Environmental Engineering Reference
In-Depth Information
the United States this is not happening because a permanent disposal site has not yet been approved.
Except for the earlier reprocessing of spent fuel (now banned in the United States), every spent fuel
rod is still stored in the water pool or dry cask at the power plants where they have been withdrawn
from the reactor. At some power plants the storage space is running out, and their reactors may
have to be shut down.
In other countries, after due decline of radioactivity, the spent fuel is reprocessed. About
96% of the original uranium in the fuel is still present, although it contains less than 1%
235
U.
Another 1% of the uranium has been converted to
239
Pu. The spent rods are chopped up and
leached in acid. Uranium and plutonium dissolve and are separated chemically from the rest of
dissolved elements. The recovered uranium is sent back to the enrichment facilities. The recovered
plutonium is mixed with natural uranium and made into fresh fuel, called mixed oxide (MOX).
France reprocesses about 2000 tons per year of spent fuel, United Kingdom 2700 tons, Russia 400
tons, India 200 tons, and Japan 90 tons, but Japan sends spent fuel abroad for reprocessing. These
amounts do not include plutonium reprocessing from defense establishment reactors for weapons
fabrication.
The liquid wastes generated in reprocessing plants are stored temporarily in cooled stainless
steel tanks surrounded by reinforced concrete. After a cooling period, the liquid wastes are
calcined
(evaporated to dry powder) and
vitrified
(encased in molten glass). The molten glass is poured into
stainless steel canisters. In the United Kingdom, France, Belgium, and Sweden the canisters are
stored in deep silos, pending permanent disposal.
6.5.4
Permanent Waste Disposal
Perhaps the largest problem facing nuclear power plants is the permanent disposal of spent fuel, or
the waste remaining after extracting the still useful fuel from the spent fuel. The level of radioactivity
of the spent fuel declines about tenfold every hundred years. After about 1000 years the level reaches
that of the original ore from whence the fuel (uranium) was extracted.
15
The only practical way of disposing of the waste would be in stable geologic formations
known not to suffer from periodic earthquakes and where the water table is either absent or very
deep beneath the formation. For example, deep salt formations have these characteristics; otherwise
the salt would have leached out long ago.
In the United States a formation at Yucca Mountain, Nevada, has been selected for permanent
disposal. Current plans call for the repository facility to be excavated from volcanic tuff at a depth
of about 300 meters beneath Yucca Crest, which is 300 meters above the local water table. When
filled, the repository is projected to store 70,000 tons of spent fuel and 8000 tons of high-level
military waste. Many studies have been undertaken to model the fate of the disposed waste. Most
studies concluded that the waste would be undisturbed for periods of 10 thousands to millions of
years. Yet some uncertainties remain in these assessments that need to be clarified before official
approval of Yucca Mountain as the permanent spent fuel disposal site in the United States. Current
plans are for a 2010 opening of this site.
16
15
“The Environmental and Ethical Basis of Geological Disposal of Long Lived Radioactive Waste,” 1995.
OECD/Nuclear Energy Agency.
16
See a series of articles by J. F. Ahearne, K. D. Crowley, W. E. Kastenberg, L. J. Gratton, and D. W. North
in
Physics Today
,
50
, 22-62, June 1997.
