Interim Storage of Spent Nuclear Fuel - Woods Hole Research Center

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dry storage is safe. The 1990 U.S. Nuclear Regulatory
Commission (NRC) Waste Confidence Decision Review,
for example, concluded that spent fuel is safe at reactors,
either in cooling pools or in dry storage systems, for at
least 30 years beyond the reactor’s licensed life of operation.
Furthermore, the NRC went on to say that dry storage
in particular is “safe and environmentally acceptable for a
period of 100 years.” 15
Of course, this is not to say dry cask storage systems
face no safety challenges. Effective regulation and safety
monitoring is essential to ensure high-quality construction
of the casks and supporting facilities, and appropriate procedures
for loading the fuel and sealing the casks. The casks
require regular monitoring after being loaded. The system
must be designed to be safe not only during normal operations,
but in the event of plausible accidents as well, including
earthquakes, tornadoes, or plane crashes.
One important concern related to the long-term
behavior of the fuel is degradation of the fuel cladding as it
is exposed to the high temperatures generated by the spent
fuel in a dry storage environment. If too much degradation
were allowed to occur, the cladding might rupture and
allow pieces of fuel to fall out into the canister; if that were
to happen, when the fuel was eventually unloaded there
would be a potential contamination risk. For this reason,
both U.S. and Japanese regulatory agencies place strict limits
on the maximum temperature for dry storage (effectively
a limit of 380 degrees C in the U.S. case). 16
A few situations have highlighted the need for effective
regulatory oversight. In 1996, for example, after the fuel
had been loaded into a cask at the U.S. nuclear plant at
Point Beach, Wisconsin, and the cask lid was being welded
INTERIM STORAGE OF SPENT NUCLEAR FUEL
into place, hydrogen inside the cask ignited, lifting the
three-tonne lid approximately 3 inches and tilting it at a
slight angle. (This was actually the second hydrogen ignition
that month at that site.) The spent fuel was not damaged,
and a NRC review concluded that no measurable
radioactivity was released. Nor were there any unanticipated
exposures of workers from this incident. Nevertheless,
had circumstances been different, some modest radioactivity
might have been released into the environment from
such an incident. It appears that the hydrogen resulted
from an electrochemical reaction between the zinc coating
used in the storage canister and the borated water in the
spent fuel pool. NRC recommended reconsidering the use
of such zinc coatings when there is a potential that the coating
will be exposed to boric acid. Nevertheless, years later
occasional hydrogen ignition incidents in loading similar
spent fuel storage casks were continuing to occur. 17 Ideally,
cask materials should be designed to avoid generation of
potentially reactive gases such as hydrogen.
Similarly, the U.S. Nuclear Regulatory Commission
took action a number of times in the mid-1990s to address
defective welds that led to cracked seals in some vendors’
storage casks, and cask quality assurance problems at other
vendors. Cracks were found in the welds of the inner lids of
some casks, and if there had been cracks in both the inner
and outer lids, helium could have leaked out and moist air
could have leaked in, increasing temperatures and causing
additional fuel corrosion. Quality assurance issues at another
vendor led to the manufacture of casks that did not meet
licensed design specifications. After the NRC blocked loading
of certain cask types for a period, these issues appear to have
been resolved, and use of these casks is again permitted. 18
15 U.S. Nuclear Regulatory Commission, Waste Confidence Decision Review, published in the Federal Register, Vol. 55, No.
181 (Washington, DC: U.S. Government Printing Office, 1990) p. 38482.
16 See, for example, the discussion in Mujid S. Kazimi and Neil E. Todreas, “Nuclear Power Economic Performance: Challenges
and Opportunities,” Annual Review of Energy and the Environment, 1999.
17 For an account of the Point Beach incident, see, for example, U.S. Nuclear Regulatory Commission, Point Beach Augmented
Inspection Team Report, July 1, 1996; Lee Bergquist, “Utility Criticized In Fire: Wisconsin Electric Misread
Flames,” Milwaukee Journal Sentinel, June 8, 1996, p. 1; and Dave Airozo, “Point Beach Cask Incident Prompts Possible
NRC Enforcement Action,” Nuclear Fuel, September 9, 1996, p. 4. For a discussion of an inspection at the Palisades reactor
in 1999 during which a hydrogen ignition occurred during loading of storage casks, see U.S. Nuclear Regulatory
Commission, Palisades Nuclear Power Plant: Inspection Report 72-00007/99002(DNMS), August 26, 1999. Both of these
NRC documents and many others are available at the NRC site on dry cask storage, http://www.nrc.gov/OPA/drycask/.
18 See, for example, Jenny Weil, “Dry Spent Fuel Storage Problems Affect Vendors, Licensees,” Nuclear Fuel, June
30,1997, and documents on the NRC site cited above.