Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555-0001
July 11, 1997
NRC INFORMATION NOTICE 97-51: PROBLEMS EXPERIENCED WITH LOADING AND UNLOADING
SPENT NUCLEAR FUEL STORAGE AND TRANSPORTATION
CASKS
Addressees
All holders of operating licenses or construction permits for nuclear power
reactors.
Designers and fabricators of independent spent fuel storage installations
(ISFSIs).
All holders of or applicants for licenses to operate ISFSIs.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to inform addressees of operating experiences and problems encountered
with the loading and unload-ing of storage and transportation casks for spent
nuclear fuel. It is expected that recipients will review this information for
applicability to their facilities and consider actions, as appropriate, to
avoid similar problems. However, suggestions contained in this information
notice are not NRC requirements; therefore, no specific action or written
response is required.
Description of Circumstances
Limitations in storage capacity of spent fuel pools at nuclear power plants
and delays in the acceptance of spent nuclear fuel by the Federal Government
have led to an increasing use of dry storage systems located in ISFSIs at
reactor sites. As licensees have implemented their ISFSI plans, problems with
the interface between the cask designers and reactor licensees using the casks
have resulted in both safety concerns and delays in the movement of spent fuel
from the spent fuel pools to ISFSIs. Information learned from the NRC
inspections indicates that many of the problems encountered have involved the
activities for loading and unloading of the dry storage casks. This
information notice describes some of the problems encountered by licensees in
preparing for or actually performing the loading or unloading of storage or
transportation casks.
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Discussion
Cask Movement
Dry storage casks, weighing in excess of 100 tons, are among the heaviest
loads that are moved within reactor facilities during power operations. NRC
Bulletin 96-02, "Movement of Heavy Loads Over Spent Fuel, Over Fuel in the
Reactor Core, or Over Safety-Related Equipment," issued April 11, 1996,
requested that licensees review their plans and capabilities for handling
heavy loads, such as dry storage casks, and determine whether such activities
were adequately addressed by the analyses in the existing licensing basis for
the affected facilities. The staff subsequently requested some licensees to
provide information related to the movement of spent fuel storage or
transportation casks without the lids on those casks being secured in place.
As a result of these inquiries, one licensee determined that an unreviewed
safety question (USQ) was introduced by the existing practice of moving
transportation casks with the lids only partially secured and with vent and
drain lines exposed as a result of the removal of protective covers. The
licensee stated that the practice involved a USQ because the actual cask
configuration differed from the configuration assumed in the cask drop
analysis that is documented in the affected facility's updated final safety
analysis report.
Licensees have also experienced problems during the movement of casks as a
result of crane interlocks, errors in the accounting for the weights of cask
components, and human error. In moving cask components at the Davis-Besse
Nuclear Power Station, both during the dry run exercises and the actual
loading of a cask, actuation of electrical thermal overloads interrupted crane
operations during slow speed operation. The licensee subsequently learned
that a creeper motor installed on the crane was intended to be used during
sustained slow speed movements instead of the main hoist motor. At Prairie
Island on May 13, 1995, a cask remained in the hoisted position above the
spent fuel pool for approximately 16 hours while the licensee developed and
implemented corrective actions to address an overload-sensing system that was
inaccurately calibrated for lifting of a loaded dry storage cask. Changes in
the lifting procedure were required at Prairie Island when it was discovered
that a dry storage cask weighed more than expected. The weight difference was
found to be the result of acceptable variations in manufacturing tolerances
that had not been accounted for in previous weight calculations. At Point
Beach, procedure changes were required during the recovery from the hydrogen
ignition event described in NRC Bulletin 96-04, "Chemical, Galvanic, or Other
Reactions in Spent Fuel Storage and Transportation Casks." The procedure
changes were necessary after the licensee discovered that the lifting
procedure failed to account for a design change that had revised the shield
lid arrangement from a two-component system to a single shield lid.
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In September 1993, an empty dry storage cask was placed in Unit 3 spent fuel
pool at the Oconee Nuclear Station and was mispositioned on the cask pit
stand. This mispositioning resulted in the cask's leaning, which caused the
lifting hook to partially slip off the cask trunnion when trying to lift the
cask. The licensee secured the cask in a position in which it was partially
resting on the stand. Fuel assemblies located in the storage racks nearest
the cask were moved in case the cask tipped over within the spent fuel pool.
The licensee was able to retrieve the cask after ensuring that the lifting
hooks were properly positioned using temporary supports and additional
rigging. At Prairie Island, the licensee identified that the wear plate in
the cask laydown area was mispositioned during the dry run exercises for cask
loadings. This mispositioning was identified after the cask had been filled
with water and was about to be lowered into the spent fuel pool. The spent
fuel pool liner might have been damaged if a cask had actually been placed on
top of the mispositioned wear plate.
Vacuum Drying
During the loading of the third dry storage cask at Calvert Cliffs, the
licensee discovered that the low-range vacuum gauge range switch had been
improperly wired, causing the low-pressure readings taken during vacuum drying
of the first two casks to be off by a factor of 10. This mistake resulted in
the failure of those casks to satisfy the technical specification requirements
for the vacuum drying pressure to be maintained at less than 3 torr for at
least 30 minutes. The licensee was able to evaluate the effects of this error
and show that the vacuum drying had adequately removed oxidizing gases from
the two loaded casks. A license amendment was requested and approved that
allowed the casks to remain in service.
During the loading of the first dry storage cask at Arkansas Nuclear One, the
initial vacuum drying of the cask was performed over a period of 86 hours,
which was much longer than had been anticipated. During the evolution, the
licensee directed considerable attention toward eliminating leaks in the
vacuum system and lines. Additional purging with helium to remove moisture
from the cask and removal of water that had accumulated in a low spot in the
line between the cask and the vacuum pump ultimately allowed the licensee to
achieve and maintain the pressure required by the cask's certificate of
compliance.
Unloading Procedures
Although experience in the unloading of spent fuel casks is limited to
transportation casks and several storage casks that were only partially
through the loading process, NRC inspections and licensee assessments have
identified some issues pertaining to the procedures for the unloading of dry
storage casks. Following the discovery of radiographic .
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indications of possible defects in a weld in a loaded storage cask, the
licensee for Palisades reviewed its unloading procedure in preparation for its
potential use. The questions raised by the licensee's review concerned the
pressurization of the cask upon initiation of refilling the cask with water,
the required venting capacity for steam generated during the refilling of the
cask with water, the possible challenge to fuel rod integrity following heatup
in the transport cask and cooling during the refilling of the cask with water,
and possible problems in the drawing of a gas sample from the cask to
determine if fuel oxidation occurred during storage. On the basis of its
review, the NRC staff concluded that had the licensee attempted to unload a
cask using the original unloading procedure, the licensee would have had to
suspend operations and revise its procedure because a continuous cooling cycle
could not have been established. An administrative limit in the original
unloading procedure maintained cask pressure to less than 10 psig. This
pressure would not have been sufficient to force steam to the outlet of the
discharge piping at the bottom of the spent fuel pool. The licensee has
revised its unloading procedure to address the identified deficiencies and has
added additional monitoring and precautions for the control of hydrogen gas in
response to NRC Bulletin 96-04 as well.
Unloading of Transportation Casks
The unloading of boiling-water reactor (BWR) fuel from the Brunswick Steam
Electric Plant after transport to the Shearon Harris Nuclear Power Plant, a
pressurized-water reactor (PWR) with borated water in its spent fuel pools,
has resulted in problems with the release of large amounts of "crud." The
material released to the spent fuel pool at Shearon Harris is thought to be a
result of exposing the BWR assemblies to the borated water in the spent fuel
pool, as well as deposited material that comes off the BWR fuel rods during
transport. The release of crud from BWR fuel has resulted in radiological
concerns because the material has contaminated the Shearon Harris spent fuel
pool and increased the concentration of airborne particles during cask
unloading operations. Although the NRC staff has confirmed that the licensee
has implemented adequate controls to limit the radiological impact associated
with the crud for the present, the licensee has stated that additional
measures may be necessary as it continues with the transfer of fuel assemblies
to the Shearon Harris spent fuel pools.
Related Generic Communications
NRC Information Notice (IN) IN 95-29, "Oversight of Design and Fabrication
Activities for Metal Components Used in Spent Fuel Dry Storage Systems,"
issued June 7, 1995 (Accession Number 9506050138).
NRC IN 96-26, "Recent Problems With Overhead Cranes," issued April 30, 1996
(Accession Number 9604260095).
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NRC Bulletin 96-02, "Movement of Heavy Loads Over Spent Fuel, Over Fuel in the
Reactor Core, or Over Safety-Related Equipment," issued April 11, 1996
(Accession Number 9604080259).
NRC Bulletin 96-04, "Chemical, Galvanic, or Other Reactions in Spent Fuel
Storage and Transportation Casks," issued July 5, 1996 (Accession Number
9607020241).
NRC IN 97-39, "Inadequate 10 CFR 72.48 Safety Evaluations of Independent Spent
Fuel Storage Installations," issued June 26, 1997 (Accession Number
9706260174).
This information notice requires no specific action or written response. If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.
signed by signed by
S. H. Weiss for
Marylee M. Slosson, Acting Director William F. Kane, Director
Division of Reactor Program Management Spent Fuel Project Office
Office of Nuclear Reactor Regulation Office of Nuclear Material
Safety and Safeguards
Technical contacts: William D. Reckley, NRR
301-415-1314
E-mail: [email protected]
Eric J. Benner, NRR
301-415-1171
E-mail: [email protected]
