Radioactive Waste 335the potential direct impact on the human environment is necessary but not sufficient,
because radionuclides can be transmitted through water, air, and land pathways for
many years and even for generations. Some radioactive waste may be retrieved and
recycled by future reprocessing, but reprocessing creates its own radioactive waste
stream. Most radioactive waste can be treated only by isolating it from the accessible
environment until its radioactivity no longer poses a threat. Isolation requirements
differ for different classes of radioactive waste.
Engineers charged with radioactive waste control must, because of technical,
political, and economic factors outside their control, focus on long-term storage tech-
nologies, i.e., disposal. Some radionuclides, particularly those that make up HLW, have
half-lives of tens of thousands of years, or even hundreds of thousands of years. It is
difficult to imagine a technology that truly offers ultimate disposal for these wastes;
thus we think in terms of very long-term storage. Many of the issues discussed in
Chapter 15 are applicable to the radioactive waste problem.High-Level Radioactive WasteA number of options were considered for HLW disposal, and it is agreed internationally
that mined geologic disposal offers the best option for isolation, although research on
transmutation (of long-lived into shorter-lived radionuclides) is continuing.
In 1979, the U.S. Geological Survey (USGS) recommended a two-stage bar-
rier system for the geologic disposal of HLW, in order to minimize the likelihood of
leakage from the repository and dispersal in the accessible environment. The waste
form itself - radioactive material dissolved and dispersed in a glass matrix - would
provide the first barrier. The second barrier would be the geologic rock formation
itself. This double-barrier scheme is being implemented for defense reprocessing HLW.
Commercial spent nuclear fuel, however, is not reprocessed and will be stored in the
geologic repository in the form in which it leaves the reactor core: spent fuel rods.
These rods will be sealed in heavy casks of steel and depleted uranium.
When the fissile uranium in a bundle of fuel rods has been used to a point where
the fission rate is too slow for efficient power generation, the rods are ejected into a
very large pool of water, where they remain until the short-lived radionuclides have
decayed away and until the rods are thermally cool enough to handle with ordinary
machinery. This takes about six months, but the lack of any other storage facility for
most spent fuel in the United States has resulted in on-site pool storage for as long as
10 years. In 1998, the U.S. Nuclear Regulatory Commission approved casks for dry
surface storage for sufficiently cooled spent nuclear fuel, and several nuclear power
plants now have dry surface storage capability.
The United States is bound by nonproliferation agreements to accept and store
spent fuel that this nation has supplied to nuclear reactors in other countries. At present,
foreign spent fuel is stored at two US. Department of Energy facilities: the Savannah
River Site in South Carolina and the Idaho National Engineering and Environmental
Laboratory in Idaho. When a repository is available, the aged and cooled spent fuel
will be loaded into casks for disposal in the repository.