ML122920722
| ML122920722 | |
| Person / Time | |
|---|---|
| Site: | South Texas  |
| Issue date: | 10/04/2012 |
| From: | Rencurrel D South Texas |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| NOC-AE-12002889, TAC ME4936, TAC ME4937 | |
| Download: ML122920722 (14) | |
Text
Nuclear Operating Company South Texas Project Electric Generating Station P.O Box289 Wadsworth, Texas 77483
__V_--
October 4, 2012 NOC-AE-12002889 10 CFR 54 STI: 33576674 File: G25 U. S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, DC 20555-0001 South Texas Project Units 1 and 2 Docket Nos. STN 50-498, STN 50-499 Response to Requests for Additional Information for the Review of the South Texas Project, Units 1 and 2, License Renewal Application - Aluminum Bronze, Set 23 (TAC Nos. ME4936 and ME4937)
References:
- 1. STPNOC letter dated October 25, 2010, from G. T. F'owell to NRC Document Control Desk, "License Renewal Application" (NOC-AE-10002607)
- 2. NRC letter dated July 26, 2012, "Requests for Additional Information for the Review of the South Texas Project, Units 1 and 2, License Renewal Application - Aluminum Bronze, Set 23 (TAC Nos. ME4936 and ME4937)"(ML12201B541)
By Reference 1, STP Nuclear Operating Company (STPNOC) submitted a License Renewal Application (LRA) for South Texas Project (STP) Units 1 and 2. By Reference 2, the NRC staff requests additional information for review of the STP LRA. STPNOC's response to the requests for additional information is provided in Enclosure 1 to this letter. Changes to LRA pages described in Enclosure 1 are depicted as line-in/line-out pages provided in Enclosure 2.
Two new regulatory commitments and two revised regulatory commitments are added to Table A4-1 of the LRA and are provided in Enclosure 3 to this letter. There are no other regulatory commitments in this letter.
Should you have any questions regarding this letter, please contact either Arden Aldridge, STP License Renewal Project Lead, at (361) 972-8243 or Ken Taplett, STP License Renewal Project regulatory point-of-contact, at (361) 972-8416.
NOC-AE-12002889 Page 2 I declare under penalty of perjury that the foregoing is true and correct.
Executed on 4e4 )s-.-
Date
ýq Chief Nuclear Officer KJT
Enclosures:
1.
2.
3.
STPNOC Response to Requests for Additional Information STPNOC LRA Changes with Line-in/Line-out Annotations Regulatory Commitments
NOC-AE-12002889 Page 3 cc:
(paper copy)
(electronic copy)
Regional Administrator, Region IV U. S. Nuclear Regulatory Commission 1600 East Lamar Boulevard Arlington, Texas 76011-4511 Balwant K. Singal Senior Project Manager U.S. Nuclear Regulatory Commission One White Flint North (MS 8B1) 11555 Rockville Pike Rockville, MD 20852 Senior Resident Inspector U. S. Nuclear Regulatory Commission P. 0. Box 289, Mail Code: MN1 16 Wadsworth, TX 77483 C. M. Canady City of Austin Electric Utility Department 721 Barton Springs Road Austin, TX 78704 John W. Daily License Renewal Project Manager (Safety)
U.S. Nuclear Regulatory Commission One White Flint North (MS 011-F1)
Washington, DC 20555-0001 Tam Tran License Renewal Project Manager (Environmental)
U. S. Nuclear Regulatory Commission One White Flint North (MS O11F01)
Washington, DC 20555-0001 A. H. Gutterman, Esquire Kathryn M. Sutton, Esquire Morgan, Lewis & Bockius, LLP John Ragan Chris O'Hara Jim von Suskil NRG South Texas LP Kevin Polio Richard Pena City Public Service Peter Nemeth Crain Caton & James, P.C.
C. Mele City of Austin Richard A. Ratliff Alice Rogers Texas Department of State Health Services Balwant K. Singal John W. Daily Tam Tran U. S. Nuclear Regulatory Commission NOC-AE-12002889 Enclosure I STPNOC Response to Requests for Additional Information Attachment A: List of References Attachment B: Calculation AES-C-1964-5, "Evaluation of the Significance of Dealloying and Subsurface Cracks on Flaw Evaluation Model," APTECH Project AES 93061964-1Q (December 1994).
NOC-AE-12002889 Page 1 of 9 SOUTH TEXAS PROJECT, UNITS 1 AND 2, REQUEST FOR ADDITIONAL INFORMATION ALUMINUM BRONZE, SET 23 (TAC NOS. ME4936 AND ME4937)
Note: References provided in the following STPNOC responses are listed in Attachment A to this Enclosure.
RAI B2.1.37-4 (111)
Background:
The May 31, 2012 response to RAI B2.1.37-3 lacked sufficient detail for the staff to complete its evaluation of the plant-specific Selective Leaching of Aluminum Bronze program. As a result, the staff has the following issues and requests for additional information.
RAI Issue 1:
It is understood that components with an external indication will be replaced at the first available opportunity; however, the staff has not been presented with sufficient information to conclude that when external leakage is identified, the applicant's external visual examinations and crack correlation methodology are sufficient to detect an internal crack in the dealloyed region and project its size. Cracks in a dealloyed region could result in a failure of the piping during a seismic event. An insufficient basis was provided for why the destructive examination of a limited number of samples provides a conservative correlation for estimating internal crack size. The staff accepts the positions that the internal dimensions of cracks can potentially be correlated to the sizes of external indications for various pipe wall thicknesses, and it may be possible to develop a valid correlation if an adequate number of samples are tested; however, the response did not address the potential for different heats of material (with varying extent of dealloying-susceptible phases) to respond differently to cracks.
The staff's question related to detection of cracks in the dealloyed region with ultrasonic testing (UT) techniques was not adequately addressed. In some instances, due to configuration, it may not be possible to perform volumetric examinations on all affected fittings; however, the staff believes that there must be some means provided in order to conclude with reasonable assurance that cracks are not approaching a critical size, beyond the currently proposed correlation. In addition, the staff acknowledges that there may be no qualified UT method to detect selective leaching; however licensees have used ultrasonic testing UT to detect selective leaching. Aspects to be considered should be:
- Using a volumetric process that provides reasonable crack sizing and, though not qualified per se, provides information on the extent of dealloying.
NOC-AE-12002889 Page 2 of 9
" When components that leak cannot be volumetrically examined, destructively examining the component when it is removed in order to size any potential cracks.
In conjunction, using trending data to project crack sizes in subsequent leaking components when volumetric methods cannot be utilized.
" Using periodic volumetric examinations on risk-informed locations that are not leaking to develop a trend of the extent of cracking in the system.
Request:
Provide bounding analyses that demonstrate that internal cracking could not grow to sufficient size to result in the failure of the component to perform its intended function(s) or propose a means to volumetrically size potential cracks associated with dealloying and commit to destructive examination of all components where volumetric examination is not possible when removed from service to develop a trend on crack size. Revise the program and UFSAR Supplement accordingly.
STPNOC Response:
STP performed analytical models documented in calculation AES-C-1 964-1 (Reference 1) and calculation AES-C-1964-5 (Reference 2). The results of a pressure test and a bending test to determine the critical bending stress for a dealloyed aluminum bronze 6-inch flange casting are documented in calculation AES-C-1964-4, (Reference 3)
Calculation AES-C-1964-1 performed a fracture mechanics analysis to determine the critical bending stress for dealloyed aluminum bronze castings as a function of dealloying length.
Through-wall cracks are also considered. The analysis methods contained in ASME Section XI for flawed piping are used to calculate critical bending stress for selected large bore piping (i.e. 3-inch, 4-inch, 6-inch, 8-inch, 14-inch, 24-inch and 30-inch nominal pipe sizes) castings in the Essential Cooling Water (ECW) system. The analysis included large-bore aluminum bronze castings (weld region under backing-ring) in the ECW system as well as flanges, tees, valves and pumps. When only dealloying is present in the component, the dealloyed region was conservatively modeled as a region not capable of carrying any load. The structural capacity of the dealloyed casting is determined by a limit load analysis. When cracks are present in the component, the structural capacity was evaluated by fracture mechanics to establish the minimum structural capacity as a function of crack length.
Calculation AES-C-1 964-5 evaluated the impact of part-through and through-wall dealloying and subsurface cracks on the through-wall flaw evaluation method for critical bending stress determination. The results of this evaluation show that dealloying and subsurface cracks (when present) do not significantly reduce structural capacity of castings below the critical bending stress computed by the flaw evaluation method (Reference 1). The analysis confirms that the primary degradation parameters for the flaw evaluation can be established from visual examination of the outside diameter. It is recognized that dealloying can also exist part-through wall at the time of external detection and may extend around the circumference of the inside diameter surface. Because the analysis assumes the crack is looking into un-dealloyed material, through-wall dealloying ahead of the crack is difficult to detect and size from visual inspection of the outside surface. Part-through cracks may exist as dealloying is progressing prior to local through-wall penetration.
NOC-AE-12002889 Page 3 of 9 As part of the evaluation, eight flanges were metallurgically examined to obtain information on crack sizes and shapes and the extent of dealloying of the cross section. The examination results were used to establish a correlation between length of a flaw on the outer diameter and the size of any internal crack and the extent of the dealloyed region of the component. Data indicate that the surface lengths of part-through cracks fall within the range of through-wall crack lengths so that the length of part-through cracks should not be more significant prior to occurring at the surface and being detected by leakage.
Calculation AES-C-1964-4 documents the pressure test and bending test performed on one of the worst case six inch flanges evaluated by calculation AES-C-1964-5 to investigate if the critical bending stresses determined in calculation AES-C-1964-1 were conservative. The bending test results are shown by Figure 6-2 in Calculation AES-C-1964-4. Although this test demonstrated that margin exists prior to catastrophic failure, the data are limited to one physical test.
The analytical methodology is based on Leak-Before-Break methodology models, destructive examinations and testing performed on components removed from the ECW system early in service life. In order to validate the analytical model and the calculations, STPNOC committed in a letter to the NRC (Reference 4) to destructively examine and test three dealloyed components (six samples total) prior to the end of 2012. See Commitment 44 in Reference 4.
In order to provide further validation of the analytical methodology as the plant ages, STPNOC committed in Reference 4 to perform metallurgical testing of 20 percent of aluminum bronze material components that demonstrate external leakage for each 10 year interval beginning 10 years prior to the period of extended operation, but at least one leaking component will be tested every five years. If at least two leaking components are not identified two years prior to the end of each 10-year testing interval, a risk-ranked approach based on those components most susceptible to degradation will be used to identify candidate components for removal and testing so at least two components are tested during the 10-year interval. See Commitment 44 in Reference 4. In addition, these components will be examined to determine the degree of dealloying and the presence of cracks. The degree of dealloying and cracking will be trended by comparing examination results with previous examination results.
In addition to the previous commitments discussed above and to address the issue that the crack size correlation methodology is based on a limited number of samples, the following additional examinations and testing will be performed. (See new Commitment 45 in )
" STPNOC will volumetrically examine aluminum bronze material components that demonstrate external leakage where the configuration supports this type of examination to conclude with reasonable assurance that cracks are not approaching a critical size.
" STPNOC will perform destructive examination of each leaking component removed from service to validate the correlation between the sizes of external indications and internal flaws. Profiling will continue until 10 percent of the susceptible components in the ECW system are examined to determine the degree of dealloying and the presence of cracks.
The total number of susceptible components is approximately 350. The determination of the extent of dealloying is described in the response to RAI Issue 3. The degree of NOC-AE-12002889 Page 4 of 9 dealloying and cracking will be trended through data plots that will compare information obtained from component examinations.
Metallurgical testing described in Commitment 44 will be expanded to test each actively leaking component that occurs in the ECW system until the following population of components is tested.
o At least three different size components of two samples each are tested, and o At least nine total samples are tested.
The components will be tested for fracture toughness that includes an in-service or lab-induced crack in the dealloyed surface and where sufficient sample size supports such testing. Additionally, the samples will be tested for chemical composition including remaining aluminum content in the dealloyed areas, mechanical properties (such as yield and ultimate tensile strength) and microstructure. Ultimate tensile strength will be trended and compared to the acceptance criterion.
The degree of dealloying and presence of cracks will be determined by destructive examination. The degree of dealloying and cracking will be trended by comparing examination results with previous examination results. provides the line-in/line-out revision to LRA Appendices A1.37 and B2.1.37. provides the line-in/line-out revision to LRA Table A4-1 for LRA Commitments 39 and 44 and new LRA Commitment 45.
RAI Issue 2:
Fracture toughness testing was not listed as an example of a parameter to be tested in the testing of the six samples discussed in Commitment No. 44 and those to be conducted starting 10 years prior to the period of extended operation and proceeding through the period of extended operation. In addition, the response to Part (i) of the RAI did not include trending of fracture toughness properties. Given that cracking has been observed in dealloyed specimens, the staff cannot conclude that the method of calculating critical bending stress is conservative without the test results including fracture toughness properties.
Request:
Revise LRA Sections A2.1.37 and B2.1.37 and Commitment Nos. 39 and 44 to state that fracture toughness properties will be obtained, or state the basis for why fracture toughness is not a critical parameter when analyzing cracked dealloyed components.
STPNOC Response:
Commitment 44 will be revised and new commitment 45 will be added to include fracture toughness testing of the composite material where sufficient sample size supports fracture toughness testing. A crack will be machined inside the dealloyed surfaces of the sample composite material if a service-induced crack is not present. The fracture toughness data will NOC-AE-12002889 Page 5 of 9 be evaluated to confirm that the original assumptions and methods of analysis remain conservative. Once fracture toughness is validated by the metallurgical testing described in Commitments 44 and 45, there is little value in continuing to determine fracture toughness.
Therefore, the testing described in Commitment 39 will not include testing for fracture toughness. provides the line-in/line-out revision to LRA Appendices, A1.37 and B2.1.37. provides the line-in/line-out revision to LRA Commitment 44 and new LRA Commitment 45.
RAI Issue 3:
No detail was included on how the percentage of dealloying has been or will be determined.
Given that past calculations have described the percentage of dealloying and evaluations of structural integrity have relied, in part, on this measurement, the staff needs to understand how the value was determined.
Request:
Describe how the percentage of dealloying is identified when testing specimens.
STPNOC Response:
Dealloyed material is characterized as selective removal of the principal alloying element of aluminum leaving a porous structure of lower strength than the original material. The shape and approximate dimension of the original component structure is maintained. A selective phase attack occurs that leaves grain boundaries altered in the dealloyed material.
A crack is characterized by surface separation with no material strength at fractured surfaces.
The degree of material dealloying through the component cross section is based on cross sectioning of the examined material. A polished surface area is etched and visually examined for discoloration that equates to the dealloying over the entire cross sectioned surface. The percent area (i.e. degree) of dealloying is quantified by measuring the sectional area of dealloyed material and comparing it to the gross cross sectional area.
The term "100% dealloying" used in Reference 4 and in this letter response is defined as follows:
- For test samples, the term refers to the ratio as a percentage of the dealloyed cross sectional area to the gross sectional area of the component surface over the total area of the cross section.
" For 100% dealloyed through-wall thickness, the term refers to a leakage location in a component where the through-wall thickness is fully dealloyed on a cross sectional area.
NOC-AE-12002889 Page 6 of 9 RAI Issue 4:
Since it is possible that the six samples from three recently removed aluminum bronze components which will be tested for chemical composition and mechanical properties may not be 100 percent dealloyed, the staff lacks sufficient information to understand how these results will be extrapolated to reflect the potential degree of degradation existing in the system.
Request:
State how the testing results of the six samples from three recently removed aluminum bronze components will be extrapolated to reflect the potential degree of degradation existing in the system. In the response, consider the potential that the selected samples may not bound the potential worst-case percentages of dealloying.
'STPNOC Response:
Extrapolation of the degree of dealloying in the system is not practical because dealloying is random and based on the amount of microstructure present in the material. Predicting the amount of microstructure present is not feasible because of variability and phase formation during fabrication. Operating experience indicates that external leakage is identified prior to the remaining circumferential section of the component wall thickness becoming 100% dealloyed.
STPNOC will examine dealloyed components removed from service as discussed in the response to RAI Issue 1 above to verify that dealloyed components retain sufficient strength after external flaws are detected.
RAI Issue 5:
Although the RAI response described the basis for the flaw size assumed in Appendix 9A of the updated final safety analysis report (UFSAR), it did not respond to the staff's question regarding the maximum tolerable flaw size. In order to respond to the staff's concern, the applicant needs to identify the maximum size flaw that would not proceed to failure or would only exhibit minor leakage until a transient occurred. Then, utilizing the transient inputs, the response needs to identify the maximum size leak path in the affected component. In addition, it is not apparent to the staff how leakage upstream of an individual component (e.g., diesel generator heat exchanger) has been addressed.
Request:
For the flooding, reduction in flow, and water loss from the essential cooling pond analyses, state the basis for why the medium energy break size flaw stated in UFSAR Appendix 9A is larger than the maximum size flaw for which the piping can still perform its intended function.
Given the maximum leak rate that could occur upstream of any individual component supplied by the essential cooling water system, state whether the affected component could still perform its intended function.
NOC-AE-12002889 Page 7 of 9 STPNOC Response:
UFSAR Appendix 9A section titled, "Reduction in ECW Flow" represents the maximum leak rate that could occur upstream of any individual component. This location at the ECW pump discharge maximizes the leakage rate. Based on this, leaks upstream of an individual component will have less than a 2% impact on flow, and individual components will continue to perform their intended functions.
The analysis in UFSAR Appendix 9A demonstrates that the additional flow rate from an equivalent rectangular opening of one-half the pipe diameter and one-half the pipe thickness (15"xl/8" opening for 30" pipe) can be supported by the ECW pump despite decreasing pump head.
STPNOC will determine leak rates that could occur upstream of any individual component supplied by the ECW system. This should validate that the maximum size flaw for which the piping can still perform its intended function does not exceed the medium energy break size flaw stated in UFSAR Appendix 9A flooding, reduction in flow, and water loss from the essential cooling pond assumptions. A summary of the results of these calculations will be submitted to the NRC for review. See new Commitment 46. provides the line-in/line-out revision to LRA Table A4-1 for new LRA Commitment 46.
RAI Issue 6:
The staff does not find the response to RAI B2.1.37-3, Part (i), acceptable because fracture toughness and yield strength properties are not listed as being trended. In addition, given that the progression rate of dealloying could change with time, the staff believes that prevalence of dealloying should be trended in order to determine if more frequent samples should be obtained.
Request:
Revise the appropriate portions of the LRA to trend fracture toughness values, yield strength values, and the degree of dealloying in addition to ultimate tensile strength.
STPNOC Response:
In addition to trending ultimate tensile strength, STPNOC will revise the LRA to trend the degree of dealloying for the samples to be examined and/or tested as described in the response to RAI Issue 1. During a public meeting (Reference 5) with the NRC, STPNOC explained that fracture toughness and yield strength properties would not be trended as trending did not provide additional value for validating the structural integrity analysis. provides the line-in/line-out revision to LRA Appendices A1.37 and B2.1.37. provides the line-in/line-out revision to LRA Table A4-1 for LRA Commitments 39 and 44 and for new LRA Commitment 45.
NOC-AE-1 2002889 Page 8 of 9 RAI Issue 7:
The staff does not find the response to RAI B2.1.37-3, Part (i), acceptable because given that the current analyses are based on a minimum 30 ksi ultimate tensile strength, the staff does not accept the position that an average value equal to or greater than 30 ksi is acceptable to demonstrate that the intended function of the susceptible components will be met. In addition, acceptance criteria for fracture toughness and yield strength values were not established.
Request:
Provide acceptance criteria for the fracture toughness and yield strength values. Amend the applicable portions of the LRA to reflect that the acceptance criteria of ultimate tensile strength is a minimum of 30 ksi, or state the basis for why when only a minimum of 12 samples will be tested, utilizing an average value is acceptable.
STPNOC Response:
LRA Sections A1.37 and B2.1.37 and Commitments 39 and 44 in Table A4.1 are revised to reflect that the acceptance criterion of ultimate tensile strength value of aluminum bronze material is equal to or greater than 30 ksi. This acceptance criterion is also provided in new Commitment 45.
The acceptance criterion for the fracture toughness is greater than or equal to 65 ksi in1 2 for aluminum bronze castings and welded joints in the heat affected zones. LRA Sections A1.37 and B2.1.37 and Commitment 44 in Table A4.1 are revised to add the additional acceptance criteria. This acceptance criterion is also provided in new Commitment 45.
The acceptance criterion for yield strength is equal to or greater than one half of the ultimate strength. LRA Sections A1.37 and B2.1.37 and Commitments 39 and 44 in Table A4.1 are revised to add the additional acceptance criteria. This acceptance criterion is also provided in new Commitment 45.
The additional testing commitments as discussed in response to RAI Issue 1 should result in a comparison of limit load stress results with material fracture stress limits to confirm the conclusion of the bounding calculations.
During the August 27, 2012 Public Meeting, STPNOC agreed to submit calculation AES-C-1964-5. See Attachment B to this Enclosure. provides the line-in/line-out revision to LRA Appendices A1.37 and B2.1.37. provides the line-in/line-out revision to LRA Commitments 39 and 44 and new LRA Commitment 45.
NOC-AE-12002889 Page 9 of 9 RAI Issue 8:
A review of plant-specific operating experience associated with the essential cooling water system demonstrates that cavitation erosion is occurring in the system. The staff does not know if any of the cavitation erosion has occurred or could occur in the vicinity of dealloying. If cavitation erosion could occur in the vicinity of dealloyed material, the staff does not know how the potential change in the rate of erosion is accounted for in the intervals between inspections of the components.
Request:
State whether cavitation erosion in the essential cooling water system has or could occur in the vicinity of dealloying. If this is the case, state how the potential change in the rate of erosion is accounted for in the intervals between inspections of the components.
STPNOC Response:
Cavitation erosion in the ECW system has occurred within the valve body of the Component Cooling Water heat exchanger discharge valve and the downstream piping. The valve body is susceptible to dealloying and cavitation erosion. The downstream piping also experiences cavitation erosion, but the pipe material is not subject to dealloying. Both locations are coated to minimize material wear and are visually inspected periodically. Adjustment to the inspection interval is performed as necessary following each inspection to account for potential changes in erosion and available margins to assure minimum wall thicknesses are not compromised between inspections. Cavitation erosion of these locations are managed by the Open-Cycle Cooling Water System aging management program as described in Reference 6.
Attachment A to Enclosure 1 NOC-AE-12002889 Page 1 of 1 List of References
- 1. Document AES-C-1 964-1, "Calculation of Critical Bending Stress for Dealloyed Aluminum Bronze Castings in the ECW System," APTECH Project AES 913061964-1Q (January 1994)
- 2.
Document AES-C-1964-5, "Evaluation of the Significance of Dealloying and Subsurface Cracks on Flaw Evaluation Method," APTECH Project AES 93061964-1Q (December 1994).
- 3.
Document AES-C-1964-4, "Evaluation of 6-Inch Flange Test," APTECH Project 93061964-1Q (June 1994)
- 4. Letter dated May 31, 2012 from D. W. Rencurrel, STPNOC, to the NRC Document Control Desk, "Response to Requests for Additional Information for the South Texas Project License Renewal Application Aging Management Program, Set 16 (TAC Nos. ME4936 and ME4937)" (NOC-AE-1 2002853) (ML12163A333)
- 5.
NRC Memorandum dated August 15, 2012, "Forthcoming Meeting with STP Nuclear Operating Company Regarding License Renewal for the South Texas Project, Units 1 and 2" (ML12226A455)
- 6.
Letter dated July 5, 2012 from G. T. Powell, STPNOC, to the NRC Document Control Desk, "Response to Requests for Additional Information (RAI) b2.1.9-4b for the South Texas Project License Renewal Application (TAC Nos. ME4936 and ME4937)"