All chemical substances consist of atoms. The most important building blocks of atoms are protons, neutrons and electrons. The number of protons in an atom is determined by the type of substance, e.g. carbon atoms have six protons and nitrogen atoms have seven protons. The number of protons in the atoms of a specific substance is fixed, but the number of neutrons may vary. Nitrogen atoms, for example, have seven or eight neutrons. All nitrogen atoms do not have the same mass, because the atom with eight neutrons will be heavier than the atom with only seven neutrons. Atoms of the same substance with different masses (due to the difference in the number of neutrons) are called isotopes. The so-called mass number of an atom is obtained by taking the sum of the number of neutrons and protons. The two isotopes of nitrogen are denoted by N-14 and N-15.

The different isotopes of certain chemical compounds have different physical properties. These different properties can be successfully utilized in a number of applications. The following applications are well-known applications of specific isotopes:

Iodine
An iodine (I-123) solution is administered to a patient by intravenous injection or as a drink. The iodine is taken up by the thyroid gland and a gamma camera is used to form functional images of the thyroid for diagnosis. Only the I-123 isotope is visible on the images.

Carbon
Another application of isotopes is the manufacturing of industrial diamonds. Diamonds consist of carbon atoms in crystal lattice. If all the carbon atoms in the lattice are of the same size (the same isotope), the lattice is more uniform and the diamond produced is harder.

Although the physical properties of isotopes of the same chemical substance may differ, the chemical properties are identical. Chemical separation processes can therefore not be used to separate isotopes. Physical properties include amongst others: boiling point, mass of the isotopes and light absorption. A number of separation processes had been developed throughout the years that exploit the differences in physical properties of isotopes. One such an example is Klydon's ASP separation process. There are also other examples: The distillation process exploits the difference in boiling points; the laser separation process exploits the difference in light absorption; the centrifuge process exploits the differences in mass and the gaseous diffusion process is based on the principle that molecules of different mass travel at different speeds, thus exploiting the speed difference to separate the isotopes.

Isotope separation is used to increase the concentration of a preferred isotope. The diagram below shows a hypothetical nitrogen isotope separation process. N-14 is the preferred isotope.

If a feed stream that contains a certain ratio of isotopes is fed to an isotope separation unit, two streams are produced. One of the streams has a higher concentration of the preferred isotope than the feed stream. This is known as the enriched stream. The other stream has been depleted of the N-14 isotope.

In many of the separation techniques used in industry, one separation step is generally insufficient to obtain the required isotopic purity. However, the required purity can be obtained by feeding the enriched material from one step to another separation step to obtain further enrichment. Distillation is a well known example where separation steps are connected in this way.

In the isotope separation industry, a number of separating steps connected to obtain a specified amount of enrichment is called an enrichment cascade. Due to the number of streams involved when multiple separation stages are used, a large number of different cascade configurations are possible. The choice of cascade configuration has a substantial influence on the capital and operating cost of the separating plant.