text: When a fuel assembly has been used in a power plant, the assembly is replaced and the old one stored under water in special ponds, or in dry concrete structures or casks.  Here, shielded from people, it is allowed to cool.\n\nAfter being sufficiently cooled, the assembly, along with others, is put into a cask for temporary storage on the nuclear power plant site.  Delaying the final disposal of high-level waste also allows its radioactivity to decay.  Forty years after removal from the reactor, less than one thousandth of its initial radioactivity remains, and it is much easier to handle.\n\nThe ultimate disposal of vitrified wastes, or of spent fuel assemblies without reprocessing, requires their isolation from the environment for long periods.  The most favored method is burial in dry, stable geological formations up to 600 m or more deep.  Several countries are investigating sites that would be technically and publicly acceptable.\n\nThe U.S. considered a number of options for high-level waste disposal, and decided that mined geologic disposal offered the best option for isolation, although research on transmutation of long-lived radionuclides is continuing.  Transmutation involving neutron bombardment has the theoretical possibility of removing isotopes that are radioactive for a long time, thus alleviating disposal issues.\n\nThe plan is that the fuel will eventually be placed in the geologic deep-level repository at Yucca Mountain in Nevada.  This is a mountain of "tuff" -- stable volcanic rock.  This site was selected after years of investigation into other rock forms including granite, layered-salt, salt domes, and basalt.  All had advantages and all would have provided satisfactory final disposal for high level wastes.  The selection of the Yucca Mountain site in Nevada was an expeditious choice based upon the political realities of the day.\n\nThe repository deep in Yucca Mountain will consist of a large number of tunnels or corridors into which the spent fuel will be placed, in the form of fuel rods.  These rods will be sealed in heavy casks of steel, and placed in the mined caverns, which are then sealed with bentonite (a clay that forms a secure seal).  The pictures below show what the repository could look like.\n\nArtist's conception of repository underground facilities and operations\n\nArtist's conception of repository facilities\nHowever, Regulatory bodies have not yet formally approved Yucca Mountain as a high-level waste repository.  Also, Congress has not adequately funded the work, even though all necessary funds have already been collected in the Nuclear Waste Fund as a surcharge on the use of electricity (at 0.1 cent per kWh - for a total of $16 billion).  So, in the meantime, used fuel assemblies are stored temporarily at the power plants in water pools or large dry casks cooled by natural convection.\n\n\nTransuranic Waste\n\nWastes arising from weapons production in the Cold War have their own repository at the Waste Isolation Pilot Project (WIPP) in New Mexico.  This is a repository constructed in layered salt formations and it is been in operation since late 1999, accepting wastes from Government weapons production sites.  The wastes stored there are presently mainly transuranic wastes.  This waste has no connection with commercial electricity generation from the nuclear option.