Chapter 1 from BASICS OF NUCLEAR PHYSICS AND OF RADIATION DETECTION AND MEASUREMENT - An open-access textbook for nuclear and radiochemistry students by Jukka Lehto

Isotope separations are difficult, since isotopes are the same element and therefore behave chemically in the same manner. The only difference between the isotopes is the mass, which is due to the variation in the number of neutrons in the nuclei. This mass difference, in a relative way, is the largest in lighter elements. The largest mass differences occur with hydrogen. The mass ratios between ¹H, ²H, and ³H are approximately 1:2:3. The relative mass differences for heavier elements, however, are smaller – in general the smaller the heavier the element. For example, the relative mass difference between the uranium isotopes, ²³⁸U and ²³⁵U, is 1.3%. Due to the mass differences, isotopes have certain physical differences that affect their behavior and this is called the isotope effect. For example, the freezing point of water (H₂O) and deuterium oxide (D₂O) differ by 3.82 degrees and boiling point by 1.43. The optical emission spectrum of hydrogen and deuterium also differ with transitions up to a 0.2 nm. The corresponding transitions for uranium are ten times lower.

Isotope separations are needed for two purposes. First, they are needed to analyze the relative abundances of isotopes, isotope ratios, and second for the preparation of isotopically pure or enriched substances. In analytical separations, the quantity required is very small, but for manufacturing isotopes large amounts, even tons, are required.

In analytics the most important method of isotope separation is mass spectrometry (Figure XVII.1). In mass spectrometry the sample is first evaporated: e.g. the solution containing the analyte is injected on top of the sample wire (filament), the sample is dried and the wire heated, wherein the sample vaporizes. The gaseous molecules are ionized by, for example, by bombarding them with electrons. The generated ions are accelerated by an electric field and separated according to their masses using a magnetic field mass separator or a quadrupole. If the resulting ions have the same charge, the lighter ion will bend more than heavier ions in the magnetic field. In this way, for example, ²³⁵U¹⁶O⁺ (mass number 251) formed in uranium ionization can be separated from ²³⁸U¹⁶O⁺ (mass number 254). The number of ions are calculated with a detector, which can be a photographic plate, in which case system is a mass spectrograph, or an ampere meter.

Figure XVII.1. Operating principle of a mass spectrometer (https://www.boundless.com/physics/textbooks/boundless-physics-textbook/magnetism-21/applications-of-magnetism-160/mass-spectrometer-564-6290/images/schematic-of-mass- spectrometer/).

Isotope analysis is also performed using nuclear spectrometry. The relative proportions of the isotopes is determined from the intensity of the particles or rays generated in nuclear decay. The alpha spectrum below is an example of this type of analyses, showing alpha spectrum of uranium after the dissolution of a rock sample and chemical separation of uranium from it. There are three peaks of naturally occurring uranium seen in the spectrum: ²³⁸U peak (4.20 MeV), ²³⁵U peak (4.68 MeV) and ²³⁴U peak (4.77 MeV). The ratio of ²³⁴U/²³⁸U and ²³⁵U/²³⁸U in the sample can be calculated from the area under the peaks. The spectrum also shows the added 232U-tracer peak (5.32 MeV), with which the chemical yield of the separation process can be determined and from which the absolute amounts of ²³⁴U, ²³⁵U and ²³⁸U in the sample can be calculated. Similar isotope analyses are even easier to perform in gamma spectrometry, because the energies of the isotopes are more readily separated due to better energy resolution.

Figure XVII.2. Alpha spectrum of naturally occurring uranium isotopes ²³⁴U, ²³⁵U and ²³⁸U and the ²³²U tracer used to determine the chemical yield in uranium separation process.

The main industrial isotope separation processes are related to nuclear power generation, specifically production of nuclear fuel materials. The methods used in the nuclear industry, were originally developed for the production of weapon grade uranium and plutonium. The most important material produced in nuclear power production is naturally the nuclear fuel. Most power-generating nuclear reactors in the world operate based on thermal neutron induced uranium fission. Of the uranium isotopes, only ²³⁵U undergo fission induced by thermal neutrons. It accounts for only 0.7% of naturally occurring uranium, however, the rest being isotope ²³⁸U (and 0.0055% of 234U). In most reactor types, this fissile uranium fraction is not enough to sustain a chain reaction, rather the portion of ²³⁵U should be increased to 3-4%. Since the relative mass difference of the two uranium isotopes is very small, a single separation only provides a relatively small enrichment.

Achieving a sufficient degree of enrichment requires a multi-stage process. Another important isotope material in nuclear industry is D₂O, which is used in certain reactor types as the primary circuit coolant.

Chemical isotope exchange is used, for example, for deuterium oxide production. The separation process utilizes the following exchange reaction

$$\mathrm{H_2O(l) + HDS(g) \leftrightarrow HDO(l) + H_2S(g)}$$

XVII.I.

The equilibrium of this reaction is dependent on temperature: k(32 ºC) = 2.32 and k(138 ºC) = 1.80. The enrichment of deuterium in water molecules is more advantageous at lower temperatures. The separation process uses two columns, one on top of the other, of which the upper column has a lower temperature (30ºC) and the lower column has a higher temperature (130 ºC). Natural water, with a deuterium proportion of 0.014%, is directed from above into the upper column. At the same time, H₂S gas, with the same proportion of deuterium, is directed into the same column from below. Since low temperatures favor the tritium exchange into water, the aqueous phase is enriched in deuterium. The deuterium-depleted of H₂S leaving from above is now passed to the lower column, where it travels against the current of natural water as it did in the upper column, but at a higher temperature (130ºC). The exchange reaction now favors the deuterium-enriched H₂S phase. The deuterium-enriched H₂S is again directed to the upper column, where the low temperature transfers the deuterium to the water phase and so on. The result is 15% deuterium-enriched water in the upper column and deuterium-depleted water in the lower column. The final fraction of D₂O is nearly 100%, which can be achieved by distillation. Such enrichment plants produce up to 1200 tons of D2O annually.

Electrolytic separation takes advantage of the fact that when the water is decomposed by electrolysis, the proportion of deuterium in the hydrogen gas generated on the cathode is less than in the original water, which is due to the slower dissociation of deuterium in water to D⁺ ions compared to H⁺ ions. Thus, the deuterium is enriched in the water. The process is no longer in widespread use.

Electromagnetic separation uses the same principle as mass spectrometry, i.e. the molecules are ionized and separated by a magnetic field. During the Second World War, the extraction method was used to separate ²³⁵U for weapons. The method is currently used only for the manufacturing of isotopically pure isotopes in gram quantities.

The gas diffusion method has been used above all in ²³⁵U enrichment for use as nuclear fuel and weapon material. The method is based on the fact that lighter molecules move faster than heavier ones in gas phase. For separation, uranium is vaporized as UF6 and directed to a separation chamber, in which there is a membrane, with a pore size of 10 to 100 nm, separating two sections. Since ²³⁵UF₆ is somewhat lighter than 238UF6 it passes more rapidly through the membrane and is enriched in the chamber on the other side of the membrane. Only a small enrichment is achieved in a single separation stage due to the small mass difference, so many successive separations are needed. The enrichment of ²³⁵uranium from the original 0.7% to 3% requires 1300 consecutive separations and 80% degree of enrichment requires 3600 separations. Gas diffusion plants have been used in the United States, Russia, France, China, and Argentina.

Gas centrifuges have replaced the gas diffusion plants in ²³⁵U enrichment. In a centrifuge the heavier particles or molecules travel towards the walls faster than lighter ones. In ²³⁵U enrichment, centrifugation is utilized by leading UF₆ gas into the center of the centrifuge chamber, wherein the ²³⁸UF₆ moves towards the walls somewhat faster than the lighter 235UF6 (Figure XVII.3). The centrifugation is continuous, so that ²³⁵U-enriched UF₆ is directed out from the middle of the chamber and ²³⁸U-enriched UF₆ from the chamber walls. Enrichment is much greater than with gas diffusion, but also in this case a series of successive separation steps are needed. After ten consecutive centrifugations 3% ²³⁵U is achieved and after forty-five 80%.

Figure XVII.3. Gas centrifuge process used to enrich ²³⁵U (https://www.euronuclear.org/info/encyclopedia/g/gascentrifuge.htm).