GB1580172A

GB1580172A - Method of isotope enrichment

Info

Publication number: GB1580172A
Application number: GB1061577A
Authority: GB
Original assignee: Philips Electronic and Associated Industries Ltd
Current assignee: Philips Electronics UK Ltd
Priority date: 1976-03-17
Filing date: 1977-03-14
Publication date: 1980-11-26
Also published as: DE2709755A1; ES456850A1; AU512495B2; FR2344326B1; IT1075674B; NL7602751A; CA1098286A; NL182451C; NL182451B; SE7702847L; IL51655A; DE2709755B2; JPS52112100A; DE2709755C3; AU2321777A; JPS544037B2; FR2344326A1; BR7701527A

Description

(54) METHOD OF ISOTOPE ENRICHMENT (71) We, PHILIPS ELECTRONIC AND ASSOCIATED INDUSTRIES LIMITED, of Abacus House, 33 Gutter Lane, London, EC2V 8AH, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The invention relates to a method of preparing an element or a compound of an element which compared with a starting material for the preparation is enriched by one of the isotopes of the element to a useful degree.

The literature discloses several methods of enriching elements or compounds thereof with given desired isotopes. In general this literature relates to methods which require a great deal of investment and/or a large quantity of energy to separate, per unit of time, a large quantity of material which has been enriched to a high degree. It has, for example, already been suggested to utilize, the difference in reaction speed of the isotopes of an element or of the compounds containing these isotopes, when separating isotopes.

However, this effect is very small with elements other than hydrogen: In addition, the effect decreases when the temperature increases so that it was not possible to adapt this method in practice.

It is an object of the invention to provide a method of preparing an element or compounds thereof enriched with a given isotope which yields the desired result in a simple and cheap manner and with a relatively low energy consumption.

The invention provides a method of preparing an element or a compound of an element which is enriched relative to a starting compound of the element with an isotope of the element, the method comprising the steps of contacting a starting material comprising the starting compound of the element in a reaction vessel with a heated substrate so as to convert part of the starting compound into the element or into another compound of the element, which element or other compound of the element is deposited on the heated substrate and contains a higher proportion of a lighter isotope of the element than the proportion of the lighter isotope of said element in the starting compound, removing the deposit from the substrate, then contacting the remaining starting material or a compound formed by chemically treating the deposit, with the heated substrate so as to convert part of the starting compound or the compound formed from the deposit into a further deposit of the element or of the other compound of the element on the heated substrate, and repeating these steps in order to produce further enrichment.

The method according to the invention is based on the recognition that owing to mass diffusion and thermal diffusion in the conversion of substances at a heated substrate while depositing onto the substrate an element or compound formed therein, at least if certain precautions are observed, enrichment of the element, or a compound thereof, with a lighter isotope will occur.

During the conversion of substances at a heated substrate, in which said substances may be present in the gaseous and/or the liquid state, a comparatively stagnant gas layer forms around the substrate whereas turbulent convection currents occur in the substance heated by the substrate in the region outside this layer. The comparatively stagnant gas layer is maintained by a steep temperature gradient which extends from the substrate to the turbulent gas or the liquid which are at a much lower temperature than the substrate.

Through the stagnant gas layer, which may consist of gaseous reaction products and any intentionally added substances, molecules diffuse from the starting material to the substrate. In general, on passing through this temperature gradient the molecules which comprise a lighter isotope of the element will diffuse more rapidly than molecules comprising a heavier isotope.

Enriching the material deposited on the substrate with a lighter isotope is promoted if it is ensured that by an intensive and rapid agitation of the starting material, the occurrence of a concentration gradient in the starting material relative to the light isotope is prevented. Owing to such an intensive agitation, of the starting material, the ratio between the isotopes at the interface between the boundary layer and the starting material is at any moment equal to the average ratio between the isotopes in the starting material. The most important difference between a process according to the invention and chemical vapour deposition processes is that a complete conversion is not aimed at but that only a part (usually 20 to 50%) of the starting material used is converted at the substrate.

So, in contrast to what usually takes place during the decomposition or the induced reaction of substances at a heated substrate, the aim must not be to have complete conversion. The portion of the starting material which is not converted is enriched with a heavy isotope, and may in some circumstances be the desired product which is to be prepared by means of a method according to the invention.

In order for the method according to the invention to proceed properly, it is desirable to choose the temperature of the substrate to be so high that the deposition rate is a function of the rate at which the molecules to be converted can reach the substrate by means of diffusion and that the deposition rate should be independent of the reaction rate or in other words: a temperature at which all molecules which reach the substrate and which may be converted are converted, so that the diffusion rate is the decisive factor for the rate of enrichment. The substrate temperature must therefore be so high that the chemical equilibrium is fully shifted to the side of the conversion product.

If in the preceding and in what follows hereinafter, mention is made of conversion, then this should not be understood to mean only a pyrolysis reaction in which a given compound decomposes into the constituent elements or into another compound, but also reactions in which various components of the starting material are involved. A starting material does not only mean a pure compound, but may be a mixture of substances, some of which or all together react together at the substrate. The mixture may contain substances which do not participate in the reaction but which consist substantially of a single isotope, such as argon and which promote the collision selectivity, whereby the mass separation is promoted. Examples of reactions are, for example, reduction reactions and double conversions, (e.g. AB+CD < AC+BD). In a reaction in which a deposit is formed by reducing a liquid compound with hydrogen, the possibility of attaining a deposition rate of 1 mm per minute is not limited by the feasibility of providing an adequate hydrogen supply rate. If elements other than hydrogen are present in the compound besides the element to be enriched, preference should be given to elements which in their natural state consist for more than 90% by weight of a single isotope.

The starting material may consist in its entirety of a gas or of a mixture of gases.

Preferably, however, the starting material consists of a liquid, at any rate at least for that part of the starting material which consists of the compound of the element to be enriched with a lighter isotope forms a part. In practice it has appeared that when a starting material is used which is present, at least partly, in the form of a liquid, a suitable enrichment can be realized in a simple and rapid manner with a method according to the invention. This is probably due to the fact that when a starting material is used which is present in the form of a liquid compound of the element to be enriched, phenomena may occur at a heated substrate which promote the enrichment of the deposited material with a lighter isotope. One of the causes may be that in the interface between the liquid starting material and the gas layer which is formed around the substrate and which consists of a vapour generated from the liquid, gaseous conversion products and any intentionally added gases, molecules which contain a lighter isotope evaporate faster than molecules which contain a heavier isotope of the element to be enriched. Also phenomena which occur when a gas-liquid interface is in violent motion, because gaseous products pass through this interface in both directions in the shape of bubbles, may play a part herein. This also promotes mixing. The rapid mixing of the starting material can be obtained and/or supported by mechanical means if the convection currents which naturally occur in the starting material during the procedure should prove to be insufficient; it is also possible when using a liquid starting material to bubble a gas, for example hydrogen through the liquid, which gas can participate in a chemical reaction, for example a reduction reaction, which results in a material which can be deposited on the substrate.

At the temperatures used the differences bctween the reaction rates of compounds containing different isotopes play no significant part, which was not to be expected on the basis of theoretical considerations.

The method can be performed in such a way that the liquid or the gas is passed along the substrate which has been heated to the decomposition or reaction temperature. It is also possible to move the substrate relative to the liquid or the gas or to combine both measures, for example in a continuous process, in which a wire-shaped substrate is passed through a reactor and a liquid moves through the reactor in the opposite direction or in which either of the two is stationary. It may be advantageous to circulate the liquid through a device, for example a filter or a sedimentator so as to separate off suspended solid by-products which may be formed during the pyrolysis at the substrate and which are not deposited thereon. Working with a liquid offers the additional advantage that the temperature thereof may be maintained at a low value so that a steeper temperature gradient is maintained through the stagnant gas layer surrounding the substrate than in a starting material which is completely gaseous and the deposition rate of the lighter isotope of an element is greater.

Working with a liquid starting material has additional important consequences which contribute to the economic applicability of the procedure, for example as regards the construction of the reactor and of the materials used therein. The reactor may, for example, consist of reinforced synthetic resin material, provided that there is no risk of corrosion thereof by the liquid.

The substrates used are made of temperature-resistant materials and may have any geometry. In their simplest form the substrates consist of a filament, a strip or a tube of a metal or an alloy, glass, quartz or a ceramic material which may be stable at the conversion temperature and which preferably does not react with the deposited products. However, in certain circumstances such a reaction could be desirable. The substrate may be a porous tube through which gases, whether or not they are involved in the reaction, can be supplied or discharged. The substrate may be a tube through which a heat-transporting medium is passed, in which case deposition will occur on the outer wall of the tube.

Liquid compounds and solutions may be used, as well as melts of substances which melt at technically acceptable temperatures and pressures.

There are several ways of heating the substrates to the desired temperature. For example, the substrates may be heated by passing an electric current through them.

However, the substrate may also be heated inductively or capacitatively or, possibly, by means of radiation. If the substrate is a tube, it can be heated, for example by means of super-heated steam, or liquid metal alloys.

It is advantageous to perform the method according to the invention at pressures which exceed one atmosphere, in many cases it appears that when the pressure is increased, the rate at which a reaction proceeds in which a lighter isotope is deposited is increased.

The product deposited on the substrate may, at the temperature used be solid or may be in the liquid state. In the latter case provisions must be made to catch the liquid deposition product separate from the starting material if the latter also consists of a liquid. In order to achieve a degree of enrichment with a given isotope which is useful, it is necessary to repeat the steps of the method a number of times in succession. In each subsequent enriching step the starting material then consists at least partly of material derived from an enriched element or compound obtained in a preceding stage, which element or compound is converted by a chemical treatment into a form suitable for performing the method of enrichment.

In this respect the invented method offers important advantage relative to non reversible processes such as diffusion processes for separating isotopes in which by far the greater portion of the power dissipated is used to maintain differences in pressure by means of pumping. Similar considerations apply to centrifugal processes. In the method according to the invention it is possible, depending on the process parameters, for example by means of a suitable choice of the reactants-to recover a significant portion of the energy used in the preceding stage-when chemically treating the deposit obtained in the preceding enrichment step to form a compound which can be used in a subsequent enrichment step. This enery can be returned to the energy source, for example by -means of super-heated steam.

It is clear that the method according to the invention can also be performed using as the starting material a compound which has already been enriched with a given isotope by means of another method. It is also possible to use a method according to the invention for producing a material enriched with a heavy isotope, for which purpose a light isotope is removed from the starting material by a method according to the invention. The method may, for example, be used for enriching uranium with a lighter isotope of that element, for enriching carbon, which may inter alia be of importance in determining the age of materials which contain carbon by determining the '4C/l3C-ratio.

In addition to the mechanisms described which result in the lighter molecules arriving in relatively larger numbers at the substrate than the heavier molecules so that at a conversion rate which is the same for both kinds of isotopes the product deposited on the substrate is enriched with the lighter isotope, it is possible to influence the conversion rate.

It is known that atoms or molecules which are, for example, excited with light of a suitable wavelength possess a greater reaction velocity in certain reactions than molecules which are not so excited. The starting material may be irradiated in the vicinity of the substrate with radiation of such a wavelength that a lighter isotope is selectively excited.

By, for example, selectively exciting 235U in molecules which contain uranium, such as UF6, with narrow-band laser light of a suitable wavelength, it is possible to ensure that a molecule which contains 235U is converted more rapidly than a molecule which contains 238U. To prevent the excited molecule losing its energy owing to collisions or reactions in the gas phase so that the desired aim is not obtained, the starting material is preferably only irradiated near the substrate so as to reduce the risk that conversion of the excited molecules at the substrate takes place before a molecule has lost its energy owing to collisions in the gas phase.

It was found that the deposition rate can be influenced in a still more positive sense by applying a high electric voltage to the substrate relative to the liquid if the liquid is not electrically conductive. The effect is independent of the sign of the voltage applied.

A particular advantage of the method according to the invention is that no auxiliary means need be used, which means are decisive for the performance of the reaction. In the known gas diffusion method for separating isotopes by means of gas diffusion membranes, the gas diffusion membranes are such auxiliary means. A method according to the invention does not require technically highgrade devices which verge on the limits of technical capability as is the case, with the gas centrifuge and earlier laser separation methods. Neither are large pumping capacities necessary.

Two embodiments of the invention will now be described with reference to the following Examples.

Example A tantalum wire having a diameter of 100 ,im whose ends are connected to current conductors was immersed in liquid Cl2H26 and the wire was heated by direct current passage to a temperature of 2000"C. The liquid was circulated by means of pumping and all suspended solid by-products present in the liquid were removed by means of a filter. After the procedure had been in progress for 1 minute it appeared that a carbon layer having a thickness of 1000Sum had been deposited on the tantalum wire.

Both the non-converted part of the starting material and the deposited product were analysed by means of mass spectroscopy. It was found that the deposited product was enriched by 0.8%+0.1% with the lighter isotope, the starting ratio 98.89/1.11 being changed into 98.09/1.91 for the ratio 12C/13C in the starting material.

When the procedure was repeated, for which the deposited carbon was first converted into a fluid compound, an increasing enrichment in 12C occurs in the deposited product.

Example 2 In a similar manner to that described in Example 1, a method according to the invention can be performed with liquid UF6 at a temperature exceeding approximately 65"C under hydrogen pressure and, optionally, while adding a gas such as argon which does not participate in the reaction, the total pressure in the system being sufficiently high for no boiling phenomena occur in the liquid at the temperature of the liquid UF6, that is to say at approximately 70"C a pressure of 2 atm. or more is used.

Then a lower fluoride of uranium, enriched with a light isotope 235U relative to 238U as compared to the starting product will be deposited on the substrate. The temperature of the substrate must then be between 2500 and 1200"C. The substrate and the wall of the reaction vessel may, for example, consist of nickel or a nickel alloy, such as a corrosion-resistant nickel-copper alloy. The reaction product HF will be taken away by the liquid flow in the form of bubbles. The lower fluoride of uranium which is deposited on the substrate can be converted with fluorine into uranium hexafluoride. If now the concentration of the required isotope in the starting material amounts to approximately 0.7%, the process must be repeated approximately 400 times, as can be calculated in a simple manner, for a required concentration of 3% and an enrichment of 0.8 /, per step.

WHAT WE CLAIM IS:- 1. A method of preparing an element or a compound of an element which is enriched relative to a starting compound of the element with an isotope of the element, the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. materials which contain carbon by determining the '4C/l3C-ratio. In addition to the mechanisms described which result in the lighter molecules arriving in relatively larger numbers at the substrate than the heavier molecules so that at a conversion rate which is the same for both kinds of isotopes the product deposited on the substrate is enriched with the lighter isotope, it is possible to influence the conversion rate. It is known that atoms or molecules which are, for example, excited with light of a suitable wavelength possess a greater reaction velocity in certain reactions than molecules which are not so excited. The starting material may be irradiated in the vicinity of the substrate with radiation of such a wavelength that a lighter isotope is selectively excited. By, for example, selectively exciting 235U in molecules which contain uranium, such as UF6, with narrow-band laser light of a suitable wavelength, it is possible to ensure that a molecule which contains 235U is converted more rapidly than a molecule which contains 238U. To prevent the excited molecule losing its energy owing to collisions or reactions in the gas phase so that the desired aim is not obtained, the starting material is preferably only irradiated near the substrate so as to reduce the risk that conversion of the excited molecules at the substrate takes place before a molecule has lost its energy owing to collisions in the gas phase. It was found that the deposition rate can be influenced in a still more positive sense by applying a high electric voltage to the substrate relative to the liquid if the liquid is not electrically conductive. The effect is independent of the sign of the voltage applied. A particular advantage of the method according to the invention is that no auxiliary means need be used, which means are decisive for the performance of the reaction. In the known gas diffusion method for separating isotopes by means of gas diffusion membranes, the gas diffusion membranes are such auxiliary means. A method according to the invention does not require technically highgrade devices which verge on the limits of technical capability as is the case, with the gas centrifuge and earlier laser separation methods. Neither are large pumping capacities necessary. Two embodiments of the invention will now be described with reference to the following Examples. Example A tantalum wire having a diameter of 100 ,im whose ends are connected to current conductors was immersed in liquid Cl2H26 and the wire was heated by direct current passage to a temperature of 2000"C. The liquid was circulated by means of pumping and all suspended solid by-products present in the liquid were removed by means of a filter. After the procedure had been in progress for 1 minute it appeared that a carbon layer having a thickness of 1000Sum had been deposited on the tantalum wire. Both the non-converted part of the starting material and the deposited product were analysed by means of mass spectroscopy. It was found that the deposited product was enriched by 0.8%+0.1% with the lighter isotope, the starting ratio 98.89/1.11 being changed into 98.09/1.91 for the ratio 12C/13C in the starting material. When the procedure was repeated, for which the deposited carbon was first converted into a fluid compound, an increasing enrichment in 12C occurs in the deposited product. Example 2 In a similar manner to that described in Example 1, a method according to the invention can be performed with liquid UF6 at a temperature exceeding approximately 65"C under hydrogen pressure and, optionally, while adding a gas such as argon which does not participate in the reaction, the total pressure in the system being sufficiently high for no boiling phenomena occur in the liquid at the temperature of the liquid UF6, that is to say at approximately 70"C a pressure of 2 atm. or more is used. Then a lower fluoride of uranium, enriched with a light isotope 235U relative to 238U as compared to the starting product will be deposited on the substrate. The temperature of the substrate must then be between 2500 and 1200"C. The substrate and the wall of the reaction vessel may, for example, consist of nickel or a nickel alloy, such as a corrosion-resistant nickel-copper alloy. The reaction product HF will be taken away by the liquid flow in the form of bubbles. The lower fluoride of uranium which is deposited on the substrate can be converted with fluorine into uranium hexafluoride. If now the concentration of the required isotope in the starting material amounts to approximately 0.7%, the process must be repeated approximately 400 times, as can be calculated in a simple manner, for a required concentration of 3% and an enrichment of 0.8 /, per step. WHAT WE CLAIM IS:-

1. A method of preparing an element or a compound of an element which is enriched relative to a starting compound of the element with an isotope of the element, the

method comprising the steps of contacting a starting material comprising the starting compound of the element in a reaction vessel with a heated substrate so as to convert part of the starting compound into the element or into another compound of the element, which element or other compound of the element is deposited on the heated substrate and contains a higher proportion of a lighter isotope of the element than the proportion of the lighter isotope of said element in the starting compound, removing the deposit from the substrate, then contacting the remaining starting material or a compound formed by chemically treating the deposit, with the heated substrate so as to convert part of the starting compound or the compound formed from the deposit into a further deposit of the element or of the other compound of the element on the heated substrate, and repeating these steps in order to produce further enrichment.

2. A method as claimed in Claim 1, characterized in that the starting material is agitated in the reaction vessel.

3. A method as claimed in Claim 1, characterized in that the starting material is continuously passed through the reaction vessel along the substrate.

4. A method as claimed in Claim 1, characterized in that the starting compound is in the form of a liquid.

5. A method as claimed in Claim 1, characterized in that the method is performed at a pressure which exceeds one atmosphere.

6. A method as claimed in Claim 1, characterized in that the starting material consists of UF6.

7. A method as claimed in Claim 1, characterized in that the starting material consists of at least two components which react with each other at the surface of the heated substrate.

8. A method as claimed in Claim 1, characterized in that in the reaction vessel contains a gas which does not participate in the conversion and which consists entirely or substantially entirely of one single isotope.

9. A method as claimed in Claim 1, characterized in that the starting material is irradiated in the vicinity of the substrate with radiation of such a wavelength that a lighter isotope is selectively excited.

10. A method of preparing a compound of an element which relative to the starting material is enriched relative to a starting compound of the element with an isotope of that element, substantially as herein described with reference to Example 1 or Example 2.

11. An element or compound of an element which is enriched with an isotope of that element, prepared by a method as claimed in any of Claims 1 to 10.