L-MT-10-023, CFR 50.55a Request No. 18: Alternative to Apply ASME Code Case N-705 to the Standby Liquid Control System Tank
| ML100920376 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 04/02/2010 |
| From: | O'Connor T Northern States Power Co, Xcel Energy |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| L-MT-10-023 | |
| Download: ML100920376 (10) | |
Text
Xcel EnergyB April 2, 2010 Monticello Nuclear Generating Plant 2807 W County Road 75 Monticello, MN 55362 L-MT-10-023 10 CFR 50.55a U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555 Monticello Nuclear Generating Plant Docket 50-263 Renewed Facility Operating License No. DPR-22
Subject:
10 CFR 50.55a Request No. 18: Alternative to Applv ASME Code Case N-705 to the Standby Liquid Control Svstem Tank Sodium pentaborate crystallization has been identified on the exposed lip of the American Society of Mechanical Engineers (ASME) Class 2 Standby Liquid Control (SLC) Tank base. Minute leakage was identified from one location. Applying several non-destructive evaluation (NDE) techniques, indications of radial cracking were identified in the SLC Tank bottom plate (transgranular stress corrosion cracking). The SLC Tank was determined operable by NDE examination and structural evaluation.
Flaw evaluation by applying ASME Code Case N-705, "Evaluation Criteria for Temporary Acceptance of Degradation in Moderate Energy Class 2 or 3 Vessels and Tanks,Section XI, Division 1," demonstrates the SLC Tank will retain its structural integrity well beyond the next refueling outage using the worst case flaw scenario.
The Northern States Power Company - Minnesota (NSPM) identified ASME Code Case N-513, "Evaluation Criteria for Temporary Acceptance of Flaws in Moderate Energy Class 2 or 3 Piping,Section XI, Division 1," as being potentially applicable. It was determined that while relevant, ASME Code Case N-513 was not applicable because it applied to piping and tubing, and not tanks. NSPM identified that ASME Code Case N-705, while not yet included within 10 CFR 50.55a, was similar to ASME Code Case N-513 and provided appropriate evaluation guidance.
NSPM pursuant to 10 CFR 50.55a, requests NRC authorization to apply ASME Code Case N-705 as an alternative to the ASME Boiler and Pressure Vessel Code,Section XI, "Rules for Inservice Inspection of Nuclear Power Plant Components," to the degradation of the SLC Tank. Authorization is requested to apply this alternative through the next refueling outage, currently scheduled to begin in March 201 1.
Document Control Desk L-MT-I 0-023 Page 2 In accordance with the requirements of ASME Code Case N-705, NSPM will take the following actions:
- 1) NSPM will restore the Standby Liquid Control Tank in accordance with Section XI of the ASME Code by startup from the next refueling outage, currently scheduled to begin in March 201 1.
- 2) Monitoring of Standby Liquid Control Tank leakage each day will be performed in accordance with Procedure 0000-J, "Operations Daily Log - Part J, Outplant" rounds until the tank is removed from service.
The 10 CFR 50.55a request is described in Enclosure 1 to this letter.
If you have any questions or require additional information, please contact Mr. Richard Loeffler at (763) 295-1247.
NSPM requests prompt authorization for this alternative.
Summaw of Commitments commitments and does not revise any existing e President, Monticello Nuclear Generating Plant n States Power Company - Minnesota Enclosure cc:
Administrator, Region Ill, USNRC Project Manager, Monticello, USNRC Resident inspector, Monticello, USNRC Minnesota Department of Commerce
ENCLOSURE 1 MONTICELLO NUCLEAR GENERATING PLANT 10 CFR 50.55a REQUEST NO. 18 PROPOSED ALTERNATIVE IN ACCORDANCE WITH 10 CFR 50.55a(a)(3)(i)
WHICH PROVIDES AN ACCEPTABLE LEVEL OF QUALITY OR SAFETY ALTERNATIVE TO APPLY ASME CODE CASE N-705 TO THE STANDBY LIQUID CONTROL SYSTEM TANK
L-MT-10-023 Page 1 of 3 10 CFR 50.55a Request No. 18 Proposed Alternative in Accordance With 10 CFR 50.55a(a)(3)(i)
Which Provides an Acceptable Level of Quality or Safety Alternative to Apply ASME Code Case N-705 to the Standby Liquid Control System Tank 1.0 ASME Code Component(s) Affected Code Class:
2 Component Numbers:
T-200 Examination Category:
C-A
==
Description:==
Alternative to ASME Section XI, Section IWC-3120 for the Standby Liquid Control (SLC) System Tank 2.0 Applicable ASME Code Edition and Addenda The Monticello Nuclear Generating Plant (MNGP) is currently in the fourth 10-year lnservice lnspection (ISI) Program interval and is committed to the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code,Section XI, "Rules for lnservice lnspection of Nuclear Power Plant Components," (ASME Section XI), 1995 Edition through 1996 Addenda.
Additionally, for ultrasonic examinations, ASME Section XI, Appendix VIII, "Performance Demonstration for Ultrasonic Examination Systems," of the 1995 Edition through 1996 Addenda is implemented, as required and modified by 10 CFR 50.55a.
3.0
Applicable Code Requirement
Section IWC-3120, "lnservice Volumetric and Surface Exams" 4.0
Reason for Request
While performing a periodic system walkdown, the system engineer identified sodium pentaborate crystallization at the bottom of the SLC Tank on the exposed lip of the tank base. Subsequent investigation identified degradation, i.e.,
indications of radial cracking in the SLC Tank bottom plate. There was also visible evidence of minute leakage at the tank base. The alternative proposed is to apply the evaluation criteria included in ASME Code Case N-705, "Evaluation
L-MT-10-023 Page 2 of 3 Criteria for Temporary Acceptance of Degradation in Moderate Energy Class 2 or 3 Vessels and Tanks,Section XI, Division 1." The proposed alternative provides an acceptable level of quality and safety.
5.0 Proposed Alternative and Basis for Use In accordance with 10 CFR 50.55a(a)(3)(i), relief is requested by the Northern States Power Company - Minnesota (NSPM) from ASME Section XI, IWC-3120, "lnservice Volumetric and Surface Exams," to implement an alternative that allows acceptance of degradation in the SLC Tank, an ASME Class 2 atmospheric pressure component, in accordance with the guidance of ASME Code Case N-705.
ASME Code Case N-705 indicates that alternatives to the requirements of IWC-3120, as specified therein, may be used to accept degradation, including through-wall degradation, in a moderate energy ASME Code Class 2 tank for a limited time not to exceed the evaluation period, determined as defined in the code case.
NSPM requests relief from IWC-3120, to allow the use of an alternate methodology to evaluate and accept through wall flaws in moderate energy Class 2 tanks. This alternative allows characterization of flaws in accordance with the applicable sections of ASME Code Case N-705, e.g., Section 2.2, "Degradation Characterization," and Section 2.4, "Bounding Flaw Evaluation," to estimate the degradation in inaccessible or uninspectable region(s) of the SLC Tank.
Section 6, "Subsequent Examinations and Surveillance," of the code case requires: 1) daily monitoring for tank leakage, and 2) examination of the degradation to verify the predicted growth at one-half of the allowed operating time (unless the time at which the degradation reaches the allowable flaw size is determined to be twice the time to reach the end of the evaluation period).
Monitoring of SLC Tank leakage each day in accordance with the operator rounds is proposed by NSPM to meet the first code case requirement (see Section 7.0 herein). NSPM has determined, based upon a flaw evaluation performed by Structural Integrity Associates, Inc. (see Enclosure 2), that the point in time where the limiting degradation reaches the allowable flaw size is far greater than twice the duration of the evaluation period,(') 26 months, and hence examination before the March 201 1 refueling outage is unnecessary. Therefore, this alternate methodology, as permitted in ASME Code Case N-705, would allow for a planned repair of the SLC Tank during the upcoming refueling outage in 201 I, vice an unnecessary emergent shutdown and repair.
- 1.
The evaluation period is the time to the next refueling outage - thirteen months.
L-MT-10-023 Page 3 of 3 NRC draft Regulatory Guide DG-1192, "lnservice Inspection Code Case Acceptability, ASME Section XI, Division 1," dated June 2009, identifies Code Case N-705 as one of the cases determined as an acceptable alternative to Section XI, for unconditional usage as indicated by its inclusion in Table 1, "Acceptable Section XI Code Cases," to this draft regulatory guide. A proposed rule revising 10 CFR 50.55a(*) to include the next revision of Regulatory Guide 1.147, i.e., Revision 16, which includes this code case, has been published in the Federal Register, and is awaiting publication as a Final Rule.
Use of ASME Code Case N-705 provides an acceptable level of quality and safety in accordance with 10 CFR 50.55a(a)(3)(i) for the evaluation and temporary acceptance of the degradation of the SLC Tank.
6.0 Duration of Proposed Alternative NSPM is requesting authorization for use of this alternative through the next refueling outage, currently scheduled to begin in March 201 I.
7.0 Interim Actions Compliance with ASME Code Case N-705, Section 6, "Subsequent Examinations and Surveillance," requires the following action to be implemented.
NSPM will restore the Standby Liquid Control Tank in accordance with Section XI of the ASME Code by startup from the next refueling outage, currently scheduled to begin in March 201 1.
Monitoring of Standby Liquid Control Tank leakage each day will be performed in accordance with Procedure 0000-J, "Operations Daily Log -
Part J, Outplant" rounds until the tank is removed from service.
- 2.
Federal Register: June 2, 2009 (Volume 74, Number 104, Proposed Rules),
"Incorporation by Reference of Regulatory Guide 1.84, Revision 35, and Regulatory Guide 1.147, Revision 16, Into 10 CFR 50.55aln pages 26303-2631 0.
ENCLOSURE 2 MONTICELLO NUCLEAR GENERATING PLANT 10 CFR 50.55a REQUEST NO. 18 PROPOSED ALTERNATIVE IN ACCORDANCE WITH 10 CFR 50.55a(a)(3)(i)
WHICH PROVIDES AN ACCEPTABLE LEVEL OF QUALITY OR SAFETY ALTERNATIVE TO APPLY ASME CODE CASE N-705 TO THE STANDBY LIQUID CONTROL SYSTEM TANK STRUCTURAL INTEGRITY ASSOCIATES, INC.
FLAW EVALUATION FOR THE MONTICELLO STANDBY LIQUID CONTROL TANK
SUMMARY
REPORT MARCH 24,2010 (3 Pages Follow)
Structural Integrity Associates, Inc.
521 5 Hellyer Ave.
Suite 210 San Jose, CA 95138-1025 Phone: 408-978-8200 Fax: 408-978-8964
.structint.com arniessi@structint.corn March 24,20 10 Report No. 1000413.401.RO Quality Program:
Nuclear Commercial Mr. Jim Bridgeman Xcel Energy Monticello Nuclear Power Plant 2807 W COUNTY ROAD 75 Monticello, MN 55362-9601
Subject:
Flaw Evaluation for Monticello Standby Liquid Control Tank
References:
- 1. ASME Boiler and Pressure Vessel Code,Section XI, Division 1, Code Case N-705, "Evaluation Criteria for Temporary Acceptance of Degradation in Moderate Energy Class 2 or 3 Vessels and Tanks."
- 2. Xcel Energy, Monticello Nuclear Generating Plant, SBLC Tank T-200 Indication Worksheet.
Dear Jim:
This summary report serves as a technical basis to support continued operation of the Monticello Standby Liquid Control (SBLC) tank until the next refueling outage under the provisions of ASME Code Case N-705 [I].
The results summarized herein are based on verified scoping analyses which utilize many conservative assumptions. The conclusions of acceptability for continued operation are not expected to change with the final analyses.
INTRODUCTION Leakage was detected at the SBLC tank during a walkdown. Subsequent inspections revealed that the leakage was emanating from a through-wall flaw in the base plate of the tank. The flaw extends radially across the base plate and appears to continue beneath the fillet weld on the outside of the cylindrical shell [2]. In addition, 16 other indications were found along the circumference of the base plate. However, only one flaw was completely through the thickness Annapolis, MD Austin, TX Centennial, CO Charlotte, NC Chattanooga, TN Oakville, Ontario, Canada South Jordan, UT Stonington, CT Uniontown, OH 41 0-571-0861 51 2-533-91 91 303-792-0077 704-597-5554 423-553-1 180 905-829-9817 801-676-0216 860-536-3982 330-899-9753
Mr. Jim Bridgeman I Xcel Energy March 24,20 10 Report No. 1000413.401.RO Page 2 of 3 Flaw Evaluation for Monticello Standby Liquid Control Tank of the base plate. It should also be noted that only five of the 17 indications span the width across the external face of the base plate. No indication was observed in the cylindrical shell.
Since the through-wall flaw at the location of the leakage extends past the attachment fillet weld, it was assumed that the flaw could be both in the base plate and also in the cylindrical shell attachment. Thus, in addition to evaluating the flaw in the base plate, a postulated circumferential flaw in the cylindrical shell is evaluated also.
TECHNICAL APPROACH The evaluation was performed in accordance with the structural requirements of ASME Section XI Code Case N-705 which has been approved by ASME but currently not approved in regulatory Guide 1.147. Allowable through-wall flaw sizes are calculated for the observed indications in the base plate and a postulated circumferential flaw in the cylindrical shell of the tank. In addition, a stress corrosion crack growth evaluation is performed to ensure that the flaws would not reach the allowable flaw size within the next operating period.
RESULTS Allowable Flaw Evaluation The bounding indication in the base plate is modeled as a through-wall flaw extending radially from the OD of the circular plate towards the center of the tank. The hoop stresses in the tank shell are conservatively applied to the base plate. Using the provisions of the structural requirements in Code Case N-705, the allowable flaw was calculated to be 85 inches.
For the tank shell, the evaluation was performed assuming a circumferential planar through-flaw in the cylindrical shell. Using the provisions of the structural requirements in Code case N-705, the allowable circumferential flaw in the tank shell was calculated to be 5.6 inches.
Stress Corrosion Crack Growth Analysis A flaw growth evaluation was also performed assuming transgranular stress corrosion cracking per the guidelines of Code Case N-705. An initial 2.7 inches long flaw (approximately twice the length from the OD of the base plate to the toe of the fillet weld inside the tank) is assumed for the crack growth analysis. This initial crack size assumption is based on postulating a leak path from the ID of the vessel through the two fillet welds and the shell wall. It was determined that it will take at least 40 years for this initial flaw to reach 3.1 inches. Based on the small crack growth, it is expected that all the indications are well below the allowable flaw sizes.
Similarly, the stress corrosion crack growth performed for the postulated circumferential through-wall flaw in the tank cylinder shows that it will take at least 18 years for a 0.35 inches long through-wall flaw to reach the allowable flaw size. No indications were observed in the vertical cylindrical shell during the current inspection. This does not imply that an indication does not exist in the vertical cylindrical shell.
Structural lntegrlfy Associates, Inc.
Mr. Jim Bridgeman 1 Xcel Energy March 24,2010 Report No. 1000413.401.RO Page 3 of 3 Flaw Evaluation for Monticello Standby Liquid Control Tank CONCLUSIONS Based on this evaluation, it is concluded that the observed indications in the base plate of the Monticello Standby Liquid Control tank are acceptable for operation until the next scheduled outage which is approximately one year away.
A postulated circumferential through-wall flaw in the cylindrical was also found to meet the acceptance criteria in Code Case N-705 for continued operation until the next scheduled outage.
Note that, per the requirements of Code Case N-705, daily monitoring is required to ensure that the leakage is within acceptable limits.
The removal of a small boat sample at the edge of the base plate, away from the fillet weld of the cylindrical shell, for metallurgical examination will not impact the conclusions of this evaluation.
Please contact us if you have any questions. Thank you.
Prepared by:
G. A. Miessi Associate Reviewed by:
S. S. Tang, P.E.
Associate Approved by:
G. A. Miessi Associate
@ Structural lntegrify Associates, Inc.