Isotope Separation

the separation of pure isotopes from mixtures of isotopes of a given element or the enrichment of a mixture with individual isotopes.

Isotope separation is an important problem of great scientific and practical significance. Attempts to separate isotopes were made from the time of their discovery until the 1930’s primarily with the aim of detecting isotopes of stable elements and measuring their mass and isotopic composition. It proved possible to separate only small (tracer) quantities of some elements insignificantly enriched with isotopes.

Fundamental research was begun in the 1930’s on atomic nuclei, nuclear reactions, interactions between particles and nuclei, and related areas. The reliability of experimental data and the interpretation of the results obtained to a large extent depended on the purity and available amount of the isotope in question. However, the preparation of even milligram quantities of isotopes proved to be a complex task. Only small quantities of enriched mixtures of isotopes, mainly of the light elements, were isolated. Deuterium was the only isotope to be produced on an industrial scale. Further development of the technology of isotope separation was stimulated in 1939 by the discovery of the fission of ²³⁵U under the action of neutrons. This opened up possibilities for the peaceful and military uses of atomic energy. The production of large quantities of uranium isotopes and other elements, which are necessary components of “nuclear fuels” and materials in nuclear technology, became an important task at this time. Huge plants were constructed for this purpose.

There are a number of methods of isotope separation. All are based on the differences in the properties of isotopes and their compounds resulting from the differences in mass of their atoms. The relative differences in mass of the isotopes are fairly small for most elements. Therein lies the complexity of the task.

The effectiveness of isotope separation is characterized by the separation factor α. For a mixture of two isotopes, α = [C’/ (1 - C’)]/[C”/(1 – C”)], where C’ and (1 - C”) are the relative contents of the light and heavy isotopes, respectively, in the enriched mixture and C” and (1 – C”) are the corresponding quantities in the starting mixture. For most methods, a is only slightly greater than unity, and therefore to obtain a high final isotopic concentration requires multiple repetitions of the single stage of isotope separation. Only electromagnetic separation is characterized by an a value of 10–1,000 per separation cycle. Selection of the method of isotope separation depends on such considerations as the properties of the substance to be separated, the required degree of separation, the desired isotope quantity, and the cost of the process (at high volumes of isotope production).

Gaseous diffusion. In the gaseous diffusion method, a gaseous compound of the element being separated is “pumped” at the rather low pressure of ~ 0.1 N/m² (~ 10~³ mm Hg) through a porous membrane containing up to 10⁶ openings per cm². Light molecules penetrate the membrane faster than heavy molecules, since the velocities of the molecules are inversely proportional to the square root of their molecular weights. Consequently, the gas is enriched in the light component on one side of the membrane and in the heavy component on the other side. If the difference in molecular weights is very small, this process must be repeated thousands of times. The number of stages of separation n is determined by the relationship q = α“, where q is the required degree of separation. The operation of huge gaseous diffusion plants for the production of U from gaseous UF₆ (α ~ 1.0043) is based on this method. Production of the required ²³⁵U concentration makes it necessary to perform about 4,000 individual stages of separation.

Diffusion in a vapor stream (countercurrent mass diffusion). In this method, the isotope separation is conducted in a cylindrical vessel (column), divided vertically by a diaphragm. The diaphragm contains about 10³ openings per cm³. The gaseous isotope mixture moves against a stream of secondary vapor. Owing to the concentration gradients of the gas and vapor within the cylinder cross section and the greater coefficient of diffusion for the light molecules, the gas, which has passed through the stream of vapor into the left-hand part of the cylinder, is enriched by the light isotope. The enriched portion is withdrawn from the upper end of the cylinder with the bulk of the vapor, and the gas remaining in the right-hand half moves along the diaphragm and is withdrawn from the apparatus. Vapor that has penetrated into the right-hand part is condensed. The isotopes of neon, argon, carbon, krypton, and sulfur are separated on a laboratory scale (up to 1 kg) using separators consisting of several dozen sequentially connected diffusion columns with vaporizing liquids, such as mercury or xylene.

Thermal diffusion. A thermal diffusion separation column consists of two vertical concentric pipes heated to different temperatures. The mixture to be separated is introduced into the space between the pipes. The temperature drop A T between the surfaces of the pipes generates a diffusion flux, which leads to a difference in the concentration of the isotopes in the cross section of the column. At the same time, the temperature drop leads to the generation of vertical convection flows of gas. Consequently, the lighter isotopes concentrate near the heated inner pipe and move upward. The separation coefficient α = 1 + γ (ΔT/T), where γ is the thermal diffusion constant, which is dependent on the relative difference in mass of the isotopes, and T = (T₁ + T₂)/2.

The thermal diffusion method makes it possible to separate isotopes in both the gas and liquid phases. The variety of isotopes that may be separated by this method is greater than the separations possible by gaseous diffusion or diffusion in a stream of vapor. However, α is small for the liquid phase. The simplicity of this method and the absence of vacuum pumps, among other features, make it a convenient method to use when separating isotopes under laboratory conditions. This method has yielded He containing 0.2 percent ³He (the natural abundance of this isotope is 1.5 X 10 ⁵ percent), as well as the isotopes ^l80, ¹⁵N, ¹³C, ²⁰Ne,²²Ne, ³⁵C1, ⁸⁴Kr, and ⁸⁶Kr in concentrations greater than 99.5 percent. Thermal diffusion was used on an industrial scale in the USA for the preliminary enrichment of ²³⁵U prior to the final separation of this isotope in an electromagnetic installation. The thermal diffusion plant consisted of 2,142 columns measuring 15 m high.

Distillation (fractional distillation). Since, as a rule, liquid isotopes have different saturated vapor pressures, for example, P₁ and p₂, and different boiling points, it is possible to separate isotopes by fractional distillation. Distilling columns with a large number of separation stages are used; a depends on the ratio P₁/P₂and its value decreases with increasing molecular weight and temperature. For this reason the process is most effective at low temperatures. Distillation has been used in the preparation of isotopes of light elements, such as ¹⁰B, “B, ¹⁸O, ¹⁵N, and ¹³C, as well as in the production of hundreds of tons of heavy water per year.

Isotope exchange. Isotope separation can also be carried out by chemical reactions in which the isotopes of the element undergoing separation change places. Thus, for example, if hydrogen chloride HC1 is brought into contact with hydrogen bromide HBr, both of which have the same initial content of deuterium D, the exchange reaction will result in a D content of HC1 that is several times higher than that of HBr. The use of several stages leads to the production of hydrogen, nitrogen, sulfur, oxygen, carbon, and lithium highly enriched in the individual isotopes.

Centrifugation. In a centrifuge rotating with a high peripheral speed (100 m/sec), the heavier molecules are concentrated near the periphery and the lighter molecules near the rotor. The stream of vapor in the outer part with the heavy isotope is directed downward, and the flow in the inner part with the light isotope is directed upward. Connecting several centrifuges in a cascade leads to the desired isotope enrichment. During centrifugation, a depends on the difference in atomic mass of the isotopes being separated rather than on the ratio of the masses. For this reason, centrifugation is suitable for separating the isotopes and heavy elements. Owing to the existence of improved centrifuges, this method has found applications in the industrial separation of the isotopes of uranium and other heavy elements.

Electrolysis. During the electrolysis of water or of aqueous solutions of electrolytes, the hydrogen evolved at the cathode contains less deuterium than the initial water. As a result, the concentration of deuterium increases in the electrolytic cell. This method has been used for the industrial production of heavy water. The separation of other isotopes of light elements (lithium, potassium) by the electrolysis of their chlorides is performed only in laboratory quantities.

Electromagnetic process. In the electromagnetic method, the substance whose isotopes are to be separated is placed in a crucible of an ion source and is evaporated and ionized. The ions are extracted from the ionization chamber by a strong electric field, and combined into a beam of ions; they then enter a vacuum separation chamber placed in a magnetic field H, oriented perpendicularly to the motion of the ions. Under the influence of the magnetic field, the ions travel in circular paths with radii of curvature that are proportional to the square root of the ratio of the ion mass M to its charge e. This leads to differences in the radii of the paths of the heavy and light ions. It is therefore possible to collect ions of various isotopes in collectors located in the focal plane of the installation.

The productivity of electromagnetic installations is determined by the magnitude of the ion current and the effectiveness of collecting the ions. In large installations the ion current varies from tens to hundreds of mA, which makes it possible to isolate up to several grams of isotopes per day (total of all isotopes). The productivity is lower in laboratory separators by a factor of 10–100.

The electromagnetic method is characterized by large a and by the possibility of the simultaneous separation of all isotopes of a given element. Usually, at large industrial installations a ~ 10–100 per stage, and in laboratory installations a is 10 to 100 times greater. In most cases, a single electromagnetic stage of separation is adequate; the repeated separation of previously enriched isotopic materials to obtain isotopes of extremely high purity is rarely done. The principal drawbacks of this method are relatively low productivity, high operating cost, and considerable losses of the separated material.

The electromagnetic method has made it possible to prepare kilogram quantities of ²³⁵U for the first time. The electromagnetic plant in Oak Ridge, Tenn. (USA), included 5,184 separation chambers called calutrons. Owing to their high universality and flexibility, electromagnetic installations are used for the separation of about 50 elements of the periodic system in quantities ranging from several milligrams to hundreds of grams and are the principal source of isotopes for scientific research and some practical applications.

Laboratory separators, like the large electromagnetic separation installations for industrial isotope production, have also found numerous applications. The laboratory separators are used in the preparation of radioactive isotopes, which are necessary for nuclear spectroscopy and for studies of the interaction between ions and solids (in ion implantation as well as for other purposes).

Other methods. In addition to the methods listed above, there are a number of other methods whose applications are limited or which are in the process of being developed or improved. These methods include preparation of ³ He based on the phenomenon of superfluidity of ⁴ He; separation by diffusion in a supersonic gas stream, which expands in a space with a decrease in pressure; chromatographic separation based on differences in the adsorption rates of isotopes; and biological separation methods.

Summary. The methods of isotope separation possess certain features that determine the areas of their mostefficient application. In the separation of light elements with mass numbers of about 40, distillation, isotope exchange, and electrolysis are the most economical and effective. Diffusion, centrifugation, and the electromagnetic method are used in the separation of the isotopes of heavy elements. Gaseous diffusion and centrifugation can, however, be used, if gaseouscompounds of these elements are available. Since such compounds are scarce, the real potential of these methods is limitedThermal diffusion permits the separation of isotopes in both the gas and liquid states, but a is small for the separation ofisotopes in the liquid phase. The electromagnetic method is characterized by high a, but it has low productivity and is used mainly in the production of isotopes on a moderate scale.

In order to provide for the scientific study and practical uses of isotopes, the State Fund of Stable Isotopes was created in the USSR. The fund contains reserves of isotopes of almost all the elements. Considerable quantities of deuterium, ¹⁰B, ¹³C, ¹⁵N, ^l8O, ²³Ne, and other isotopes are regularly produced. The production of various chemical compounds labeled with stable isotopes has also been set up.