CNL-21-081, 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

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

ML22014A344
Person / Time
Site:	Browns Ferry
Issue date:	01/14/2022
From:	Polickoski J Tennessee Valley Authority
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
CNL-21-081
Download: ML22014A344 (25)

Text

1101 Market Street, Chattanooga, Tennessee 37402 CNL-21-081 January 14, 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:

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 In accordance with Title 10 of the Code of Federal Regulations 50.55a, Codes and standards, paragraph (z)(1), the enclosure to this submittal contains the Tennessee Valley Authority (TVA) request for alternative (RFA) BFN-21-ISI-02 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. The RFA relates specifically to the successive examination requirements of paragraph IWB-2420(b) of the ASME Code. to the enclosure provides justification that, although required by IWB-2420(b),

successive examinations of the indication identified in the subject weld are not needed as shown by the methodology found in the technical basis document for ASME Code Case N-526 and using the inside diameter surface to apply the proximity rule. The justification provided in to the enclosure includes a plant-specific ASME Code Case N-526 assessment of the subject weld flaw utilizing the flaw's outside diameter surface proximity, plant-specific operating stresses, and as-found flaw aspect ratio, and following the same procedure used in the technical basis of ASME Code Case N-526, it demonstrates that the intent of ASME Code Case N-526 is met.

TVA requests approval of this alternative within one year of submittal.

U.S. Nuclear Regulatory Commission CNL-21-081 Page 2 January 14, 2022 There are no new regulatory commitments contained in this letter. If you have any questions regarding this submittal, please contact Kimberly D. Hulvey, Senior Manager, Fleet Licensing at 423-751-3275.

Respectfully, Digitally signed by Carla Edmondson Date: 2022.01.14 13:55:43

-05'00' James T. Polickoski Director, Nuclear Regulatory Affairs

Enclosure:

Request for Alternative, 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 cc:

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

Browns Ferry Nuclear Plant, Unit 2, Request for Alternative, 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 1.0 ASME CODE COMPONENTS AFFECTED Code Class: Class 1

Description:

Reactor Pressure Vessel Vertical Shell Weld Examination Category: B-A - Pressure Retaining Welds in Reactor Vessel Item Number: B1.12 - Longitudinal Shell Welds Component ID: V-3-A 2.0 APPLICABLE CODE EDITION AND ADDENDA The Code of Record for the fifth inservice inspection (ISI) interval for Browns Ferry Nuclear Plant (BFN), Unit 2, is the 2007 Edition with 2008 Addenda of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (ASME Code),Section XI, Division 1, Rules for Inservice Inspection of Nuclear Power Plant Components.

3.0 APPLICABLE CODE REQUIREMENT 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.

4.0 REASON FOR REQUEST Background Information During the Unit 2 Cycle 21 refueling outage at BFN in spring 2021, an indication was identified in a weld (Weld No. V-3-A) of the reactor pressure vessel (RPV) (Reference 1). The ultrasonic examination results were documented as unacceptable to the requirements of ASME Code,Section XI, 2007 Edition with 2008 Addenda, as modified by the performance demonstration initiative (PDI) program description and 10 CFR Part 50.55a for Category B-A RPV Assembly Welds.

BFN Unit 2 Weld No. V-3-A received 100 percent coverage during Unit 2 Cycle 21 in accordance with the Appendix VIII Performance Demonstration program. Automated scanning was performed from the vessel inside surface. Automated transverse and parallel scans were performed in accordance with qualified procedures using 45-degree shear wave and 60-degree refracted longitudinal (RL) search units. The automated examination was restricted due to the proximity of the feedwater spargers and core spray piping. Manual transverse and parallel scans were performed in accordance with qualified procedures using 60-degree RL search units in the area restricted for the automated examination.

The flaw is located between the 434.45-inch and 438.2-inch elevations. Figure 1 provides an approximation of the flaw location on a BFN Unit 2 vessel roll-out schematic. The flaw is a linear planar flaw oriented parallel to the vertical seam weld. The as-found flaw in the vertical weld of the BFN RPV was characterized as subsurface flaw per IWA-3320 with a half depth (a) of 1.6 inches and length (l) of 3.8 inches. This flaw was evaluated and found to be CNL-21-081 E1 of 8

Enclosure acceptable for continued service per the rules of ASME Code,Section XI, paragraph IWB-3600 (Reference 2). Based on this acceptance, IWB-2420(b) would therefore require successive inspections for each of the next three inspection periods.

Figure 1: Approximate Location of Flaw CNL-21-081 E2 of 8

Enclosure Technical Information ASME Code Case N-526 provides alternative requirements for the successive inspection requirements of IWB-2420(b), but its use is limited based on the proximity of the flaw to the vessel surface. ASME Code Case N-526 is endorsed by the NRC in Regulatory Guide 1.147 Revision 19, in Table 1, Acceptable Section Code Cases, with no conditions. 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 ASME Code Case N-526 further restricts subsurface flaws to those whose surface proximity is greater than the flaw half depth (S > a).

The IWB-3600 flaw evaluation for the BFN Unit 2 V-3-A weld flaw concluded that the flaw meets the criteria set forth in ASME Code Case N-526 to not require the three successive inspections required by IWB-2420 when considering the proximity of the flaw to the inside diameter (ID) surface. However, when the evaluation was performed using the distance from the outside diameter (OD) surface, the flaw does not meet the criteria set forth in ASME Code Case N-526.

Because ASME Code Case N-526 does not specify which surface is to be used for proximity evaluation, TVA assumed that the calculation should apply to the lesser of these two distances, similar to the conditions required by ASME Code,Section XI, paragraph IWA-3320. However, upon review of the ASME Code Case N-526 technical basis referenced in Attachment 1, it is evident that the basis for the revised proximity criteria 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 is not the case in this application.

Additionally, as discussed in the notes for Table 1 of this enclosure, conservative assumptions were identified in the analysis supporting the technical basis of ASME Code Case N-526 regarding stresses and flaw aspect ratio. To address the impact of these conservative assumptions, an evaluation was performed by Structural Integrity Associates (SI) that demonstrates the flaw will meet the technical basis and intent of ASME Code Case N-526 when plant-specific materials, stresses, and flaw aspect ratio are utilized in the analysis. The evaluation is provided as Attachment 1 of this enclosure.

5.0 PROPOSED ALTERNATIVE AND BASIS FOR USE Proposed Alternative As an alternative to ASME Code,Section XI, paragraph IWB-2420(b), TVA is requesting to defer the periodic successive examinations of the identified flaw in Weld V-3-A at BFN, Unit 2, until the next regularly scheduled inservice inspection. 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. Periodic system pressure tests and 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.

CNL-21-081 E3 of 8

Enclosure Basis for Use The location and orientation of the BFN Unit 2 V-3-A indication align with a fabrication repair documented in the General Electric (GE) vessel fabrication discrepancy report. General Electric-Hitachi (GE-Hitachi) was able to image the repair area from the outside surface through the top of the reported indication. The image from the inspection provided a repair length and depth that correlates to the recorded length and depth in the fabrication records for repair area MRA-021. The indication appears to be a series of intermittent reflectors that combine ultrasonically. Although the proximity rules defined in ASME Code,Section XI, paragraph IWA-3300 required the indication to be defined as a continuous discontinuity, the ultrasonic testing (UT) results appeared to be multiple aligned fabrication flaws and not representative of a service induced flaw. Based on this assessment, the flaw is not expected to grow. However, the calculations performed in support of this alternative request have conservatively evaluated the indication as a true service induced crack-like flaw.

SI performed an evaluation (Reference 2) in accordance with IWB-3600 accepting the flaw for continued service. The flaw evaluation applies fracture mechanics solutions for bounding flaw geometry and loading conditions and uses linear elastic fracture mechanics methods consistent with the requirements of ASME Code,Section XI, paragraph IWB-3600 and Nonmandatory Appendix A. The SI fracture mechanics software pc-CRACK 5.0 was utilized for these calculations. In order to evaluate acceptability of the flaw through the license renewal period, the projected 48-effective-full-power-years fluence value was established and used to determine the limiting fracture toughness of the BFN Unit 2 vessel material. SI used the as-found sub-surface flaw geometry depicted in Figure 2, and reactor vessel parameters to perform flaw stability and fatigue crack growth calculations. Using structural factors imposed by IWB-3612, the allowable fracture toughness for normal conditions was calculated to be 69.57 kilopound per square inch (ksi) square root inches (in) to prevent brittle failure. The calculated allowable flaw half depth is 1.7156 inch. The flaw evaluation was performed to determine when the crack would propagate to the allowable half depth, assuming that the crack can grow in either direction. Reference 2 determined that it will take 70 years to grow the initial crack from half depth (ai) of 1.6 inch to the allowable half depth (af) 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. 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.

The technical basis for this request is the plant-specific analysis of the flaw described below and in Attachment 1 to this enclosure. This evaluation uses the same methodology as the technical basis of ASME Code Case N-526 with the plant-specific material properties, operating stresses, and as-found flaw aspect ratio to develop a site-specific proximity curve using the distance of the flaw from the OD surface. The result of this plant-specific analysis demonstrates that this alternative provides an acceptable level of quality and safety for exemption from the successive examinations requirement in IWB-2420(b).

Note, when the flaw is evaluated using the distance from the inside surface, which is the stated intent of the ASME Code Case N-526 surface proximity criteria per the ASME Code Case N-526 technical basis, ASME Code Case N-526 could be used in lieu of the successive examination requirements of IWB-2420(b), and a Request for Alternative would not be required.

CNL-21-081 E4 of 8

Enclosure Methodology 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.

The methodology used for the BFN, Unit 2, plant-specific assessment develops the proximity criteria from the OD surface for subsequent successive re-examinations for the BFN Unit 2 V-3-A RPV weld flaw by following the same procedure used in the technical basis document for ASME Code Case N-526, but using site-specific material properties, operating stresses, and as-found flaw aspect ratio.

Design Inputs Table 1 provides a comparison of the design inputs used in the ASME Code Case N-526 technical basis to the plant-specific design inputs applied in the BFN Unit 2 V-3-A analysis.

Table 1, Comparison of Design Input Data BFN, Unit 2, Code Case N-526 plant-specific technical basis flaw analysis Primary Membrane plus Bending Stress 45 ksi (1) 26.66 ksi (2)

(Pm+Pb)

Flow Strength (sf) 65 ksi (3) 61.1 ksi (4)

Flaw Aspect Ratio (a/l) ~0.2 (5) 0.421 (6)

1. As noted in the ASME Code Case N-526 technical basis document, the flaw proximity criterion for vessels was determined using the primary membrane stress (Pm) equal to the ASME Code,Section III, allowable stress intensity (Sm = 30 ksi). The primary membrane plus bending stress (Pm+Pb) was assumed to be equal to 1.5Sm (Pm+Pb = 45 ksi).

2. For the BFN Unit 2 V-3-A flaw, the membrane (hoop) stress due to pressure is 21 ksi, calculated using plant-specific vessel geometry and operating parameters. Additional membrane stress due to the bounding thermal transient (plant startup) is calculated as 3.61 ksi. The maximum bending stress due to the bounding thermal transient was calculated as 2.05 ksi. The resulting maximum combined membrane and bending stresses at the vessel OD surface equates to 26.66 ksi.

3. A flow strength value of f = 65 ksi for vessel materials was used for the ASME Code Case N-526 technical evaluation. This value represents a mid-range value for various vessel materials at 70°F.

CNL-21-081 E5 of 8

Enclosure

4. The plate material of the BFN Unit 2 RPV is SA-302 Grade B. Using the BFN Unit 2 vessel material specification, the bounding flow strength of RPV material at an operating temperature of 600°F was determined by averaging the yield and ultimate strengths of the material. As per technical basis document of ASME Code Case N-526, the proximity rule was developed using flow strength at room temperature only. However, the bounding flow strength at operating temperature (600°F) is used in the current analysis for additional conservatism.

5. As shown in ASME Code Case N-526, Figure 1, a subsurface flaw has surface proximity distance greater than the flaw half depth (S>a). When plotted on a curve, this line closely relates to the calculated a/l curve for ligament yielding. Note this value is an approximation, provided for comparative purposes only.

6. The as-found flaw in the vertical weld of the BFN Unit 2 RPV was characterized as subsurface flaw per IWA-3320 with a half depth (a) of 1.6 and length (l) of 3.8. Therefore, the aspect ratio of the as-found flaw is (a/l =) 0.421. Also shown in Attachment 1 to this enclosure, the flaw geometry is depicted in Figure 2.

Figure 2: Flaw Geometry Results Following the methodology described in ASME Code Case N-526 Technical Basis, the plant-specific surface proximity curve is determined using the as-found flaw aspect ratio, applied plant-specific stresses, and flow strength of RPV material as shown in Figure 3.

IWA-3320 surface proximity rules and ASME Code Case N-526 surface proximity rules are also added in Figure 3 for reference. As shown in Figure 3, the plant-specific surface proximity curve falls below both IWA-3320 surface proximity rules and ASME Code Case N-526 surface proximity rules, indicating that the ASME Code Case N-526 surface proximity rule is overly conservative for the as-found flaw in the vertical weld of the BFN RPV.

CNL-21-081 E6 of 8

Enclosure Figure 3: Development of BFN Plant-Specific Proximity Rule Using the plant-specific surface proximity rule, the identified flaw in the BFN Unit 2 V-3-A weld can be characterized as subsurface, with adequate technical conservatism to ensure the flaw will not grow to an unacceptable size, without application of the periodic successive examinations required by ASME Code,Section XI, paragraph IWB-2420(b). Because the as-found flaw must meet the requirements of IWA-3320 to be considered as subsurface per ASME Code,Section XI, the S>0.4a line (dotted black line) will be used to define the proposed alternative surface proximity criteria for the as-found flaw in the vertical weld of the BFN RPV.

This evaluation demonstrates that the proposed alternative will provide an acceptable level of quality and safety in accordance with 10 CFR 50.55a(z)(1), as compared to both ASME Code,Section XI, paragraph IWA-3320 and ASME Code Case N-526 for determining the need for periodic successive examinations.

Conclusion The indication characteristics in BFN Unit 2 Weld No. V-3-A align with a major repair area MRA-021 documented in the BFN Unit 2 RPV fabrication records.

Based on the BFN, Unit 2, RPV vertical weld flaw evaluation (Reference 2) using ASME Code,Section XI, paragraph IWB-3600, it will take 70 years for an as-found flaw with the initial half depth to increase to the allowable half depth.

The plant-specific surface proximity rule, developed for the as-found flaw in vertical weld V-3-A of the BFN, Unit 2, RPV using the plant-specific stresses, flaw aspect ratio, and bounding flow strength of RPV material, alongside the surface proximity curves from both ASME Code,Section XI, paragraph IWA-3320 and ASME Code Case N-526 demonstrate that the observed flaw can be classified as a subsurface flaw, and that ligament tearing will not occur. This evaluation demonstrates that the flaw meets the intent of the requirements of ASME Code Case CNL-21-081 E7 of 8

Enclosure N-526 to exempt the successive re-examinations required by IWB-2420(b) for vessels containing subsurface flaws, and therefore provides an acceptable level of quality and safety.

Therefore, successive examinations of the observed flaw in Weld V-3-A BFN Unit 2 RPV in accordance with IWB-2420(b) are not required.

6.0 DURATION OF PROPOSED ALTERNATIVE 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.

7.0 PRECEDENT TVA has not identified any similar alternative requests from ASME Code,Section XI, paragraph IWB-2420(b), related to the ASME Code,Section XI, Category B-A, Item No. B1.12 (Shell Longitudinal Weld) and ASME Code N-526.

8.0 REFERENCES

1. TVA letter to NRC, American Society of Mechanical Engineers,Section XI, Fifth 10 Year Inspection Interval, Inservice Inspection, System Pressure Test, Containment Inspection, and Repair and Replacement Programs, Owners Activity Report for Browns Ferry Nuclear Plant, Unit 2, Cycle 21 Operation, dated July 21, 2021 (ML21202A242)

2. TVA letter to NRC, Submittal of Browns Ferry Unit 2 Reactor Pressure Vessel Vertical Weld Flaw Evaluation, dated October 4, 2021(ML21277A123)

CNL-21-081 E8 of 8

Attachment 1 2100312.401.Revision 1, BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526 CNL-21-081

muddin@structint.com 11515 Vanstory Drive, Suite 125 l Huntersville, NC 28078 l 614-717-5887 September 20, 2021 REPORT NO. 2100312.401 REVISION: 1 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.

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.

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

© 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.

MCG00700 08.21.20

September 20, 2021 Adam Keyser BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526 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 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.

Report No. 2100312.401 R1 PAGE l 2

September 20, 2021 Adam Keyser BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526 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 (y) was used synonymous with the flow stress (f). 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 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 f = 65 ksi for vessel materials was used for the evaluation which is a mid-range value for various vessel materials at Report No. 2100312.401 R1 PAGE l 3

September 20, 2021 Adam Keyser BFN V-3-A Weld Flaw Plant-Specific Assessment per the Technical Basis for ASME Code Case N-526 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 R1 PAGE l 4

September 20, 2021 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 72 to 74.25 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 R1 PAGE l 5

September 20, 2021 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 R1 PAGE l 6

September 20, 2021 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 b =

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

Membrane stress due to thermal, m-t = 0.6392 3.2 + 1.5691 = 3.61 ksi Max. bending stress due to thermal, b-t = 0.6392 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 R1 PAGE l 7

September 20, 2021 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 R1 PAGE l 8

September 20, 2021 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.

f (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 R1 PAGE l 9

September 20, 2021 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 R1 PAGE l 10

September 20, 2021 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.

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September 20, 2021 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.

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September 20, 2021 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.

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September 20, 2021 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:

09/20/2021 09/20/2021 Moses Taylor Date Mo Uddin Date Senior Associate Senior Consultant Approved by:

09/20/2021 Mo Uddin Date Senior Consultant Report No. 2100312.401 R1 PAGE l 14