NOC-AE-06002070, Request for Relief from ASME Boiler and Pressure Vessel Code, Section XI Requirements for the Essential Cooling Water System (Relief Request RR-ENG-2-47)
| ML063250344 | |
| Person / Time | |
|---|---|
| Site: | South Texas  |
| Issue date: | 11/15/2006 |
| From: | Rencurrel D South Texas |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| NOC-AE-06002070, RR-ENG-2-47 | |
| Download: ML063250344 (7) | |
Text
Nuclear Operating Company South TeAvs Project Ekctric Genem1n$ Statoio? PO Box289 Wadswodrth Texas 77483 *A/\ --
November 15, 2006 NOC-AE-06002070 File No.: G25 10CFR50.55a U. S. Nuclear Regulatory Commission Attention: Document Control Desk One White Flint North 11555 Rockville Pike Rockville, MD 20852-2746 South Texas Project Unit 2 Docket No. STN 50-499 Request for Relief from ASME Boiler and Pressure Vessel Code,Section XI Requirements for the Essential Cooling Water System (Relief Request RR-ENG-2-47)
In accordance with the provisions of 10 CFR 50.55a(g)(5)(iii), the South Texas Project requests relief from IWA-5250 of Section Xl of the ASME Boiler and Pressure Vessel Code. Approval will allow deferral of code repair of a flaw recently identified in the Unit 2 Essential Cooling Water (ECW) Class 3 piping. Repair of the flaw with a code repair at this time is impractical. In accordance with the guidance provided in Generic Letter 90-05 and subject to Nuclear Regulatory Commission approval, code repairs will be implemented no later than the next scheduled Unit 2 refueling outage.
An indication of through-wall dealloying was identified on the cast aluminum-bronze ECW tee connecting lines 10"EW-2106-WT3 (cooling water supply header to standby diesel generator
- 21) and 4"EW-2129-WT3 (Lube Oil Cooler Supply Line). The dealloying indication is a spot with residue buildup at the bottom of the tee. Evaluation of the flaw using fracture mechanics methodology provided by NRC Generic Letter 90-05 determined that the structural integrity of the ECW piping is not adversely affected.
The attached relief request addresses the present condition of the pipe section, and implementation of compensatory and corrective actions in accordance with the guidelines provided in Generic Letter 90-05. Operability and functionality of the system have been maintained, and deferring repair of the flaw will not affect the health and safety of the public.
A list of commitments in the request is attached.
If there are any questions, please contact either Mr. P. L. Walker at (361) 972-8392 or me at (361) 972-7867.
vDiW. Rencurrel Vice President, Engineering
& Strategic Projects PLW Attachments: 1) Request for Relief from ASME Boiler and Pressure Vessel Code, Section X1 Requirements for the Essential Cooling Water System (Relief Request RR-ENG-2-47)
- 2) List of Commitments STI No. 32064448
NOC-AE-06002070 Page 2 of 2 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 NRG South Texas LP Austin, TX 78756-3189 Senior Resident Inspector J. J. Nesrsta U. S. Nuclear Regulatory Commission R. K. Temple P. O. Box 289, Mail Code: MN116 E. Alarcon Wadsworth, TX 77483 City Public Service C. M. Canady C. Kirksey City of Austin City of Austin Electric Utility Department 721 Barton Springs Road Jon C. Wood Austin, TX 78704 Cox Smith Matthews
Attachment 1 NOC-AE-06002070 Page 1 of 4 SOUTH TEXAS PROJECT UNIT 2 REQUEST FOR RELIEF FROM ASME BOILER AND PRESSURE VESSEL CODE, SECTION XI REQUIREMENTS FOR THE ESSENTIAL COOLING WATER SYSTEM (RELIEF REQUEST RR-ENG-2-47)
Component for Which Relief is Requested (a)
Description:
Cast aluminum ECW tee connecting lines 10"EW-2106-WT3 (cooling water supply header to standby diesel generator #21) and 4"EW-2129-WT3 (Lube Oil Cooler Supply Line).
(b) Function:
The Essential Cooling Water System is designed to supply cooling water to various safety-related systems for normal plant operation, normal shutdown, and during and after postulated design-basis accidents.
(c) Class:
ASME Code Class 3 (d) Description of the flaw:
An indication of through-wall dealloying was found on the cast aluminum-bronze ECW tee connecting lines 10"EW-2106-WT3 (cooling water supply header to standby diesel generator #21) and 4"EW-2129-WT3 (Lube Oil Cooler Supply Line). The dealloying indication is a spot with residue buildup at the bottom of the tee. The flaw appears to be porous, dealloyed pipe material, with no measurable leakage.
- 2. Applicable Code Edition and Addenda:
ASME Boiler and Pressure Vessel Code,Section XI, 1989 Edition
- 3. Applicable Code Requirement:
Relief from the requirements of ASME Section Xl IWA-5250(a)(3) is requested so that code repair of the through-wall flaw at this location may be deferred until the next outage of sufficient duration but not later than the next refueling outage provided the conditions of Generic Letter 90-05, "Guidance for Performing Temporary Non-Code Repair of ASME Code Class 1, 2 and 3 Piping," are met.
- 4. Flaw Detection The flaw was identified on July 6, 2006, during the periodic examination of ECW large bore piping. Unit 2 was in Mode 1 at 100% power.
- 5. Impracticality Determination As stated in Generic Letter 90-05, an ASME Code repair is required for Code Class 1, 2, and 3 piping unless specific written relief is granted by the NRC. Relief is appropriate when performing the repair at the time of discovery is determined to be impractical.
A repair is considered to be impractical if:
Attachment 1 NOC-AE-06002070 Page 2 of 4
- The flaw detected during plant operation is in a section of Class 3 piping that cannot be isolated to complete a code repair within the time period permitted by the limiting condition for operation of the affected system as specified in the plant Technical Specifications, and
- Performance of code repair necessitates a plant shutdown.
Performance of code repairs within the allowed outage time for the ECW system, as permitted by the limiting condition for operation, is not practical due to the amount of time required to implement the repair, and the potential for fit-up problems during repair.
A plant shutdown may be necessary to complete the repair. Therefore, relief is requested on the basis of impracticality.
- 6. Proposed Alternative and Basis for Use 6.1 Proposed Alternative Repair of the defect will be deferred until adequate time is available for the repair, but no later than the next Unit 1 refueling outage, provided the condition continues to meet the acceptance criteria of Generic Letter 90-05. The next Unit 2 refueling outage is currently scheduled to begin in March 2007. Compensatory action has been implemented to detect changes in the condition of the flaw.
6.2 Basis for Use 6.2.1 Scope An indication of through-wall dealloying in Unit 2 was identified on the cast aluminum-bronze ECW tee connecting lines 10"EW-2106-WT3 (cooling'water supply header to standby diesel generator #21) and 4"EW-2129-WT3 (Lube Oil Cooler Supply Line). The flaw is measured approximately 1/2" long at outside surface of the ECW pipe. The flaw appears to be porous dealloyed structure as the leakage is not readily measurable.
There is no active measurable leakage.
6.2.2 Specific Considerations Consequences of potential system interactions, including flooding, spray on equipment, and loss of flow to the system, are addressed in Appendix 9A of the South Texas Project Updated Final Safety Analysis Report, "Assessment of the Potential Effects of Through-Wall Cracks in ECWS Piping".
The ECW system is a low-pressure system with normal operating pressures of approximately 50 psig and a design pressure of 120 psig. Therefore, the consequences associated with failure of high-enrergy lines are not applicable to this relief request.
The structural integrity is monitored by the following methods:
" Monthly monitoring for qualitative assessment of leakage (quantitative if measurable leaks are observed). Currently there is no measurable leakage.
" Continuation of large bore ECW piping periodic walkdowns. These walkdowns are regularly scheduled VT-2 examinations. The inspection technique has proven to be an effective means of identifying dealloyed/cracked components prior to deterioration of structural integrity margins below ASME Section XI requirements.
Structural integrity and the monitoring frequency will be re-evaluated if significant changes in the condition of the dealloyed area are found during this monitoring.
Attachment 1 NOC-AE-06002070 Page 3 of 4 6.2.3 Root Cause Determination The root cause of dealloying is a combination of corrosion and stress. The dealloying process normally initiates from a crevice such as the area behind a backing ring, a fabrication-induced flaw, or a casting flaw. Dealloying in this case is believed to be similar to dealloying seen in other susceptible aluminum-bronze components. The process by which dealloying of aluminum-bronze occurs has been described in previous communications with the NRC (Reference 8.11).
6.2.4 Flaw Evaluation In assessing the structural integrity of partially dealloyed aluminum-bronze piping components, a conservative evaluation has been performed to assure that adequate margins remain. This was accomplished by evaluating the condition where the dealloyed region is assumed to have lost its load carrying capacity and will behave like a crack-like flaw. Under these conditions, flaw evaluation procedures similar to Section XI of the ASME Code have been applied.
Unlike some carbon steels and low-alloy steels, aluminum bronze is inherently ductile and tough. This stems from its crystal structure which is like that of Type 304 stainless steel. Thus, the fracture resistance of aluminum bronze is expected to be high and the affected fittings are relatively insensitive to material flaws such as cracks.
Aluminum-bronze is not expected to behave in a non-ductile manner; however, linear elastic fracture mechanics (LEFM) have established the load carrying capacity of partially dealloyed fittings when treating the dealloyed region as a crack-like flaw. When LEFM principles are applied, flaw tolerance can be quantified in terms of applied stress, flaw size and shape, and the material fracture toughness. Conservative values for fracture toughness and conservative representation of the size and extent of dealloying as a flaw give a conservative determination of the structural capacity.
The structural integrity of the flanged piping was assessed using the "through-wall flaw" evaluation approach provided in Section C.3.a of Enclosure 1 to NRC Generic Letter 90-
- 05. This approach evaluates the flaw stability by LEFM methodology. To summarize the results:
s = predicted bending stress at the flaw location s = 4.8 ksi K = stress intensity factor K = 8.1 ksi-in1/2 For flaw stability, this methodology specifies "K" should be less than the critical stress intensity factor which represents the fracture toughness of the material. The fracture toughness of the aluminum-bronze ranges from 63.5 to 95.1 ksi-in11 2.
Attachment 1 NOC-AE-06002070 Page 4 of 4 Stresses Pressure + Faulted Thermal Dead Weight Stress (psi) 1187 951 2489 Allowable Stress (psi) 7500 18000 11250 Safety Margin 6.3 18.92 4.5 The calculated safety margins are adequate for the various loading conditions.
6.2.5 Auqmented Inspection Augmented monthly inspections are performed to detect changes in the size of the discolored area or leakage. Structural integrity and the monitoring frequency will be re-evaluated if significant changes in the condition of the dealloyed area are found during this monitoring.
6.2.6 Conclusion The South Texas Project has analyzed through-wall flaws in ECW piping and found that degradation progresses slowly. Dealloying produces detectable leakage before flaws reach a limiting size that would affect the operability of the Essential Cooling Water System. Rapid or catastrophic failure due to dealloying is not a concern. Flaws are monitored and inspected to ensure detection of leakage. Continued inspection provides assurance that changes in the condition of the flaws will be identified and assessed for further action as needed. Evaluation of the flaw using fracture mechanics methodology provided by NRC Generic Letter 90-05 concludes that the structural integrity of the ECW piping is not adversely affected. Operability and functionality of the system have been maintained, and deferring repair of the flaw will not affect the health and safety of the public.
- 7. Duration of Proposed Alternative Repair of the defect will be deferred until adequate time is available for the repair, but no later than the next Unit 2 refueling outage, provided the condition continues to meet the acceptance criteria of Generic Letter 90-05. The next Unit 2 refueling outage is currently scheduled to begin in March 2007.
- 8.
Reference:
8.1 Status of Corrective Actions in the ECW System, M. A. McBurnett to Document Control Desk, dated November 1, 1988 (ST-HL-AE-2748)
Attachment 2 NOC-AE-06002070 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 Expected CR Action No.
Completion Date Repair of the defect will be deferred until adequate time is available for the repair, but no 04/25/2007 06-8594-8 later than the next Unit 2 refueling outage, provided the condition continues to meet the acceptance criteria of Generic Letter 90-05.
Perform monthly walkdowns of dealloying location to detect changes in size of the discolored area or leakage until a code repair is performed. 05/01/2007 06-8594-5 Structural integrity and the monitoring frequency will be re-evaluated if significant changes in the condition of the dealloyed area are found during this monitoring.