As a reactor operates, the uranium 235 is consumed and fission products are built up. These fission products are highly radioactive and emit significant amounts of energy. In a typical reactor, the radioactive fission products account for about 10% of the energy produced during operation. The uranium 238 also fissions since there are neutrons with high enough energy born in the reactor core. However, the uranium 238 is also converted into plutonium 239 through the absorption of a neutron in a non-fission producing reaction. Like uranium 235, plutonium 239 is able to fission with a slow or fast moving neutron. The build up of plutonium is significant, so much so that the fissioning of plutonium eventually produces about one-third of the energy produced in the reactor.
As the amount of uranium 235 in the fuel is used up and drops below about one and one-half percent, the fuel must be replaced. Since the fissioning of the uranium has produced highly radioactive fission products, the spent fuel must be stored and cooled for many years. Initially, the fuel is immersed in a swimming pool like structure called the spent fuel pool where it may remain for many years. After it has
4 The use of heavy water enriched in the deuterium isotope shifts the burden of enrichment from enriching uranium to enriching water.
cooled, the spent fuel is moved to a shielded cask and stored, dry-cooled by air that circulates around the cask. Since the fuel contains material that will remain radioactive for hundreds of thousands of years, the spent fuel must be isolated from the environment throughout that time. As currently planned, the spent fuel is to be disposed of in a deep geological repository.
The spent fuel, however, still contains much of its energy in the form of unburned uranium 235 and plutonium. It is possible to recycle the unburned uranium and plutonium through a process called reprocessing. In reprocessing, the uranium and plutonium are separated from the fission products, formed into new fuel, and then recycled through the reactor. Although the United States currently does not reprocess of spent fuel, other countries that do not have significant domestic supplies of uranium, most notably France and Japan, are currently reprocessing spent fuel and producing new fuel utilizing the unburned uranium and plutonium. Reprocessing has the advantage of removing one of the isotopes with the longest period of radioactivity, plutonium 239.
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