Enriched uranium, with a high percentage of U-235, is used for nuclear energy and weapons. Separating U-235 from U-238 is difficult and expensive. Techniques include electromagnetic isotope separation, gaseous diffusion enrichment, and centrifuges. Other methods include aerodynamic, laser, plasma, and chemical separation.
Enriched uranium is uranium with a high percentage of the isotope U-235, which makes up only about 72% of natural uranium. Regular uranium is referred to as U-238, where the number indicates the amount of nucleons (protons and neutrons) in its atomic nucleus. U-235 has an uneven amount of protons and neutrons, making it slightly unstable and susceptible to fission (splitting) by thermal neutrons. Making the fission process proceed as a chain reaction is the basis of nuclear energy and nuclear weapons.
Since U-235 has identical chemical properties to regular uranium and is only 1.26% lighter, separating the two can be quite a challenge. The processes are generally quite energy-intensive and expensive, which is why only a few countries have been able to implement them on an industrial scale so far. To produce reactor-grade uranium, U-235 percentages of 3-4% are needed, while weapons-grade uranium must consist of 90% U-235 or more. There are at least nine techniques for separating uranium, although some work significantly better than others.
During World War II, a number of techniques were used in the United States when researchers were pursuing isotope separation. The first stage consisted of thermal diffusion. By introducing a subtle temperature gradient, scientists could induce the lighter U-235 particles towards a warm region and the heavier U-238 molecules towards a cooler region. This was just the preparation of the feed for the next stage, the separation of the electromagnetic isotopes.
Electromagnetic isotope separation involves the vaporization of uranium and subsequent ionization to produce positively charged ions. The ionized uranium was then accelerated bent by a strong magnetic field. The lighter U-235 atoms were deflected slightly more, while the U-238 atoms slightly less. By repeating this process many times, the uranium could be enriched. This technique was used to produce some of the enriched uranium for the Little Boy bomb, which destroyed Hiroshima.
During the Cold War, electromagnetic isotope separation was abandoned in favor of the technique of gaseous diffusion enrichment. This approach forced uranium hexafluoride gas through a semipermeable membrane, which slightly separated the two isotopes from each other. Like the previous art, this process would have to be done many times to isolate a substantial amount of U-235.
Modern enrichment techniques use centrifuges. The lighter U-235 atoms push slightly preferentially towards the outer walls of the centrifuges, concentrating them where they can be extracted. Like all other techniques, it has to be done many times to work. Complete systems that purify uranium in this way use many centrifuges and are called centrifuge cascades. The Zippe centrifuge is a more advanced variation of the traditional centrifuge that uses heat and centrifugal force to separate the isotope.
Other uranium separation techniques include aerodynamic processes, various laser separation methods, plasma separation, and a chemical technique, which exploits a minimal difference in the propensity of the two isotopes to change valence in oxidation/reduction reactions.
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