Inside the core of a typical nuclear reactor are pencil-thin fuel rods, each about 12 feet long, which are grouped by the hundreds in bundles called fuel assemblies. Inside each fuel rod, pellets of uranium are stacked end to end. Less than one percent of natural uranium is uranium 235 (U-235), which can undergo nuclear fission and can fuel nuclear reactors. The remaining 99 percent is a non-fissile variant called U-238.

Most nuclear reactors use uranium that has been artificially enriched to contain three percent U-235. As fissionable fuel is used up in a reactor, fission by-products accumulate in the fuel rods. Some fission by-products absorb neutrons, meaning they are capable of stopping the chain reaction that nuclear power generation relies on. Thus, to keep the reactor running, used rods must be removed and replaced with fresh ones every two to four years.

Spent fuel rods may be temporarily stored in water-filled pools or dry steel casks until a permanent home can be found for them. Or, since only a small percentage of the original uranium fuel pellets is actually used up, the uranium and plutonium -- a fission by-product that is also fissionable -- may be reprocessed for future use.

Spent fuel assemblies are extremely hot and highly radioactive when first removed. Therefore, they must be transferred from the reactor core to a water-filled cooling pond by remote handling equipment. The water prevents high levels of gamma-ray radiation from escaping. After about four years, the assemblies are somewhat safer to handle and can be sent either to long-term storage or -- as in countries such as France, Britain, Japan, and Russia -- to a reprocessing plant, where the plutonium, fission products, and unused uranium are chemically separated from each other.

There are several advantages to reprocessing over a "once-through" use of nuclear materials. By recycling the uranium, the amount of mining and milling of new uranium ore is reduced and the finite uranium reserves are conserved for future use. Recycling reduces waste, since fewer fission products must be disposed of, and it minimizes exposure risks because reprocessing uses materials that would otherwise need to be transported and stored. Disadvantages include the extra costs of building and operating reprocessing facilities, radiation exposure to facility workers, and the increased availability of plutonium, a fissionable by-product of uranium fission that is needed for building nuclear weapons and is in demand by terrorist organizations.