ML070430378

Mitigation of Alloy 600/82/182 Pressurizer Butt Welds

ML070430378
Person / Time
Site:	Palo Verde
Issue date:	01/31/2007
From:	Mauldin D Arizona Public Service Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
102-05640-CDM/TNW/RJR
Download: ML070430378 (10)

Text

Z-A M A subsidiaryof Pinnacle West CapitalCorporation David Mauldin Mail Station 7605 Palo Verde Nuclear Vice President Tel: 623-393-5553 PO Box 52034 Generating Station Nuclear Engineering Fax: 623-393-6077 Phoenix, Arizona 85072-2034 102-05640-CDM/TNW/RJR January 31, 2007 Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001

Reference:

"Industry Actions Associated with Potential Generic Implications of Wolf Creek Inspection Finding," Nuclear Energy Institute to U.S. Nuclear Regulatory Commission, letter dated January 26, 2007

Subject:

Palo Verde Nuclear Generating Station (PVNGS)

Units 1, 2, and 3 Docket Nos. STN 50-52815291530 Mitigation of Alloy 600182/182 Pressurizer Butt Welds In October of 2006, while performing inspections of its pressurizer Alloy 600/82/182 butt welds in accordance with the Electric Power Research Institute (EPRI) Material Reliability Program (MRP), "Primary System Piping Butt Weld Inspection and Evaluation Guideline (MRP-1 39)," a PWR licensee discovered several circumferential indications in its pressurizer surge, safety and relief nozzles. Because of the potential importance of this issue, Arizona Public Service Company (APS) is notifying the NRC of the actions that have been taken or are planned for inspecting or mitigating Alloy 600/82/182 butt welds on pressurizer spray, surge and relief lines for Palo Verde Units 1, 2 and 3.

This letter contains new commitments as described in Table 1. Table 2 provides an inspection and mitigation summary of pressurizer Alloy 600/82/182 butt welds and Table 3 provides the results of inspections completed within the preceding two operating cycles of each unit.

Palo Verde Units I and 3 Mitigation of pressurizer Alloy 600/82/182 butt welds at Palo Verde Units 1 and 3 will be completed during U1R13, scheduled to begin in May 2007 and U3R13, scheduled to begin in September 2007. Palo Verde has also performed Bare Metal Visual (BMV) inspections on all the pressurizer Alloy 600/82/182 butt welds as required by MRP-1 39 (Table 3, VT-2 inspection). Future inspections of pressurizer butt welds at Palo Verde Units 1 and 3 will be performed in accordance with industry guidance issued by the MRP. Post mitigation inspections will be performed and results reported to the NRC within 60 days after startup following the inspections.

AN/ID

NRC Document Control Desk Page 2 Inspection and Mitigation of Alloy 82/182 Pressurizer Butt Welds Palo Verde Unit 2 Mitigation activities at Palo Verde Unit 2 are scheduled to be completed during U2R14 which is scheduled to start March 29, 2008. This represents an extension of approximately 3 months beyond December 31, 2007, the MRP-1 39 deadline. This deviation from the MRP-1 39 deadline applies to the following pressurizer weld locations; spray nozzle (1), surge nozzle (1) and safety/relief nozzles (4). Our scheduled date is justifiable based on the information provided below.

Previous Inspection Results These welds are inspected per American Society of Mechanical Engineers (ASME)

Boiler and Pressure Vessel Code Section XI. The Unit 2 surge line nozzle dissimilar metal weld (DMW) received an ASME Section XI Appendix VIII Code inspection in the spring of 2005. The inspections were conducted using ASME Section XI techniques and no recordable indications were found. Palo Verde also performed BMV inspections on all the pressurizer Alloy 600/82/182 butt welds as required by MRP-139 during U2R13 (September 30, 2006) with no evidence of leakage. Table 3 tabulates the inspections performed within the preceding two operating cycles by scope, method, qualification, coverage, limitations and findings.

Fabrication Records Review Plants with significant Alloy 600/82/182 butt weld indications or leaks have a history of extensive weld repairs. Construction and fabrication records for the Palo Verde Unit 2 piping and pressurizers were reviewed to find inspection records such as rejection notices which would indicate a repair to a weld made with nickel based alloy weld filler material. The results of this search showed no evidence of any inside diameter weld repairs done during fabrication. The only repair vwas in the butter of one of the safety/relief valve nozzles. Weld repairs performed to the butter of this nozzle are not considered relevant because this repair received post weld heat treatment (PWHT).

Leak Detection Capability Palo Verde's Nuclear Administrative and Technical Manual (NATM) 40ST-9RC02, "ERFDADS (Preferred) Calculation of RCS Water Inventory" contains instructions for reactor coolant system leakage monitoring. The procedure directs reactor operators in the performance of site specific calculations and the initiation of investigations when established acceptance criteria are exceeded. NATM 40OP-9RC03, "RCS Leakage Source Determination" provides instructions for determination of the leakage out of the RCS or a connecting system. The Shift Technical Advisor tracks and trends RCS leakage on a periodic basis and it is reported daily in the PVNGS Daily Status Report. RCS leakage monitoring capabilities, methods, action levels, and responses are described below:

NRC Document Control Desk Page 3 Inspection and Mitigation of Alloy 82/182 Pressurizer Butt Welds Current monitoring capabilities:

o PVNGS uses a linear regression algorithm to calculate RCS mass balance and from it derives Total Leak Rate, Identified Leak Rate, and Unidentified Leak Rate. The surveillance 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 /> but is scheduled to be performed every 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br />.

o Containment Sump rate of change is checked by linear regression every 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> in accordance with Technical Requirements Manual (TRM)

Surveillance Requirement (SR) 3.4.203.1.

o The Containment Building Atmosphere radiation monitors are trended shiftly.

Containment atmosphere is routinely sampled every 7 to 10 days as part of the Containment vent process (containment pressure control). Response to containment building atmosphere radiation monitor alarms requires performance of an RCS water balance and analysis of a containment atmosphere grab sample.

The following describes our sensitivity to RCS leakage. The average unidentified leak rates and standard deviation for the current fuel cycle in each unit is shown below:

Unidentified Leak Rates Unit Average (gpm) Std 1 0.053 0.030 2 0.057 0.035 3 0.042 0.036 o 40ST-9RC02 directs the operator to determine the source of leakage when unidentified RCS leakage exceeds the Alert level of 0.12 gpm. Also, this surveillance procedure directs interconnected system leakage that exceeds 0.3 gpm to be corrected because of its potential to mask Unidentified Leakage.

Currently Palo Verde has no procedurally prescribed escalation based on increasing leak rates, but will adopt the general approach in Pressurized Water Reactor Owners Group (PWROG) WCAP-1 6465-NP, "Pressurized Water Reactor Owners Group Standard RCS Leakage Action Levels and Response Guidelines for Pressurized Water Reactors," and revise plant procedures by June 30, 2007.

APS is currently evaluating the PVNGS leak monitoring processes and program to determine if any changes should be made. APS will inform the NRC by March 31, 2007, if any additional changes result from this evaluation.

NRC Document Control Desk Page 4 Inspection and Mitigation of Alloy 82/182 Pressurizer Butt Welds Diverse Leakage Detection Capabilities:

APS is evaluating the feasibility of plant modifications to install diverse leakage detection capability. Plans for any additional capability which reliably and meaningfully add to APS' ability to diagnose primary system leakage, as well as installation schedules as appropriate, will be submitted to the NRC by May 31, 2007.

Susceptibility The susceptibility to primary water stress corrosion cracking (PWSCC) of Alloy 600/82/182 is largely a function of time at temperature when all other variables are constant. Due to the high temperature, the pressurizer is the most highly susceptible location in an operating plant. However, Palo Verde Unit 2 is a relatively young plant. At the start of U2R14 outage, scheduled for March 29, 2008, PVNGS Unit 2 will have operated for approximately 17.2 EFPYs. Unit 2 is ranked 5 6 th out of 69 operating PWRs in operating hours, which is considered relatively low (MRP-48).

Supplemental Justification MRP-1 09, "Alloy 82/182 Pipe Butt Weld Safety Assessment for US PWR Plant Designs" provides the bounding analysis for circumferential crack growth for Combustion Engineering (CE) fleet plants. This analysis shows that for all nozzles, with the exception of the pressurizer spray nozzle, an assumed 1 gpm through-wall flaw would take more than one operating cycle (18 months) to reach critical flaw length. The analysis shows that for the pressurizer spray nozzle, the time for an assumed 1 gpm through-wall flaw to reach critical flaw length is less than 1 year.

This analysis was performed based on the limiting data which was particular to Plant "R". APS Engineering verified that Palo Verde Unit 2 was not Plant "R".

To evaluate the impact on the Palo Verde Unit 2 pressurizer spray nozzle, the crack growth curve for Plant "R" was used to extrapolate backwards to smaller crack sizes.

The extrapolation showed it would take longer than 18 months for a 0.1 gpm crack size to grow to the critical size. APS performed a BMV inspection and a digital radiography examination (RT) of the pressurizer spray nozzle weld during U2R1 3 (Fall 2006). The results were a leak free BMV and a digital RT with no evidence of through wall or significant subsurface flaws.

Since there is no existing through wall flaws in the pressurizer spray nozzle, there is reasonable assurance of the weld joint integrity of this limiting weld until the application of a full structural weld overlay during the spring 2008.

Additionally, EPRI Report MRP2007-003, Implications of Wolf Creek Pressurizer Butt Weld indications Relative to Safety Assessment and Inspection Requirements,"

dated January 2007, determined that there is nothing regarding the Wolf Creek indications that would invalidate the safety assessment of MRP-1 39 butt weld inspection requirements.

NRC Document Control Desk Page 5 Inspection and Mitigation of Alloy 82/182 Pressurizer Butt Welds Contingency Planning APS is continuing to evaluate the basis for performing inspections after December 31, 2007, and is following the industry actions outlined in the Nuclear Energy Institute (NEI) letter dated January 26, 2007 (Reference).

If analytical results do not demonstrate that current schedules are adequate or if new information is obtained during upcoming inspections that challenge current assumptions, APS will accelerate the Unit 2 weld overlays currently scheduled for the spring of 2008.

APS will revise plant procedures by March 31, 2007, to state that if a shutdown of a unit should occur due to excessive primary system unidentified leakage, and if the leakage cannot be confirmed to originate from a source other than the pressurizer, perform a bare metal visual examination of the Alloy 600/82/182 butt weld locations on the pressurizer to determine whether the leakage originated from those locations.

The justification to deviate from the MRP-1 39 UT inspection deadline by approximately 12 weeks meets the intent of performing timely dissimilar metal weld mitigation with a reasonable degree of margin before PWSCC degradation would lead to a pressure boundary leak or a safety issue. APS is committing to mitigate all six (6) Alloy 600/82/182 pressurizer butt welds in U2R1 4 (Spring 2008) to come into full compliance with MRP-139. Mitigation will be implemented by applying a full structural weld overlay.

Post mitigation inspections will be performed and results reported to the NRC within 60 days after startup following the inspections.

NRC Document Control Desk Page 6 Inspection and Mitigation of Alloy 82/182 Pressurizer Butt Welds Table I - Commitments Made by this Letter Committed COMMITMENT Date/Outage/Condition Mitigation of pressurizer Alloy 600/82/182 butt welds at Palo Scheduled Start Verde Unit I will be completed by UIRI3 (Spring 07) May 2007 Mitigation of pressurizer Alloy 600/82/182 butt welds at Palo Scheduled Start Verde Unit 2 will be completed by U2R14 (Spring 08) March 2008 Mitigation of pressurizer Alloy 600/82/182 butt welds at Palo Scheduled Start Verde Unit 3 will be completed by U3RI 3 (Fall 07), September 2007 U1 post mitigation inspections will be performed and results Estimated Date reported to the NRC within 60 days after startup following 08/19/2007 the inspections.

U2 post mitigation inspections will be performed and results Estimated Date reported to the NRC within 60 days after startup following 07/06/2008 the inspections.

U3 post mitigation inspections will be performed and results Estimated Date reported to the NRC within 60 days after startup following 02/11/2008 the inspections.

Evaluate the PVNGS leak monitoring processes and program to determine if any changes should be made. APS March 31, 2007 will inform the NRC by March 31, 2007, if any additional changes result from this evaluation.

Adopt the general approach in Pressurized Water Reactor Owners Group (PWROG) WCAP-16465-NP, "Pressurized Water Reactor Owners Group Standard RCS Leakage June 30, 2007 Action Levels and Response Guidelines for Pressurized Water Reactors," and revise plant procedures.

Revise plant procedures to state that if a shutdown of a unit should occur due to excessive primary system unidentified leakage, and if the leakage cannot be confirmed to originate from a source other than the pressurizer, perform a bare June 30, 2007 metal visual examination of the Alloy 600/82/182 butt weld locations on the pressurizer to determine whether the leakage originated form those locations.

Evaluate the feasibility of plant modifications to install diverse leakage detection capability. Plans for any additional capability which reliably and meaningfully add to APS' ability to diagnose primary system leakage, as well as May 31, 2007 installation schedules as appropriate, will be submitted to the NRC.

If analytical results do not demonstrate that current schedules are adequate or if new information is obtained Notification by industry during upcoming inspections that challenge current assumptions, APS will accelerate the Unit 2 weld overlays currently scheduled for the spring of 2008

NRC Document Control Desk Page 7 Inspection and Mitigation of Alloy 82/182 Pressurizer Butt Welds Table 2- Inspection and Mitigation Summary for Alloy 82/182 Pressurizer Butt Welds MRP-139 Volumetric Susceptible Inspection Requirement Mitigation to Be Unit Weld Component Material to be Met after Mitigation Completed Comments DWC p tesriptn (Outage Description Outage Scheduled Designation)

Designation Start Date PZR Surge (PZR Side), U11113 1 PZR Safety/Relief (4) 82/182 UIR13 May 2007 FSWO*

PZR Spray PZR Surge (PZR Side), September U3R13 PZR Safety/Relief (4) 82/182 U3R13 2007FSWO*

PZR Spray 2007 PZR Surge (PZR Side) U2R14 2 PZR Safety/Relief (4) 82/182 U2R14 March 2008 FSWO*

PZR Spray

• Full structural Weld Overlay

NRC Document Control Desk Page 8 Inspection and Mitigation of Alloy 82/182 Pressurizer Butt Welds Table 3 - Results of Inspections Completed within the Preceding Two Operating Cycles of Each Unit Unit I Refuel 11 date 4/3/04 Inspection PDI Coverage Limitations Findings Weld Scope Method Qualified Obtained 4" Spray Sec. XI VT-2 N/A 100% None None 6"

S/ Sec. XI VT-2 N/A 100% None None Safety/Relief Sae6/ele Sec. X1 VT-2 N/A 100% None None 6"

Safety/Relief 6" Sec. Xl VT-2 N/A 100% None None Safety/Relief S/ Sec. XI VT-2 N/A 100% None None 12" Surge Sec. X1 VT-2 N/A 100% None None Unit I Refuel 12 date 10/01/05

NRC Document Control Desk Page 9 Inspection and Mitigation of Alloy 82/182 Pressurizer Butt Welds Table 3 - (continued)

Unit 2 Refuel 12 date 4/2/05 Weld Inspection Scopn Method PDI Qualified Coverage Obtained Limitations Findings 4" Spray Sec. X UT VT- No N/A 100% N/A None None 6"

6" Sec. XA UT RT No N/A 100% N/A None None Safety/Relief VT-2 100% None 6"

6" Sec. Xl UT RT No N/A 100% N/A None None Safety/Relief VT-2 100% None 6"

6" Sec. Xl UT RT No N/A 100% N/A None None Safety/Relief VT-2 100% None 6" UT N/A N/A Safety/Relief Sec. XA RT No 100% None None VT-2 100% None 12" Surge Sec. X UT VT-2 Yes >90%

100% None None None Unit 2 Refuel 13 date 9/30/06 Weld Inspection Method PDI Coverage Limitations Findings Scope Qualified Obtained RT 4" Spray Sec. XI T2 N/A 100% None None 6"

Safety/Relief Sec. XI VT-2 N/A 100% None None 6"

S/ Sec. Xl VT-2 N/A 100% None None Safety/Relief 6"

S/ Sec. XI VT-2 N/A 100% None None Safety/Relief 6"

Safe /Relief Sec. XA VT-2 N/A 100% None None 12" Surge Sec. Xl VT-2 N/A 100% None None

NRC Document Control Desk Page 10 Inspection and Mitigation of Alloy 82/182 Pressurizer Butt Welds Table 3 - (continued)

Unit 3 Refuel 11 date 10/2/04 Weld Inspection Method PDI Coverage Limitations Findings Scope Qualified Obtained UT N/A N/A 4" Spray Sec. XI RT 100% None None VT-2 100% None UT No N/A N/A 6"

Sec. Xl PT 100% OD None None Safety/Relief VT-2 100% None 6" UT No N/A N/A Safety/Relief Sec. Xl PT 100% OD None None VT-2 100% None 6" UT No N/A N/A Safety/Relief Sec. XA PT 100% OD None None VT-2 100% None UT No N/A N/A 6"

Sec. XA PT 100% OD None None Safety/Relief VT-2 100% None 12" Surge Sec. XI VT-2 N/A 100% None None Unit 3 Refuel 12 date 4/01/06 Weld Scope Inspection Mto Qualified PDI Obtained Coverage Limitations Findings 4" Spray Sec. XI VT-2 N/A 100% None None 6"

Safety/Relief Sec. Xl VT-2 N/A 100% None None 6"

6" Sec. XA VT-2 N/A 100% None None Safety/Relief 6". Sec. XI VT-2 N/A 100% None None Safety/Relief 6"

S/ Sec. Xl VT-2 N/A 100% None Safety/Relief None 12" Surge Sec. Xl VT-2 N/A 100% None None If you have any questions, please contact Thomas N. Weber at (623) 393-5764.

Sincere" ,

CDM/SAB/RJR cc: B. S. Malleft NRC Region IV Regional Administrator M. B. Fields NRC NRR Project Manager M. T. Markley NRC NRR Project Manager G. G. Warnick NRC Senior Resident Inspector for PVNGS