Information Notice 2013-07, Premature Degradation of Spent Fuel Storage Cask Structures and Components from Environmental Moisture: Difference between revisions

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{{#Wiki_filter:ML12320A697 April 16, 2013 NRC INFORMATION NOTICE 2013-07: PREMATURE DEGRADATION OF SPENT FUEL STORAGE CASK STRUCTURES AND COMPONENTS FROM ENVIRONMENTAL MOISTURE
{{#Wiki_filter:ML12320A697 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==
==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."
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==
==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 operation The NRC expects recipients to review the information for applicability to their facilities and consider actions, as appropriate, to avoid similar problem However, suggestions contained in this IN are not NRC requirements; therefore, no specific action or written response is require
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==
==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 ISFS Cask TN-
Peach Bottom Atomic Power Station ISFSI
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 200 Figure 1 shows a cross sectional view of the lid region for a TN-68 cas A protective cover was installed on the lid region to protect the system from external weatherin 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 bolt 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 li 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 specification The licensee returned the spent fuel assemblies to the spent fuel pool to perform additional inspections on the cask lid and seal 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 bolt The inner portion of the cask lid seal remained intact; therefore, the cask's primary confinement was not compromise 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 cove 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 surfac 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 sea 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 cove 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 integrit 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 installatio Figure 1 Additional information is available in "Peach Bottom Atomic Power Station-NRC Inspection Report 05000277/12010010," dated July 8, 2011 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML111890441).
 
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 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 cask's 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 Station-NRC 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, ISFSI-NRC Inspection of the


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 component 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 HS The prefabricated modules consist of a body and a roof joined together by anchor bolt All sections were a minimum of 0.6-meters (2-feet) thic In 2000, the licensee noted cracks in the HSMs, and concluded they were cosmetic and insignifican However, in 2007, the licensee observed continued cracking, crazing and spalling as well as increased efflorescence on the HSM surface The efflorescence was a solid, whitish crystalline material which was determined through sampling and analysis to be calcium carbonat The licensee performed an evaluation in 2007, during which it determined that the HSMs were capable of performing their design basis function 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).
Independent Spent Fuel Storage Installation-Inspection Report 07200020/2012-001," dated


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 lif 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 HSM 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 degradatio 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 200 The evaluation included a field investigation and laboratory analysis to evaluate the concrete material quality, strength, and long-term durability potentia The conclusion reached was that water had entered the anchor bolt blockout holes on the roof of the HSM Subsequent freeze and thaw cycles initiated the crack formatio Repetition of the process resulted in both continued crack growth and the efflorescence growth identified in 200 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 action Additional information is available in "Three Mile Island, Unit 2, ISFSI-NRC Inspection of the Independent Spent Fuel Storage Installation-Inspection Report 07200020/2012-001," dated August 14, 2012 (ADAMS Accession No. ML12228A457).
August 14, 2012 (ADAMS Accession No. ML12228A457).


==DISCUSSION==
==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 syste 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 syste In another instance, water contributed to an accelerated aging process of concrete structures of the spent fuel storage syste 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 concret 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 efflorescenc 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 condition The effects of weathering and environmental moisture may lead to degradation of structures, systems, and component Several phenomena are discussed in NUREG-1536, "Standard Review Plan for Spent Fuel Dry Storage Systems at a General License Facility," NUREG-1567,  
The instances described above illustrate how the intrusion of water can potentially decrease the
"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 safet Adequate drainage of the base mat (i.e., ISFSI pad) may also prove advantageous for the following reason First, pooled water may cause premature degradation of the base ma 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 syste These examples show the importance of periodically monitoring the physical condition of a spent nuclear fuel storage syste 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 implemente Such information may prove useful in assessing aging management in license renewal application
 
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==
==CONTACT==
This IN requires no specific action or written respons Please direct any questions about this matter to the technical contacts listed below or to the appropriate Spent Fuel Storage and Transportation (SFST) project manage /RA/
This IN requires no specific action or written response.  Please direct any questions about this
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 Librar
 
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.


==CONTACT==
==CONTACT==
This IN requires no specific action or written respons Please direct any questions about this matter to the technical contacts listed below or to the appropriate Spent Fuel Storage and Transportation (SFST) project manage /RA/  
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.
 
ADAMS Accession No. ML12320A697 OFC: SFST Technical Editor BC:RIV/NMSS/RSFS BC:RI/NMSS/Dec NAME: WAllen KAzariah-Kribbs      via e-mail BSpitzberg  via e-mail MFerdas 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  OFFICIAL RECORD COPY


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 Librar ADAMS Accession No. ML12320A697 OFC: SFST Technical Editor BC:RIV/NMSS/RSFS BC:RI/NMSS/Dec NAME: WAllen KAzariah-Kribbs via e-mail BSpitzberg via e-mail MFerdas 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 OFFICIAL RECORD COPY}}
}}


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Revision as of 09:51, 28 March 2018

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 C
References
IN-13-007
Download: ML12320A697 (6)
v • d • e


ML12320A697 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 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 cask's 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 Station-NRC 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, ISFSI-NRC Inspection of the

Independent Spent Fuel Storage Installation-Inspection 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.

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.

ADAMS Accession No. ML12320A697 OFC: SFST Technical Editor BC:RIV/NMSS/RSFS BC:RI/NMSS/Dec NAME: WAllen KAzariah-Kribbs via e-mail BSpitzberg via e-mail MFerdas 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 OFFICIAL RECORD COPY


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