13 Back end management
Back end management is a blanket term for after the fuel is taken out of the reactor; i.e., the fuel is burned and has to be removed. Typically, the fuel is stored in a large cooling pool while the short lived fission products decay. The pools serve dual purpose – heat dissipation due to decay of the fission products and shielding. After a varying period of time, anywhere from 3 to 20 years, fuel is moved to dry casks. Several assemblies are placed in each cask depending on the design.
The back end management ‘policy’ in the United States is to store the used fuel on the dry cask pads onsite until a repository is constructed for disposal.
The United States does have an actively operating repository – the Waste Isolation Pilot Plant operated by the Department of Energy in Carslbad, New Mexico, for TRU waste stemming from nuclear weapons production. The WIPP facility is a salt formation.
Yucca Mountain, about 100 miles north of Las Vegas was designated the repository for commercial high-level waste; i.e., used fuel, in 1987, but legal issues and local opposition has held it up since. Unfortunately, many administrations since have used it as a political wedge issue. Ironically, based on the used fuel inventory in the United State, Yucca Mountain would fill up if it opened tomorrow. The capacity is only set by statute, but collectively, it is agreed that a second repository is needed.
How the repository is supposed to work
Whether high-level waste or used fuel, the repository is a geologic formation that is first meticulously characterized in terms of hydrology, geology, chemistry, meteorology, etc. If there is an -ology, it will be studied. Characterization will take at least ten years to even thirty years.
The design of the repository is an example of defense-in-depth. Waste is contained in a matrix – borosilicate glass, or the uranium oxide itself. This, in turn, is then placed in a canister. For an unsaturated repository; i.e., above the water table, like Yucca Mountain, the cans are placed underneath drip shields. For the rest of the world, the repository concepts are for saturated conditions; i.e., under the water table. Typically, the canisters would then be surrounded by bentonite. This is a clay material that absorbs water and expands. Ever see kitty litter clump? That is basically the physical process.
Eventually, the canisters will degrade. Modelers assume canisters last about 1000 years. With the borosilicate glass matrix, dissolution of that takes even longer. Radionuclides will leak out, but some, due to geochemistry will precipitate out and remain there. Others will dissolve in water or seep through the vadose zone to the water table. The approach is called ‘dilute and disperse’ in that the water volume is large enough to render the concentration of any particular radionuclide to be low.
Operate
Most repositories are expected to ‘operate’; i.e., people actually working there, for up to 300 years. After that, the repository enters the ‘post closure period’ where everyone leaves. There is considerable debate as to whether some sort of signage should be installed.
Additional notes