NOC-AE-06002099, Inspection and Mitigation of Alloy 82/182 Pressurizer Butt Welds

Inspection and Mitigation of Alloy 82/182 Pressurizer Butt Welds

ML070370319
Person / Time
Site:	South Texas
Issue date:	01/30/2007
From:	Rencurrel D South Texas
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
G25, NOC-AE-06002099
Download: ML070370319 (10)

Text

Nuclear Operating Company South Toxas Pro/ectElectric Generatin$Station P.. Box 289 Wadsworth, Texas 77483 ,V\ - -

January 30, 2007 NOC-AE-06002099 File No.: G25 10 CFR 50.55a U. S. Nuclear Regulatory Commission Attention: Document Control Desk One White Flint North 11555 Rockville Pike Rockville, MD 20852-2738 South Texas Project Units 1 and 2 Docket No. STN 50-498, STN 50-499 Inspection and Mitigation of Alloy 82/182 Pressurizer Butt Welds In October 2006, while performing inspections of pressurizer Alloy 82/182 butt welds in accordance with MRP-1 39, a PWR licensee discovered several circumferential indications in the pressurizer surge, safety, and relief nozzles. Because of the potential importance of this issue, STP Nuclear Operating Company (STPNOC) commits to the following actions taken or planned at the South Texas Project (STP) for inspecting or mitigating Alloy 600/82/182 butt welds on pressurizer spray, surge and relief lines.

Inspection/Mitigation Schedule The affected Alloy 600/82/182 butt weld in the STP Unit 1 pressurizer surge line has been mitigated and inspected. Details concerning the mitigated location are provided in the attached Table 1. Results of completed inspections are attached as Table 3.

Remaining STP Unit 1 pressurizer steam space mitigation activities will be completed after December 31, 2007, during the Spring 2008 refueling outage. This scheduled date is acceptable as justified below. The inspection of pressurizer steam space butt welds at STP Unit 1 will be performed in accordance with industry guidance (MRP-139). The results of the STP Unit 1 inspections or mitigation of pressurizer steam space Alloy 600/82/182 butt weld locations will be reported to the NRC within 60 days of startup from the outage in which they were performed.

Inspection of pressurizer Alloy 600/82/182 butt welds at STP Unit 2 in accordance with industry guidance (MRP-139) has not yet been completed. STPNOC intends to complete all of the mitigation and inspection activities on these locations in refueling outage 2RE12 (Spring 2007).

Details concerning STP Unit 2 inspection and mitigation activities are provided in the attached Table 2. Results of completed inspections are attached as Table 4. The inspection of pressurizer butt welds at STP Unit 2 will be performed in accordance with industry guidance (MRP-139). The results of the STP Unit 2 inspections or mitigation of pressurizer Alloy 600/82/182 butt weld locations will be reported to the NRC within 60 days of startup from the outage in which they were performed.

Justification for Inspection/Mitigation Schedule Previous Inspection Results The results of previous inspections of pressurizer butt welds at STP Units 1 and 2 are provided in Tables 3 and 4, respectively. There have been no relevant indications.

STh 32103821 /4-//

NOC-AE-06002099 Page 2 of 5 Assessment of Original Fabrication Welds An assessment of the original fabrication radiography documentation for the STP Unit 1 pressurizer upper head nozzle safe end welds was performed to identify welds that were reworked during construction. The review determined that only Unit 1 Safety Nozzle RC2009NSS incurred any weld repairs during fabrication. The repairs to RC2009NSS passed the final radiography, pre-service inspection, and inservice inspection (Table 4).

Leak Detection Capability STP Unit 1 and Unit 2 incorporate design provisions supported by procedures that enable detection of leaks originating from the reactor coolant system.

Monitoring Methods Employed Reactor coolant system unidentified leakage is determined every 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> by plant surveillance procedure. A Reactor Coolant inventory is required to be performed every 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> in accordance with Technical Specification 3.4.6.2. Reactor Coolant inventory performs a primary system mass balance with a 0.12 gpm unidentified leakage sensitivity.

Containment airborne monitors and area radiation monitors have demonstrated early detection of RCS leakage. RCS inventory checks are required daily by Technical Specification 3.4.6.1 when the airborne radiation monitor is not in service.

Overall Sensitivity to RCS Leakage Reactor containment building airborne radiation monitor, particulate channel, can generally detect small changes in RCB atmospheric radioactivity due to increases in fission gas daughter products, water and corrosion activation products, and fission products. Previous occurrences of low level fluid leakage demonstrate that unidentified leakage from the pressurizer steam space can be detected at leakage levels of < 0.10 gpm.

Reactor Containment Sump inflow calculations can detect a change in inflow of about 0.10 gpm. However, an estimated eight-hour delay can occur between initiation and detection of a leak. The operator records sump unidentified leakage each shift using the plant computer or manual calculation.

The overall sensitivity of daily reactor coolant inventories, reactor containment building airborne radiation monitoring, and reactor containment sump inflow calculations is expected to be 0.10 gpm.

If an RCS leak is suspected, the condition is tracked using the site corrective action program. A detailed plan of action and a monitoring plan are developed to identify the source of the leak.

Preventive Maintenance activities performed bi-weekly include review of primary system trends to detect RCS leakage at low levels. Review of ICS trends can detect a 0.02 gpm change in containment sump inflow rates. These methods have previously demonstrated their capability to detect an RCS leak measuring 0.025 gpm.

Action Levels and Responses to Action Levels Exceeded In accordance with site procedures, the RCS unidentified leakage alert limits and actions are:

Greater than or equal to 0.2 gpm Perform operational troubleshooting and walkdowns of the CVCS and RCS in the MAB and RCB based on recent evolutions, alternate indications such as

NOC-AE-06002099 Page 3 of 5 increasing sump or tank levels, or increasing radiation levels. Operational troubleshooting may involve isolation of specific portions of the CVCS or RCS in accordance with normal operating procedures.

Notify the duty management team [All action levels]

" Greater than or equal to 0.3 gpm or 0.2 gpm for 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> Perform inspections of the areas inside the missile barrier.

" Expected to exceed 1 gpm within 24 Hrs Form a team to perform operational troubleshooting and walkdowns (24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> a day until leak is located).

If STP Unit 1 or Unit 2 is shut down due to excessive primary system unidentified leakage, and if the leakage cannot be confirmed to originate from a source other than the pressurizer, a bare metal visual examination of Alloy 600/82/182 butt weld locations on the pressurizer is performed to determine if the leakage originated at those locations.

Historical Performance The STP leakage detection program is robust and STPNOC has a consistent history of prompt and thorough response to indications of RCS leakage. For example:

• In May, 1990, Unit 2 was shut down to repair reactor coolant pressure boundary leakage from a small leak in the drain line from the "C" Steam Generator. The leak rate was estimated to be about 10 ml/minute and was initially detected by the containment atmosphere radiation monitors. [LER 2-90-008]

• In April, 2003, STPNOC identified very small boron deposits on two bottom-mounted instrumentation penetrations in Unit 1 during restart from a refueling outage. STPNOC determined that the deposits were due to pressure boundary leakage. The plant was cooled down and defueled. All 58 Unit 1 BMI penetrations were ultrasonically examined and no other penetration was found to be affected. The two affected penetrations were repaired prior to restart of the unit. The minute leakage from the affected penetrations was well below the detection capability of the on-line leakage detection systems and could only be identified by visual inspection. STPNOC's prompt action in resolution of the BMI leakage issue demonstrated the station's overall ability to identify and respond to very small leaks. [LER 1-03-003]

" In December 2004, a leak of about 0.12 gpm was discovered in a fitting on a Unit 1 Train A RCS hot leg primary system 3/8" sample line (i.e., not RCS pressure boundary leakage). The initial indication of the leak was from the particulate channel of the containment atmosphere radiation monitor.

• In February, 2005, STP Unit 2 was shut down to repair a primary pressure boundary leak from a 3/4 inch vent line off of the "A" Cold Leg Safety Injection line.

At the time of discovery, the total unidentified reactor coolant system leak rate was determined to be 0.13 gallons per minute. There were no challenges to plant safety. [LER 2-05-001]

Plant Age The susceptibility of Alloy 600/82/182 to PWSCC is largely a function of time at temperature when all other variables are constant. STP Unit 1 is a relatively young plant compared to other domestic nuclear power plants. EPRI MRP-48, Table 2-1 lists the effective full power years (EFPY) of operation for each of the 69 operating PWRs.

NOC-AE-06002099 Page 4 of 5 The EFPY data from the MRP-48 survey ranked STP 64th out of 69 with 9.2 EFPY. The list was current as of February 28, 2001. Upon entry into the most recent refueling outage (1RE13), STP Unit 1 had operated for approximately 14 EFPY. STP's relative ranking generally will not change as time passes since the listed plants will have gained similar operating time since the table was compiled.

Design Assessment STPNOC will review the STP leak monitoring processes, program, and equipment to determine if changes should be made. STPNOC will inform the NRC by May 31, 2007, of the status of the review.

Advanced Non-Linear Finite Element Analysis Additional refined crack growth calculations of the limiting pressurizer nozzle Alloy 600/82/182 weld configuration are being developed using three-dimensional finite element analysis as described in correspondence from Marvin D. Fertel, Nuclear Energy Institute, to Luis A. Reyes, Executive Director of Operations, NRC, dated January 26, 2007. Modeling a changing crack shape, rather than a semi-elliptical flaw shape, is expected to result in a significant increase in the estimated time between through-wall penetration and rupture for the limiting case nozzle.

The status of this effort will be provided in the May 31, 2007, submittal.

Contingency Planning STPNOC will perform a mid-cycle outage for mitigation and inspection of the Unit 1 pressurizer butt welds if analytical results do not demonstrate to the NRC that current schedules are adequate. The schedule could also be accelerated if results obtained during upcoming inspections challenge current assumptions.

Commitments made in this letter are listed in attachment 5. Changes to any of the information contained in this letter will be provided to the NRC for review prior to making a formal revision.

STPNOC staff is available to meet with the NRC to discuss any of the information in this letter.

If there are any questions, please contact either Mr. Philip L. Walker at (361) 972-8392 or me at (361) 972-7867.

David W. Rencurrel Vice President, Engineering and Alliances PLW

Attachment:

1. Table 1, South Texas Project Unit 1 Inspection and Mitigation Summary for Alloy 600/82/182 Pressurizer Butt Welds

2. Table 2, South Texas Project Unit 2 Inspection and Mitigation Summary for Alloy 600/82/182 Pressurizer Butt Welds

3. Table 3, South Texas Project Unit 1 Previous Inspection Results

4. Table 4, South Texas Project Unit 2 Previous Inspection Results

5. List of Commitments

NOC-AE-06002099 Page 5 of 5 cc:

(paper copy) (electronic copy)

Regional Administrator, Region IV A. H. Gutterman, Esquire U. S. Nuclear Regulatory Commission Morgan, Lewis & Bockius LLP 611 Ryan Plaza Drive, Suite 400 Arlington, Texas 76011-8064 Mohan C. Thadani U. S. Nuclear Regulatory Commission Richard A. Ratliff Steve Winn Bureau of Radiation Control Christine Jacobs Texas Department of State Health Services Eddy Daniels 1100 West 49th Street Marty Ryan Austin, TX 78756-3189 NRG South Texas LP Senior Resident Inspector J. J. Nesrsta U. S. Nuclear Regulatory Commission R. K. Temple P.O. Box 289, Mail Code: MN116 Kevin Polio Wadsworth, TX 77483 City Public Service C. M. Canady Jon C. Wood City of Austin Cox Smith Matthews Electric Utility Department 721 Barton Springs Road C. Kirksey Austin, TX 78704 City of Austin

Attachment 1 NOC-AE-06002099 Page 1 of 1 Table 1: South Texas Project Unit I Inspection and Mitigation Summary for Alloy 600/82/182 Pressurizer Butt Welds MRP-139 Volumetric Mitigation Nozzle Inspection Requirement Completed or to Met or to be Met be Completed Comments Susceptible Outage Start Date Outage Function / Designation Material Description Designation (MM/YYYY) Designation 1RE14 04/2008 To be completed Mitigate by Full Structural Weld Spray RC1003BB 1RE14 Overlay and inspect. This outage will occur after 12/31/2007.

1RE13 10/2006 Completed Mitigated by Full Structural Weld Surge! RC1412NSS 1RE13 Overlay and inspected. Mitigation is complete.*

1RE14 04/2008 To be completed Mitigate by Full Structural Weld Safety! RC1004NSS 1RE14 Overlay and inspect. This outage Nozzle-to-safe will occur after 12/31/2007.

end weld 1RE14 04/2008 To be completed Mitigate by Full Structural Weld Safety/RC1009NSS 1RE14 Overlay and inspect. This outage will occur after 12/31/2007.

1RE14 04/2008 To be completed Mitigate by Full Structural Weld Safety! RC1012NSS 1RE14 Overlay and inspect. This outage will occur after 12/31/2007.

1RE14 04/2008 To be completed Mitigate by Full Structural Weld Relief!/ RC115NSS 1RE14 Overlay and inspect. This outage will occur after 12/31/2007.

STPNOC completed the ultrasonic examination for the STP Unit 1 pressurizer surge nozzle weld overlay on October 24, 2006. No relevant indications were found. Two non-relevant indications were detected that were found to be acceptable per the standards of ASME Section X1, IWB-3514-2 and/or IWB-3514-3 criteria for their respective size and position. These indications are laminar-type flaws located in the first weld layer at the base metal interface. No repairs to the overlay material and/or base metal were required or performed. The results were submitted to the NRC November 1, 2006 (ML063170071).

Attachment 2 NOC-AE-06002099 Page 1 of 1 Table 2: South Texas Project Unit 2 Inspection and Mitigation Summary for Alloy 600/82/182 Pressurizer Butt Welds MRP-139 Volumetric Mitigation Nozzle Inspection Requirement Completed or to Met or to be Met be Completed Comments Susceptible Outage Start Date Outage Function / Designation Material Description Designation (MM/YYYY) Designation Spray / RC2003BB 2RE12 04/2007 To be completed Mitigate by Full Structural Weld 2RE12 Overlay and inspect Surge / RC2412NSS 2RE12 04/2007 To be completed Mitigate by Full Structural Weld 2RE12 Overlay and inspect Safety / RC2004NSS Nozz/e-to-safe end 2RE12 04/2007 To be completed Mitigate by Full Structural Weld SwNe-2RE12 Overlay and inspect weld Safety / RC2009NSS 2RE12 04/2007 To be completed Mitigate by Full Structural Weld 2RE12 Overlay and inspect Safety / RC2012NSS 2RE12 04/2007 To be completed Mitigate by Full Structural Weld 2RE12 Overlay and inspect Relief/ RC2015NSS 2RE12 04/2007 To be completed Mitigate by Full Structural Weld 2RE12 Overlay and inspect

Attachment 3 NOC-AE-06002099 Page 1 of 1 Table 3: South Texas Project Unit 1 Previous Inspection Results Nozzle Refueling Outage Inspection Type(s) Inspection Results Spring 1995 UT & PT Spray /RC1003BB Spring 2005 VT-1 & BMV No relevant Indications Fall 2006 VT-1 & BMV Spring 1991 UT & PT Surge / RC1412NSS Spring 2005 VT-1 & BMV No relevant Indications Fall 2006 VT-1 & BMV Fall 1989 UT & PT Safety / RC1004NSS Spring 2005 VT-1 & BMV No relevant Indications Fall 2006 VT-1, BMV & PT Fall 1992 UT & PT Safety /RC1009NSS Spring 2005 VT-1 & BMV No relevant Indications Fall 2006 VT-1 & BMV Fall 1992 UT & PT Safety 6RC1012NSS Spring 2005 VT-1 & BMV No relevant Indications Fall 2006 VT-1 & BMV Fall 1989 UT & PT Relief/6RC1015NSS Spring 2005 VT-1 & BMV No relevant Indications Fall 2006 VT-1 & BMV

Attachment 4 NOC-AE-06002099 Page 1 of 1 Table 4: South Texas Project Unit 2 Previous Inspection Results Nozzle Refueling Outage Inspection Type(s) Inspection Results Spring 1997 UT & PT Spray / RC2003BB Spring 2004 VT-1 & BMV No relevant Indications Fall 2005 VT-1 & BMV Spring 1997 UT & PT Surge / RC2412NSS Fall 2005 VT-1 & BMV No relevant Indications Fall 1990 UT &PT Safety / RC2004NSS Spring 2004 VT-1 & BMV No relevant Indications Fall 2005 VT-1, BMV Fall 1995 UT &PT Safety / RC2009NSS Spring 2004 VT-1 & BMV No relevant Indications Fall 2005 VT-1 & BMV Fall 1995 UT & PT Safety/6RC2012NSS Spring 2004 VT-1 & BMV No relevant Indications Fall 2005 VT-1 & BMV Fall 2002 UT & PT Relief/6RC2015NSS Spring 2004 VT-1 & BMV No relevant Indications Fall 2005 VT-1 & BMV

Attachment 5 NOC-AE-06002099 Page 1 of 1 LIST OF COMMITMENTS The following table identifies the actions in this document to which the STP Nuclear Operating Company has committed. Statements in this submittal with the exception of those in the table below are provided for information purposes and are not considered commitments. Please direct questions regarding these commitments to Philip Walker at (361) 972-8392.

Commitment CR Action Expected Completion The inspection of pressurizer butt welds at STP Unit 1 will be performed in 05-15744-79 05/01/2008 accordance with industry guidance (MRP-1 39).

The inspection of pressurizer butt welds at STP Unit 2 will be performed in 05-15744-80 05/01/2007 accordance with industry guidance (MRP-1 39).

The results of future inspections or mitigation of pressurizer Alloy 600/82/182 Unit 1: 06/23/2008 butt weld locations will be reported to the NRC within 60 days of startup from 05-15744-81 the outage in which they were performed.

The results of future inspections or mitigation of pressurizer Alloy 600/82/182 Unit 2: 06/26/2007 butt weld locations will be reported to the NRC within 60 days of startup from 05-15744-82 the outage in which they were performed.

Remaining STP Unit 1 mitigation activities will be completed after December 05-15744-83 04/24/2008 31, 2007, during the Spring 2008 refueling outage.

If STP Unit 1 or Unit 2 should shut down due to excessive primary system Unit 1: 03/30/2008 unidentified leakage, and if the leakage cannot be confirmed to originate from a 05-15744-84 source other than the pressurizer, a bare metal visual examination of Alloy Unit 2: 03/25/2007 600/82/182 butt weld locations on the pressurizer will be performed to 05-15744-85 determine if the leakage originated at those locations.

STPNOC will review the STP leak monitoring processes, program, and 05-15744-86 05/31/2007 equipment to determine if any changes should be made. STPNOC will inform the NRC by May 31, 2007, of the status of the review.

Additional refined crack growth calculations of the limiting pressurizer nozzle 05-15744-88 05/31/2007 Alloy 600/82/182 weld configuration are being developed using three-dimensional finite element analysis. The status of this effort will be provided in the May 31, 2007, submittal.

STPNOC will perform a mid-cycle outage for mitigation and inspection of the 05-15744-89 03/30/2008 Unit 1 pressurizer butt welds if analytical results do not demonstrate to the NRC that current schedules are adequate. The schedule could also be accelerated if results obtained during upcoming inspections challenge current assumptions.

Changes to any of the information contained in this letter will be provided to the 05-15744-87 04/24/2008 NRC for review prior to making a formal revision.