Information Notice 2013-07, Premature Degradation of Spent Fuel Storage Cask Structures and Components from Environmental Moisture
| ML12320A697 | |
| Person / Time | |
|---|---|
| Issue date: | 04/16/2013 |
| From: | Mark Lombard NRC/NMSS/SFST |
| To: | |
| Allen W | |
| References | |
| IN-13-007 | |
| Download: ML12320A697 (6) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR MATERIAL
SAFETY AND SAFEGUARDS
WASHINGTON, DC 20555-0001 April 16, 2013 NRC INFORMATION NOTICE 2013-07: PREMATURE DEGRADATION OF SPENT FUEL
STORAGE CASK STRUCTURES AND
COMPONENTS FROM ENVIRONMENTAL
MOISTURE
ADDRESSEES
All holders of, applicants for, and registered users of spent fuel storage system certificates of
compliance (CoCs) as well as all holders of and applicants for an independent spent fuel
storage installation (ISFSI) license under Title 10 of the Code of Federal Regulations (10 CFR)
Part 72, Licensing Requirements for the Independent Storage of Spent Nuclear Fuel, High- Level Radioactive Waste, and Reactor-Related Greater Than Class C Waste.
PURPOSE
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to inform
addressees of recent operating experience on environmental moisture causing premature
degradation of structures and components important to safety during spent nuclear fuel storage
operations. The NRC expects recipients to review the information for applicability to their
facilities and consider actions, as appropriate, to avoid similar problems. However, suggestions
contained in this IN are not NRC requirements; therefore, no specific action or written response
is required.
DESCRIPTION OF CIRCUMSTANCES
Peach Bottom Atomic Power Station ISFSI
On October 11, 2010, a cask seal pressure monitoring system low pressure alarm was received
for Cask TN-68-01 during storage at the Peach Bottom Atomic Power Station ISFSI. Cask TN-
68-01 is a bolted closure cask system with a double mechanical O-ring seal (cask lid seal) that
provides confinement between the lid and cask interface. The cask had been in service at
Peach Bottom since June 2000. Figure 1 shows a cross sectional view of the lid region for a
TN-68 cask. A protective cover was installed on the lid region to protect the system from
external weathering. During disassembly and removal of the protective cover, the licensee
found streaks of rust on the underside of the cover, a pronounced pattern of rust directly under
the access plate, and water or signs of moisture around most of the bolt lid holes and bolts. The
licensee found the elastomer O-ring seal on the bottom of the protective cover to be completely
intact and sealed against the top of the cask lid. After performing a sequence of helium leak
tests, the licensee identified that the outer sealing surface of the main cask lid seal was leaking
at a rate greater than allowed by the CoC technical specifications. The licensee returned the
ML12320A697 spent fuel assemblies to the spent fuel pool to perform additional inspections on the cask lid and
seals. The initial evaluation revealed corrosion of the outer portion of the cask lid seal, lower
than expected torque on some of the main lid bolts, and corrosion on the threads of the lid bolts.
The inner portion of the cask lid seal remained intact; therefore, the casks primary confinement
was not compromised.
A root cause evaluation concluded that the seal leakage was caused by corrosion of the outer
portion of the cask lid seal from water infiltration through the access plate in the protective
cover. The water infiltration caused galvanic corrosion of the outer portion of the cask lid seal
due to the presence of moisture at the interface of the aluminum-clad cask lid seal and the
stainless steel clad cask body sealing surface. The presence of the moisture at the interface of
the two dissimilar metals set up a galvanic cell that caused the aluminum to corrode and
allowed helium to leak through the outer portion of the cask lid seal. The root cause evaluation
further stated that the helium leak was attributed to inadequate sealing of the access plate in the
protective cover and a lack of any verification of the integrity of the water-tight cover. The
primary corrective actions developed by the cask vendor and the licensee involved improving
the access plate design and developing a method for verifying protective cover seal integrity.
Additional corrective actions, which were incorporated into operating procedures immediately
after the event, included a change to the lid bolt torquing process and ensuring access plate
gaskets and O-rings were inspected at installation.
Figure 1 Additional information is available in Peach Bottom Atomic Power StationNRC Inspection
Report 05000277/12010010, dated July 8, 2011 (Agencywide Documents Access and
Management System (ADAMS) Accession No. ML111890441).
Three Mile Island, Unit 2 ISFSI at the Idaho National Laboratory Site
The Three Mile Island, Unit 2 ISFSI uses NUHOMS-12T horizontal storage modules (HSMs).
The HSMs were delivered to the Idaho National Laboratory site in 1999 as precast concrete components. The storage system consists of an external rectangular reinforced concrete vault
(i.e., HSM) with a storage canister resting horizontally on internal rails inside the HSM. The
prefabricated modules consist of a body and a roof joined together by anchor bolts. All sections
were a minimum of 0.6-meters (2-feet) thick. In 2000, the licensee noted cracks in the HSMs, and concluded they were cosmetic and insignificant. However, in 2007, the licensee observed
continued cracking, crazing and spalling as well as increased efflorescence on the HSM
surfaces. The efflorescence was a solid, whitish crystalline material which was determined
through sampling and analysis to be calcium carbonate. The licensee performed an evaluation
in 2007, during which it determined that the HSMs were capable of performing their design
basis functions. In 2008, the licensee noted that 28 of the 30 HSMs had cracks, mostly
emanating from the anchor bolt blockout holes with widths up to 0.95 centimeters (0.38 inches).
At that time, the licensee determined that the HSMs appeared to be prematurely deteriorating
and that continued crack growth could impact the ability of the HSMs to fulfill their originally
planned 50-year design service life. Subsequent evaluations by the licensee initiated the
development of an annual inspection plan for the HSMs and base mat as well as an
examination of the inside of the HSMs. The evaluation also recommended that the licensee
retain the services of a company experienced and qualified in testing and evaluating concrete to
determine the degradation mechanism and make recommendations both for repairs and to
prevent further degradation. Although the cracking was discussed with the storage system
vendor, the licensee chose an independent vendor to perform an evaluation of the HSMs and
base mat concrete in 2009. The evaluation included a field investigation and laboratory analysis
to evaluate the concrete material quality, strength, and long-term durability potential. The
conclusion reached was that water had entered the anchor bolt blockout holes on the roof of the
HSMs. Subsequent freeze and thaw cycles initiated the crack formation. Repetition of the
process resulted in both continued crack growth and the efflorescence growth identified in 2007.
In addition to identifying the root cause of the cracking, the report also suggested repairs
(injecting resin into the cracks), preventative actions (e.g., installing caps over the anchor bolt
blockout holes), and monitoring (use of crack gauges). The licensee incorporated the
suggested corrective actions.
Additional information is available in Three Mile Island, Unit 2, ISFSINRC Inspection of the
Independent Spent Fuel Storage InstallationInspection Report 07200020/2012-001, dated
August 14, 2012 (ADAMS Accession No. ML12228A457).
DISCUSSION
The instances described above illustrate how the intrusion of water can potentially decrease the
effective life of both the structures and components of a spent fuel storage system. In one
instance, the presence of water not only caused chemical degradation through oxidation of one
metal, but it also facilitated the formation of a galvanic cell between two dissimilar metals that
contributed to the degradation of the secondary confinement barrier of the storage system. In
another instance, water contributed to an accelerated aging process of concrete structures of
the spent fuel storage system. Water entered cracks and crevices around the anchor bolt
blockout holes in the concrete structure, and when subjected to freezing temperatures, generated mechanical forces that produced cracks in the concrete. These cracks provided
additional and larger pathways for water to enter the interior of the concrete which resulted in
larger cracks from subsequent freezing temperatures and promoted efflorescence. If remedial
actions had not been taken, this accelerated aging process could have inhibited the ability of the
concrete structure to perform its design function of protecting the canister system containing the radioactive material, as well as protecting personnel from ionizing radiation, during normal and
accident conditions.
The effects of weathering and environmental moisture may lead to degradation of structures, systems, and components. Several phenomena are discussed in NUREG-1536, Standard
Review Plan for Spent Fuel Dry Storage Systems at a General License Facility, NUREG-1567, Standard Review Plan for Spent Fuel Dry Storage Facilities, and Table D-1 of NUREG-1927, Standard Review Plan for Renewal of Spent Fuel Dry Cask Storage System Licensees and
Certificates of Compliance (ADAMS Accession Nos. ML101040620, ML003686776, and
ML111020115, respectively). Identifying potential moisture entry points, such as cracks, crevices and joints in both vertical and horizontal storage systems, can facilitate the
incorporation of gaskets and sealing materials into both the design of and maintenance of spent
nuclear fuel storage systems to minimize premature degradation of structures and components
important to safety. Adequate drainage of the base mat (i.e., ISFSI pad) may also prove
advantageous for the following reasons. First, pooled water may cause premature degradation
of the base mat. Second, since humidity and deliquescence have been shown to contribute to
stress corrosion cracking in marine environments (see Information Notice IN2012-20, ADAMS
Accession Nos. ML12139A440), the combination of pooled water and heat from canisters
containing irradiated spent nuclear fuel could produce humid conditions within the storage
system.
These examples show the importance of periodically monitoring the physical condition of a
spent nuclear fuel storage system. By obtaining baseline measurements and performing
periodic evaluations, accelerated degradation can be detected before the structures and
components of a storage system are unable to perform their intended function, and corrective
actions can be implemented. Such information may prove useful in assessing aging
management in license renewal applications.
CONTACT
This IN requires no specific action or written response. Please direct any questions about this
matter to the technical contacts listed below or to the appropriate Spent Fuel Storage and
Transportation (SFST) project manager.
/RA/
Mark Lombard, Director
Division of Spent Fuel Storage
and Transportation
Office of Nuclear Material Safety
and Safeguards
Technical Contacts: Chris Allen, NMSS/SFST
301-492-3148 E-mail: william.allen@nrc.gov
John Nicholson, R-I/DNMS
610-337-5236 E-mail: john.nicholson@nrc.gov
Lee Brookhart, R-IV/DNMS
817-200-1549 E-mail: lee.brookhart@nrc.gov
Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under NRC Library.
ML12320A697 OFC: SFST Technical Editor BC:RIV/NMSS/RSFS BC:RI/NMSS/Dec
KAzariah-Kribbs BSpitzberg MFerdas
NAME: WAllen
via e-mail via e-mail via e-mail
DATE: 11/9/12 11/14/12 1/18/13 2/14/13 OFC: BC:SFST/RIO BC:SFST/LB D:SFST
NAME: EBenner MSampson MLombard
DATE: 3/4/13 3/22/13 4/ 10 /13, 4/16/2013