While reprocessing is essential to the production of weapons-usable plutonium, plutonium is not created by reprocessing. Plutonium is first produced in a nuclear reactor from uranium-238 in fuel rods during the course of a controlled nuclear chain reaction in the reactor core. In uranium-fueled reactors, uranium-238 in the fuel rods is converted into fissile plutonium-239 as a result of neutron absorption. 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 build-up as well. Higher plutonium isotopes, notably plutonium-241 and plutonium-242, also build up with longer irradiation time.

Plutonium is produced in both civilian and military reactors. Spent fuel in civilian plants is typically "high burn-up" spent fuel -- that is, it has been irradiated for extended periods at high power output in the reactors so as to generate a large amount of energy. 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. Plutonium from spent fuel with less than 7% plutonium-240 content is considered "weapons grade" by the Department of Energy, but it is possible to make a nuclear bomb with less pure grades, including plutonium from high burn-up commercial reactor fuel. This is why any consideration of the separation and transportation of plutonium raises significant security concerns, regardless of the stated purpose of the reprocessing.

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, "The Purex Process.")

Other reprocessing techniques that have been used in the past, notably at Hanford, are the Butex process, the Redox (for REDuction OXidation) process, and the original bismuth phosphate process used to build the first U.S. atom bomb. DOE is also developing new reprocessing methods. The farthest along is electrometallurgical processing, which uses electrolysis to separate spent fuel components.

Source: Office of Scientific and Technical Information, integrated Data Base Report -- 1994: U.S. Spent Nuclear fuel and radioactive Waste Inventories, projections, and Characteristics. DOE/RW-0006, Rev. 11, (Oak Ridge National laboratory, oak ridge, TN, September, 1995), p. 15.