ML22298A285
| ML22298A285 | |
| Person / Time | |
|---|---|
| Site: | Browns Ferry  |
| Issue date: | 11/14/2022 |
| From: | David Wrona Plant Licensing Branch II |
| To: | Jim Barstow Tennessee Valley Authority |
| Green K | |
| References | |
| EPID L-2022-LLR-0008 | |
| Download: ML22298A285 (1) | |
Text
November 14, 2022 Mr. James Barstow Vice President, Nuclear Regulatory Affairs and Support Services Tennessee Valley Authority 1101 Market Street, LP 4A-C Chattanooga, TN 37402-2801
SUBJECT:
BROWNS FERRY NUCLEAR PLANT, UNIT 2 - REQUEST FOR ALTERNATIVE TO AMERICAN SOCIETY OF MECHANICAL ENGINEERS BOILER AND PRESSURE VESSEL CODE, SECTION XI, PARAGRAPH IWB-2420(B) AND USE OF CODE CASE N-526 (EPID L-2022-LLR-0008)
Dear Mr. Barstow:
By letter dated January 14, 2022, as supplemented by letter dated September 12, 2022, the Tennessee Valley Authority (TVA) submitted Request BFN-21-ISI-02 to the U.S. Nuclear Regulatory Commission (NRC) for the use of an alternative to American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code),Section XI, paragraph IWB-2420(b) and the use of ASME Code Case N-526 at the Browns Ferry Nuclear Plant (Browns Ferry),
Unit 2.
Specifically, pursuant to Title 10 of the Code of Federal Regulations (10 CFR)
Section 50.55a(z)(1), TVA requested to use the alternative on the basis that the alternative provides an acceptable level of quality and safety.
The NRC staff has reviewed the subject request and concludes, as set forth in the enclosed safety evaluation, that TVAs proposed alternative provides an acceptable level of quality and safety. Accordingly, the NRC staff concludes that TVA has adequately addressed all the regulatory requirements set forth in 10 CFR 50.55a(z)(1). Therefore, the NRC authorizes the use of alternative BFN-21-ISI-02 for the fifth 10-year inservice inspection interval at Browns Ferry, Unit 2.
All other ASME BPV Code,Section XI, requirements for which an alternative was not specifically requested and authorized in the subject request remain applicable, including third-party review by the Authorized Nuclear Inservice Inspector.
If you have any questions, please contact the Project Manager Kimberly Green at (301) 415-1627 or by email at Kimberly.Green@nrc.gov.
Sincerely, David J. Wrona, Chief Plant Licensing Branch II-2 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-260
Enclosure:
Safety Evaluation cc: Listserv David J.
Wrona Digitally signed by David J. Wrona Date: 2022.11.14 13:49:41 -05'00'
Enclosure SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION REQUEST FOR ALTERNATIVE TO AMERICAN SOCIETY OF MECHANICAL ENGINEERS BOILER AND PRESSURE VESSEL CODE, SECTION XI TENNESSEE VALLEY AUTHORITY BROWNS FERRY NUCLEAR PLANT, UNIT 2 DOCKET NO. 50-260
1.0 INTRODUCTION
By letter dated January 14, 2022 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML22014A344), as supplemented by letter dated September 12, 2022 (ML22256A045), the Tennessee Valley Authority (TVA or the licensee) submitted Request BFN-21-ISI-02 to the U.S. Nuclear Regulatory Commission (NRC) for the use of an alternative to American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code),Section XI, paragraph IWB-2420(b) and the use of ASME Code Case N-526, Alternative Requirements for Successive Inspections of Class 1 and 2 Vessels,Section XI, Division 1, at the Browns Ferry Nuclear Plant (Browns Ferry), Unit 2 for the fifth 10-year inservice inspection (ISI) interval.
Specifically, pursuant Title 10 of the Code of Federal Regulations (10 CFR) 50.55a(z)(1), as an alternative to ASME Code,Section XI, paragraph IWB-2420(b), the licensee requested to defer the periodic successive examinations of the identified flaw in weld V-3-A at Browns Ferry, Unit 2, until the next regularly scheduled ISI. The licensee proposed to apply the alternative requirements of ASME Code Case N-526 to the identified flaw, except it proposed to use the IWA-3320 surface proximity rules in lieu of the ASME Code Case N-526 proximity rules.
2.0 REGULATORY EVALUATION
The regulations in 10 CFR 50.55a(g) require that the ISI of ASME Code Class 1, 2, and 3 components be performed in accordance with Section XI of the ASME Code and applicable addenda.
Pursuant to 10 CFR 50.55a(g)(4), Inservice inspection standards requirement for operating plants, components (including supports) that are classified as ASME Code Class 1, Class 2, and Class 3 must meet the requirements in 10 CFR 50.55a throughout the service life of a boiling-or pressurized-water reactor. The exception is the design and access provisions and preservice examination requirements set forth in Section XI of editions and addenda of the ASME Code that become effective subsequent to editions specified in 10 CFR 50.55a(g)(2) and (3), which are incorporated by reference in 10 CFR 50.55a(a)(1)(ii) to the extent practical within the limitations of design, geometry, and materials of construction of the components.
The regulations in 10 CFR 50.55a(g)(4)(ii) state, in part, that inservice examination of components and system pressure tests conducted during successive 120-month intervals must comply with the requirements of the latest edition and addenda of the ASME Code incorporated by reference in 10 CFR 50.55a(a)(1)(iv) 18 months before the start of the 120-month interval (or the optional ASME Code Cases listed in NRC Regulatory Guide (RG) 1.147, Revision 20, Inservice Inspection Code Case Acceptability, ASME Section XI, Division 1 (ML21181A222),
as incorporated by reference in 10 CFR 50.55a(a)(3)(ii) and (iii), respectively), subject to the conditions listed in 10 CFR 50.55a(b).
The regulation in 10 CFR 50.55a(z)(1) states that alternatives to the requirements of paragraphs (b) through (h) of 10 CFR 50.55a or portions thereof may be used when authorized by the NRC prior to implementation. The licensee must demonstrate that the proposed alternative would provide an acceptable level of quality and safety.
Regulatory Guide (RG) 1.147 contains ASME Code Cases and revisions to existing Code Cases that the NRC staff has approved for use, as listed in Tables 1 and 2 of the RG. The RG also states the requirements governing the use of Code Cases. Licensees may voluntarily use Code Cases approved by the NRC as an alternative to compliance with the ASME Code provisions that have been incorporated by reference into 10 CFR 50.55a. ASME Code Case N-526 is listed in Table 1 as an acceptable ASME Code Case.
3.0 TECHNICAL EVALUATION
3.1 Licensees Proposed Alternative 3.1.1 ASME Code Components Affected The affected component is the ASME Code Class 1, reactor pressure vessel (RPV) vertical shell weld, number V-3-V. The weld is classified under Examination Category, B-A, Pressure Retaining Welds in Reactor Vessel, and Item Number, B1.12, Longitudinal Shell Welds, of ASME Code,Section XI, Table IWB-2500-1.
3.1.2 Applicable Code Edition and Addenda The code of record for the fifth ISI interval for Browns Ferry, Unit 2, is the 2007 Edition with 2008 Addenda of the ASME Code,Section XI, Division 1, Rules for Inservice Inspection of Nuclear Power Plant Components.
3.1.3 Applicable Code Requirement The ASME Code,Section XI, paragraph IWB-2420(b) requires areas containing flaws or relevant conditions to be reexamined during the next three inspection periods listed in the schedule of the Inspection Program of subarticle IWB-2400 if a component is accepted for continued service in accordance with subparagraphs IWB-3132.3 or IWB-3142.4.
3.1.4 Reason for Request During the Browns Ferry, Unit 2, Cycle 21 (U2R21) refueling outage in spring 2021, TVA identified an indication in weld V-3-A of the RPV by an ultrasonic examination. This inspection was performed as part of the second period of the fifth ISI interval as stated in TVAs supplement dated September 12, 2022. TVA dispositioned the indication as unacceptable in accordance with the acceptance standards of the ASME Code,Section XI, 2007 Edition with 2008 Addenda, as modified by the performance demonstration initiative program and 10 CFR 50.55a for Examination Category B-A components.
TVA performed automated transverse and parallel scans from the vessel inside surface in accordance with qualified procedures using 45-degree shear wave and 60-degree refracted longitudinal (RL) search units. TVA stated that the automated examination was restricted due to the proximity of the feedwater spargers and core spray piping. To compensate for the restriction, TVA also performed manual transverse and parallel scans in accordance with qualified procedures using 60-degree RL search units in the area restricted for the automated examination. As a result of the automated and manual inspections, TVA achieved 100 percent examination coverage for weld V-3-A during the Browns Ferry, Unit 2, Cycle 21 refueling outage in accordance with the ASME Code,Section XI, Appendix VIII Performance Demonstration Program.
TVA stated that the RPV indication/flaw is located between the 434.45-inch and 438.2-inch elevations as shown in Figure 1 of the proposed alternative. The linear planar flaw is oriented parallel to the vertical seam weld. TVA characterized the flaw in the vertical weld as a subsurface flaw per the ASME Code,Section XI, paragraph IWA-3320, with a half depth, a, of 1.6 inches and length, l, of 3.8 inches. Although the flaw was found unacceptable by the acceptance standards, TVA accepted the flaw for continued service per the analytical evaluation rules of ASME Code,Section XI, subarticle IWB-3600. Based on the acceptance by analytical evaluation, the ASME Code,Section XI, paragraph IWB-2420(b) requires successive examinations for each of the next three ISI periods.
TVA stated that ASME Code Case N-526 provides alternative requirements for the successive inspection requirements of paragraph IWB-2420(b), but its use is limited based on the proximity of the flaw to the vessel surface. The NRC has determined that Code Case N-526 is acceptable for use as shown in Table 1 of RG 1.147, Revision 20. The ASME Code,Section XI, paragraph IWA-3320 and Figure IWA-3320-1 define subsurface flaws as those whose surface proximity is greater than 0.4 times the flaw half depth (S > 0.4a, where S is the distance from the flaw to the vessel surface and 2a is the measured flaw depth), while Code Case N-526 further restricts subsurface flaws to those whose surface proximity is greater than the flaw half depth (S > a).
Based on its evaluation of the flaw in the V-3-A weld, TVA stated that the flaw meets the criteria set forth in ASME Code Case N-526 to forgo the three successive inspections when considering the proximity of the flaw to the inside diameter (ID) surface. TVA further stated that when the proximity evaluation was performed using the distance from the outside diameter (OD) surface, the flaw does not meet the criteria set forth in Code Case N-526. TVA explained that because Code Case N-526 does not specify which surface is to be used for proximity evaluation, TVA assumed that the proximity calculation should apply to the lesser of these two distances, similar to the conditions required by ASME Code,Section XI, paragraph IWA-3320. TVA further stated that the basis for the revised proximity criteria in the code case was developed to address a flaws proximity to the ID surface. The revised criteria were developed to address a flaw, located very close to the inside surface, that has the possibility of yielding in the remaining ligament between the flaw and the inside surface, thus exposing the flaw to the reactor coolant and potentially to accelerated crack growth, which TVA stated is not the case in this application.
3.1.5 Proposed Alternative As an alternative to ASME Code,Section XI, paragraph IWB-2420(b), TVA requested to defer the periodic successive examinations of the identified flaw in weld V-3-A until the next regularly scheduled ISI. TVA proposed to apply the alternative requirements of ASME Code Case N-526 to the identified flaw, except that the surface proximity rules of ASME Code,Section XI, paragraph IWA-3320 will be used in lieu of the Code Case N-526 proximity rules.
In addition, TVA stated that periodic system pressure tests with associated VT-2 visual examinations will continue to be performed on Class 1 pressure retaining welds and items following each refueling outage in accordance with Examination Category B-P of ASME Code,Section XI, Table IWB-2500-1.
3.1.6 Basis for Use TVA used fracture mechanics for bounding flaw geometry and loading conditions and used linear elastic fracture mechanics methods consistent with the requirements of ASME Code,Section XI, subarticle IWB-3600 and Nonmandatory Appendix A. To evaluate acceptability of the flaw through the license renewal period, TVA used the projected 48-effective-full-power-years fluence value to determine the limiting fracture toughness of the Browns Ferry, Unit 2, vessel material. TVA used the as-found sub-surface flaw geometry, and reactor vessel parameters to perform flaw stability and fatigue crack growth calculations. Using structural factors imposed by ASME Code,Section XI, paragraph IWB-3612, TVA calculated the allowable fracture toughness for normal conditions to be 69.57 kilopound per square inch (ksi) square root inches (in) to prevent brittle failure. TVA stated that the calculated allowable flaw half depth is 1.7156 inch. TVA determined the time when the detected flaw would propagate to the allowable half depth, assuming that the flaw can grow in either direction. TVAs calculation showed that it will take 70 years to grow the initial crack size from half depth of 1.6 inch to the allowable half depth of 1.7156 inch. The limiting factor for flaw growth is the time at which the crack driving force (KI) would exceed the allowable fracture toughness for brittle failure. This limiting flaw size is much smaller than the crack size required for the flaw to reach either the OD or ID surfaces.
According to TVA, after 70 years, each crack tip would still be at least 0.8844 inch from the OD surface and 2.08 inches from the ID surface of the RPV.
TVA explained that the technical basis for the reduced surface proximity criteria in ASME Code Case N-526 is intended to prevent yielding of the average net-section area of the remaining ligament by ensuring the nominal membrane and bending stresses do not exceed the flow strength of the material.
TVA stated that when the flaw is evaluated using the distance from the inside surface of the RPV, the flaw would satisfy the acceptance criterion of ASME Code Case N-526. As such, TVA would not need to perform the successive examinations, and a request for alternative would not be required. However, when the flaw is evaluated using the distance from the outside surface of the RPV, the flaw would not satisfy the acceptance criterion of Code Case N-526. Therefore, TVA submitted the subject alternative to defer the successive examinations.
3.1.7 Duration of Proposed Alternative TVA proposed the alternative for the next two subsequent inspection periods, following identification of the flaw during the U2R21 refueling outage in spring 2021. TVA stated that after such time, the examinations of the subject weld will resume in the sixth ISI interval in accordance with the schedule of the ASME Code,Section XI, Table IWB-2500-1. In the supplement dated September 12, 2022, TVA stated that it will inspect weld V-3-A during the second period of the sixth ISI interval.
3.2
NRC Staff Evaluation
The NRC staff evaluated TVAs flaw characterization, its flaw evaluation, and its proposed alternative use of the methodology consistent with ASME Code Case N-526. The following are the key topics in the submittal that support the proposed alternative.
3.2.1 Flaw Characterization In the supplement dated September 12, 2022, TVA stated that it has ultrasonically examined weld V-3-A during the Browns Ferry, Unit 2, spring 2001 (U2R11), spring 2011 (U2R16), and spring 2021 (U2R21) refueling outages in the third, fourth, and fifth 10-year inspection intervals, respectively.
TVA explained that the examination conducted during the second 10-year ISI interval was prior to development of inspection tooling to conduct ultrasonic examinations from the inside surface in boiling water reactors; therefore, an examination was not possible in the area of the indication at weld V-3-A.
The NRC staff noted that TVA has used automated and manual ultrasonic testing to characterize the flaw size and orientation. TVA has achieved the maximum 100 percent of examination coverage in accordance with the provisions of ASME Code,Section XI. In addition, TVA performed the ultrasonic (UT) inspection based on the performance demonstration program of the ASME Code,Section XI, Appendix VIII. The NRC staff finds that TVAs characterization of the flaw satisfies the provisions of the ASME Code,Section XI; therefore, TVAs flaw characterization is acceptable.
3.2.2 Flaw Growth Calculation When a UT indication was detected in a Class 1 component, TVA was required to disposition the indication in accordance with the acceptance standards of ASME Code,Section XI, subarticle IWB-3400. Weld V-3-A is part of Examination Category B-A; therefore, the acceptance standards are specified in ASME Code,Section XI, paragraph IWB-3510. As TVA reported, the flaw in weld V-3-A exceeded the acceptance standards of ASME Code,Section XI. The ASME Code,Section XI, subparagraph IWB-3132.3 permits TVA to accept the flaw for continued service by analytical evaluation performed in accordance with provisions of subarticle IWB-3600. The NRC staff determined that TVAs flaw evaluation is acceptable because TVA followed the methodology required by subarticle IWB-3600.
The NRC staff noted that the detected flaw in weld V-3-A may affect the pressure-temperature (P-T) limit curves in the plant technical specifications. The P-T limit curves control the operation of the reactor coolant system, and they are developed based on a postulated 1/4T flaw (T is the wall thickness of the RPV shell) in the RPV shell. To address the NRC staffs concern, in the supplement dated September 12, 2022, TVA provided an evaluation to address the following:
(1) neutron fluence at weld V-3-A compared to that for the postulated 1/4T flaw; (2) the stress intensity factor for the V-3-A indication compared to that for the 1/4T flaw; and (3) comparison of the P-T limit curves for the pressure test and heatup/cooldown curves for the V-3-A indication and for the 1/4T flaw.
The NRC staff reviewed TVAs P-T limit curve evaluation and verified that (a) the highest neutron fluence at the weld V-3-A is lower than the neutron fluence at the 1/4T flaw evaluated in developing the P-T limit curves; (b) the stress intensity factor for the V-3-A indication is lower than that for the postulated 1/4T flaw evaluated in the P-T limit curves; and (c) the existing P-T limit curves bound that for the weld V-3-A indication. The NRC staff verified that, based on TVAs P-T curve evaluation, the fracture consequences of the postulated 1/4T flaw bound that of the weld V-3-A indication. Therefore, the NRC staff determined that the P-T limit curves in the Browns Ferry, Unit 2, technical specifications do not need to be revised.
3.2.3 Applicability and Use of ASME Code Case N-526 The ASME Code Case N-526 eliminates successive examinations of a flaw that is accepted by analytical evaluation if the flaw is characterized as subsurface. Figure 1 of Code Case N-526 presents the criterion that classifies the surface flaw and subsurface flaw based on the proximity rule of ASME Code,Section XI, subarticle IWA-3300. An embedded flaw could be characterized as a surface flaw if it is embedded near the surface of the component because the remaining ligament between the flaw and the component surface may cause yielding and the flaw may reach the surface by growth. The significance of classifying a flaw is that a surface flaw is required to be analyzed for stress corrosion cracking and fatigue, whereas a subsurface flaw is required to be analyzed for fatigue only. As such, a surface flaw will grow faster than a subsurface flaw because the crack growth rate for stress corrosion cracking is higher than fatigue crack growth rate. To monitor its flaw growth, a component containing a surface flaw may require to be inspected more often than a component having a subsurface flaw.
Based on the information submitted, the NRC staff determined that the subject embedded flaw satisfies the proximity rule of ASME Code,Section XI, paragraph IWA-3320 based on the ID and OD surfaces. However, based on ASME Code requirements, TVA would have to perform three successive volumetric examinations of weld V-3-A. ASME Code Case N-526 permits elimination of the three successive examinations, but the subsurface flaw needs to be located further away from the weld surfaces. The assumption in the Code Case is that the flaw that is located away from the surfaces, if it grows, will not likely grow toward the weld surface.
The NRC staff noted that ASME Code Case N-526 does not specify whether its acceptance criterion is based on the distance of the embedded flaw location to the inside surface or to the outside surface of the component. As such, the NRC staff determined that TVA should classify an embedded flaw based on the distance from both the inside surface and outside surface of the affected component.
The NRC staff determined that when the location of the embedded flaw in weld V-3-A is measured from the ID surface of the weld, the flaw is acceptable to be characterized as a subsurface flaw per ASME Code Case N-526. As such, TVA would not need to perform the successive examinations. However, the NRC staff determined that when the subject flaw is measured from the OD surface of the weld, the flaw exceeds the acceptance criterion and should be characterized as a surface flaw. In this case, TVA is required to perform three successive examinations because the flaw could not satisfy the provisions of Code Case N-526.
TVA proposed an alternative acceptance criterion based on a plant-specific analysis. The NRC staff evaluated TVAs analysis, which used the plant-specific operating stresses and as-found flaw aspect ratio to develop a plant-specific criterion to classify the subject flaw. The NRC staff determined that the plant-specific analysis does demonstrate that the subject flaw can be classified as a subsurface flaw based on distance from the OD and ID surfaces.
In the supplement dated September 12, 2022, TVA stated that according to the technical basis document of ASME Code Case N-526, only primary membrane and bending stresses are required to be included to develop the plant-specific proximity rule, and secondary stresses due to thermal transients and weld residual stresses are not required. Therefore, the licensees original analysis considered service level A/B (normal and upset, respectively) loading conditions to develop the plant-specific proximity curves. TVA performed an additional plant-specific analysis for service level C/D (emergency and faulted, respectively) loadings using the same methodology as the technical basis of ASME Code Case N-526 with the plant-specific material properties, operating stress (only internal pressure for service level C/D loadings), and as-found flaw aspect ratio to develop a site-specific proximity curve as presented in Enclosure 3 to the supplement dated September 12, 2022.
Based on the results of TVAs analysis, the NRC staff determined that because the plant-specific proximity curve remains applicable for both service level A/B and C/D loadings and the as-found flaw is bounded by the plant-specific proximity rule, the subject flaw is exempt from the successive re-examinations required by paragraph IWB-2420(b).
In summary, the NRC staff finds that the flaw in weld V-3-A satisfies Code Case N-526 when the flaw is measured from the ID surface. The flaw does not satisfy the Code Case when it is measured from the OD surface. However, the NRC staff finds that the proposed alternative contains the following technical basis to eliminate the three successive examinations: (1) TVA has demonstrated that the subsurface flaw is acceptable per the proximity rule of the ASME Code,Section XI, paragraph IWA-3320; (2) Per the plant-specific analysis, TVA has considered the distance from the ID and OD surfaces to the flaw tip; (3) TVA will perform a system leakage test prior to startup with associated VT-2 visual examination to monitor any potential leakage from weld V-3-A as required by Examination Category B-P of the ASME Code,Section XI, Table IWB-2500-1; (4) TVA will re-examine weld V-3-A during the second period of the sixth ISI interval; and (5) TVA performed a flaw growth calculation showing that it will take 70 years for the existing flaw to grow to the allowable flaw size. Based on these findings, the NRC staff determines that the proposed alternative is acceptable.
4.0 CONCLUSION
As set forth above, the NRC staff determines that TVAs proposed alternative provides an acceptable level of quality and safety. Accordingly, the NRC staff concludes that the licensee has adequately addressed all the regulatory requirements set forth in 10 CFR 50.55a(z)(1).
Therefore, the NRC authorizes the use of alternative BFN-21-ISI-02 for the fifth 10-year ISI interval at Browns Ferry, Unit 2.
All other ASME Code,Section XI, requirements for which an alternative was not specifically requested and authorized in the subject request remain applicable, including third-party review by the Authorized Nuclear Inservice Inspector.
Principal Contributors:
I. Anchondo-Lopez, NRR J. Tsao, NRR Date: November 14, 2022
ML22298A285 OFFICE NRR/DORL/LPL2-2/PM NRR/DORL/LPL2-2/LA NRR/DNLR/NVIB/BC NRR/DORL/LPL2-2/BC NAME KGreen RButler ABuford DWrona DATE 10/21/22 10/26/22 10/20/22 11/14/22