Lecture Notes On Separation Of Stable Isotopes

Methods for the separation of stable isotopes Karanam L. Ramakumar India (This lecture notes was initially meant for the participants of Workshop on “Radiochemistry and Application of radioisotopes” regularly organized by Indian Association of Nuclear Chemists and Allied Scientists (IANCAS), a non-profit organization based in Mumbai, India. Major portions of this document were included in the IANCAS lecture notes brought out in 1992 and subsequently in a book “Principles of Radiochemistry” published by IANCAS in 1993-94. This enlarged document is uploaded for interested readers. Some figures were downloaded from Internet.)

Introduction Although the isotopes of an element have very similar chemical properties, they behave as completely different substances in nuclear reactions. In addition, many of the stable isotopes find wide spread applications in chemical, industrial, agricultural and clinical research to elucidate and understand reaction pathways, mechanisms and kinetics, effect of trace elements on physico-chemical properties, up-take and plant metabolism studies and the behaviour of trace elements from toxicological and human metabolism point of view respectively. Table 1 lists separated isotopes that are being produced on a significant industrial scale for various applications. Table 1 Separated isotopes and their uses Isotope U 2 H

Abundance in nature (at%) 0.7205 0.015

6

7.5

7

92.5

10

19.61 1.107 0.366 0.037 0.204 2.20

235

Li Li

B C 15 N 17 O 18 O 57 F 13

Use Fuel for nuclear fission reactors D2 moderator for natural uranium reactors Fuel for thermonuclear reactors Source for tritium Fuel for thermonuclear reactors As LiOH, water conditioner for water cooled reactors As metal, possible high temperature reactor coolant Neutron absorber in control rods Isotope tracer in living systems Nuclear magnetic resonance studies of molecular structure Mossbauer studies

In addition to these, separated isotopes of practically all natural elements are being produced for applications in research. Very small differences that are manifested in the Physico-chemical properties of isotopes of element, because of different masses, such as the boiling points, diffusion coefficients, rate of electrolysis, and difference in optical properties are

exploited to achieve their separation. Almost all the methods used for separation in conventional industries can be used for separation of isotopes also with varying degree of success. The methods can be broadly classified into physical and chemical methods. Physical methods are those in which difference in physical properties e.g. inertia, density, electrical properties etc. Chemical methods are those in which difference in chemical properties e.g. chemical potential is harnessed Table 2 lists some of the processes used to separate isotopes. Table 2 Different methods employed in isotope separation Method Electromagnetic Electrolysis Distillation Chemical exchange Ion migration Gaseous diffusion Thermal diffusion Gas centrifuge Aerodynamic Photochemical Ion exchange

Applied to U, all others D, Li D, 10B, 13C, 15N, 18O D, Li, 10B, 13C, 15N, 18O Li 235 U 18 O, 15N, inert gas isotopes 235 U 235 U 235 U, other isotopes Lighter isotopes, 235U 235

Chemical methods are more suitable for light isotopes and are extensively used for separation of hydrogen isotopes e.g. production of heavy water. Physical methods are in general more suitable for separation of heavy isotopes e.g. uranium enrichment.

Separation Factor in a Typical Enrichment Process Irrespective of the process employed, the extent of separation of isotopes is given by “separation factor". Two types of separation factor are generally in vogue; (i) separation factor (α) and (ii) enrichment factor (ε). The two separation factors are a measure of the efficiency and capability of the particular process in its basic operation to achieve the separation of isotopes. Separation factors are different for different elements for the same process and also different for same elements in different processes. The sample material, which is fed into an enrichment plant, is known as feed. After enrichment, a portion of the sample material will be obtained as the enriched fraction in the desired isotope. This fraction is known as product. Correspondingly there will be another fraction known as tails, which are depleted in the desired isotope. The two separation factors are given as follows:

α=

y /(1 - y) z /(1 - z)

ε=

y /(1 - y) x /(1 - x)

where 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. These separation factors are

derived from different mass dependent physico-chemical properties of the isotopes of the elements based on which the separation process is selected. Table 3 gives representative separation factors and the physico-chemical property for different isotope separation processes. Table 3 Separation factors in typical enrichment processes Process Distillation Mono-thermal chemical exchange Gaseous diffusion Gas centrifuge

Property R K

(m2 /m1 )

Separation factors for 14 H2 – HD N – 15N 1.5 1.033 3.6 1.055

1.225

(m2-m1)v2 1.056 e 2RT R = relative volatility, K = equilibrium constant

235

UF6 – 238UF6 1.00002 1.0016

1.017

1.00429

1.056

1.162

A value of α close to unity indicates that the separation is difficult, a value far from unity, easier. For gaseous diffusion of UF6 for example, α is so close to unity (I.00429) that the processes must be repeated many times for useful degree of separation. This is achieved by employing stages and cascades.

Separating Unit, Stage and Cascade The smallest element of an isotope separation plant that effects some separation of the process material is called the separating unit. Examples of a single separating unit are one gas centrifuge, or one electrolytic cell etc. No single separating unit can enrich uranium from its natural concentration of about 0.7 percent uranium-235 to the 3 to 5 percent required to fuel a nuclear reactor. In addition, the throughput of a unit is 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. A group of parallely connected separating units, all fed with material of same composition and producing partially separated product streams of the same composition, is known as a stage. A series-connected group or stages is known as a cascade. Examples of a cascade are a complete distillation column, or a battery of solvent extraction mixed-settlers, or series of electrolytic cells etc. A cascade that has the same number of units in all stages or in a group is known as squared-off cascade. A cascade in which number of units in each stage decreases as the product and waste ends of the cascade are approached is called a tapered cascade. A single multi-plate distillation column is an example of a squaredoff cascade; a gaseous diffusion plant for uranium separation is an example of a tapered cascade. The pattern of that connection is determined by the properties of the individual separating units and the required quantity and concentration of the final product. An enrichment plant usually holds thousands of cascades.

The cascade theory of isotope separation was developed by P.A.M.Dirac, and R.Peierls in England and by K.Cohen and I.Kaplan in the United States. The cascade theory is an involved one and has been extensively covered in the references (1) and (2). Cascades are generally of two types. Simple cascade: a cascade in which no attempt is made to reprocess the partially depleted waste streams leaving each stage is called a simple cascade. In the simple cascade 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.

Feed

Waste

Waste

Waste

Product Fig.1 An example of simple cascade Counter current or recycle cascade: a cascade in which the feed for each stage consists of heads from the next lower stage and wastes from the next higher stage is called a counter current or recycle cascade. This is by far the most commonly employed cascade type. In general as (α-l) < < < 1, in most of the cases, these are also known as close-separation cascades. A recycling cascade has two sections: the enriching section, consisting of the stages above the point at which the feed enters the cascade and the stripping section below the feed point. The stripping section increases the recovery of the material, whereas the enriching section produces material of increased concentration. 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. A common

asymmetric cascade feeds the product stream into the next stage and sends the tails stream back two stages back. This is called a two-up, one-down cascade. Asymmetric cascades were used in aerodynamic nozzle separation processes. While asymmetric cascades are rarely used for gas centrifuges, some argue that an asymmetric cascade can increase centrifuge cascade performance by about ten percent relative to a symmetric arrangement.

Feed

Waste

Product Fig.2 An example of tapered counter-current symmetric cascade An ideal cascade, a type of tapered cascade, is the most efficient arrangement of separating units. It minimizes the total inter-stage flow rate and as a result uses the least number of units to achieve the required separation by maximizing the separative work per separating element. An ideal cascade has two important characteristics: it has the same separation factor for each stage and streams of different levels of enrichment are never mixed. The first condition makes the use of identical units possible throughout the cascade. The no mixing condition lowers energy consumption because no separative power is wasted by mixing flows of different concentrations. To achieve this, the cuts of the stages are adjusted so the waste flow passed down to a stage has exactly the same isotopic concentration as the product flow being passed up from the preceding stage. The ideal cascade has the shortest equilibrium time and the smallest inventory. An ideal cascade is never achieved in practice. Not every machine will perform exactly the same so the no-mixing rule is broken even within a stage. Theoretically, the cut must be varied for each stage to keep different concentrations from mixing. For an ideal cascade, the cut fluctuates very slightly around 0.5 but, in practice, these small fine adjustments in flow

rates cannot be achieved. Moreover, the theoretically optimal flow rate for every stage may correspond to a non-integer number of machines because units are optimized for a particular cut and throughput. Obviously, it is not possible to operate a fraction of unit, so separating units will either have to work at non-optimum conditions to balance flow rates or there will be mixing of different isotope concentrations and, therefore, wasted separative work. In practice, engineering tradeoffs among flow rate, cut, and separation factor are made to bring a real cascade as close as possible to ideal performance.

Feed

Waste

Product Fig.3 An example of squared-off cascade A square cascade has the same flow rates in all stages and therefore the same number of machines per stage. Square cascades are 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. The overall performance characteristics of enrichment plant are expressed in terms of two factors: Separation capacity and separative work. The separation capacity is a measure of the rate at which the cascade performs separation. In many isotope separation plants the initial cost of plant is proportional to the separative capacity the plant. Separative capacity has the dimensions of the flow rate (Kg of material/day). It is also useful to have a measure of the amount of separation performed by a cascade. This measure is provided by the separative work units (SWU). In an enrichment plant, the annual operating costs are proportional to the amount of separative work done per year. Separative work unit, S has the dimensions of the amount of material E. More number of SWUs is required to produce 1 Kg. of 90% compared to 1 Kg. of 3%

235

UF6 as

235

UF6. Accordingly cost of enriched uranium increases with the total

number of SWUs and enrichment value. As an example, to get 1 Kg of 90 % enriched

235

UF6

we need 178 Kg of natural uranium in UF6 feed. To get 1Kg of 3% enriched

235

UF6 we need 5.6

Kg of natural uranium in UF6 feed. Concept of Separative Power, Separative capacity and Separative Work Unit In conventional industrial separations, where the level of separation is almost 100% (or some standard level), 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. However it may be observed that unlike in heavy water production, varied degree of separation of isotopes is required in case of uranium. These varying levels of enrichment required for different applications ranging from 3% for LWRs to 90% & above for weapon grade is typical of uranium isotope separation. So when the enrichment levels are different in different plants then a question arises as to how to compare the capacities of two different plants. Because 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. Therefore an additional parameter, which would be a combined function of quality & quantity of separation performed by a separating element or a plant is defined. At the same time it should be independent of the level of concentration of feed material. This additional parameter is called separative power or separative capacity particularly for uranium enrichment. The separative power is defined as 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 is expressed in terms of the number of separative work units needed, given by the expression:

WSWU = P.V(xp ) + T.V(xt ) - F.V(xf )

It should be noted that SWUs required for enrichment increase with decreasing levels of desired isotope in the waste streams. But the amount of feed material needed will decrease with decreasing levels of the desired isotope in the waste streams. Thus they operate in the opposite direction. For example in uranium enrichment we have the following expressions: 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   xf − xw 

 xp −xf  W = P   xf −xw 

WSWU function after substituting for value function is written as

 xp   xw   xf  WSWU = P(2xp -1)ln   + W(2x w -1)ln   - F(2xf -1)ln   1-xw   1-xf   1-xp  SWU required per kg of product (P) produced is given by

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

It is seen that 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. Various methods are employed for the separation of stable isotopes. Among these, gaseous diffusion, gas centrifuge, aerodynamic methods, distillation, electrolysis and chemical exchange methods, electromagnetic separation, thermal diffusion, laser methods of separation and ion migration are described in this chapter.

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 = [M2 / M1 ]1 / 2 This ideal value can be realised only at the beginning of the diffusion process. As diffusion proceeds, the concentration of the lighter constituent in the diffusate increases and there is a tendency for back diffusion to occur. As a result, the effective value of the separation factor decreases. On the other hand, if diffusion were allowed to proceed long enough, the composition of the gas would remain same on both sides of the barrier. In practice, about half of the gas is allowed to pass through the barrier. As the value of α is so close to unity. to obtain a useful degree of separation. the process must be repeated many times in a counter-current cascade of gaseous diffusion stages. Obviously the feed should be available in the form of gas, which should be compatible with the materials used. Otherwise special precautions (as in the case of highly corrosive gas UF6) have to be taken.

Tails

Pump A

B

C

D

Feed

Product

Fig.3 Typical diffusion counter-current cascade

Diffusion barrier materials should have pore sizes between 0.01-0.05 µm. They should be chemically inert towards the operating gas. One way in which such barriers could be made is by etching a thin sheet of silver-zinc alloy with HCl acid. The acid would then dissolve out zinc atoms leaving a large number of pores in the sheet of metal. Other materials like sintered alumina, nickel, anodically oxidised aluminium, teflon powder pressed into nickel gauge etc. can also be used.

Gas Centrifugal Methods The use of centrifugal fields for isotope separation was first suggested in 1919; but efforts in this direction were unsuccessful until 1934, when J.W. Beams and co-workers at the University of Virginia applied a vacuum ultracentrifuge to the separation of chlorine isotopes. 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 walls and up around the central axis. It should then be possible to draw off a product richer in the lighter isotope at the top of the apparatus, near the centre, whereas the heavier species would be removed at the bottom near the periphery. The separation factor α for centrifugal method along the radius is given by

r2  (M -M )ω2a2 α= exp  2 1 (1- 2 2RT a 

 ) 

where 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 and r is the radius at any given location in the centrifuge. R is the molar gas constant and T is the absolute temperature. The separation factor increases with the length of the rotor, the peripheral speed and also with the radius. It may be seen that α depends on the difference between the masses and not on their ratio. Thus better separation would be obtainable, in principle, of the

235

U and

238

U isotopes of

uranium than of the isotopes of hydrogen with masses 1 and 2. Also, 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. A long, thin vertical cylinder made of material with high strength and relatively low density is used in the construction of centrifuges. In a typical centrifuge rotating at a peripheral speed of 500 m/s, the abundance ratio of heavier isotope to lighter isotope with mass difference = 3 at the outer radius is greater than at the centre by a factor of 1.162 and the pressure at the outside is greater than at the centre by a factor of 4.6 x 107. Countercurrent flow between the stream near the outer radius and the stream near the axis is induced either with the help pf internal scoops and baffles or by establishing a temperature gradient along the wall. This leads to higher separating factors resulting in higher enrichment. The main subsystems of the centrifuge are (1) rotor

and

end

caps;

(2)

top

and

bottom

bearing/suspension system; (3) electric motor and power supply (frequency changer); (4) center post, scoops and baffles; (5) vacuum system; and (6) casing. Because of the corrosive nature of UF6, all components that come in direct contact with UF6 must be must be fabricated from, or lined with, corrosionresistant materials. The

materials

used

for

constructing

the

centrifuge should withstand tremendous centrifugal forces resulting due to high speed revolutions of the vertical

cylinder

(50000

to

100000

rpm).

The

peripheral speeds reached under these conditions are more than 500 m/s. The primary limitation on rotor wall speed is the strength-to-weight ratio of the rotor material. The materials therefore should have high tensile strength, low density and high modulus of elasticity. Further, the material should be inert to the gas being centrifuged. Aluminium alloys, high tensile steels, titanium, glass fibres are some of the materials preferred. Another limitation on rotor speed is the lifetime of the bearings at either end of the rotor. Rotor length is limited by the vibrations a rotor experiences as it spins. The rotors can undergo vibrations similar to those of a guitar string, with characteristic frequencies of vibration. Balancing of rotors to minimize their vibrations is especially critical to avoid early failure of the bearing and suspension systems. Because perfect balancing is not possible, the suspension system must be capable of damping some amount of vibration. The casing is needed both to maintain a vacuum and to contain the rapidly spinning components in the event of a failure. If the shrapnel from a single centrifuge failure is not contained, a “domino effect” may result and destroy adjacent centrifuges. A single casing may enclose one or several rotors.

One of the key components of a gas centrifuge enrichment plant is the power supply (frequency converter) for the gas centrifuge machines. The power supply must accept alternating current (ac) input at the 50- or 60-Hz line frequency available from the electric power grid and provide an ac output at a much higher frequency (typically 600 Hz or more). The high-frequency output from the frequency changer is fed to the high-speed gas centrifuge drive motors (the speed of an ac motor is proportional to the frequency of the supplied current). The centrifuge power supplies must operate at high efficiency, provide low harmonic distortion, and provide precise control of the output frequency. Although the separation factors obtainable from a centrifuge are large compared to gaseous diffusion [ranging from 1.01 to over 1.10], several cascade stages are still required to produce even LEU material. Separation factors depend on the absolute mass difference between isotopes (not the mass ratio) and the square of the peripheral speed. Separation factors for U-235/238 range from 1.026 for a 250 m/sec centrifuge to over 1.233 for a 600 m/sec centrifuge. The power consumption of a centrifuge plant per unit separative capacity is less than that of a gaseous diffusion plant. For a given mass difference between the isotopes, the stage separation factor is more than in gaseous diffusion plant. This means far fewer stages in series would be needed with the centrifuge to arrive at a given enrichment. For example, to get 3 % enriched uranium, 13 stages are needed as compared to about 1300 stages required in the gaseous diffusion plant. This advantage is however, partly off set by a lower yield per stage compared to the process of gaseous diffusion. This means a large number of centrifuges have 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 100-120,000 SWU is required to enrich the annual fuel loading for a typical 1000 MWe light water reactor. Advantages of gas centrifuge over gas diffusion processes of enrichment 1. Higher separation factor hence requires less number of stages. 2. 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. 3. 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. 4. 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.

5. 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 leadtime. 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. 6. 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.

Aerodynamic Process Processes in which isotopic composition changes are produced when a flowing gas mixture experiences large linear or centrifugal acceleration are termed aerodynamic processes. These processes are characterised by high speed flow of gases along a stream lined curvature. Again, like in a centrifuge, the heavier species tend to concentrate on the outer side of the curvature. One of the important aerodynamic processes is the jet nozzle separation process. This process has basically been developed for uranium enrichment. In the jet nozzle process, a 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. The 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. The separation factor is higher for (i) higher pressure ratios between the inlet and outlet streams and (ii) lower expansion ratio (flow rate of the light fraction to that of the feed). Separation factor for the dimensions given above is around 1.015 for U isotopes. About 740 stages are required to produce 3% enriched uranium. Dilution of UF6 with hydrogen has two beneficial effects: it helps to increase the speed of flow and it delays the establishment of a hypsometric distribution of Uf6 density. This delay reduces the remingling by diffusion of the isotopes of uranium already separated by the centrifugal forces.

The Electromagnetic Method Electromagnetic separation of isotopes of elements makes use of principle underlined in mass spectrometry. In short, whenever an ion of charge ne (n is number of charges) and mass m (amu) is accelerated in an electric potential of V (volts), it gains kinetic energy, which is given by

K.E. =

1 mv2 = neV 2

where v is the velocity of ions. If these accelerated ions pass through a magnetic field of strength H (Tesla), in a direction normal to the application of magnetic field, then a force equivalent to Hev is experienced by the ions. The direction of this force is mutually perpendicular to the direction of the ions as well as magnetic field. This force is balanced by the centripetal force given by mv2/r. So we have

Hev =

mv2 r

Where r is the radius of the path described by ions under the influence of the magnetic field. Eliminating velocity (v) term from equations 1 and 2, we get the well known mass spectrometric equation

m H2r2 = ne 2V Thus, ions of different masses but with same kinetic energy and charge describe different radii in the applied magnetic field and can be resolved according to their mass to charge ratio. For a given constant magnetic field and kinetic energy,

m α r2 ne Electromagnetic separation of isotopes is therefore possible. Historical Perspective Electromagnetic separation of uranium isotopes was one of the three major processes considered by USA under the “Manhattan Project” during World War II period. The other two were gaseous diffusion and centrifugal separation. Of the many electromagnetic schemes suggested, three soon were recognized as being the most promising: the “calutron” (a name representing

a

contraction

of

“California

University

cyclotron”)

mass

separator,

the

magnetron-type separator later developed into the “ionic centrifuge,” and the “isotron” method of “bunching” a beam of ions. The first two of these approaches were followed at California and the third at Princeton. Calutrons are basically 1800 deflection magnetic analysers. The isotron is an electromagnetic mass separator using extended source of ions, in contrast to the slit sources used in ordinary mass spectrographs. The ions from the extended source are first accelerated by a constant, high-intensity, electric field and are then further accelerated by a low-intensity electric field varying at radio frequency and in “saw tooth” manner. The effect of the constant electric field is to project a strong beam of ions down a tube with uniform kinetic energy and

therefore with velocities inversely proportional to the square root of the masses of ions. The varying electric field, on the other hand, introduces small, periodic variations in ion velocity, and has the effect of causing the ions to “bunch” at a certain distance down the tube. (This same principle is used in the klystron high frequency oscillator, where the electrons are “bunched” or “velocity-modulated.”). The bunches of ions of different mass travel with different velocities and therefore become separated. Separation of isotopes using Magnetron or ionic centrifuge was based on the fact that the strong centripetal acceleration in a rotating plasma typically of the order 5 x 1010m/sec2, can be used for separating difference species of elements and isotopes. Isotope Separators Electromagnetic separators are large-radius (e.g., 24 in.), 1800 deflection mass spectrometers. Ions are normally obtained in low voltage (100-150 V) arc discharge sources. The arc current is about 3-5 A, while the ion current obtained is of the order of a few hundred mA. Ionization efficiency is usually of the order of 5%. Gaseous materials, often in the form of chlorides SnCl4, SiCl4) are continuously fed into the arc chamber through a controlled leak. The pressure inside the arc chamber must be kept constant to obtain a steady ion beam. Compounds which can be volatilized in the 100-6000C-temperature range (chlorides, bromides of many elements) are introduced through a heated stainless steel “charge bottles”. Temperatures as high as 12000C can be obtained with graphite “charge bottles”. The pumping systems of large separators are impressive indeed. The main requirement is pumping capacity rather than ultimate vacuum. In a large instrument, total pumping capacity as high as 20,000 liters/sec is employed to maintain a vacuum of 10-5 torr. The collection of the separated isotopes presents many problems. First of all, considerable cooling is necessary. When an ion beam of 100 mA, accelerated to 35 kV, impinges on a metallic receiver, the heat generated is of the order of 3.5 kW. Other problems which must be considered in the design of a collector system are erosion, sputtering, efficiency, and technique of isotope removal. Graphite and copper are the most frequently used collector materials. The collected material is often removed from graphite by simple scraping or by the burning of the entire pocket in oxygen. Dissolution in nitric acid is a frequent removal method with copper pockets. Mercury isotopes may be collected in silver pockets from which removal is easily accomplished by heating in vacuum. Enrichment factor and production rate are two important performance parameters in electromagnetic separators. The enrichment factor is the ratio of the ratios of the isotopic concentrations in the product and the starting material, i.e., A/B in the product over A/B in the feed gas, where A is the “wanted” isotope. Enrichment factors are normally in the 30-50 range; in special applications enrichment may be as high as 1000. The production rate of the separators greatly depends on the throughput. A current of 1 mA corresponds to 6.3 X 1015 unit charges per second. A current of 100 mA will produce about 10 g of a nuclide of mass 100 in a full day’s operation; about one-half of this amount may be recovered from the collector.

Since the efficiency of ionization, as mentioned, is below 10%, at least 100 g of charge material is required. When the “wanted” isotope is a rare nuclide, production rate may be very poor; for example,

4O

K can only be produced at a rate of 0.5 mg/day. In cases like this, other

separation methods such as chemical exchange, thermal diffusion, and distillation should be used if at all possible. Principal Considerations of Electromagnetic Separators They are threefold: First, it is difficult to produce large quantities of gaseous ions. Second, a sharply limited ion beam is usually employed so that only a fraction of the ions produced are used. Third, too great densities of ions in a beam can cause space-charge effects which interfere with the separating action. Electromagnetic methods developed before 1941 had very high separation factors but very low yields and efficiencies. Since that time it has been shown that the limitations are not insuperable. In fact, the first appreciable-size samples of pure U-235 were produced by an electromagnetic separator. In addition to the foregoing factors, one has to consider the ensuing factors: •

The Number of Stages: As in all methods, a compromise must be made between yield and separation factor. In the electromagnetic system, the separation factor is much higher than in other systems so that the number of stages required is small. There was a possibility that a single stage might be sufficient. Early studies indicated that attempts to push the separation factor so high as to make single-stage operation feasible cut the yield to an impracticably small figure.

•

The size of a unit: As determined by the radius of curvature of the ion path the length of the source slit, and the arrangement of source and receivers;

•

The maximum intensity of magnetic field required;

•

Whether or not to use large divergence of ion beams;

•

The number of ion sources and receivers per unit;

•

Whether the source should be at high potential or at ground potential;

•

The number of accelerating electrodes and the maximum potentials to be applied to them;

•

The power requirements tar arcs, accelerating voltages, pumps, etc.;

•

Pumping requirement;

•

Number of units per pole gap

Calutron Separation of Isotopes It should be mentioned that much of the work on electromagnetic separation of isotopes was carried out during Manhattan Project in USA and only in the fall of 1945 it became possible to examine the electromagnetic isotope separation equipment to see if it could be adapted to the separation of isotopes of elements other than uranium and which find wide spread applications in fundamental or applied scientific research. This program is

operated now almost entirely for the isotopes produced and the scientific findings which they make possible. Fundamentally, there is little difference between a laboratory mass spectrometer and its production counterpart, the calutron. The primary difference is that the calutron is designed to collect useable quantities of desired isotopes, while the laboratory spectrometer is used almost entirely as an instrument for detection or analysis. Of course, if one desired, very small quantities of virtually any isotope could be collected in the laboratory instrument, but the collection rates would be exceedingly small. The electromagnetic separation of isotopes by the calutron involves the following phases: •

preparation of suitable charge material for the calutron ion source,

•

operation of the calutron in separating the isotopes,

•

quantifative recovery from the collectors and chemical purification.of the separated isotopes,

•

spectrochemical analysis of refined isotopes, and

•

preparation of suitable sample and mass spectrometer analysis of enriched isotopes. The flexibility which makes possible the electromagnetic separation of isotopes is

derived primarily from the magnetic field which can be set at a predetermined value to focus ions of mass ranging from lithium to uranium at the collector. Also, ion accelerating and focusing voltages are variable. Calutron Ion Sources Usually these are arc discharges. Calutron ion source units should possess a certain degree of flexibility as to temperature range, accelerating electrode arrangement, arc number and size, and cooling system. In processing a variety of elements and compounds having widely varying vapour pressures, it is desirable to have source units operable over temperature ranges required for realizing sufficient vapour pressures. Earlier investigations carried with uranium indicated that the temperature range of uranium-type ion source units could be extended sufficiently to cover approximately half of the elements having more than one nuclide if the charge compound of the element under consideration is carefully chosen. Charge Materials For charge materials occurring as liquids or gases at room temperature, sample introduction systems external to the ion source can be employed. The flow of feed material to the arc region should be controllable with the help of a sensitive needle valve. In the case of Solid materials, usually they are introduced in a contained inside the ion source and are heated. Usually it is desirable to have a small flow of gas such as nitrogen into the arc is beneficial, and with some elements necessary, for stabilizing the arc and promoting production. The problem of producing ions of all isotopes for calutron operation is not

completely solved; however, methods and equipment now in use are adequate to produce ions, with varying degrees of efficiency, from virtually any element in the periodic chart. Magnetic Analysers A special class of shaped magnetic lenses, used successfully in beta-ray spectroscopy have found wide spread applications in isotope separations. The fundamental concept of this focusing system relates to the use of magnetic field that decreases in intensity with increasing radius. The approximate magnetic field intensity distribution across the mid plane in these “inhomogeneous systems” is given by the relationship

r  βr = β0  0  r 

n

where βr is the mid plane magnetic field as a function of radius, r0 is the radius of the central orbit, β0 is the mid plane magnetic field strength at r0 and n is the inhomogeneous filed index. Siegbahn and Svartholm have shown that under these conditions, both radial (φr) and azimuthal (φz) focusing occurs when φr = φz = √2n= 2550 and at this focusing angle and for n = 0.5, spherical aberration is minimized. Both the radial and azimuthal or axial trajectories converge to the same image point. As a result, the transmission of a 2550 analyser can approach 100%. for reasonable ion source limits relating to angular divergence and ion energy spread. In addition, increase in mass or energy dispersive power is realized. The increased dispersion is extremely valuable for it permits the use of a magnet smaller than a homogeneous type. For a magnet having a basic radius of curvature r0, with the object and image symmetrically located, the dispersion D is given by

D=

r0 ∆m m0 (1 − n)

where n is the index of inhomogeneity of the magnetic field. For a 0.5 index the dispersion is doubled. Such an isotope analyser has been constructed. It had a transmission approaching 100% for several types of ion sources. The mean radius of curvature of this 75 ton electromagnet is 6 cm, the pole-piece gap width at this radius is 10 cm, and the mass dispersion at mass 100 is about 1.5 cm. Isotope Collectors For each element processed, an isotope collector must be designed, providing properly spaced receptacles for whatever isotopes may be present. Isotope collector structural materials, structural design, and cooling, all require considerable attention and have to be chosen depending on the system being studied. Since one mass unit separation at the collector decreases as the mass of the ions increase, each element requires especially designed collectors. In addition to this consideration, which is essentially a problem in pocket placement, suitable pocket materials and cooling techniques must be chosen in order to retain the deposited material. The temperature of receiving surfaces, which must necessarily be

bombarded With ions, must be kept sufficiently low that deposited material Will be retained and not lost to cooler surfaces such as walls of the vacuum chamber through the vacuum system. The problem of building a collector for any element then consists of calculating proper pocket spacing for the desired isotopes, and from vapor pressure considerations, determining suitable pocket material and cooling techniques. Usually the design of a collector is dictated by the desire for having maximum flexibility so that collectors could be constructed for any element from a minimum number of parts. The all-purpose type collector, in which pockets up to 10 can be assembled at predetermined spacings has been fabricated for this purpose. This basic structure permits the placing of collectors to receive the isotopes of any element in the periodic chart; pockets may be replaced easily if desired, and cooling may employed as required. Collector Materials These vary with the element being processed, and the receiver parts may or may not be water-cooled, depending on the energy to be dissipated and the vapor pressure of the collected material. Furthermore, with some elements, special retention techniques are required. For example: Sulfur is allowed to combine chemically with copper shavings packed in the collector pocket. Mercury is retained by allowing it to amalgamate with silver collector pockets, and by cooling the pockets with a circulating refrigerant. In the processing of mercury, the problem of high vapor pressure in the calutron and the contamination of collected isotopes by the condensation of neutral mercury vapor in the collector pockets, needs to be eliminated by refrigerating the, vacuum chamber. Charge Materials for Production of Ions The availability or synthesis of suitable calutron charge materials is essential. Several requirements must be met, such as suitable vapor pressure, at calutron operating temperatures, simplicity of molecular structure, stability of the compound and absence of water of crystallization. If the element itself has a suitable vapor pressure, it is generally used as charge material since its use greatly reduces" extraneous ions or sidebands. For the same reason, simple compounds generally are preferred over the more complex compounds; thus, halides are used widely. Anhydrous compounds are necessary because the water of crystallization gives rise to increased vacuum chamber base pressure and prolonged start-up time. The ultimate choice of the compound best suited for use in the electromagnetic separation of the isotopes of an element in the calutron is often made only after the systematic testing of a number of compounds. The criterion of suitability of such compounds is whether or not they give rise to a satisfactory rate of production of the separated isotopes of the given element. While a number of properties of the compounds are considered, chief consideration is usually the vapor pressure-temperature relationship, since such compounds must be sublimed into the arc chamber where positive ions are formed. Usually laboratory

studies involving mass spectrometer are a more satisfactory method for the evaluation of compounds. The compound under consideration is introduced into a mass spectrometer, and the ion current due to each of the fragments or species formed is plotted as a function of the mass of the fragment. This gives the relative abundance of the ions formed due to dissociation and ionization by the slow electrons used. The appearance potential is also determined for the singly charged metal ion involved, It then follows that if the number of singly charged metal ions formed is a fair proportion of all of the positive ions thus produced from a compound, and if the appearance potential of the metal ions is not greatly in excess of the ionization potential of the metal, then the rate of positive ion production, and therefore the rate of production of the isotopes of the element in question, will be high. A wealth of basic inorganic chemistry has resulted from these charge material developments. Suitable charge materials of some 43 elements have been supplied for calutron separations of isotopes, including the synthesis of many unusual compounds and, the development of methods of purification to meet special requirements. Calutron Operational Procedures After suitable ion sources, collectors, and charge materials are identified and assembled, the components are placed in an evacuated volume between the poles of an electromagnet. Metered ion currents at the collector are necessary to properly monitor the isotope beams and afford a means for estimating total isotope collection. Thus, for a singly charged ion, 1 amp.-hr, at the collector represents approximately 0.04 gm.-atom of the monitored isotope. The length of time allowed for a collection is determined from the metered production rates on the desired isotopes, with some allowance for collector rejection and chemical losses. Chemical Purification of Separated Isotopes From the collector pockets, the isotope contents are removed by one of several methods depending on the element and pocket material. Extreme care is exercised during this phase to prevent isotope contamination and to assure quantitative removal of the product isotope. Chemical purification procedures vary widely, depending upon the element being processed. Reactions must be complete, filtrations or centrifugations must be carefully performed, and care must be exercised to minimize losses caused by discarding reaction side products. It may be mentioned that by the beginning of 195I, adequate purification techniques had been worked out in USA for every element which had been processed in the calutron. These included radioactive materials such as beryllium-10 and potassium-40, rare earths such as cerium, neodymium, and samarium, and toxic materials such as selenium and mercury. Procedures for separation of elements with similar chemical properties have also been worked out. For example, a method for the production of pure zirconium and hafnium fluorides was

developed. It was found that indium may be separated from gallium by the use of mandelic acid. It was also found that mandelic acid precipitates some of the rare earths and is useful in their separation. The physical and chemical properties of the mandelates of zirconium and hafnium were thoroughly investigated. Also, it was found that the homogeneous precipitation of the rare earths by hydrolysis of methyl oxalate is a selective process. Chemical and Mass Analysis of Separated Isotopes Upon completion of chemical purification, product isotopes are subjected to chemical and mass analyses. Methods for handling small samples of precious isotope materials are being continuously improved. In mass spectrometry, experience and development have made it possible to mass analyze isotope collections from all elements thus far processed in the calutron. More than 700 samples of isotopes of elements were prepared as compounds suitable for mass analysis. The interest in using certain stable isotopes as tracers has stimulated the development of microwave methods for isotopic analyses. The resolution of the microwave spectrometer is so great that theoretically there is room for more than 50 million rotational lines in the present experimental region. Although considerable progress, has been made, isotopic analysis by microwave techniques do not yet meet the high precision of a mass spectrometer. Isotopes Enriched Electromagnetically By December 31, 1950, a total of 43 elements from lithium to lead comprising 177 isotopes had been separated in the calutron in milligram to multigram quantities.

Thermal Diffusion 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. The mixture to be separated is confined in a long, vertical tube, cooled externally and heated internally by a hot wire at the axis of the tube. Concentric annular type cylinders can also be used. In both types, the mixture to be separated is confined in a narrow space between the inner heated and an outer cooled surface. As a result of thermal diffusion

effect, the lighter isotope usually concentrates in the inner zone at higher temperature. At the same time, convection currents are set up, with the lighter isotope adjacent to the inner wall moving upward and the heavier isotope adjacent to the outer wall moving downward. Thus it is possible to achieve substantial degrees of separation in a practical length of column. It is preferable to work with gases rather than liquids because the higher diffusion coefficients result in higher separative capacity. The optimum space between hot and cold surfaces is a few mm for gases and fraction of mm for liquids. A series of very long columns (tens of meters) are essential to achieve reasonable degree of enrichment. The separation factor a is given by

T  ln α = r ln  1  T 2  where r is thermal diffusion factor proportional to (M2-M1)/(M1+M2), T1 = Hot wall temperature and T2 = cold wall temperature. The thermal diffusion factor r can be calculated from kinetic theory of gases. If r is positive, lighter isotope concentrates at the higher temperature. The thermal diffusion factor, r is strictly not a constant and varies with temperature for certain types of gases. Table 4 gives examples of the highest reported concentrations of separated isotopes that have been obtained by thermal diffusion. Table 4 Isotopes separated by thermal diffusion method Working fluid (Gas) HCl Kr Kr O2 N2 Xe He Ar Ne CH4

Isotope separated Cl 37 Cl 84 Kr 86 Kr 78 Kr 86 Kr 17 O 18 O 15 N 134 Xe 136 Xe 3 He 36 Ar 38 Ar 20 Ne 22 Ne 13 C 12 C 35

% in product 99.6 99.4 98.2 99.5 10.0 96.1 0.5 99.5 99.8 1.0 99.0 10.0 99.8 23.8 99.99 99.99 90.0 4.4

Laser Isotope Separation The possibility of using the slight differences that exist in the absorption spectra of isotopes of an element for isotope separation has been recognised ever since isotopes were discovered. Initially photochemical separation of isotopes was restricted to very few elements (Cl, Hg). But the advent of lasers provided the intense, monochromatic, tunable light source needs to make photochemical isotope separation applicable to all elements, at least on a laboratory scale. Two general methods have been proposed for separating uranium isotopes: (i) Laser isotope separation of uranium metal vapour and (ii) Laser isotope separation of UF6 Laser isotope separation of uranium: then separated from unionised

238

235

U in uranium metal vapour is ionised selectively and

U by deflection in electric or magnetic fields. The absorption

spectrum of uranium metal vapour is very complex, with over 3,00,000 lines at visible wavelengths. Many of these absorption lines are sharp, with sufficient displacement between a 238

U absorption line and the

the 235

235

U line with the

238

235

U line for the corresponding transition and without overlap of

U line for a different transition, to permit selective excitation of the

U atoms. In actual practice two lasers, one a narrow frequency laser supplying visible light

at 502.72 nm to excite ionise the excited

235

235

U followed by another supplying ultra violet light at 262.5 nm to

U atoms, are employed.

238

U does not absorb 502.74nm line and absorb

only at 262.5 nm, thereby achieving the selective ionisation. The selectively ionised

235

U ions are deflected by electric or magnetic fields or collector

plates. Even though photon absorption process selectively ionises between

235

U ions and neutral

238

235

U, charge exchange

U atoms and atomic collisions deflect enough

238

U atoms to

the collector plates limiting the maximum enrichment possible to attain. Pressures of uranium metal vapours are usually kept below 1 torr, to minimise the collisions. Laser isotope senaration of UF6: In this method,

235

UF6 in UF6 vapour is excited selectively and

caused to react chemically to produce a solid lower fluoride, which is then separated from unreacted

238

UF6 vapour.

The absorption spectrum of UF6 is far more complex than that of uranium metal, because the spectrum of the UF6 molecule involves transitions between many vibrational and rotational states that are absent in uranium atom. Further, absorption bands of the molecule overlap those of

238

by O.55 cm

from the peak in the

However, the absorption by

UF6

UF6 so that highly selective absorption by one isotope is seldom

found. For instance, at room temperature, the peak in the -1

235

238

238

235

UF6 absorption band is displaced

UF6 absorption band at a wave number of 625 cm-1.

UF6 at the peak absorption by

235

UF6 is so great as to preclude

selective absorption under these conditions. However, it was shown theoretically that if all transitions were to occur from the lowest vibrational levels, the fine structure of the absorption bands would be such that absorption maximum might be found at a

238

235

UF6

UF6 absorption minimum. Then a tuned laser

beam with a frequency spread narrower than line spacing of 0.16 cm-1 might be able to excite 235

UF6 to the first vibrational level without exciting

238

UF6. Such selective absorption from the

lowest vibrational level is not possible at room temperature. Further, at room temperature,

even with

238 235

UF6 molecules will be in excited states and many of these would get dissociated atoms

UF6 molecules during the chemical reactions. At sufficiently lower temperature (< 77K)

it is thus may be possible to realise the selective excitation of

235

UF6 molecules from the lowest

vibrational levels. But at such low temperatures the vapour pressure of UF6 is expected to be very low 5 x l0-25 torr. What is normally done is to cool a dilute mixture of UF6 in hydrogen to 30K by expansion to high speed through a hypersonic nozzle. The sub cooled UF6 molecules can remain uncondensed to assume the low temperature energy distribution and display an absorption spectrum in which

235

UF6 do not overlap with the

238

UF6 lines. This high speed sub-

cooled gas mixture is first irradiated by 16 µm light of a frequency absorbed by by

238

235

UF6 and not

UF6 and then by additional light of sufficient energy to dissociate the excited

insufficient to dissociate the unexcited undissociated

238

238

UF6. The dissociated lower fluoride of

235 235

UF6 but

UF6 and

UF6 are then separated by chemical means.

Efforts have also been made to separate deuterium isotope from hydrogen containing species. In general, two methods are being pursued; (i) photon dissociation of formaldehyde with UV lasers and (ii) infrared photolysis of methane derivatives by multiphoton dissociation. In UV photon dissociation, HCHO is irradiated with UV laser of about 325 nm when the photon dissociation reaction HCDO → HD + CO occurs. The reaction is carried out at a temperature of 350 K and a relatively higher pressure of about 100 torr. This is to prevent the polymerisation of formaldehyde, though at high pressures absorption selectivity is somewhat reduced. In the second method, freon 123 (CF3DCCl2) or fluoroform (CDF3) is subjected to photolysis with 10.65 or 10.2 µ IR laser. In practice, formaldehyde route is preferred as it involves only a single photon conversion and also easily separable gaseous products are formed. In addition, corrosion problems are also less. Table 5 lists some of the isotopes separated using lasers. Table 5 Isotopes separated using lasers System H2CO BCl3

Isotopes separated H, D 10 B, 11B

CF2Cl2 CS2 O2 UF6

12

C, 13C C, 13C, 32S, 34S 16 , 17 , 18 O O O 235 , 238 U U 12

Lasers used 319 nm frequency doubled dye 325 nm HeCd laser 10 µm CO2 + UV flash lamp 10 µm CO2 10 µm CO2 193 nm ArF 193 nm ArF 16 µm IR = UV

Distillation methods In general, isotopic species have different vapour pressures (and boiling points), so that partial separation by fractional distillation can be employed for their separation. For example, the boiling point of D2O is about 1.4° higher than that of H2O. This difference, though small, should make separation by distillation practical, provided efficient columns are

used. Deuterium can also be separated by fractional distillation of hydrogen gas at low temperatures (< -250°C). Heavy hydrogen gets concentrated in liquid phase. The separation factor α, for the desired component is given as

α =

x/(1-x) y/(1-y)

where x is the fraction of the component in the liquid phase and y is the fraction in the vapour phase. Deuterium in hydrogen or in water is always in equilibrium according to the reactions H2 +D2 ↔ 2HD and H2O + D2O ↔ 2HDO If the liquid and vapour phases form ideal solutions, the separation factor α can be written as

 π(H 2 )  α(H /D ) =   π (D )  2 

1/2

 π(H 2 O )  α(H 2O /D 2O ) =   π (D O )  2 

1/2

where π is the vapour pressure. The separation factors for H2/HD and H2O/D2O at their boiling points are 1.81 and 1.03 respectively. The elementary separation factor in the distillation process can be multiplied by inducing a countercurrent flow in the distillation columns (or towers). These columns are cylindrical containers within which contact is made between the two phases; (i) the liquid phase that flows by gravity from top to bottom and (ii) the vapour phase that, produced in a reboiler at the base of the column, flows from bottom to top. Distillation columns can be divided into two basic categories; plate columns and packed columns. In the former, there is repeated contact between the two phases, whereas the latter operates on the principle of continuous contact.

Electrolysis During the electrolysis, the gas discharged at the cathode of an electrolytic cell is distinctly poorer in heavier isotope content than in the electrolyte. Separation factor is defined as

α =

(M 1 /M 2 ) electrolyticgas (M 1 /M 2 ) electrolyte

It has been found that α depends on the cathode material, electrolyte composition and cell temperature. In the case of water for example, a high value of 8.1 has been reported. Separation factor is higher for an alkaline electrolyte than for acid. With KOH at 15°C, a pure iron cathode gave the highest value of 13.2. Minimum is expected to be about 3.8 as dictated by the equilibrium constant for the reaction H2O(l) + HD(g) <= = > HDO(l) + H2(g)

A typical electrolysis set-up consists of a vertical cylindrical cell wherein the central anode is separated by a porous diaphragm of nickel wire-reinforced asbestos from the outer casing metal (steel) which serves as cathode. The electrolyte is generally a solution of 25 % by weight of potassium carbonate. It would appear that a cascade of a number of stages, about 15, would be sufficient to arrive at 99.9 % D2O. But practical difficulties such as loss of a vast amount of deuterium in the early stages of the cascade come in the way. Stage losses of hydrogen with sufficient deuterium content can be prevented by incorporating the steamhydrogen exchange process. The reaction HD + H2O < = = > H2 + HDO attains equilibrium at elevated temperatures (> 80°C) over a catalyst (supported nickel or nickel on chromia or platinum on charcoal). The equilibrium constant of the above reaction is 2.8 at 80°C. Hence rapid exchange of deuterium occurs with hydrogen in steam and after condensation the deuterium rich water can be re-electrolysed for further enrichment.

Chemical Exchange Methods The deuterium exchange reaction between water and hydrogen and other compounds is one of the group of deuterium exchange reactions that have been extensively studied and are the basis for the most of the world's heavy water production. Table 6 lists deuterium separation factors between water and gaseous compounds of hydrogen. In all the systems deuterium tends to concentrate in the liquid phase except in ammonia-water at high temperature. Separation factors in chemical exchange are much higher than separation factors in distillation. The high value of these separation factors and their strong dependence on temperature give chemical exchange processes their importance for separation of deuterium and isotopes of other elements. Table 6 Separation factors for different exchange reactions Reactants Liquid Gas H2O NH2D H2O PH2D H2O HDS H2O HD NH3 HD CH3NH2 HD *at 60°

Products Liquid HDO HDO HDO HDO NH2D CH3NHD

#at -25°

Gas NH3 PH3 H2S H2 H2 H2 @at 40°

Separation factor (α) at different temperatures 00 25° 50° 100° 125° 1.02 1.00 1.00 0.99 0.99 2.71 2.44 2.27 2.04 1.96 2.60 2.37 2.19 1.94 1.84 4.53 3.81 3.30 2.65 2.43 4.25 3.62 2.99* 2.55 2.34 4.85 3.60@

200 0.99 1.78 1.64 1.99 5.19# 7.90$

$at-50°

The deuterium exchange reaction between water and ammonia, water and hydrogen sulphide proceeds rapidly in the liquid without catalysis, because of ionic dissociation. On the other hand, deuterium exchange between water and hydrogen, ammonia and hydrogen or methylanline and hydrogen does not proceed without catalysis. The water-hydrogen reaction can be catalysed by nickel or platinum metal catalyst, ammonia-hydrogen reaction by potassium amide dissolved in liquid ammonia and the methylamine-hydrogen reaction by potassium methylamide.

Solutions used in the ammonia-water, water-hydrogen and ammonia-hydrogen processes are relatively non-corrosive and may be handled in ordinary steel equipment. Solutions used in all of the other processes are relatively corrosive, and require use of stainless steel or other expensive construction materials. For example, in water-hydrogen sulphide exchange reaction (also known as Guilder-sulphide (GS) process), H2S is highly corrosive. Carbon steel coated with iron sulphide and bubble trap trays made of SS-304 are used. In all the above processes, to achieve a multiplication effect for the overall separation factor, the light and isotopic fractions collecting at the two ends of the column are reconverted to the initial chemical forms and returned to the exchange column to recycle countercurrently. To illustrate, consider the equilibrium reaction H2O(l)+HD(g) <==> HDO(l)+H2(g), The hydrogen gas, depleted in deuterium, escaping at the top is burnt with oxygen and the water formed flows back down the column again while the water, enriched in deuterium, collecting at the bottom is electrolysed into hydrogen and the gas is made to re-ascend the exchange column for further exchange with water flowing down. Alternately, a dual temperature exchange or bi-thermal exchange process can be set up to avoid the cumbersome exercise of Feed: Natural water

Recycle D2S Blower

reconverting

the

products

into

initial

reactants to achieve the multiplication effect in the separation factor. The dual

Cold tower T = 320C α = 2.32

temperature

exchange

takes

the

temperature dependent property of the equilibrium constant for the exchange

Depleted water

Product

D2S flow D2O flow Heat exchangers

reaction

under

consideration.

For

example, the values for the K for the reaction H2O(l) + HD(g) < = = > HDO(I) + H2(g) for the temperatures 250C and 800C are

Hot tower T = 1280C α = 1.80

3.78 and 2.80 respectively. Similarly for the reaction H2O(l) + HDS(g) < = = > HDO(l) + H2S(g), the value of K changes from 2.32 at 32°C to 1.30 at 138°C. By carrying out the equilibrium exchange process

in

one

column

(cold

tower)

maintained at a lower temperature say at Dual temperature Water – H2S Exchange Girdler – Spevack Process (GS Process)

32°C

for

water-H2S

reaction

and

transferring the enriched products to a second

column

maintained

at

higher

temperature (hot tower) at 138°C, the proportion of the reactants to products change. Thus

by carrying out the exchange reaction alternately in the two columns at different temperatures, the necessary multiplication effect on the separation factor is realised. Much of the D2O is produced this way. Chemical-exchange isotope separation requires segregation of two forms of an element into separate but contacting streams. Since many contacts are required to achieve the desired separation, the contacting process must be fast and achieve as much separation as possible. For heavy elements such as uranium, achieving a suitable separation factor involves contact between two valence (oxidation state) forms such as hexavalent [U6+ as in uranyl chloride (UO2Cl2)] and the quadrivalent [U4+ as in uranium tetrachloride (UCl4)]. The

235

U isotope

exhibits a slight preference for the higher valence, for example, the hexavalent over the quadrivalent in the Asahi process or the quadrivalent over the trivalent (U3+) in the French solvent-extraction process. The chemical-exchange process, developed by the French, is commonly referred to as CHEMEX. It uses the exchange reaction that takes place between two valence states (U3+ and U4+) of uranium ions in aqueous solution. Isotopic enrichment results from the tendency of 238

U to concentrate in the U3+ compound while

235

U concentrates in the U4+ compound. It is

therefore possible to obtain enriched uranium by removing the U4+ ions with an organic solvent that is immiscible with the aqueous phase (concentrated hydrochloric acid). Several possible extractants are available; however, tributyl phosphate (TBP), the choice of the French, is typically used. TBP is diluted with an aromatic solvent, and this organic phase moves countercurrent to the aqueous phase through a series of pulsed columns. In the pulse column, the heavier aqueous phase is fed into the top of the column, and the lighter organic phase is fed into the bottom of the column. A rapid reciprocating motion is applied to the contents of the column, providing efficient and intimate contact of the two phases. In an HEU plant, centrifugal contactors might be employed particularly for the higher assay sections, since the stage times and corresponding specific uranium inventory could be reduced significantly. After passing through the column, the enriched and depleted uranium streams must be chemically treated so that they can be recirculated through the column again (refluxed) or sent to another column for additional enrichment. This requires complicated refluxing equipment at both ends of the column. The ion-exchange process was developed by the Asahi Chemical Company in Japan and uses the chemical isotope effect between two valences (U4+ and U6+) of uranium. In this process, the organic phase is replaced by a proprietary ion-exchange resin. The aqueous phase flows through the stationary resin held in a column, and the net effect of all the chemical reactions is a “band” of uranium that moves through the ion-exchange column. The exchange between the unadsorbed uranium flowing through the band and that adsorbed on the resin enhances the isotopic separation. In this continuous separation system,

235

U and

238

U

tend to accumulate respectively at the entrance and exit ends of the adsorption band. In this process, it is economical to regenerate many of the chemicals by reaction with oxygen and hydrogen in separate equipment.

The development and manufacture of the appropriate adsorbent beads are based on technology and know-how gained by Asahi in over 25 years of ion-exchange membrane development and manufacture. The adsorbent is a spherical bead of porous anion-exchange resin with a very high separation efficiency and an exchange rate over 1,000 times faster than the rates obtained in most commercially available resins. The two exchange processes discussed here are representative of exchange processes now under study in several countries. At present, no country has built or operated a full-scale uranium enrichment plant based on an exchange process. The primary proliferation concern is that they are based on standard chemical engineering technology (except for the proprietary ion-exchange resins). Chemical exchange processes have also been investigated to separate other isotopes such as C, N, 0, U. But none could go beyond laboratory scale only. Recently a pilot plant for the enrichment of

235

U using cation exchange resins has been set up by Japanese. Difference

in the kinetics of redox reaction between the isotopes

235

U and

238

U is exploited for getting

enrichment. The reaction 238

UO2+ +

235

U(IV) →

235

UO2+ +

238

with an equilibrium constant of 1.0015 is exploited to enrich

U(IV) 235

U. Rate of exchange is first

order with respect to U(IV). Feed composition employed was U concentration = O.1M and free acid = 0.24 to 0.4 M. A cation exchange resin has been employed and the process is carried out in H2SO4 medium. Enrichment is effected by repeated oxidation and reduction of uranium on the column. It has been reported by Japanese that uranium enriched up to 3% U-235 could be obtained after 20 days of continuous operation.

Ion Migration Slight differences in velocities of isotopic ions in solution under an electric field, can also be exploited for the separation of isotopes. 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 have also to be taken into consideration. The 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. The major advantage of the fused salt medium is the absence of ion solvation resulting in larger mass effects in the migration of isotopic ions. The separation factor in ion migration process is proportional to ∆v/v where ∆v is difference in velocities between the isotopes and v is the mean velocity. 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,

39

K and

41

K were

separated by the electromigration of potassium chloride solution in a U-shaped tube using platinum gauge electrodes, with an arrangement for controlled counter flow of HCl acid from a reservoir above the cathode compartment. The tube was packed with glass beads or sand held under pressure. The counterflows were so adjusted that the speed of H+ ions was intermediate between those of

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K+ and

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K+ ions.

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K+ ions were washed back while the faster moving

ions moved ahead thereby building an isotopic concentration. Heavy Water plants in India

Kota More than 90% of the cumulative world production of heavy water (D20) is by the Hydrogen-sulphide (H2S) water (H20) exchange process, known as the GS process, with the major contribution being from the Canadian plants. Heavy Water Plant at Kota is a solely indigenous effort and is based on the Bithermal H20-H2S exchange process. The plant is located at a distance of 65 KM from Kota Railway Station, adjacent to Rajasthan Atomic Power Plant (RAPP). The Heavy Water Plant is integrated with RAPP for its supply of power and steam. Water from the nearby Rana Pratap Sagar lake, purified of suspended and dissolved impurities forms the process feed with the D20 enriched from 150 ppm (0.015%) in the feed to 15% D20 by chemical exchange with H2S and later by vacuum distillation to produce 99.8% D20.

Baroda Heavy Water Plant at Baroda is the first plant set up in the country for the production of heavy water by employing Monothermal Ammonia-Hydrogen exchange process. This facility also has a Potassium metal plant for supply of Potassium metal for preparing catalyst solution for all Monothermal Ammonia-Hydrogen exchange plants. In 1999, M/S GSFC commissioned their new technology based low-pressure Ammonia plant (A4), operating at around 140 kg/cm² pressure, and stopped operation of their old high pressure (640kg/cm²) ammonia plants. Operation of HWP had to be suspended simultaneously due to non-availability of synthesis gas at high pressure. Facing the challenge, HWP is modified to reorient its operation independent of GSFC. The Ammonia Water Exchange Front-End unit is set up which eliminates dependency of HWP on Ammonia plants of M/S GSFC for its deuterium feed stock. It uses water as the source of deuterium in the process and ammonia works as carrier gas only.

Hazira Heavy Water Plant at Hazira, employs the ammonia-hydrogen exchange monothermal process. The plant is located at a distance of about 16 km from Surat city. Work on HWP (Hazira) commenced in August 1986 and the plant was commissioned in February 1991.

Thal The plant comprises of two streams consisting of two separate isotopic exchange units, final enrichment units, final production units & cracker units, but a common ammonia synthesis unit. Feed synthesis gas (a mixture of one part of nitrogen and three parts of hydrogen containing deuterium from the

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K+

Ammonia plant is routed through the plant at a flow rate of about 96 T/Hr. at a pressure of about 180 kg/cm². The deuterium enriched ammonia from the bottom of the exchange tower is then fed to the second isotopic exchange tower where it gets further enriched by coming in contact with the enriched synthesis gases obtained by cracking of enriched ammonia. A part of the enriched gas and liquid from the second isotopic exchange tower is then taken to the final enrichment section where the concentration of deuterium in the ammonia can be further increased as desired upto 99.8%. Finally, the enriched ammonia so obtained is made free of the catalyst and is cracked. A portion of this enriched synthesis gas is burnt to produce heavy water. However, for reasons of better recovery efficiency the concentration of deuterium in ammonia in the final enrichment section is kept low so as to produce heavy water of about 60% which is then vacuum distilled to produce heavy water of nuclear grade.

Manuguru The Heavy Water Plant at Manuguru, Andhra Pradesh is based on the Bithermal Hydrogen Sulphide-Water (H2S-H20) Exchange Process. This plant with a capacity of 185 MTY is the second plant based on this process, the earlier one being at Kota, Rajasthan for which the complete technology has been developed indigenously by BARC and HWB. Water from the nearby Godavari river, purified from both suspended and dissolved impurities forms the process feed and the D20 content is enriched from 150 ppm (0.015%) to 15% D20 by chemical exchange with H2S and later vacuum distilled to produce 99.8% D20.

SUGGESTED READING 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) QUESTIONS 1. Mention two important and widely used methods each for the separation of uranium isotopes and hydrogen isotopes. 2. Bring out clearly the difference between the separation factor and enrichment factor. 3. Mention the physico-chemical property on which the following methods for separation of isotopes are based: (a) gaseous diffusion (b) distillation (c) thermal diffusion (d) chemical exchange 4. Define the terms separating unit, stage and cascade.

5. Why do we need to employ series of stages and cascades in the separation of isotopes? 6. Name some of the important applications of stable isotopes? 7. Better separation of uranium isotopes is possible in principle with gas centrifuge methods than with gaseous diffusion methods. Why? 8. Why is UF6 diluted with hydrogen in jet nozzle process? 9. Can one employ distillation methods for the separation of uranium isotopes? Justify your answer. 10. Explain briefly the Guilder-Sulphide (GS) process in the separation of hydrogen isotopes? 11. For deuterium enrichment, chemical exchange methods are preferred to distillation methods. Why? 12. What is the difference between squared-off cascade and tapered cascade? Give an example of each. 13. Which type of the cascade is preferred for obtaining high recovery of the product: Simple cascade or counter current cascade? Justify your answer. 14. Calculate the number of stages required to get enrichment of U-235 from its natural value of 0.77 % to 3% employing gaseous diffusion method. 15. Can a mass spectrometer be used for the separation of isotopes? Justify your answer. 16. What are the limiting factors deciding the maximum enrichment possible in laser isotope separation of isotopes, even though very large separation factors are possible?