This map shows the location of major reprocessing facilities, both commercial and military. Design capacities are given for commercial plants. Military and commercial reprocessing use an identical process of plutonium separation; it is the type of spent fuel used that differentiates one from the other.

In a reactor, uranium-238 in the fuel rods is converted into fissile plutonium-239 as a result of neutron absorption and subsequent nuclear reactions. Gradually, some of the plutonium-239 is converted into non-fissile plutonium-240 upon absorption of another neutron. As the reactor continues to operate, more uranium-238 is converted into plutonium-239, leading to more plutonium-240 as well. Higher plutonium isotopes, notably plutonium-241 and plutonium-242, also build up with longer irradiation time.

Spent fuel in civilian plants is typically "high burn-up" spent fuel-that is, it has been irradiated for extended periods in the reactors so as to generate a large amount of energy. Spent fuel from pressurized water reactors (the most common type of civilian reactor) typically contains approximately 0.7% plutonium-239 and plutonium-241 (the fissile isotopes), and 0.2% non-fissile plutonium isotopes. Uranium irradiated for the extraction of plutonium for weapons is "low burn-up" fuel, which has been irradiated to minimize production of plutonium-240 and other undesirable higher plutonium isotopes. Spent fuel from military reactors contains a small fraction of a percentage of plutonium, almost all of it plutonium-239. Plutonium from spent fuel with less than 6-7% plutonium-240 content is considered "weapons grade," but it is possible to make a nuclear bomb with plutonium from high burn-up commercial reactor fuel as well.

Plutonium in irradiated fuel cannot be used until it is recovered from the spent fuel through reprocessing. The most common kind of reprocessing is called the "Purex" process, which stands for Plutonium-URanium EXtraction (see diagram). All reprocessing facilities which are currently operating use the Purex process. Other reprocessing techniques have been used in the past, including the Butex (for diBUTyl carbitol EXtraction) process, the Redox (for REDuction OXidation) process, and the original bismuth phosphate process used to build the first US atom bomb. The US is also developing a new reprocessing method, commonly known as "pyroprocessing" which is an electrolytic method of separating spent fuel into three different streams (see The International Civlian Reprocessing Business).