Another concern that is frequently cited is what to do with the spent fuel that is generated. To optimize the burnup of the uranium 235, the reactor is shutdown about every 18-24 months and some of the fuel replaced and the remaining fuel rearranged. A typical fuel assembly stays in the reactor a total of about 6 years before the uranium 235 content becomes too low for its continued use. It is then transferred to the spent fuel storage pool where the faster decaying fission products are allowed to decay and the assembly cools. It may remain there for many years before being transferred to a dry fuel storage cask. Even after removal from the spent fuel pool, the fuel assembly still contains very large amounts of highly radioactive material that must be kept from the environment. Under the National Waste Policy Act, the US Department of Energy is to take custody of the spent fuel and dispose of it through deep geological disposal.
After an extensive scientific assessment, the Department of Energy chose Yucca Mountain, located about 90 miles north of Las Vegas, Nevada, as the site for the repository. In June of 2008, the Department of Energy submitted a construction license application for the repository to the United States Nuclear Regulatory Commission, the licensing authority [12, 13]. The construction and operation of the proposed repository is highly controversial and is strongly opposed by many critics including Nevada's congressional delegation. Even if the licensing and construction of the Yucca Mountain repository continues, it is not likely to be in operation until 2020 or later. Despite assurances that the spent fuel currently at reactor sites and anticipated to be produced in the future may be safely stored there for at least 20 years beyond the life of the reactor, critics are concerned that the nuclear industry has yet to develop and demonstrate a safe long term solution to the issue of spent fuel disposal.
Outside the United States, the approach to the disposal of spent fuel is somewhat different. Through the use of reprocessing, the plutonium and unburned uranium are removed from the waste stream. Since plutonium is one of the materials in the fuel that remains radioactive the longest, its removal reduces the time the waste must be isolated from hundreds of thousands of years to a few thousand years. Although even safe storage of spent fuel for this amount of time has yet to be demonstrated, it is believed to pose much less of a technological challenge and has in fact been implemented at the Waste Isolation Pilot Plant or WIPP . Waste from the U.S.'s nuclear weapons program is presently being disposed of at WIPP in salt domes. The waste consists only of the fission products and not the plutonium or uranium that would be in spent fuel sent directly for disposal without reprocessing.
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