Information Notice 2013-18, Refueling Water Storage Tank Degradation: Difference between revisions

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| issue date = 09/13/2013
| issue date = 09/13/2013
| title = Refueling Water Storage Tank Degradation
| title = Refueling Water Storage Tank Degradation
| author name = Cheok M C, Kokajko L E
| author name = Cheok M, Kokajko L
| author affiliation = NRC/NRO, NRC/NRR/DPR
| author affiliation = NRC/NRO, NRC/NRR/DPR
| addressee name =  
| addressee name =  
Line 9: Line 9:
| docket =  
| docket =  
| license number =  
| license number =  
| contact person = Klos L J
| contact person = Klos L
| document report number = IN-13-018
| document report number = IN-13-018
| document type = NRC Information Notice
| document type = NRC Information Notice
| page count = 6
| page count = 6
}}
}}
{{#Wiki_filter:ML13128A118 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION OFFICE OF NEW REACTORS WASHINGTON, DC 20555-0001 September 13, 2013 NRC INFORMATION NOTICE 2013-18: REFUELING WATER STORAGE TANK DEGRADATION
{{#Wiki_filter:UNITED STATES
 
NUCLEAR REGULATORY COMMISSION
 
OFFICE OF NUCLEAR REACTOR REGULATION
 
OFFICE OF NEW REACTORS
 
WASHINGTON, DC 20555-0001 September 13, 2013 NRC INFORMATION NOTICE 2013-18:               REFUELING WATER STORAGE TANK
 
DEGRADATION


==ADDRESSEES==
==ADDRESSEES==
All holders of an operating license or construction permit for a nuclear power plant issued under Title 10 of the
All holders of an operating license or construction permit for a nuclear power plant issued under
 
Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Domestic Licensing of
 
Production and Utilization Facilities, except those who have permanently ceased operations
 
and have certified that fuel has been permanently removed from the reactor vessel.


Code of Federal Regulations (10 CFR) Part 50, "Domestic Licensing of Production and Utilization Facilities," except those who have permanently ceased operations and have certified that fuel has been permanently removed from the reactor vessel.
All holders of and applicants for a power reactor early site permit, combined license, standard


All holders of and applicants for a power reactor early site permit, combined license, standard design certification, standard design approval, or manufacturing license under 10 CFR Part 52,  
design certification, standard design approval, or manufacturing license under 10 CFR Part 52, Licenses, Certifications, and Approvals for Nuclear Power Reactors.
"Licenses, Certifications, and Approvals for Nuclear Power Reactors."


==PURPOSE==
==PURPOSE==
The U.S. Nuclear Regulatory Commission (NRC) is is
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to inform
 
addressees of potential issues associated with leakage due to flaws in refueling water storage


suing this information notice (IN) to inform addressees of potential issues associated with leakage due to flaws in refueling water storage
tanks. The NRC expects recipients to review the information for applicability to their facilities


tanks. The NRC expects recipients to review the information for applicability to their facilities
and consider taking action, as appropriate. Suggestions contained in this IN are not NRC


and consider taking action, as appropriate.  Suggestions contained in this IN are not NRC requirements; therefore, no specific action or written response is required.
requirements; therefore, no specific action or written response is required.


==DESCRIPTION OF CIRCUMSTANCES==
==DESCRIPTION OF CIRCUMSTANCES==


===Kewaunee Power Station===
===Kewaunee Power Station===
The refueling water storage tank (RWST) at Kewaunee Power Station is a stainless steel tank of
welded construction with a capacity of 272,500 gallons that is located in the auxiliary building.


The refueling water storage tank (RWST) at Kewaunee Power Station is a stainless steel tank of welded construction with a capacity of 272,500 gallons that is located in the auxiliary building.  On July 10, 2012, the licensee identified a boric acid deposit adjacent to the weld connecting
On July 10, 2012, the licensee identified a boric acid deposit adjacent to the weld connecting


the tank wall to the floor plate. No actual leakage of water from the tank was observed. The
the tank wall to the floor plate. No actual leakage of water from the tank was observed. The


licensee postulated that the boric acid deposit was due to a pinhole leak that resulted from an
licensee postulated that the boric acid deposit was due to a pinhole leak that resulted from an


original weld defect or some form of stress corrosion cracking, e.g., chloride stress corrosion cracking. The licensee justified continued operation of the tank through the use of American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (Code) Case N-705,  
original weld defect or some form of stress corrosion cracking, e.g., chloride stress corrosion
"Evaluation Criteria for Temporary Acceptance of Degradation in Moderate Energy Class 2 or 3 Vessels and Tanks," until permanent repairs could be made to the tank. Palisades Nuclear Plant
 
cracking. The licensee justified continued operation of the tank through the use of American
 
Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (Code) Case N-705, Evaluation Criteria for Temporary Acceptance of Degradation in Moderate Energy Class 2 or 3 Vessels and Tanks, until permanent repairs could be made to the tank.
 
===ML13128A118 Palisades Nuclear Plant===
The Palisades Nuclear Plant safety injection refueling water tank (SIRWT1) is an aluminum tank
 
of welded construction with a capacity of 300,000 gallons that is located on the roof of the
 
auxiliary building. The tank was constructed in accordance with ASME Standard B96.1, Welded


The Palisades Nuclear Plant safety injection refueling water tank (SIRWT
Aluminum-Alloy Storage Tanks, which is inactive and has no replacement. Part of the tank is


1) is an aluminum tank of welded construction with a capacity of 300,000 gallons that is located on the roof of the
located directly above the control room. Numerous pipes exit the floor of the tank through


auxiliary building.  The tank was constructed in accordance with ASME Standard B96.1, Welded Aluminum-Alloy Storage Tanks, which is inactive and has no replacement.  Part of the tank is located directly above the control room.  Numerous pipes exit the floor of the tank through nozzles.
nozzles.


Leakage into the auxiliary building concrete roof structure (catacombs) was observed beginning in the spring of 2011.  Despite repair efforts in 2012, leakage from the tank, sometimes accompanied by rainwater leaking through the roof around the tank, was present until the majority of the tank floor was replaced during the summer of 2013.
Leakage into the auxiliary building concrete roof structure (catacombs) was observed beginning


Inspection and repair efforts revealed the following issues:
in the spring of 2011. Despite repair efforts in 2012, leakage from the tank, sometimes


1. Inspections were conducted using visual, surface and electromagnetic techniques.  These techniques revealed numerous flaws in the tank floor welds and floor to shell weld
accompanied by rainwater leaking through the roof around the tank, was present until the


seam. Some flaws in the floor plates were also identified. These flaws were generally
majority of the tank floor was replaced during the summer of 2013.
 
Inspection and repair efforts revealed the following issues:
1.      Inspections were conducted using visual, surface and electromagnetic techniques.
 
These techniques revealed numerous flaws in the tank floor welds and floor to shell weld
 
seam. Some flaws in the floor plates were also identified. These flaws were generally


associated with arc strikes and/or spatter, were not through wall, and may have been
associated with arc strikes and/or spatter, were not through wall, and may have been
Line 64: Line 101:
original construction defects.
original construction defects.


2. Through wall defects (leaks) were located in the tank floor welds. Some of these defects were associated with tank floor to penetration nozzle welds. As part of the repair
2.     Through wall defects (leaks) were located in the tank floor welds. Some of these defects
 
were associated with tank floor to penetration nozzle welds. As part of the repair


process the penetrations were redesigned to improve nozzle strength and to permit the
process the penetrations were redesigned to improve nozzle strength and to permit the
Line 70: Line 109:
nozzles to move in response to flexing of the tank floor.
nozzles to move in response to flexing of the tank floor.


3. ASME Standard B96.1 does not contain specific guidance for the construction of tank floor penetration as used in this tank.
3.     ASME Standard B96.1 does not contain specific guidance for the construction of tank
 
floor penetration as used in this tank.
 
4.      Leakage from the tank recurred in 2013, requiring plant shutdown. This leakage was
 
attributed to the failure of a nozzle repair weld made in 2012. A destructive evaluation of
 
the weld indicated significant lack of fusion that could be the result of inadequate


4. Leakage from the tank recurred in 2013, requiring plant shutdown.  This leakage was attributed to the failure of a nozzle repair weld made in 2012. A destructive evaluation of
cleaning of the weld surfaces, and/or low weld heat input.


the weld indicated significant lack of fusion that could be the result of inadequate cleaning of the weld surfaces, and/or low weld heat input.
5.      Design drawings for the tank indicated that the tank shell was supported by a concrete


5. Design drawings for the tank indicated that the tank shell was supported by a concrete ring foundation and that the tank floor rested on a sand bed. During tank floor
ring foundation and that the tank floor rested on a sand bed. During tank floor


replacement in 2013 it was determined that neither the ring foundation nor the sand bed was present. Given that the entire tank bottom sits on the concrete roof structure of the auxiliary building, these differences in the "as designed" and "as built" tank are not
replacement in 2013 it was determined that neither the ring foundation nor the sand bed
 
was present. Given that the entire tank bottom sits on the concrete roof structure of the
 
auxiliary building, these differences in the as designed and as built tank are not


structurally significant.
structurally significant.


6. Some of the nozzles which were encased in concrete exhibited denting. Denting appears to be the result of a two-step process. In the first step aluminum corrodes as a result of contact with moist concrete. In the second step, the resulting corrosion products, which are more voluminous than the metal from which they are formed, force
6.     Some of the nozzles which were encased in concrete exhibited denting. Denting
 
appears to be the result of a two-step process. In the first step aluminum corrodes as a
 
result of contact with moist concrete. In the second step, the resulting corrosion
 
products, which are more voluminous than the metal from which they are formed, force


the nozzle inward.
the nozzle inward.


1 SIRWT is a site-specific term that is essentially equivalent to the generic term RWST. South Texas Project, Unit 1
1 SIRWT is a site-specific term that is essentially equivalent to the generic term RWST. South Texas Project, Unit 1 The RWST at South Texas Project, Unit 1, is an ASME Code, Section III, Class 2 seismically- qualified stainless steel tank of welded construction with a capacity of 550,000 gallons. It is


The RWST at South Texas Project, Unit 1, is an ASME Code, Section III, Class 2 seismically- qualified stainless steel tank of welded construction with a capacity of 550,000 gallons.  It is
located inside the Unit 1 auxiliary building and has drains connected to the radioactive drain


located inside the Unit 1 auxiliary building and has drains connected to the radioactive drain
waste collection system. In September 1997, the licensee observed a spot of rust, traces of
 
moisture, and some boric acid crystals at the RWST floor to tank wall weld. Since no actual


waste collection system.  In September 1997, the licensee observed a spot of rust, traces of moisture, and some boric acid crystals at the RWST floor to tank wall weld.  Since no actual leakage of water was observed, it was initially suspected the boric acid residue may have come
leakage of water was observed, it was initially suspected the boric acid residue may have come


from a tank connection above this location. Based on an engineering evaluation, the licensee
from a tank connection above this location. Based on an engineering evaluation, the licensee


initiated a periodic monitoring program of the RWST but took no additional action.
initiated a periodic monitoring program of the RWST but took no additional action.


In February 1999, the licensee observed boric acid crystals in the same location as identified in 1997. The licensee performed a failure analysis using in-situ replication metallography and determined that the observed boric acid crystals were the result of a through wall crack at the
In February 1999, the licensee observed boric acid crystals in the same location as identified in
 
1997. The licensee performed a failure analysis using in-situ replication metallography and
 
determined that the observed boric acid crystals were the result of a through wall crack at the
 
base plate weld (tank floor to tank shell). Additionally, the licensee determined that the crack


base plate weld (tank floor to tank shell).  Additionally, the licensee determined that the crack was transgranular and branched, consistent with chloride stress corrosion cracking.
was transgranular and branched, consistent with chloride stress corrosion cracking.


In 2001, the licensee performed a visual examination from the inside of the RWST using a video
In 2001, the licensee performed a visual examination from the inside of the RWST using a video


camera on a remotely controlled submersible device. No evidence of base plate or side wall
camera on a remotely controlled submersible device. No evidence of base plate or side wall


cracking was observed, and no repairs were perform
cracking was observed, and no repairs were performed. Periodic monitoring of the exterior of


ed.  Periodic monitoring of the exterior of the tank continued.
the tank continued.


In September 2011, boric acid residues were discovered under the base lip of the RWST in two
In September 2011, boric acid residues were discovered under the base lip of the RWST in two


new locations near the RWST discharge line. Following discussions with the NRC, STP conducted the necessary calculations to demonstrate the structural adequacy of the tank in
new locations near the RWST discharge line. Following discussions with the NRC, STP
 
conducted the necessary calculations to demonstrate the structural adequacy of the tank in


accordance with ASME Code Case N-705. The licensee made repairs to the tank in
accordance with ASME Code Case N-705. The licensee made repairs to the tank in


accordance with the ASME Code, Section XI, in the fall of 2012.
accordance with the ASME Code, Section XI, in the fall of 2012.
Line 123: Line 190:
operations and supplying water to the emergency core cooling system and the containment
operations and supplying water to the emergency core cooling system and the containment


spray system in emergency situations. RWSTs are fabricated from stainless steel or aluminum to minimize corrosion resulting from the borated water stored in the tank. These tanks are generally classified as ASME Class 2 for purposes of inspection. Leaks from the tanks may
spray system in emergency situations. RWSTs are fabricated from stainless steel or aluminum
 
to minimize corrosion resulting from the borated water stored in the tank. These tanks are
 
generally classified as ASME Class 2 for purposes of inspection. Leaks from the tanks may


result in failure to meet the tank's intended safety function due to loss of inventory, contamination of soil and or groundwater due to release of tritium to the environment, and/or
result in failure to meet the tanks intended safety function due to loss of inventory, contamination of soil and or groundwater due to release of tritium to the environment, and/or


damage to safety related equipment due to water spray or flooding.
damage to safety related equipment due to water spray or flooding.


==DISCUSSION==
==DISCUSSION==
The above events demonstrate that leaks from RWSTs can result in disruption of plant operations. Based on the above events, the NRC believes that the following observations may
The above events demonstrate that leaks from RWSTs can result in disruption of plant
 
operations. Based on the above events, the NRC believes that the following observations may
 
be of value to licensees:
1.      Tank inspections have determined that all of the leaks have occurred in welds which are
 
in or near the bottom of the tank.
 
2.        Although additional degradation modes appear possible, the subject leaks have been
 
attributed to weld fabrication flaws, stress corrosion cracking, and/or high stress low


be of value to licensees:
cycle fatigue. 3.      The degradation mechanisms which have been identified are not those described for
1. Tank inspections have determined that all of the leaks have occurred in welds which are in or near the bottom of the tank.


2.  Although additional degradation modes appear possible, the subject leaks have been attributed to weld fabrication flaws, stress corrosion cracking, and/or high stress low cycle fatigue. 3. The degradation mechanisms which have been identified are not those described for above ground tanks in NUREG 1801, the Generic Aging Lessons Learned (GALL)  
above ground tanks in NUREG 1801, the Generic Aging Lessons Learned (GALL)
Report (ADAMS Accession No. ML103490041). Based on the events cited, the NRC is
        Report (ADAMS Accession No. ML103490041). Based on the events cited, the NRC is


considering whether modifications to the GALL Report are warranted.
considering whether modifications to the GALL Report are warranted.


4. Although the events described in this IN resulted in leakage, none of the events has challenged the structural integrity of the tank.
4.     Although the events described in this IN resulted in leakage, none of the events has
 
challenged the structural integrity of the tank.
 
5.      As demonstrated by two of the events described above and documented in IN 2012-20,
        Potential Chloride-Induced Stress Corrosion Cracking of Austenitic Stainless Steel and
 
Maintenance of Dry Cask Storage System Canisters (ADAMS Accession No.
 
ML12319A440), cracking of 300 series stainless steel may occur in environments


5. As demonstrated by two of the events described above and documented in IN 2012-20, "Potential Chloride-Induced Stress Corrosion Cracking of Austenitic Stainless Steel and Maintenance of Dry Cask Storage System Canisters" (ADAMS Accession No. ML12319A440), cracking of 300 series stainless steel may occur in environments consisting of moist air and chloride containing surface deposits at temperatures less
consisting of moist air and chloride containing surface deposits at temperatures less


than 140 degrees F.
than 140 degrees F.


6. Due to the age of some of the tanks under consideration, differences in the "as designed" and "as built" tank may exist. An understanding of the existence and extent of
6.     Due to the age of some of the tanks under consideration, differences in the as
 
designed and as built tank may exist. An understanding of the existence and extent of


these differences may be significant in determining the susceptibility of the tank to
these differences may be significant in determining the susceptibility of the tank to
Line 152: Line 244:
degradation.
degradation.


7. The interaction of aluminum and moist concrete may play a role in the long term performance of aluminum components.
7.     The interaction of aluminum and moist concrete may play a role in the long term
 
performance of aluminum components.


In summary, recent events have indicated that RWSTs may undergo degradation which may
In summary, recent events have indicated that RWSTs may undergo degradation which may
Line 158: Line 252:
lead to disruption of plant operations and could result in release of tritium to the environment.
lead to disruption of plant operations and could result in release of tritium to the environment.


These events to date have been attributed to a variety of causes and have resulted in leakage issues, but have not challenged the structural integrity of the tanks. Licensees are encouraged to consider a wide range of environmental and mechanical degradation modes for these tanks
These events to date have been attributed to a variety of causes and have resulted in leakage
 
issues, but have not challenged the structural integrity of the tanks. Licensees are encouraged
 
to consider a wide range of environmental and mechanical degradation modes for these tanks


when developing or revising inspection or aging management programs.
when developing or revising inspection or aging management programs.


==CONTACT==
==CONTACT==
S This IN requires no specific action or written response. Please direct any questions about this
S
 
This IN requires no specific action or written response. Please direct any questions about this
 
matter to the technical contact listed below or the appropriate project managers in the Office of
 
Nuclear Reactor Regulation (NRR) and Office of New Reactors (NRO).
 
/RA/ (Sher Bahadur Acting for)                /RA/
Lawrence E. Kokajko, Director                  Michael Cheok, Acting Director


matter to the technical contact listed below or the appropriate project managers in the Office of Nuclear Reactor Regulation (NRR) and Office of New Reactors (NRO).
Division of Policy and Rulemaking              Division of Construction Inspection


/RA/ (Sher Bahadur Acting for)  /RA/
Office of Nuclear Reactor Regulation          and Operational Programs
Lawrence E. Kokajko, Director  Michael Cheok, Acting Director


Division of Policy and Rulemaking  Division of Construction Inspection Office of Nuclear Reactor Regulation    and Operational Programs      Office of New Reactors
Office of New Reactors


===Technical Contact:===
===Technical Contact:===
David Alley, NRR/DE/EPNB 301-415-2178 E-mail: David.Alley@nrc.gov
David Alley, NRR/DE/EPNB
 
301-415-2178 E-mail: David.Alley@nrc.gov


Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under the NRC Library.
Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under the NRC Library.


ML13128A118     TAC No. MF1357 OFFICE NRR:DE:EPNB NRR:DE:EPNB BC:RIII/DRP/RPB4 Tech Editor NAME JTsao DAlley JGiessner CHsu DATE 8/29/2013 8/23/2013 8/29/2013 5/8/2013 OFFICE RES:/DE/CIB/BC BC:DE:EPNB BC:DE:EVIB DE:SLS NAME ACsontos TLupold SRosenberg RHardies DATE 8/30/2013 8/29/2013 8/30/2013 8/29/2013 OFFICE DLR:RAPB BC:NRR:DLR: RAPBD:NRR:DE D:NRO/DCIP (Acting) NAME WHolston KGreen PHiland MCheok DATE 5/7/2013 5/7/2013 8/29/2013 9/3/2013 OFFICE PM:NRR:PGCB LA:NRR:PGCB BC:NRR:PGCB DD:NRR:DPR D:NRR:DPR NAME JKlos CHawes (TAlexion for) SStuchell SBahadur (S.Bahadur for) LKokajko DATE 9/3/2013 9/4/2013 9/11/2013 9/12/2013 9/ 13   /2013}}
ML13128A118                                             TAC No. MF1357 OFFICE   NRR:DE:EPNB     NRR:DE:EPNB         BC:RIII/DRP/RPB4 Tech Editor
 
NAME     JTsao           DAlley               JGiessner         CHsu
 
DATE     8/29/2013       8/23/2013           8/29/2013         5/8/2013 OFFICE   RES:/DE/CIB/BC BC:DE:EPNB           BC:DE:EVIB       DE:SLS
 
NAME     ACsontos       TLupold             SRosenberg       RHardies
 
DATE     8/30/2013       8/29/2013           8/30/2013         8/29/2013 OFFICE   DLR:RAPB       BC:NRR:DLR: RAPB    D:NRR:DE         D:NRO/DCIP (Acting)
NAME     WHolston       KGreen               PHiland           MCheok
 
DATE     5/7/2013       5/7/2013             8/29/2013         9/3/2013 OFFICE   PM:NRR:PGCB     LA:NRR:PGCB         BC:NRR:PGCB       DD:NRR:DPR         D:NRR:DPR
 
NAME     JKlos           CHawes               (TAlexion for)   SBahadur           (S.Bahadur for)
                                              SStuchell                            LKokajko
 
DATE     9/3/2013       9/4/2013             9/11/2013         9/12/2013           9/ 13 /2013}}


{{Information notice-Nav}}
{{Information notice-Nav}}

Latest revision as of 19:37, 4 November 2019

Refueling Water Storage Tank Degradation
ML13128A118
Person / Time
Issue date: 09/13/2013
From: Michael Cheok, Kokajko L
Office of New Reactors, Division of Policy and Rulemaking
To:
Klos L
References
IN-13-018
Download: ML13128A118 (6)
v • d • e


UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

OFFICE OF NEW REACTORS

WASHINGTON, DC 20555-0001 September 13, 2013 NRC INFORMATION NOTICE 2013-18: REFUELING WATER STORAGE TANK

DEGRADATION

ADDRESSEES

All holders of an operating license or construction permit for a nuclear power plant issued under

Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Domestic Licensing of

Production and Utilization Facilities, except those who have permanently ceased operations

and have certified that fuel has been permanently removed from the reactor vessel.

All holders of and applicants for a power reactor early site permit, combined license, standard

design certification, standard design approval, or manufacturing license under 10 CFR Part 52, Licenses, Certifications, and Approvals for Nuclear Power Reactors.

PURPOSE

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to inform

addressees of potential issues associated with leakage due to flaws in refueling water storage

tanks. The NRC expects recipients to review the information for applicability to their facilities

and consider taking action, as appropriate. Suggestions contained in this IN are not NRC

requirements; therefore, no specific action or written response is required.

DESCRIPTION OF CIRCUMSTANCES

Kewaunee Power Station

The refueling water storage tank (RWST) at Kewaunee Power Station is a stainless steel tank of

welded construction with a capacity of 272,500 gallons that is located in the auxiliary building.

On July 10, 2012, the licensee identified a boric acid deposit adjacent to the weld connecting

the tank wall to the floor plate. No actual leakage of water from the tank was observed. The

licensee postulated that the boric acid deposit was due to a pinhole leak that resulted from an

original weld defect or some form of stress corrosion cracking, e.g., chloride stress corrosion

cracking. The licensee justified continued operation of the tank through the use of American

Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (Code) Case N-705, Evaluation Criteria for Temporary Acceptance of Degradation in Moderate Energy Class 2 or 3 Vessels and Tanks, until permanent repairs could be made to the tank.

ML13128A118 Palisades Nuclear Plant

The Palisades Nuclear Plant safety injection refueling water tank (SIRWT1) is an aluminum tank

of welded construction with a capacity of 300,000 gallons that is located on the roof of the

auxiliary building. The tank was constructed in accordance with ASME Standard B96.1, Welded

Aluminum-Alloy Storage Tanks, which is inactive and has no replacement. Part of the tank is

located directly above the control room. Numerous pipes exit the floor of the tank through

nozzles.

Leakage into the auxiliary building concrete roof structure (catacombs) was observed beginning

in the spring of 2011. Despite repair efforts in 2012, leakage from the tank, sometimes

accompanied by rainwater leaking through the roof around the tank, was present until the

majority of the tank floor was replaced during the summer of 2013.

Inspection and repair efforts revealed the following issues:

1. Inspections were conducted using visual, surface and electromagnetic techniques.

These techniques revealed numerous flaws in the tank floor welds and floor to shell weld

seam. Some flaws in the floor plates were also identified. These flaws were generally

associated with arc strikes and/or spatter, were not through wall, and may have been

original construction defects.

2. Through wall defects (leaks) were located in the tank floor welds. Some of these defects

were associated with tank floor to penetration nozzle welds. As part of the repair

process the penetrations were redesigned to improve nozzle strength and to permit the

nozzles to move in response to flexing of the tank floor.

3. ASME Standard B96.1 does not contain specific guidance for the construction of tank

floor penetration as used in this tank.

4. Leakage from the tank recurred in 2013, requiring plant shutdown. This leakage was

attributed to the failure of a nozzle repair weld made in 2012. A destructive evaluation of

the weld indicated significant lack of fusion that could be the result of inadequate

cleaning of the weld surfaces, and/or low weld heat input.

5. Design drawings for the tank indicated that the tank shell was supported by a concrete

ring foundation and that the tank floor rested on a sand bed. During tank floor

replacement in 2013 it was determined that neither the ring foundation nor the sand bed

was present. Given that the entire tank bottom sits on the concrete roof structure of the

auxiliary building, these differences in the as designed and as built tank are not

structurally significant.

6. Some of the nozzles which were encased in concrete exhibited denting. Denting

appears to be the result of a two-step process. In the first step aluminum corrodes as a

result of contact with moist concrete. In the second step, the resulting corrosion

products, which are more voluminous than the metal from which they are formed, force

the nozzle inward.

1 SIRWT is a site-specific term that is essentially equivalent to the generic term RWST. South Texas Project, Unit 1 The RWST at South Texas Project, Unit 1, is an ASME Code,Section III, Class 2 seismically- qualified stainless steel tank of welded construction with a capacity of 550,000 gallons. It is

located inside the Unit 1 auxiliary building and has drains connected to the radioactive drain

waste collection system. In September 1997, the licensee observed a spot of rust, traces of

moisture, and some boric acid crystals at the RWST floor to tank wall weld. Since no actual

leakage of water was observed, it was initially suspected the boric acid residue may have come

from a tank connection above this location. Based on an engineering evaluation, the licensee

initiated a periodic monitoring program of the RWST but took no additional action.

In February 1999, the licensee observed boric acid crystals in the same location as identified in

1997. The licensee performed a failure analysis using in-situ replication metallography and

determined that the observed boric acid crystals were the result of a through wall crack at the

base plate weld (tank floor to tank shell). Additionally, the licensee determined that the crack

was transgranular and branched, consistent with chloride stress corrosion cracking.

In 2001, the licensee performed a visual examination from the inside of the RWST using a video

camera on a remotely controlled submersible device. No evidence of base plate or side wall

cracking was observed, and no repairs were performed. Periodic monitoring of the exterior of

the tank continued.

In September 2011, boric acid residues were discovered under the base lip of the RWST in two

new locations near the RWST discharge line. Following discussions with the NRC, STP

conducted the necessary calculations to demonstrate the structural adequacy of the tank in

accordance with ASME Code Case N-705. The licensee made repairs to the tank in

accordance with the ASME Code,Section XI, in the fall of 2012.

BACKGROUND

RWSTs serve the dual purpose of supplying water to flood the refueling cavity during refueling

operations and supplying water to the emergency core cooling system and the containment

spray system in emergency situations. RWSTs are fabricated from stainless steel or aluminum

to minimize corrosion resulting from the borated water stored in the tank. These tanks are

generally classified as ASME Class 2 for purposes of inspection. Leaks from the tanks may

result in failure to meet the tanks intended safety function due to loss of inventory, contamination of soil and or groundwater due to release of tritium to the environment, and/or

damage to safety related equipment due to water spray or flooding.

DISCUSSION

The above events demonstrate that leaks from RWSTs can result in disruption of plant

operations. Based on the above events, the NRC believes that the following observations may

be of value to licensees:

1. Tank inspections have determined that all of the leaks have occurred in welds which are

in or near the bottom of the tank.

2. Although additional degradation modes appear possible, the subject leaks have been

attributed to weld fabrication flaws, stress corrosion cracking, and/or high stress low

cycle fatigue. 3. The degradation mechanisms which have been identified are not those described for

above ground tanks in NUREG 1801, the Generic Aging Lessons Learned (GALL)

Report (ADAMS Accession No. ML103490041). Based on the events cited, the NRC is

considering whether modifications to the GALL Report are warranted.

4. Although the events described in this IN resulted in leakage, none of the events has

challenged the structural integrity of the tank.

5. As demonstrated by two of the events described above and documented in IN 2012-20,

Potential Chloride-Induced Stress Corrosion Cracking of Austenitic Stainless Steel and

Maintenance of Dry Cask Storage System Canisters (ADAMS Accession No.

ML12319A440), cracking of 300 series stainless steel may occur in environments

consisting of moist air and chloride containing surface deposits at temperatures less

than 140 degrees F.

6. Due to the age of some of the tanks under consideration, differences in the as

designed and as built tank may exist. An understanding of the existence and extent of

these differences may be significant in determining the susceptibility of the tank to

degradation.

7. The interaction of aluminum and moist concrete may play a role in the long term

performance of aluminum components.

In summary, recent events have indicated that RWSTs may undergo degradation which may

lead to disruption of plant operations and could result in release of tritium to the environment.

These events to date have been attributed to a variety of causes and have resulted in leakage

issues, but have not challenged the structural integrity of the tanks. Licensees are encouraged

to consider a wide range of environmental and mechanical degradation modes for these tanks

when developing or revising inspection or aging management programs.

CONTACT

S

This IN requires no specific action or written response. Please direct any questions about this

matter to the technical contact listed below or the appropriate project managers in the Office of

Nuclear Reactor Regulation (NRR) and Office of New Reactors (NRO).

/RA/ (Sher Bahadur Acting for) /RA/

Lawrence E. Kokajko, Director Michael Cheok, Acting Director

Division of Policy and Rulemaking Division of Construction Inspection

Office of Nuclear Reactor Regulation and Operational Programs

Office of New Reactors

Technical Contact:

David Alley, NRR/DE/EPNB

301-415-2178 E-mail: David.Alley@nrc.gov

Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under the NRC Library.

ML13128A118 TAC No. MF1357 OFFICE NRR:DE:EPNB NRR:DE:EPNB BC:RIII/DRP/RPB4 Tech Editor

NAME JTsao DAlley JGiessner CHsu

DATE 8/29/2013 8/23/2013 8/29/2013 5/8/2013 OFFICE RES:/DE/CIB/BC BC:DE:EPNB BC:DE:EVIB DE:SLS

NAME ACsontos TLupold SRosenberg RHardies

DATE 8/30/2013 8/29/2013 8/30/2013 8/29/2013 OFFICE DLR:RAPB BC:NRR:DLR: RAPB D:NRR:DE D:NRO/DCIP (Acting)

NAME WHolston KGreen PHiland MCheok

DATE 5/7/2013 5/7/2013 8/29/2013 9/3/2013 OFFICE PM:NRR:PGCB LA:NRR:PGCB BC:NRR:PGCB DD:NRR:DPR D:NRR:DPR

NAME JKlos CHawes (TAlexion for) SBahadur (S.Bahadur for)

SStuchell LKokajko

DATE 9/3/2013 9/4/2013 9/11/2013 9/12/2013 9/ 13 /2013


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