Final concentration of deuterium by distillation of liquid hydrogen Karanam L. Ramakumar

48

Counter-current process

Electrolysis Once-through process Feed water

≈

≈

Partially enriched water

≈ ≈

Karanam L. Ramakumar

≈

49

Three-stage cascade of electrolytic cells and exchange towers C7

C8

T1, T2, T3 Exchange towers

T

F

B

T1 600C

E1, E2, E3 Electrolytic cells F Feed water 10000 moles, 0.0148% D

T3

T2

T 9999 moles of depleted water 0.005# D

C1

C3

C2

E1

B

C9

C4

E2

C10

C5

B Burners

C6

P E3

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

Karanam L. Ramakumar

50

Chemical exchange methods HD + 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.

Karanam L. Ramakumar

51

Depleted water Waste Sulphur H2S + ½ O2 →H2O + S

Air

Sulphur recovery unit

320C α = 2.32 H2S

Water

Mono-thermal water – H2S exchange 2Al + 3S → Al2S3

Heavy water product

D2S generator

Al2S3 producer Al2O3

G.S. Process

Al

3D2O + 2Al2S3 → 3D2S + Al2O3

Karanam L. Ramakumar

52

Feed: Natural water

Recycle D2S Blower

Cold tower T = 320C α = 2.32

Depleted water

Product

D2S flow D2O flow Heat exchangers

Hot tower T = 1280C α = 1.80

Dual temperature Water – H2S Exchange Girdler – Spevack Process (GS Process)

Karanam L. Ramakumar

Dual temperature exchange or bi-thermal exchange process Avoids 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 250C 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. 53

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 α ∆v/v ∆v is difference in velocities between the isotopes and v is the mean velocity Karanam L. Ramakumar

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The extent of separation effect between the two isotopes can also be expressed in terms of relative mass effect given as

-µ = ∆v /v ∆m/m

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, while electromigration 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 the electromigration of potassium chloride solution in a Ushaped tube using platinum gauge electrodes,

Karanam L. Ramakumar

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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 Graw Hill, New York (1955) Karanam L. Ramakumar

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