ML073330255

Pressurizer Nozzle Full Structural Weld Overlay Evaluation

ML073330255
Person / Time
Site:	Vogtle
Issue date:	12/06/2007
From:	Siva Lingam NRC/NRR/ADRO/DORL/LPLII-1
To:	Tynan T Southern Nuclear Operating Co
Lingam, Siva NRR/DORL 415-1564
References
TAC MD5234
Download: ML073330255 (7)

Text

December 6, 2007

Mr. Tom E. Tynan Vice President - Vogtle Vogtle Electric Generating Plant 7821 River Road Waynesboro, GA 30830

SUBJECT:

VOGTLE ELECTRIC GENERATING PLANT, UNIT 2, PRESSURIZER NOZZLE FULL STRUCTURAL WELD OVERLAY EVALUATION (TAC NO. MD5234)

Dear Mr. Tynan:

By letter dated April 13, 2007, Southern Nuclear Operating Company, Inc. (the licensee),

submitted the completion of the ultrasonic examination (UT) of Vogtle Electric Generating Plant, Unit 2 (Vogtle 2) pressurizer nozzle full structural weld overlays on April 1, 2007. This activity was accomplished in accordance with the licensee's alternative ISI-GEN-ALT-06-03, Rev. 2, which was approved per the Nuclear Regulatory Commission (NRC) safety evaluation (SE) dated March 8, 2007 (Agencywide Documents Access and Management System (ADAMS) Accession Number ML070600246). In the NRC's SE, the staff requested the licensee to evaluate residual stresses and flaw growth of the repaired weldments to demonstrate that the pressurizer nozzles after the weld overlay installation will perform their intended design function.

On the basis of information submitted, the NRC staff concludes that the Vogtle 2 pressurizer nozzle full structural weld overlay designs have been demonstrated to meet the requirements in the alternative ISI-GEN-ALT-06-03, Rev. 2 through a finite element analysis and fracture mechanics evaluation. The NRC staff concludes that the full structural weld overlay designs for all the Vogtle 2 pressurizer nozzles are adequate before the next in-service inspection interval because the final UT examinations did not detect any flaw in the upper 25 percent of the original Alloy 82/182 weld and stainless steel weld materials in any of the pressurizer nozzles. A summary of the NRC staff's review of the licensee's weld overlay evaluation is enclosed.

Sincerely,

/RA/ Siva P. Lingam, Project Manager Plant Licensing Branch II-1 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation

Docket No. 50-425

Enclosure:

Summary of Evaluation

cc w/encl: See next page

ML073330255 *transmitted by memo dated OFFICE NRR/LPL2-1/PM NRR/LPL2-1/LA NRR/DCI/CPNB/BC NRR/LPL2-1/BC NAME SLingam:nc MO

=Brien TChan EMarinos DATE 12/5/07 12/5/07 10/5/07* 12/6/07

Enclosure

SUMMARY

OF FULL STRUCTURAL WELD OVERLAY EVALUATION FOR PRESSURIZER NOZZLES VOGTLE ELECTRIC GENERATING PLANT, UNIT 2 DOCKET NO. 50-425

1.0 INTRODUCTION

By letter dated April 13, 2007 (Agencywide Documents Access and Management System (ADAMS) Accession Number ML071080311), Southern Nuclear Operating Company, Inc. (the licensee), completed the last performance demonstration initiative (PDI) qualified ultrasonic examination (UT) of Vogtle Electric Generating Plant, Unit 2 (Vogtle 2), pressurizer nozzle full structural weld overlays on April 1, 2007. This activity was accomplished in accordance with the licensee's alternative ISI-GEN-ALT-06-03, Rev. 2, which was approved per the Nuclear Regulatory Commission (NRC) safety evaluation (SE) (ADAMS Accession Number ML070600246). In the NRC's SE, the staff requested the licensee to evaluate residual stresses and flaw growth of the repaired weldments to demonstrate that the pressurizer nozzles after the weld overlay installation will perfo rm their in tended design function.

2.0 DISCUSSION

Weld Overlay Design

Based on the Vogtle 2 specific loadings at the nozzles, the licensee determined the minimum required full structural weld overlay thickness in accordance with the requirements of Alternative ISI-GEN-ALT-06-03, Rev. 2. Per the alternative, the licensee assumed a flaw to be 100 percent through the original wall thickness for the entire circumference. The thickness of the full structural weld overlay applied meets the criteria of IWB-3640 of the American Society of Mechanical Engineers, Boiler and Pressure Vessel Code (ASME Code),Section XI. Due to a concern for potential weld dilution, the licensee also applied a sacrificial layer prior to the addition of the required full structural weld overlay thickness to ensure that the Chromium content in the first layer exceeded 24 percent for primary water stress corrosion cracking (PWSCC) resistance. The minimum full-structural weld overlay thickness did not take credit for the sacrificial layer.

The full structural weld overlay length was based conservatively on the length of 0.75 Rt per ISI-GEN-ALT-06-03, Rev. 2, where R and t are the outer radius and wall thickness of the pipe/nozzle, respectively. In addition, the full structural weld overlay was extended to include the stainless steel butt weld region. The ability to examine the weld overlay was a controlling factor in the structural weld overlay design. Therefore, additional weld metal was added to improve the ability to examine the overlay, beyond that required for repair and/or mitigation. The licensee stated that as a result, the final full structural weld overlay length and thickness exceeded the requirements for a full structural weld overlay designed in accordance with ISI-GEN-ALT-06-03, Rev. 2. Because the full structural weld overlay was applied before any UT examinations were performed, the licensee considered the possibility of discove ring an almost through-wall flaw at the Alloy 82/182 weld during the final UT examination of the completed weld overlay. To allow for this possibility, the licensee incr eased the required full structural weld overlay thickness over the Alloy 82/182 weld to account for at least 10 years of crack growth into the weld overlay.

The licensee stated that this approach of increasing overlay thickness was conservative because no PWSCC indications were detected in the upper 25 percent of the original weld material during the PDI qualified examinations after the structural weld overlay was applied. The licensee stated that no increase in the full structural weld overlay thickness was necessary over the stainless steel weld because PWSCC is not an active mechanism in stainless steel.

The NRC staff finds that the weld overlay design for the Alloy 82/182 welds at Vogtle 2 satisfies the requirements in the NRC staff-approved alternative ISI-GEN-ALT-06-03, Rev. 2, in terms of thickness and length. The NRC staff agrees with the licensee that the weld overlay thickness covering the stainless steel weld does not need to be increased because the weld overlay is primary for the Alloy 82/182 weld metal, not the stainless steel metal. Primary stress corrosion cracking is not an active degradation mechanism in stainless steel welds.

ASME Section Ill Stress Evaluation

The NRC staff finds that the licensee evaluated the effects of the full structural weld overlay to demonstrate that the mitigated pressurizer nozzles continue to meet the applicable ASME Code, Section Ill, Subsections NB-3200 and NB-3600 requirements and the conclusions documented in the existing piping and pressurizer nozzle stress reports remain valid. The licensee stated that the limiting stress intensity and fatigue usage factor, reflecting the impact of the full structural weld overlay for the mitigated pressurizer nozzles, were located at the tapered end of the weld overlay on the original stainless steel piping. The NRC staff finds that the weld overlaid pressurizer nozzles are acceptable because their stresses meet the applicable ASME Code, Section Ill requirements.

Weld Overlay Residual Stress Evaluation

The licensee performed finite element analyses to determine the residual stresses in the pressurizer nozzle dissimilar metal butt weld regions resulting from the structural weld overlay in order to support the subsequent crack growth evaluations. The licensee grouped weld passes into weld areas as has been done in most weld simulation analyses in the industry. Each weld area applied represents one or more weld beads.

For the structural weld overlay finite element models, the licensee modeled each nozzle to include the final nozzle configuration with the structural weld overlay. All the finite elements used to model the structural weld overlay are present in the model at the start of the weld overlay simulation analysis. The resulting residual weld stresses for the mitigated pressurizer nozzles are compressive on the inside surface of the nozzles, over the entire length of the PWSCC-susceptible material, thereby minimizing the potential for any future PWSCC crack initiation and/or crack propagation. The NRC staff finds that the licensee has confirmed, by analysis, that the installed weld overlay has induced favorable compressive stresses at the inside surface of the nozzle to minimize the potential for future PWSCC.

Crack Growth Evaluation

The objective of the crack growth analysis was to determine the service life required for the flaw to propagate to an allowable flaw depth without adversely impacting the integrity of the structural weld overlay. The licensee postulated a 100-percent through-wall flaw in the original weld (versus the 75 percent required in the alternative) and only fatigue crack growth was considered in the weld overlay material since it is PWSCC-resistant. The crack growth analysis was performed in accordance with the IWB-3640 of ASME Code,Section XI.

The licensee used the through-wall stress distribution consisting of residual stresses resulting from the full structural weld overlay, thermal transient stresses, and applicable mechanical loadings to calculate the crack growth. The postulated flaw was subjected to cyclic loading due to the applicable plant-specific thermal transients including the residual stresses resulting from the structural weld overlay mitigation process. The thermal transients considered in the analysis were distributed equally over the plant design life. The crack growth rate reference curves used in the crack growth evaluation for the austenitic nickel alloy and stainless steel materials were obtained from (1) NUREG/CR-6721, Chopra, O. K., Soppet, W. K., and Shack, W. J., "Effects of Alloy Chemistry Cold Work and Water Chemistry on Corrosion Fatigue and Stress Corrosion Cracking of Nickel Alloys and Welds," U.S. Nuclear Regulatory Commission, May 2001, and (2) Bamford, W. H., "Fatigue Crack growth of Stainless Steel Piping in a Pressurized Water Reactor Environment," Transaction ASME, Journal of Pressure Vessel Technology, February 1979.

The licensee stated that because the full structural weld overlay was applied before any UT examinations were performed, the possibility of discovering an al most through-the-original-wall flaw during the final UT examination of the completed weld overlay was considered in the crack growth evaluation. The required full structural weld overlay thickness for the pressurizer nozzles has taken into account at least 10 years of fatigue crack growth into the weld overlay material resulting from a postulated 100 percent initial through-wall flaw. The licensee confirmed that the full structural weld overlay designs for all the mitigated pressurizer nozzles are adequate for at least 10 years even for postulated 100-percent initial through-wall flaws in the Alloy 82/182 weld.

The licensee stated that because the final UT examination for all the mitigated pressurizer nozzles did not identify any unacceptable indications in the outer 25 percent of the original wall thickness, the assumptions and the results of the crack growth calculations are conservative. The NRC staff finds that the 100-percent through-wall crack assumed in the licensee's crack growth calculations is conservative because the actual UT examination did not find any crack in the upper 25 percent of the original wall thickness of the nozzles.

The crack growth results indicate that small crack growth is expected in 10 years in the Alloy 82/182 welds for the spray and surge nozzle, while there is no expected crack growth for the safety/relief nozzles. In accordance with ISI-GEN-ALT-06-03, Rev. 2, all overlaid pressurizer nozzles will be examined within two refueling outages after the implementation of the full structural weld overlay at Vogtle 2. The licensee maintained that because small flaw growth is expected in the spray and surge nozzles, these two nozzles will be included in the 25-percent sample to be examined approximately every 10 years. The NRC staff finds that the licensee is proactive in managing PWSCC by placing the spray and surge nozzles in the 25-percent sample examination population.

3.0 CONCLUSION

On the basis of information submitted by the licensee, the NRC staff concludes that the licensee has demonstrated that Vogtle 2 pressurizer nozzle full structural weld overlay designs meet the requirements in the alternative ISI-GEN-ALT-06-03, Rev. 2 through finite element analysis and fracture mechanics evaluation. In its final UT examinations, the licensee did not detect any flaw in the upper 25 percent of the original Alloy 82/182 and stainless steel weld material in any of the pressurizer nozzles. Therefore, the NRC staff concludes that the full structural weld overlay designs for all the Vogtle 2 pressurizer nozzles are adequate for the current in-service inspection interval.

Principal Contributor: J. Tsao, DCI/CPNB

Date: December 6, 2007

Vogtle Electric Generating Plant, Units 1 & 2

cc:

Mr. Tom E. Tynan Vice President - Vogtle Vogtle Electric Generating Plant 7821 River Road Waynesboro, GA 30830

Mr. N. J. Stringfellow Manager, Licensing Southern Nuclear Operating Company, Inc.

P.O. Box 1295 Birmingham, AL 35201-1295 Mr. Jeffrey T. Gasser Executive Vice President Southern Nuclear Operating Company, Inc.

P.O. Box 1295 Birmingham, AL 35201-1295

Mr. Steven M. Jackson Senior Engineer - Power Supply Municipal Electric Authority of Georgia 1470 Riveredge Parkway, NW Atlanta, GA 30328-4684

Mr. Reece McAlister Executive Secretary Georgia Public Service Commission 244 Washington St., SW Atlanta, GA 30334

Mr. Harold Reheis, Director Department of Natural Resources 205 Butler Street, SE, Suite 1252 Atlanta, GA 30334

Attorney General Law Department 132 Judicial Building Atlanta, GA 30334

Mr. Laurence Bergen Oglethorpe Power Corporation 2100 East Exchange Place P.O. Box 1349 Tucker, GA 30085-1349 Arthur H. Domby, Esquire Troutman Sanders Nations Bank Plaza 600 Peachtree Street, NE Suite 5200 Atlanta, GA 30308-2216

Resident Inspector Vogtle Plant 8805 River Road Waynesboro, GA 30830

Office of the County Commissioner Burke County Commission Waynesboro, GA 30830