NOC-AE-08002293, Response to Request for Additional Information: Request for Relief from ASME Boiler and Pressure Vessel Code, Section XI Requirements for the Essential Cooling Water System (Relief Request RR-ENG-2-49)
| ML081440426 | |
| Person / Time | |
|---|---|
| Site: | South Texas  |
| Issue date: | 05/14/2008 |
| From: | Rencurrel D South Texas |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| NOC-AE-08002293, RR-ENG-2-49, TAC MD7495 | |
| Download: ML081440426 (10) | |
Text
Nuclear Operating Company South Texas Pro/ect Electric Generatng Station PO Box289 Wadsworth, Texas 77483 -
May 14, 2008 NOC-AE-08002293 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 Response to Request for Additional Information:
Request for Relief from ASME Boiler and Pressure Vessel Code,-
Section XI Requirements for the Essential Cooling Water System (Relief Request RR-ENG-2-49) (TAC No. MD7495)
Reference:
Correspondence from David W. Rencurrel to NRC Document Control Desk, "Request for Relief from ASME Boiler and Pressure Vessel Code, Section Xl Requirements for the Essential Cooling Water System (Relief Request RR-ENG-2-49)," dated September 6, 2007 (NOC-AE-07002189) (ML072600151)
In accordance with the provisions of 10 CFR 50.55a(g)(5)(iii), the South Texas Project requested relief from IWA-5250 of Section XI of the ASME Boiler and Pressure Vessel Code to allow deferral of code repair of a flaw identified in the Unit 2 Essential Cooling Water (ECW)
Class 3 piping, as referenced above. Repair of the flaw with a code repair at this time is impractical. In accordance with the guidance provided in NRC 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. Attached are responses to an NRC request for additional information provided via email on February 12, 2008.
There are no commitments in this submittal.
If there are any questions, please contact either Mr. P. L. Walker at (361) 972-8392 or me at (361) 972-7867.
Vice President, Engineering & Strategic Projects PLW
Attachment:
Request for Relief from ASME Boiler and Pressure Vessel Code,Section XI Requirements for the Essential Cooling Water System (Relief Request RR-ENG-2-49) -7, STI: 32299507
NOC-AE-08002293 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 612 East Lamar Blvd., Suite 400 Arlington, Texas 76011-4125 Mohan C. Thadani U. S. Nuclear Regulatory Commission Mohan C. Thadani Thad Hill Senior Project Manager Catherine Callaway U.S. Nuclear Regulatory Commission Jim von Suskil One White Flint North (MS 7 D1) NRG South Texas LP 11555 Rockville Pike Rockville, MD 20852 Richard A. Ratliff J. J. Nesrsta Bureau of Radiation Control R. K. Temple Texas Department of State Health Services E. Alarcon 1100 West 49th Street Kevin Polio Austin, TX 78756-3189 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 Senior Resident Inspector U. S. Nuclear Regulatory Commission P. 0. Box 289, Mail Code: MN1 16 Wadsworth, TX 77483
Attachment NOC-AE-08002293 Page 1 of 8 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-2-49)
- 1. In section 6.2.1 Scope of the subject relief request, the licensee stated that an indication of through-wall dealloying was identified on SBDG #23 cast aluminum-bronze ECW return flow balance throttle valve 2-EW-0204. The licensee also stated that the dealloying indication is a spot with residue buildup on the seat retainer.
Please provide the following additional information:
(a) Provide a sketch to show how the seat retainer was connected to the valve 2-EW-0204 and the ECW piping.
Figure 1 shows the configuration of the specific valve. Figure 2 is a typical representation of a valve showing how a seat retainer is installed. Figure 3 is a photograph of the actual valve showing its position relative to the attached ECW piping.
(b) Provide a sketch to show the dimensions of the referenced seat retainer (ID, OD, wall thickness and the length of component).
The dimensions of the parts illustrated in Figure 1 are as follows:
Outside diameter of seat retainer =: 8.6" Inside diameter of seat retainer = -5.9" to 6.2" (the ID is chamfered)
Thickness (radial) of seat retainer = -1'.2" (the ID is chamfered)
Thickness (longitudinal) of seat retainer = -0.55" (c) From the photographs you provided in the e-mail, it appears that the materials of the components adjacent to the seat retainer are different from that of the seat retainer. Identify the adjacent components and its materials.
The adjacent components are a red rubber gasket, pipe flange (SB-271 CA-952 cast aluminum-bronze), and the valve body (SB 148 CA 954). The seat retainer material is also SB 148 CA 954. The difference in appearance between the seat retainer and the valve body may be due to machining of the seat retainer surface while the valve body is closer to its original as-cast appearance.
(d) Is the referenced valve made of cast aluminum-bronze material? Also discuss the previous service experiences of such valves and associated ECW piping in both units regarding the degradation due to dealloying.
The valve body is cast aluminum-bronze (SB 148 CA 954).
Attachment NOC-AE-08002293 Page 2 of 8 Dealloying previously occurred in a 6-inch Posi-Seal valve in 2000/2001. Incidents of apparent through-'wall dealloying are not known to have previously occurred in Hills-McCanna valves at the South Texas Project.
- 2. Provide detailed discussion regarding the compensatory action -mentioned in 6.1 Proposed Alternative.
Normally, walkdowns of ECW piping are performed at intervals of six months. In the event a flawed area is discovered, augmented monthly inspections are performed to monitor the flaw to detect changes in the size ,of the discolored area or leakage rate. Inspectors look for: change from residue buildup to active dripping; new indication at a different area on the component; or, a substantial change (about 2x or more) in the area of the original indication.
Structural integrity and the monitoring frequency are re-evaluated if monitoring identifies significant changes in the condition of the dealloyed area.
- 3. Provide details regarding the limiting condition for operation of the affected system as specified in the plant technical specification as discussed in section 5.0 Impracticality Determination.
ASME Section XI, IWA-5250, requires that leakage be evaluated for corrective action and implies that any component with through-wall leakage must be repaired or replaced regardless of the leakage rate. The expectation of ASME Section Xl is that through-wall leaks will be repaired at the time of discovery. Repairs of dealloyed section of piping require that the affected ECW loop be made inoperable. However, repairs can not be initiated immediately upon discovery of a flaw due to time required for obtaining parts, staging materials, and repair crew preparation, with the time for actual repair beyond that. The amount of time needed for resolution will vary depending upon individual circumstances, but could require months from time of discovery.
As stated in the South Texas Project Technical Specifications, the three independent essential cooling water loops shall be operable in modes 1, 2, 3, and 4. With only two of the essential cooling water loops operable, all three are to be operable within seven days. If only one loop remains in service, one loop is to be returned to service within one hour. If these requirements are not met, the affected unit is to be in Hot Standby within the next six hours, or the requirements of the Configuration Risk Management Program are to be applied.
The South Texas Project applies risk-managed Technical Specifications in accordance with the Configuration Risk Management Program. If there is a need to extend the allowed outage time for the affected ECW loop, risk analysis techniques can be applied that take into account real-time plant status to keep overall risk below 1OE-5 up to a maximum of 30 days. However, taking an otherwise operable ECW loop out of service while at power not only increases overall risk to the plant, but also limits flexibility in dealing with other plant equipment issues that may arise in the interim.
- 4. Provide detailed discussion to support the statement that this assessment is bounding for the condition under consideration as stated in 6.2.2 Specific Considerations.
Attachment NOC-AE-08002293 Page 3 of 8 Flooding in a given area due to the ECW system is enveloped by worst case flow from an opening in a local pipe due to a "critical crack," with an area equivalent to a rectangle of length one-half the pipe diameter and a width equal to one-half the pipe wall thickness.
This assessment is bounding for the condition under consideration.
Flooding analyses have been performed for specific regions of the South Texas Project, taking into consideration the local worst case flood source and the pote6tial impact on plant equipment. Flooding is addressed in Updated Final Safety Analysis Report Appendix 9A.
Water in a diesel generator bay can be tolerated up to a maximum level of 4 inches above the floor. Should that level be reached, the affected diesel would be declared inoperable.
Technical Specifications require three diesel generators to be operable in Modes 1-4 and two diesel generators to be operable in Modes 5 and 6. The Diesel Generator Building design precludes flooding in one bay from impacting the other two bays. Plant operators monitor the diesel sump levels for increases that could be indicative of ECW leakage.
Flooding due to ECW dealloying was reviewed by NRC inspectors during an inspection conducted January 25 through February 12, 1999. This is documented in NRC Inspection Report No. 50-498/98-19; 50-499/98-19, dated March 26, 1999.
- 5. Provide detailed discussion regarding the methodology used in flaw evaluation in section 6.2.4, particularly with regard to the methodology described in GL 90-05.
Also provide detailed quantitative information regarding the calculated safety margins for the various loading conditions.
The methodology used in flaw evaluations is described in Section 5.0 of ATPECH analysis document AES-C-1964-1, "Calculation of Critical Bending Stress for Dealloyed Aluminum-Bronze Castings in the ECW System." This was previously submitted to the NRC as an attachment to STPNOC correspondence dated August 10, 2000 (ML003742174).
Safety margins are determined from stresses on the affected region using postulated stresses due to pipe breaks as well as loads transferred from the pipe to the valve body.
Postulated Pipe Break Postulated pipe break stresses are determined using ASME Section III, subsection ND-3652.2, equation 9, and subsection ND-3652.3, equation 10, for occasional loads and thermal expansion, respectively. The seat retainer is treated as a section of 6-inch diameter schedule 40 pipe.
Postulated Pipe Break Stresses Eq. 9 (PSI) Eq. 10 (PSI) Eq. 9+10 (PSI) Allowable (PSI) Safety Margin 978 3593 4571 19440 4.25*
- NOTE: Dealloying reduces material strength. However, the body of the valve is not degraded; therefore, safety margins for the valve are not reduced. The retaining ring
Attachment NOC-AE-08002293 Page 4 of 8 evaluation is performed separately and accounts for the impact of reduced material strength.
Pipe-to-Valve Body Loads Valve body loads are determined according to ASME Code Section III, Subsection ND (for Class 3 piping), where:
Ma = torsional moment Mb, M, = bending moment(s)
Safety margins are determined by comparing allowable moment (6),to the calculated load (5).
The results are summarized below:
Valve Body Loads Loading Moment Ma Moment Mb Moment M, (Mb 2+Mc 2 ) 11 2 Allowable Safety Condition (Ft-Lb) (Ft-Lb) (Ft-Lb) (Ft-Lb) Moment Margin (Ft-Lb) [(6)/(5)]
(1) (2) (3) (4), (5) (6) (7)
Normal 1531 1293 1662 2106 4149 1.97 Upset 1666 1550 1958 2498 8297 3.32 Faulted 1702 1557 1958 2502 14007 5.59
,Although dealloying reduces the strength of the affected material, safety margins for the valve are not reduced because the body of the valve is not degraded.
- Seat Retainer Design For evaluation purposes, the seat retainer is treated as 6-inch diameter schedule 40 pipe.
The combined primary membrane stresses due to pressure plus bending loads are compared to the primary bending stress at incipient plastic collapse. The combined primary stresses as determined as follows:
The seat retainer is fabricated from SB 148 CA 954.. Tensile strength and yield strength are listed as 86 ksi and 36 ksi, respectively.
Taking into account the circumferential bolt holes, the effective diameter of the retainer is 7.905 inches, with an effective thickness of 0.925 inch. The seat retainer has significant radial thickness available to resist pressure. The pressure stress determined by PD/2T (640 psi) is small compared to a dealloyed material allowable value of 7500 psi.
Attachment NOC-AE-08002293 Page 5 of 8 Primary membrane stress due to bending is calculated from the resultant moment. Using square root of the sum of the squares (SRSS), the resultant moment is calculated from orthogonal faulted moment loads, which are those loads used in qualifying flange in the pipe stress calculation. These loads are normal (thermal plus dead weight) plus seismic plus waterhammer combined as an absolute sum.
Loads: Ma = 1702 ft-lb Mb = 1557 ft-lb Mc = 1958 ft-lb Combined load (SRSS): Mr = 3026 ft-lb Section modulus of pipe: Z = 8.5 in3 Membrane stress: Mr*12/Z = 4272 psi (due to bending loads)
Combined primary membrane stress due to pressure plus bending loads = 4912 psi.
The code allowable stress for aluminum-bronze is 18,700 ksi. Dealloyed aluminum-bronze has an allowed stress value of 7,500 ksi. These allowed values are significantly higher than the calculated combined primary membrane stress given above.
- 6. Provide detailed discussion to support your findings in section 6.2.6 Conclusion regarding the statement that the referenced degradation that progresses slowly.
Periodic monitoring and inspection by STPNOC provide confidence in the ability to detect changes in the leakage rate before leakage becomes a safety issue. Flawed areas are inspected monthly, instead of every six months, and none has shown sufficient change from the time of discovery to warrant accelerated implementation of corrective measures.
NOC-AE-08002293 Page 6 of 8 FIGURE 1: LINE DRAWING OF ROCKWELL VALVE
NOC-AE-08002293 Page 7 of 8 FIGURE 2: EXPLODED DRAWING OF TYPICAL VALVE EXPLODED VIEW Approximate location of dealloyed region KA-C' A NIN Il.A.Il4IN C LIr1X' DLrIou 1 nrI Ll V-%LV MODEL II CONFIGURATION
NOC-AE-08002293 Page 8 of 8 FIGURE 3: PHOTOGRAPH OF AFFECTED VALVE Area of dealloying 7