Text

Response to Request for Additional Information Regarding Browns Ferry Nuclear Plant, Unit 2, Request for Alternative, BFN-21-ISI-02, Alternative to American Society of Mechanical Engineers, Boiler and Pressure Vessel Code, Section Xi,.
	ML22256A045
Person / Time
Site:	Browns Ferry
Issue date:	09/12/2022
From:	Rymer S; Tennessee Valley Authority
To:	; Document Control Desk, Office of Nuclear Reactor Regulation
References
	CNL-22-075, EPID L-2022-LLR-0008
	Download: ML22256A045 (53)
	v • d • e

1101 Market Street, Chattanooga, Tennessee 37402 CNL-22-075 September 12, 2022 10 CFR 50.55a ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001 Browns Ferry Nuclear Plant, Unit 2 Renewed Facility Operating License No. DPR-52 NRC Docket No. 50-260

Subject:

Response to Request for Additional Information Regarding Browns Ferry Nuclear Plant, Unit 2, Request for Alternative, BFN-21-ISI-02, Alternative to American Society of Mechanical Engineers, Boiler and Pressure Vessel Code,Section XI, Paragraph IWB-2420(b) and the Use of Case N-526 (EPID L-2022-LLR-0008)

References:

1. TVA Letter to NRC, CNL-21-081, Browns Ferry Nuclear Plant, Unit 2, Request for Alternative, BFN-21-ISI-02, Alternative to American Society of Mechanical Engineers, Boiler and Pressure Vessel Code,Section XI, Paragraph IWB-2420(b) and the Use of Case N-526, dated January 14, 2022 (ML22014A344)

2. NRC Email to TVA, Request for Additional Information Related to TVA Relief Request BFN-21-ISI-02 (CNL-21-081) (EPID L-2022-LLR-0008), dated July 15, 2022 (ML22208A217)

In Reference 1, Tennessee Valley Authority (TVA) submitted a request for alternative (RFA) for the Browns Ferry Nuclear Plant (BFN), Unit 2, from the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code),Section XI, Code Case N-526.

Specifically, the RFA proposed an alternative to successive examination requirements of paragraph IWB-2420(b) of the ASME Code.

In Reference 2, the Nuclear Regulatory Commission (NRC) issued a request for additional information (RAI) and requested that TVA respond by September 13, 2022. Enclosure 1 to this letter provides the response to the RAI.

U.S. Nuclear Regulatory Commission CNL-22-075 Page 2 September 12, 2022 Additionally, TVA identified an administrative error in Attachment 1 of the enclosure to Reference 1 regarding location of the BFN V-3-A weld flaw after submittal to the NRC. to this letter provides a revised weld flaw assessment by Structural Integrity Associates (SI) with the corrected location, and this assessment replaces and supersedes of Reference 1.

Enclosures 3 and 4 provide the underlying analyses for the responses to RAI-4 and RAI-5, respectively.

There are no new regulatory commitments contained in this letter. Please address any questions regarding this submittal to slrymer@tva.gov.

Respectfully, Digitally signed by Rymer, Stuart Loveridge Date: 2022.09.12 22:44:04 -04'00' Stuart L. Rymer Director (Acting), Nuclear Regulatory Affairs

Enclosures:

1. Response to NRC Request for Additional Information

2. SI 2100312.401, Revision 2, BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526

3. SI 2200769.401, Revision 0, BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526 under Service Level C/D loadings

4. SI 2200769.301, Revision 0, Resolution of RAI-5 (P-T Limits Inquiry) cc (with Enclosures):

NRC Regional Administrator - Region II NRC Senior Resident Inspector - Browns Ferry Nuclear Plant NRC Project Manager - Browns Ferry Nuclear Plant

Enclosure 1 Response to NRC Request for Additional Information

==

Introduction:==

By letter dated January 14, 2022, (Agencywide Documents and Access Management System (ADAMS) Accession No. ML22014A344), Tennessee Valley Authority (TVA) submitted request for alternative BFN-21-ISI-02 for the fifth 10-year inservice inspection (ISI) interval at Browns Ferry Nuclear Plant (Browns Ferry or BFN), Unit 2. Pursuant to Title 10 of the Code of Federal Regulations (10 CFR), Part 50, Paragraph 50.55a(z)(1), TVA proposed an alternative to the requirements of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (Code),Section XI, paragraph IWB-2420(b), for deferring the applicable successive examinations of the flaw identified in Weld V-3-A at Browns Ferry, Unit 2, until the normally scheduled inspection during the sixth 10-year ISI interval. The U.S. Nuclear Regulatory Commission (NRC) staff needs additional information to complete its review of the proposed alternative.

Regulatory Basis:

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. Paragraph 10 CFR 50.55a(g)(4) states, in part, that ASME Code Class 1, 2, and 3 components will meet the requirements, except the design and access provisions and the preservice examination requirements, set forth in the ASME Code,Section XI.

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 IWB-2400 if a component is accepted for continued service in accordance with IWB-3132.3 or IWB-3142.4.

Paragraph 10 CFR 50.55a(z) 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 Director, Office of Nuclear Reactor Regulation. A proposed alternative must be submitted and authorized prior to implementation. The applicant or licensee must demonstrate that: (1) the proposed alternative would provide an acceptable level of quality and safety; or (2) compliance with the specified requirements of this section would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.

Requests for Additional Information:

RAI-1

Background Section 5 of the proposed alternative states, in part, TVA proposes to apply the alternative requirements of ASME Code Case N-526 to the identified flaw, except that the IWA-3320 surface proximity rules will be used in lieu of the ASME Code Case N-526 proximity rules. The other conditional requirements of ASME Code Case N-526 will be met.

CNL-22-075 E1 of 7

Enclosure 1 Issue The staff lacks clarity regarding what is meant by other conditional requirements.

Request Clarify what is meant by The other conditional requirements of ASME Code Case N-526 will be met.

TVA Response to RAI-1 The reference to The other conditional requirements of ASME Code Case N-526 will be met, was intended to address the following requirements of ASME Code Case N-526, which states that re-examinations in accordance with IWB-2420(b) or IWC-2420(b) of vessels examination volumes containing subsurface flaws are not required, provided the following are met:

(a) The flaw is characterized as subsurface in accordance with Figure 1.

(b) The nondestructive examination technique and evaluation that detected and characterized the flaw, with respect to both sizing and location, shall be documented in the flaw evaluation report.

(c) The vessel containing the flaw is acceptable for continued service in accordance with IWB-3600, and the flaw is demonstrated acceptable for the intended service life of the vessel.

The proposed alternative request seeks to utilize ASME Code Case N-526 to eliminate the periodic re-examinations of the subject weld using an alternative to the proximity rules of Figure 1. Therefore, condition (a) of Code Case N-526 will not be met, instead using the subsurface characterization of IWA-3320 as noted in the proposed alternative. The statement in question is an attestation that the conditions (b) and (c) of Code Case N-526 have been met.

RAI-2

Background Section 6.0 of the proposed alternative states:

The proposed alternative is requested for BFN, Unit 2 for the next two subsequent inspection periods, following identification of the flaw during the Unit 2, Cycle 21 refueling outage in spring 2021. After such time, the successive examinations of the subject weld will resume in the sixth inservice examination interval, in accordance with the schedule of ASME Code,Section XI, Table IWB-2500-1.

It is the NRC staffs understanding that TVA proposes to defer the required successive examinations of weld V-3-A during the next two inspection periods instead of three successive periods, as required by IWB-2420(b), because a third successive examination would resume in the sixth ISI interval.

CNL-22-075 E2 of 7

Enclosure 1 Issue Subsection IWB-2420(b) of Section XI (2007 Edition with 2008 Addenda), states If a component is accepted for continued service in accordance with IWB-3132.3 or IWB-3142.4, the areas containing flaws or relevant conditions shall be reexamined during the next three inspection periods listed in the schedule of the Inspection Program of IWB-2400. Alternatively, acoustic emission may be used to monitor growth of existing flaws in accordance with IWA-2234.

The NRC staff interprets the requirements of IWB-2420(b), regarding the required reexaminations, as performing three consecutive inspections (one inspection per period) irrespective of the ISI interval. Therefore, the third successive inspection of weld V-3-A would be required to be performed in the first period of the sixth ISI interval because it seems that the weld was examined during the first period of the fifth ISI interval.

Request

1. Confirm whether the flaw in weld V-3-A was identified during the first period of the fifth ISI interval.

2. Clarify whether the intent of the proposed alternative is to not perform the first two successive examinations but perform the third required successive examination during the first period of sixth ISI interval. If not, provide the timeframe of when the inspection of weld V-3-A is planned to be performed and justify why this inspection plan is compliant with the ASME Code,Section XI.

TVA Response to RAI-2

1. The flaw in weld V-3-A was identified during the second period of the fifth ISI interval.

Inspection was not required during the first period of the fifth ISI interval.

2. This request for alternative proposes to omit the first two successive examinations, which would have been required in the third period of the fifth interval and in the first period of the sixth interval. Weld V-3-A will then be subsequently examined in the second period of the sixth interval. This schedule meets the requirement of the third successive examination per IWB-2420(b) and is also the next scheduled periodic examination requirement per ASME Code Section XI, Table IWB-2500-1 examination category B1.12. This request does not propose any alternatives to the normal periodic examination requirements of Table IWB-2500-1.

RAI-3

Background TVA states that the flaw in weld V-3-A at BFN, Unit 2, was identified in the fifth ISI interval during the Cycle 21 refueling outage. Furthermore, the weld was ultrasonically examined in accordance with Appendix VIII, Performance Demonstration for Ultrasonic Examination Systems, of Section XI of the ASME Code as incorporated by referenced in 10 CFR 50.55a.

Specifically, TVA performed automated scanning from the reactor vessel inside surface using CNL-22-075 E3 of 7

Enclosure 1 45-degree shear wave and 60-degree refracted longitudinal (RL) search units. TVA states that a portion of the automated examination was restricted due to the proximity of the feedwater spargers and core spray piping. Consequently, a manual ultrasonic examination was performed using a qualified 60-degree RL search units in the area restricted for automated examination.

Issue TVA did not provide details regarding previous inspections of weld V-3-A. The NRC staff notes that weld V-3-A is classified as ASME examination category B-A, Pressure Retaining Welds in Reactor Vessel, which are required to be examined once per ASME Code ISI interval (i.e., 10 years). The NRC staff lacks clarity whether the indication in weld V-3-A had been previously identified.

Request

1. Provide details regarding the examination and results of weld V-3-A during the previous four 10-year ISI intervals. In your response, include details on the ultrasonic capabilities for each examination that was performed.

2. Provide details on whether an evaluation (e.g., root cause) was performed to identify why the flaw was not identified in previous examinations but was identified during the Cycle 21 refueling outage.

TVA Response to RAI-3

1. The V-3-A weld in the area of the indication has undergone ultrasonic examination three times previously during the BFN 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. The examination performed in U2R11 was conducted using conventional ultrasonic techniques with 100% coverage with no recordable indications. The examination performed in U2R16 was conducted using phased array ultrasonic techniques with 100% coverage with no recordable indications. The examination performed in U2R21 was conducted by a different vendor using conventional ultrasonic techniques with 100%

coverage.

The examination conducted during the second 10-year inspection 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 V-3-A indication.

2. The following explanation addresses why the flaw was not identified in previous examinations but was identified during the Cycle 21 refueling outage.

The three ultrasonic examinations performed on the V-3-A weld were conducted by two vendors using three different examination methodologies, which were qualified to ASME Section XI, Appendix VIII criteria in accordance with the performance demonstration initiative (PDI) process. The recording and evaluation criteria specified in accordance with the qualified PDI procedures varied in all three examination efforts.

The results, even when different techniques are deployed, will be fairly consistent and repeatable. However, it is not uncommon for variations to occur even when deploying the CNL-22-075 E4 of 7

Enclosure 1 same techniques with the same vendors. A causal evaluation was not performed, and the definitive reasons for why the unacceptable indication in the V-3-A weld was not found during previous examination cannot be ascertained. However, it could be surmised that contributing factors to the failure to identify the weld flaw in previous examination could be coupling, recording criteria, examination methodologies, and differing resolution capabilities.

RAI-4

Background In Attachment 1 of the proposed alternative, the plant-specific assessment states, The Service Level A/B RPV transient in Table 4 of Reference [12] with the highest T was chosen as the bounding transient which is the Start-up Transient.

Issue The NRC staff notes that the Browns Ferry Final Safety Analysis Report (FSAR), Appendix C, Structural Qualification of Subsystems and Components, describes the basic structural loading criteria and qualification methods used in the original design of BFN, Unit 2, components and piping subsystems. These conditions include normal, upset, emergency, and faulted loading conditions. Furthermore, the staff notes that these loading conditions are the equivalent to the ASME Code,Section III, Level A, B, C, and D service limits. Based on the above statement in of the proposed alternative, the staff is not clear if loading conditions applicable to service levels C and D were considered in the licensees analysis provided in the submittal.

Request Discuss the applicability of all service loadings (i.e., normal, upset, emergency, and faulted) and any combination of them that were taken into consideration in the licensees analysis in the submittal. Discuss whether loading conditions applicable to service levels C and D were considered in the licensees analysis. If not considered, provide a justification for not considering the loading conditions applicable to service level C and D in the analysis in the submittal.

TVA Response to RAI-4 As noted in the staffs question, the ASME Code,Section III, service limits A, B, C, and D are equivalent to the loading conditions described in the BFN Updated FSAR. For simplicity, this response will use the ASME service limits. 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 loading conditions to develop the plant-specific proximity curves. Because the as-found flaw is an axial flaw type, bending and axial stresses from service level C/D loadings such as safe shutdown earthquake and loss of coolant accident do not contribute to the crack driving force. Only membrane hoop stress (due to pressure) contributes to the crack-driving force. An additional plant-specific analysis was performed in Enclosure 3 for service level C/D 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.

CNL-22-075 E5 of 7

Enclosure 1 The results of the analysis in Enclosure 3 demonstrate that the plant-specific surface proximity curves for service level C/D loading is slightly higher than that for service level A/B loading (due to higher internal pressure for C/D loadings). However, each of these curves falls below both the IWA-3320 proximity rule and the Code Case N-526 surface proximity rule. Because the as-found flaw must meet the requirements of IWA-3320 to be considered as subsurface, the IWA-3320 rule (S > 0.4a) would be the plant-specific proximity rule for the as-found flaw under both service level A/B and C/D loadings.

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 (i.e., above the IWA-3320 curve), it meets the intent of the requirements of ASME Code Case N-526 to exempt the successive re-examinations required by IWB-2420(b), ensuring an acceptable level of performance and safety.

Reference:

N. G. Cofie, P. C. Riccardella, J. H. Merkle, and H. Do, Technical Basis for Alternate Successive Inspection Requirements for Vessels and Piping Welds as Prescribed in Code Cases N-526 and N-735, Paper No. PVP2008-61412, Proceedings of PVP2008, 2008 ASME Pressure Vessels and Piping Division Conference, July 27-31, 2008, Chicago, Illinois, USA

RAI-5

Background The pressure-temperature (P-T) limit curves for the operation of the reactor vessel during heat up and cooldown evolution as shown in the plant technical specifications are constructed based on a postulated flaw that has a depth of 1/4 of the wall thickness of the reactor vessel shell. The flaw is postulated to initiate from the inside surface and outside surface of the reactor vessel shell.

Issue TVA has not indicated if the flaw in weld V-3-A affects the P-T limit curves.

Request Discuss whether the flaw in weld V-3-A affects the existing P-T limit curves in the plant technical specifications. If yes, indicate when TVA plans to revise the P-T limit curves to reflect the impact of the flaw on the P-T limit curves in the technical specifications via a separate license amendment request.

CNL-22-075 E6 of 7

Enclosure 1 TVA Response to RAI-5 provides an evaluation that was performed to address the following: (i) fluence at weld V-3-A compared to that for the Appendix G quarter-T flaw; (ii) the stress intensity factor for the V-3-A indication compared to that for the Appendix G quarter-T flaw; and (iii) comparison of the P-T limit curves for the pressure test (Curve A) and heatup/cooldown (Curves B and C) for the V-3-A indication and for the Appendix G quarter-T flaw. The evaluation confirmed the following.

x The highest fluence at the weld V-3-A is lower than the influence at the quarter-T flaw evaluated in developing the P-T curves per Appendix G.

x The stress intensity factor for the V-3-A indication is lower than that for the Appendix G quarter-T flaw.

x The P-T curve for postulated Appendix G quarter-T flaw bounds that for the weld V-3-A indication.

Based on the analysis, the fracture consequences of the postulated quarter-T flaw bound that of the weld V-3-A indication. Therefore, no revision of the P-T limit curves in the BFN Unit 2 Technical Specifications is required.

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Enclosure 2 SI 2100312.401, Revision 2, BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526 CNL-22-075

muddin@structint.com 11515 Vanstory Drive, Suite 125 l Huntersville, NC 28078 l 614-717-5887 July 7, 2022 REPORT NO. 2100312.401 REVISION: 2 PROJECT NO. 2100312.00 Quality Program: Nuclear Commercial Adam Keyser Tennessee Valley Authority Browns Ferry Nuclear Plant, Unit 2 PO Box 2000 Decatur, AL 35609

Subject:

BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526 Revision 1:

Removed Proprietary Information Notice and proprietary information markers from information previously identified as proprietary. Removed P from Report No. Updated Reference 1 to Revision 2.

Revision 2:

Revised Section 4.0, 1: Incorporated the correct elevation of the axial flaw per CAR 22-041.

Dear Adam,

1.0 INTRODUCTION

During the spring 2021 outage at Browns Ferry Nuclear Plant, Unit 2 (BFN), an indication was identified in a vertical weld (Weld No. V-3-A) of the reactor pressure vessel (RPV). This flaw was evaluated by Structural Integrity Associates, Inc. (SI) in SI Calculation 2100264.301P [1]

and determined to be acceptable for continued service per the rules of Section XI, IWB-3600 [2].

This evaluation concluded that the flaw met the criteria set forth in ASME Code Case N-526 for exemption of the three successive inspections required by IWB-2420, when considering the proximity of the flaw to the inside (ID) surface (which is the most critical since it is in contact with the reactor coolant). However, when the evaluation was performed using the distance from the outside (OD) surface, the flaw does not meet the criteria set forth in ASME Code Case N-526 [3]

for exemption from three successive IWB-2420 examinations.

© 2022 by Structural Integrity Associates, Inc. All rights reserved. No part of this document or the related files may be reproduced or transmitted in any form, without the prior written permission of Structural Integrity Associates, Inc.

MCG0070008.21.20

July 7, 2022 Adam Keyser BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526 Since ASME Code Case N-526 does not specify which surface is supposed to be used for proximity evaluation, it was conservatively assumed that the calculation should apply the lesser of these two distances. However, upon review of the ASME Code Case N-526 technical basis

[4], it is evident that the calculational basis was developed based on the flaws proximity to the ID surface. Additionally, conservative assumptions were identified in the technical basis of Code Case N-526 regarding stresses and flaw aspect ratio. In accordance with Reference [5], SI was contracted to determine if the BFN RPV weld flaw will meet the intent of Code Case N-526 if plant-specific materials, stresses, and aspect ratio are utilized, using the technical basis of Code Case N-526 [3] as guidance.

2.0 BACKGROUND

In Reference [1], it was demonstrated that successive examinations in accordance with IWB-2420 (b) and (c) of the indication identified in Weld V-3-A are not required per the technical basis document [4] for Code Case N-526 [3] using the ID surface to apply the proximity rule.

However, since Code Case N-526 does not specify to use the ID surface to apply the proximity rule, the successive examinations are required when the most conservative interpretation of the Code Case using the OD surface to apply the proximity rule is used. However, upon review of the ASME Code Case N-526 technical basis [4], it was noted that some conservative assumptions were made regarding stresses and flaw aspect ratio in developing the Code Case N-526. This report provides a plant-specific Code Case N-526 assessment of the Browns Ferry V-3-A weld flaw, conservatively utilizing the flaw's OD surface proximity, plant-specific operating stresses, and as-found flaw aspect ratio, following the same procedure used in the technical basis of Code Case N-526 to determine if the intent of Code Case N-526 is met.

3.0 METHODOLOGY The methodology used for this plant-specific assessment develops the proximity rule from the OD surface for subsequent augmented re-examinations for the BFN RPV weld flaw by following the same procedure used in the technical basis document [4] for ASME Code Case N-526 but using the operating stresses and as-found flaw aspect ratio. A summary of the procedure used in the technical basis document [4] is described below followed by the procedure used for the plant-specific assessment.

3.1 Methodology Used in Code Case N-526 Technical Basis The intent of the technical basis for Code Case N-526 [4] is to provide alternate proximity criteria for distinguishing subsurface from surface defects in Class 1 and 2 vessels to eliminate the need for the successive inspection requirements in these components. This is based on the argument that if a flaw is very close to inside surface, then there is a possibility of yielding of the remaining ligament between the flaw and the inside surface, thus exposing the flaw to the reactor coolant, and potentially to accelerated crack growth. As noted in the technical basis that the concern is only regarding the flaws near the ID surface due to reactor coolant exposure for potential accelerated crack growth. However, since Code Case N-526 does not specify to use

Report No.2100312.401 R2 PAGE l 2

July 7, 2022 Adam Keyser BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526 ID surface to apply the proximity rule, a conservative assumption is made to apply the proximity rule in Code Case N-526 to both the ID and OD surfaces.

In developing the proximity rule, the basic premise for the criterion is that the average stress in the remaining ligament should not exceed the material flow strength, which would potentially create a risk for rupture of the remaining ligament. It should be noted that in the technical basis

[4], the terminology yield strength (Vy) was used synonymous with the flow stress (Vf). The approach is illustrated in Figure 1. Yielding of the average net-section area (considering flaw) defined as An in Figure 1 will occur when the nominal membrane and bending stresses (as given by Equation 1 below) exceeds the flow strength of the material.

Eq. 1 f

Figure 1. Model for ligament tearing [4]

In the technical basis document [4], the flaw proximity criterion (using Equation 1) for vessels was determined using the primary membrane stress (Pm) to be equal to the ASME Section III

Report No.2100312.401 R2 PAGE l 3

July 7, 2022 Adam Keyser BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526 allowable stress intensity, Sm = 30 ksi and the primary membrane plus bending stress (Pm + Pb) was assumed to be equal to 1.5Sm = 45 ksi. A flow strength value of Vf = 65 ksi for vessel materials was used for the evaluation which is a mid-range value for various vessel materials at 70F. Figure 2 presents the locus of flaw parameters (a/l and S) which would produce net section yielding. The ASME Code,Section XI surface proximity rule [2], as described in IWA-3320 is also shown in Figure 2 (S=0.4a). There are three curves shown in Figure 2 for different flaw aspect ratios, a/l of 0, 0.1 and 0.2. Flaws which lie above these curves for their respective aspect ratio would not cause yielding in the remaining ligament between the flaws and the vessel surface, even for vessel stress all the way to ASME Code,Section III primary membrane and bending stress allowables. As the trend shows, curves for higher aspect ratios would lie below these. The successive examination surface proximity rule per Code Case N-526 is also shown in Figure 2 (S>a line) which closely corresponds to yielding curve for a/l of 0.2.

According to Code Case N-526, the successive examination is not required if the flaw can be characterized as subsurface flaw in accordance with Code Case N-526 rule (heavy line in Figure 2).

Figure 2. Development of Proximity Rules for Code Case N-526 [4]

Report No.2100312.401 R2 PAGE l 4

July 7, 2022 Adam Keyser BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526 3.2 Methodology Used for BFN Plant-Specific Assessment As described in the previous section, the successive examination surface proximity rule per Code Case N-526 is based on preventing a flaw from causing yield in the remaining ligament of the ID surface by ensuring that the applied stress in the component (vessel) remains below the flow strength of the materials. In that regard, a plant-specific surface proximity rule can be developed based on the same technical requirements. To develop the plant-specific proximity rule from the OD surface for subsequent augmented re-examinations for the BFN RPV weld flaw, the plant-specific applied stresses, flaw dimension (i.e., aspect ratio) and flow strength of the RPV materials are required input.

Using Eq. 1 above and the plant-specific input, the plant-specific proximity rule will be developed based on preventing a flaw from causing yield in the remaining ligament of the OD surface. The flaw proximity curve for the plant-specific flaw aspect ratio (similar to curves shown in Figure 2) with plant-specific input will be developed. Any flaw that falls above this plant-specific proximity curve would be considered as subsurface flaw for successive examination.

4.0 DESIGN INPUTS As reported in the Reference [1] flaw evaluation, the following design inputs are used:

1. The indication is reported as being subsurface, separated from the vessel base metal/clad interface by 2.2 inches [6]. The cross-flaw depth dimension in the vessel radial direction (2a) is 3.2 inches [6]. The flaw is in the vertical weld V-3-A at 107° azimuth, or 42.95 to 46.7 inches above the circumferential weld C-2-3 [6,7]. Details of the geometric parameters of the subsurface flaw are summarized in Figure 3.

2. The plate material of the RPV is SA-302 Grade B [8].

3. From Reference [9, Table Y-1], the yield strength, Sy for SA-302 Grade B, is 50 ksi and 42.1 ksi at 70°F and 600°F respectively. 600°F bounds the plant operating temperature.

4. From Reference [9, Table U], the ultimate strength, Su for SA-302 Grade B, is 80 ksi for the temperature range of 70°F-600°F. 600°F bounds the plant operating temperature.

5. The vessel has an inside radius (centerline to base metal) of 125.6875 inches [8]. At the flaw location, the vessel wall thickness is 6.4 inches [6].

Report No.2100312.401 R2 PAGE l 5

July 7, 2022 Adam Keyser BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526 S = SOD = 1.0 in.

SID = 2.2 in.

Figure 3. Flaw geometry [6]

5.0 CALCULATIONS 5.1 Flaw Characterization and Flaw Evaluation The as-found flaw in the vertical weld of the BFN RPV was evaluated as per the requirements of IWB-3600 as reported in Reference [1]. The flaw was characterized as subsurface flaw as per IWA-3320 with a half-depth, a of 1.6-inch and length, l of 3.8-inch (see Figure 3). Therefore, the aspect ratio of the as-found flaw is (a/l=) 0.421. During the evaluation of the flaw, it was reported that the observed flaw is acceptable for continued operation per the requirements of ASME Code,Section XI, IWB-3640 and it would take 70 years for the as-found flaw to propagate to the allowable depth.

5.2 Stress Calculation In the following sections, membrane and bending stresses due to pressure and thermal are calculated.

Report No.2100312.401 R2 PAGE l 6

July 7, 2022 Adam Keyser BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526 5.2.1 Stress Due to Pressure Load The observed flaw is in the RPV shell in a vertical weld. It is therefore subjected to the membrane (hoop) stress due to the internal pressure in this cylindrical location. The membrane stress is calculated as:

Membrane stress due to pressure, m-p = PRm/t = 21,024 psi = 21 ksi where:

P = Maximum pressure = 1044 psig [10]

t = thickness = 6.4 inches [6]

Rm = Mean radius = 128.8875 inches (Di/2 + t/2), Di = 251.375 inches [8]

5.2.2 Membrane and Bending Stresses Due to Thermal Transient To determine the thermal stress in the RPV wall, a thermal transient analysis was performed using the ANSYS finite element software [11]. The details of the thermal analysis are given in Reference [1]. The Service Level A/B RPV transient in Table 4 of Reference [12] with the highest T was chosen as the bounding transient which is the Start-up Transient. Hoop stresses are extracted at OD surface and both crack tips, i.e., 2.2-inch and 5.4-inch depth (from ID) during the transient as shown in Figure 4. As seen in Figure 4, hoop stresses at both OD and crack-tip near OD (5.4-inch depth) reached maximum at around the same time (~19,872 seconds). A hoop stress distribution along the RPV thickness is shown in Figure 5 for the time step corresponding to maximum hoop stress (at OD). The total stress is then converted to linear stress distribution which is tangent to the point corresponds to crack-tip near OD as shown in Figure 5 (red line). The linear stress distribution due to thermal transient is given by Vb =

0.6392x+1.5691 from which membrane and bending stresses are deconstructed as follows.

Membrane stress due to thermal, m-t = 0.6392u 3.2 + 1.5691 = 3.61 ksi Max. bending stress due to thermal, b-t = 0.6392u 6.4 + 1.5691 - 3.61 = 2.05 ksi Note that maximum stresses at OD surface is used to calculate the bounding maximum bending stress.

Report No.2100312.401 R2 PAGE l 7

July 7, 2022 Adam Keyser BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526 Figure 4. Hoop Stress During Start-up Transient at OD surface, crack-tip near ID and OD Figure 5. Start-up, Maximum Stress Distribution at Time = 19872 seconds 5.2.3 Total Stresses Below shows the total membrane and bending stresses due to pressure and thermal transient.

Total membrane stress, m = 21 + 3.61 = 24.61 ksi Total bending stress, b = 2.05 ksi

Report No.2100312.401 R2 PAGE l 8

July 7, 2022 Adam Keyser BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526 5.3 Material Properties Below shows the bounding flow strength of RPV material at operating temperature of 600F which were determined by averaging the yield and ultimate strengths of the material as given in Section 4.0 Design Input.

Vf (at 600F) = (42.1+80)/2 = 61.1 ksi 5.4 The Plant-Specific Surface Proximity Rule Following the methodology described in Section 3.0, the plant-specific surface proximity curve is determined using the as-found flaw aspect ratio (of 0.421), applied plant-specific stresses and flow strength of RPV material as shown in Figure 6. As per technical basis document of Code Case N-526, the proximity rule was developed using flow strength at room temperature only.

However, the bounding flow strength at operating temperature (600F) is used in the current analysis for additional conservatism. IWA-3320 proximity and Code Case N-526 surface proximity rules are also added in Figure 6 for reference. As can be seen in the figure, the plant-specific surface proximity curve falls below both IWA-3320 proximity and Code Case N-526 surface proximity rules indicating that the Code Case N-526 surface proximity rule is overly conservative for the as-found flaw in the vertical weld of the BFN RPV. Use of room temperature flow strength in the analysis would provide even more favorable results. However, the as-found flaw must meet the requirements of IWA-3320 to be considered as subsurface and therefore, the S>0.4 line (dotted black line) would be the plant-specific proximity rule for the as-found flaw in the vertical weld of the BFN RPV for the exemption of successive examination.

For reference, the chart also includes the Code Case N-526 proximity analysis result, considering only the distance of the flaw from the ID surface. This data point, in red, shows that the as-found flaw is acceptable per the existing conservative requirements of Code Case N-526, when the Code Case technical basis is applied, which would consider the flaw's proximity to only the ID surface.

Report No.2100312.401 R2 PAGE l 9

July 7, 2022 Adam Keyser BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526 Figure 6. Development of BFN Plant-Specific Proximity Rule

Report No.2100312.401 R2 PAGE l 10

July 7, 2022 Adam Keyser BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526 6.0 RESULTS The as-developed plant-specific surface proximity rule has been applied on the observed flaw in the vertical weld of the BFN RPV. Using the plant-specific proximity curve, developed using the same technical evaluation methods as the Code Case N-526 curve, this graph demonstrates that the as-found flaw is still bounded by the Code Case N-526 technical basis, even when considering the flaw's proximity to the OD surface. As shown in Figure 6 (blue square marker),

the minimum distance to the OD surface for the as-found flaw is 1 inch, and the half-flaw depth is a = 1.6 inch. The observed flaw (from OD surface) is above the as-developed plant-specific surface proximity curve in Figure 6 as discussed in Section 5.4 and therefore, it can be classified as a subsurface flaw for the purpose of applying Code Case N-526 successive examination exemption requirements. For completeness, the assessment of the observed flaw from the ID surface is also added in Figure 6 (red circle marker) where the minimum distance to the ID surface for the as-found flaw is 2.2 inch, and the half-flaw depth is a = 1.6 inch. This data point shows that the as-found flaw is acceptable per the existing conservative requirements of Code Case N-526, when the Code Case technical basis is applied, which would consider the flaw's proximity to only the ID surface. As seen in the figure, the observed flaw (from ID surface) meets the requirements of both the Code Case N-526 and as-developed plant-specific proximity rules to be classified as subsurface flaw.

Using either the as-developed plant-specific surface proximity rule, or the ID surface proximity as intended by the Code Case N-526 technical basis, the following requirements specified in Code Case N-526 to exempt the re-examinations in accordance with IWB-2420(b) of vessel volumes containing subsurface flaws are met. Code Case N-526 is accepted without condition in Regulatory Guide 1.147 Revision 19 [13].

(a) The flaw is characterized as subsurface according to the as-developed plant-specific surface proximity rule (shown in Figure 6).

(b) The NDE technique and evaluation that detected and characterized the flaw, with respect to both sizing and location, shall be documented in the flaw evaluation report.

(c) The vessel containing the flaw is acceptable for continued service in accordance with IWB-3600, and the flaw is demonstrated acceptable for the intended service life of the vessel.

Report No.2100312.401 R2 PAGE l 11

July 7, 2022 Adam Keyser BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526

7.0 CONCLUSION

A plant-specific assessment surface proximity rule was developed for an as-found flaw in a vertical weld of the BFN RPV using the plant-specific stresses, actual flaws aspect ratio and bounding flow strength of RPV material.

Using the as-developed plant-specific surface proximity rule, it is demonstrated in Section 6.0 that the observed flaw is classified as a subsurface flaw and meets the intent of the requirements of Code Case N-526 to exempt the re-examinations in accordance with IWB-2420(b) of vessel volumes containing subsurface flaws. Therefore, successive examinations in accordance with IWB-2420 (b) and (c) of the observed flaw in Weld V-3-A BFN Unit 2 RPV are not required.

Report No.2100312.401 R2 PAGE l 12

July 7, 2022 Adam Keyser BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526

8.0 REFERENCES

1. SI Calculation No. 2100264.301, Revision 2, Browns Ferry Unit 2 Reactor Pressure Vessel Vertical Weld Flaw Evaluation.

2. ASME Boiler and Pressure Vessel Code, 2007 Edition with 2008 Addenda,Section XI, Rules for Inservice Inspection of Nuclear Power Plant Components.

3. ASME Boiler and Pressure Vessel Code, Code Case N-526, Alternative Requirements for Successive Inspections of Class 1 and 2 Vessels,Section XI, Division 1, Approved by ASME Code Committee August 9, 1996. Approved in Regulatory Guide 1.147, Revision 19.

4. N. Cofie, P. Riccardella, J. Merkle and H. Do, Technical Basis for Alternate Successive Inspection Requirements for Vessels and Piping Welds as Prescribed in Code Cases N-526 And N-735, Proceedings of ASME Pressure Vessel and Piping Conference, Chicago, IL, July 2008.

5. Tennessee Valley Authority (TVA) Purchase Order Number 6831497, Revision 1.

6. General Electric Hitachi Energy NOI Form No. U2R21-R003, BFN/2, V-3-A, Ultrasonic Flaw Evaluation in Accordance with ASME Section XI, 2007 Edition, 2008 Addenda. SI File No. 2100264.201.

7. TVA Drawing No. 2-CHM-2046-C-01, Revision 3, Browns Ferry Nuclear Plant Unit1, Reactor Pressure Vessel, Shell Course Weld/Nozzle Locations (Outside View) SI File No. 2100264.201.

8. TVA Document No. Form N-1, Unit #2, Manufacturers Data Report for Nuclear Vessels, SI File Number 2100264.201.

9. ASME Boiler and Pressure Vessel Code, 2007 Edition with 2008 Addenda,Section II, Materials-Part D (Customary).

10. General Electric Hitachi Energy Document No. 24A5890, Revision 7, Reactor Pressure Vessel - Extended Power Uprate PROPRIETARY, SI File No. 2100264.201P.

11. ANSYS Mechanical APDL (UP20170403) and Workbench (March 31, 2017), Release 18.1, SAS IP, Inc.

12. Structural Integrity Associates, Inc. Calculation Package 1201256.301, Revision 0, Loading for Browns Ferry Feedwater Piping Limiting Fatigue Location, 06/03/2013.

13. Regulatory Guide 1.147, Inservice Inspection Code Case Acceptability, ASME Section XI, Division 1, Revision 19, Nuclear Regulatory Commission, October 2019.

Report No.2100312.401 R2 PAGE l 13

July 7, 2022 Adam Keyser BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526 Prepared by: Verified by:

07/07/2022 07/07/2022 Garivalde Dominguez Date Mohammed Uddin Date Senior Associate Senior Consultant Approved by:

07/07/2022 Mohammed Uddin Date Senior Consultant

Report No.2100312.401 R2 PAGE l 14

Enclosure 3 SI 2200769.401, Revision 0, BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526 under Service Level C/D loadings CNL-22-075

muddin@structint.com 11515 Vanstory Drive, Suite 125 l Huntersville, NC 28078 l 614-717-5887 September 8, 2022 REPORT NO. 2200769.401 REVISION: 0 PROJECT NO. 2200769.00 Quality Program: Nuclear Commercial Adam Keyser Tennessee Valley Authority Browns Ferry Nuclear Plant, Unit 2 PO Box 2000 Decatur, AL 35609

Subject:

BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526 under Service Level C/D loadings

Dear Adam,

1.0 INTRODUCTION

During the spring 2021 outage at Browns Ferry Nuclear Plant, Unit 2 (BFN), an indication was identified in a vertical weld (Weld No. V-3-A) of the reactor pressure vessel (RPV). This flaw was evaluated by Structural Integrity Associates, Inc. (SI) in SI Calculation 2100264.301P [1]

and determined to be acceptable for continued service per the rules of Section XI, IWB-3600 [2].

In order to support exemption of the three successive inspections required by IWB-2420 as per ASME Code Case N-526 [3], an additional evaluation was also performed in Reference [5] using the technical basis documents [4] of Code Case N-526 to show that the BFN RPV weld flaw met the intent of Code Case N-526 under Service Level A and B loadings when plant-specific materials, stresses, and aspect ratio are utilized. In the current evaluation, a calculation will be performed to determine if the BFN RPV weld flaw will meet the intent of Code Case N-526 under Service Level C and D loadings.

2.0 METHODOLOGY The methodology used in Reference [5] for the plant-specific assessment developed the proximity rule from the OD surface for subsequent augmented re-examinations for the BFN RPV weld flaw by following the same procedure used in the technical basis document [4] for ASME Code Case N-526 but using the operating stresses for Service A/B loadings and as-found flaw

© 2021 by Structural Integrity Associates, Inc. All rights reserved. No part of this document or the related files may be reproduced or transmitted in any form, without the prior written permission of Structural Integrity Associates, Inc.

MCG0070008.21.20

September 8, 2022 Adam Keyser BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526 under Service Level C/D loadings aspect ratio. A brief description of the procedure used in the technical basis document [4] is also provided in Reference [5].

In developing the proximity rule, the basic premise for the criterion is that the average stress in the remaining ligament should not exceed the material flow strength, which would potentially create a risk for rupture of the remaining ligament. The approach is illustrated in Figure 1.

Yielding of the average net-section area (considering flaw) defined as An in Figure 1 will occur when the primary membrane (Vm) and primary bending stresses (Vb) exceeds the flow strength of the material as given by Equation 1 below.

Eq. 1 f

Figure 1. Model for Ligament Tearing [4]

Report No.2200769.401 R0 PAGE l 2

September 8, 2022 Adam Keyser BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526 under Service Level C/D loadings 2.1 Methodology Used for BFN Plant-Specific Assessment The successive examination surface proximity rule per Code Case N-526 is based on preventing a flaw from causing yield in the remaining ligament of the ID surface by ensuring that the applied stress in the component (vessel) remains below the flow strength of the materials. In that regard, a plant-specific surface proximity rule can be developed based on the same technical requirements. To develop the plant-specific proximity rule from the OD surface for subsequent augmented re-examinations for the BFN RPV weld flaw, the plant-specific applied stresses for Service Level C and D, flaw dimension (i.e., aspect ratio), and flow strength of the RPV materials are required input.

Using Eq. 1 above and the plant-specific input, the plant-specific proximity rule will be developed based on preventing a flaw from causing yield in the remaining ligament of the OD surface. The flaw proximity curve for the plant-specific flaw aspect ratio with plant-specific input will be developed. Any flaw that falls above this plant-specific proximity curve would be considered as a subsurface flaw for successive examination.

3.0 DESIGN INPUTS As reported in the Reference [1] flaw evaluation, the following design inputs are used:

1. The indication is reported as being subsurface, separated from the vessel base metal/clad interface by 2.2 inches [6]. The cross-flaw depth dimension in the vessel radial direction (2a) is 3.2 inches [6]. The flaw is in the vertical weld V-3-A at 107° azimuth, or 42.95 to 46.7 inches above the circumferential weld C-2-3 [6,7]. Details of the geometric parameters of the subsurface flaw are summarized in Figure 2.

2. The plate material of the RPV is SA-302 Grade B [8].

3. From Reference [9, Table Y-1], the yield strength, Sy for SA-302 Grade B, is 50 ksi and 42.1 ksi at 70°F and 600°F respectively. 600°F bounds the plant operating temperature.

4. From Reference [9, Table U], the ultimate strength, Su for SA-302 Grade B, is 80 ksi for the temperature range of 70°F-600°F. 600°F bounds the plant operating temperature.

5. The vessel has an inside radius (centerline to base metal) of 125.6875 inches [8]. At the flaw location, the vessel wall thickness is 6.4 inches [6].

Report No.2200769.401 R0 PAGE l 3

September 8, 2022 Adam Keyser BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526 under Service Level C/D loadings S = SOD = 1.0 in.

SID = 2.2 in.

Figure 2. Flaw Geometry [6]

4.0 CALCULATIONS 4.1 Flaw Characterization and Flaw Evaluation The as-found axial flaw in the vertical weld of the BFN RPV was evaluated as per the requirements of IWB-3600 as reported in Reference [1]. The flaw was characterized as subsurface flaw as per IWA-3320 with a half-depth, a of 1.6-inch and length, l of 3.8-inch (see Figure 2). Therefore, the aspect ratio of the as-found flaw is (a/l=) 0.42. During the evaluation of the flaw, it was reported that the observed flaw is acceptable for continued operation per the requirements of ASME Code,Section XI, IWB-3640 and it would take 70 years for the as-found flaw to propagate to the allowable depth.

4.2 Stress Calculation As noted in Section 3.0 and the technical basis document [4], only primary membrane and primary bending stresses are included in the evaluation to develop the proximity rule. As such, secondary stresses due to thermal transients and weld residual stresses are not included in the current evaluation. Note that secondary stress due to thermal transient was conservatively used in Reference [5] evaluation for service level A/B loadings. Also, since the flaw is an axial flaw, only hoop stresses due to primary loading contribute to the driving force. In that regard, stresses due to deadweight, safe shutdown Earthquake (SSE), Loss of Coolant Accident (LOCA) are not included and only stresses due to pressure is used in the current evaluation. The pressure for Service Level C/D is 1375 psi from the Reactor Overpressure Event as documented in Reference [10, Table 4-1]

Report No.2200769.401 R0 PAGE l 4

September 8, 2022 Adam Keyser BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526 under Service Level C/D loadings 4.2.1 Stress Due to Pressure Load The membrane (hoop) stress due to the internal pressure of 1375 psi is calculated as:

Membrane stress due to pressure, m-p = PRm/t = 27,691 psi = 27.7 ksi where:

P = Maximum pressure (Service Level C/D) = 1375 psig [10]

t = thickness = 6.4 inches [6]

Rm = Mean radius = 128.8875 inches (Di/2 + t/2), Di = 251.375 inches [8]

4.3 Material Properties Below shows the bounding flow strength of RPV material at operating temperature of 600F which were determined by averaging the yield and ultimate strengths of the material as given in Section 4.0 Design Input.

Vf (at 600F) = (42.1+80)/2 = 61.1 ksi 4.4 The Plant-Specific Surface Proximity Rule Following the methodology described in Section 3.0, the plant-specific surface proximity curve is determined using the as-found flaw aspect ratio (of 0.42), applied plant-specific stresses for Service Level C/D and flow strength of RPV material as shown in Figure 3. IWA-3320 proximity rule and Code Case N-526 surface proximity rule are also added in Figure 3 for reference. As can be seen in the figure, the plant-specific surface proximity curve for Service Level C/D loading is slightly higher than that for service level A/B loading; however, both curves fall below both IWA-3320 proximity and Code Case N-526 surface proximity rules indicating that the Code Case N-526 surface proximity rule is overly conservative for the as-found flaw in the vertical weld of the BFN RPV. Since the as-found flaw must meet the requirements of IWA-3320 to be considered as subsurface, the S>0.4 line (dotted black line) would be the plant-specific proximity rule for the as-found flaw in the vertical weld of the BFN RPV for the exemption of successive examination.

Report No.2200769.401 R0 PAGE l 5

September 8, 2022 Adam Keyser BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526 under Service Level C/D loadings Figure 3. Development of BFN Plant-Specific Proximity Rule 5.0 RESULTS The as-developed plant-specific surface proximity rule for Service Level C/D has been applied on the observed flaw in the vertical weld of the BFN RPV. Using the plant-specific proximity curve, developed using the same technical evaluation methods as the Code Case N-526 curve, demonstrates that the as-found flaw is still bounded by the Code Case N-526 technical basis, even when considering the flaw's proximity to the OD surface. As shown in Figure 3 (blue square marker), the minimum distance to the OD surface for the as-found flaw is 1 inch, and the half-flaw depth is a = 1.6 inch. The observed flaw (from OD surface) is above the as-developed plant-specific surface proximity curve in Figure 3 and therefore, it can be classified as a subsurface flaw for the purpose of applying Code Case N-526 successive examination exemption requirements. For completeness, the assessment of the observed flaw from the ID surface is also added in Figure 3 (red circle marker) where the minimum distance to the ID surface for the as-found flaw is 2.2 inch, and the half-flaw depth is a = 1.6 inch. This data point shows that the as-found flaw is acceptable per the existing conservative requirements of Code Case N-526, when the Code Case technical basis is applied, which would consider the flaw's proximity to only the ID surface. The observed flaw (from ID surface) meets the requirements of both the Code Case N-526 and as-developed plant-specific proximity rules to be classified as subsurface flaw.

Report No.2200769.401 R0 PAGE l 6

September 8, 2022 Adam Keyser BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526 under Service Level C/D loadings Using either the as-developed plant-specific surface proximity rule, or the ID surface proximity as intended by the Code Case N-526 technical basis, the following requirements specified in Code Case N-526 to exempt the re-examinations in accordance with IWB-2420(b) of vessel volumes containing subsurface flaws are met. Code Case N-526 is accepted without condition in Regulatory Guide 1.147 Revision 19 [13].

(a) The flaw is characterized as subsurface according to the as-developed plant-specific surface proximity rule (shown in Figure 3).

(b) The NDE technique and evaluation that detected and characterized the flaw, with respect to both sizing and location, shall be documented in the flaw evaluation report.

(c) The vessel containing the flaw is acceptable for continued service in accordance with IWB-3600, and the flaw is demonstrated acceptable for the intended service life of the vessel.

6.0 CONCLUSION

A plant-specific assessment surface proximity rule was developed for an as-found flaw in a vertical weld of the BFN RPV using the plant-specific stresses for Service Level C/D loadings, actual flaws aspect ratio and bounding flow strength of RPV material.

Using the as-developed plant-specific surface proximity rule, it is demonstrated in Section 6.0 that the observed flaw is classified as a subsurface flaw and meets the intent of the requirements of Code Case N-526 to exempt the re-examinations in accordance with IWB-2420(b) of vessel volumes containing subsurface flaws. Therefore, successive examinations in accordance with IWB-2420 (b) and (c) of the observed flaw in Weld V-3-A BFN Unit 2 RPV are not required.

7.0 REFERENCES

1. SI Calculation No. 2100264.301, Revision 3, Browns Ferry Unit 2 Reactor Pressure Vessel Vertical Weld Flaw Evaluation, July 8, 2022.

2. ASME Boiler and Pressure Vessel Code, 2007 Edition with 2008 Addenda,Section XI, Rules for Inservice Inspection of Nuclear Power Plant Components.

3. ASME Boiler and Pressure Vessel Code, Code Case N-526, Alternative Requirements for Successive Inspections of Class 1 and 2 Vessels,Section XI, Division 1, Approved by ASME Code Committee August 9, 1996. Approved in Regulatory Guide 1.147, Revision 19.

4. N. Cofie, P. Riccardella, J. Merkle and H. Do, Technical Basis for Alternate Successive Inspection Requirements for Vessels and Piping Welds as Prescribed in Code Cases N-526 And N-735, Proceedings of ASME Pressure Vessel and Piping Conference, Chicago, IL, July 2008.

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September 8, 2022 Adam Keyser BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526 under Service Level C/D loadings

5. SI Calculation No. 2100312.401, Revision 2, BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526.