ML22300A100

End of Cycle 21 Steam Generator Tube Inspection Report

ML22300A100
Person / Time
Site:	Millstone
Issue date:	10/27/2022
From:	O'Connor M Dominion Energy Nuclear Connecticut
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
22-274
Download: ML22300A100 (1)

Text

Domin ion En ergy Nucl ea r Conn ecti cut, Inc.

Mill st one Powe r Station 3 14 Rope Ferry Road, Wa terford, CT 06385 OCT *2 7 2022 ft Dominion j a,, Energy Domini onEn ergy.co m U. S. Nuclear Regulatory Commission Serial No .22-274 Attention: Document Control Desk . NSSL/TFO RO Washington, DC 20555 Docket No. 50-423 License No. NPF-49 DOMINION ENERGY NUCLEAR CONNECTICUT, INC.

MILLSTONE POWER STATION UNIT 3 END OF CYCLE 21 STEAM GENERATOR TUBE INSPECTION REPORT In accordance with the Millstone Power Station Unit 3 Technical Specification (TS)

Section 6.9.1.7, Dominion Energy Nuclear Connecticut, Inc. hereby submits the End of Cycle 21 (EOC21) Steam Generator (SG) Tube Inspection report. The report is submitted within 180 days after initial entry into MODE 4 following completion of the spring 2022 SG inspections performed in accordance with TS 6.8.4.g, "Steam Generator (SG) Program." Initial entry into Mode 4 occurred on May 18, 2022. contains the EOC21 SG Tube Inspection report. Attachment 2 contains a list of acronyms.

The report addresses the following reporting requirements:

a. The scope of inspections performed on each SG,

b. Degradation mechanisms found,

c. Nondestructive examination techniques utilized for each degradation mechanism,

d. Location, orientation (if linear), and measured sizes (if available) of service induced indications,

e. Number of tubes plugged during the inspection outage for each degr~dation mechanism,

f. The number and percentage of tubes plugged to date and the effective plugging percentage in each steam generator,

g. The results of condition monitoring, including the results of tube pulls and in-situ testing,

h. The primary to secondary LEAKAGE rate observed in each SG (if it is not practical to assign the LEAKAGE to an individual SG, the entire primary to secondary LEAKAGE should be conservatively assumed to be from one SG) during the cycle preceding the inspection which is the subject of the report,

i. The calculated accident induced leakage rate from the portion of the tubes below 15.2 inches from the top of the tubesheet for the most limiting accident in_ the most limiting SG. In addition, if the calculated accident induced leakage rate from the most limiting accident is less than 2.49 times the maximum operational primary to secondary leakage rate, the report should describe how it was determined; and

j. The results of monitoring for tube axial displacement (slippage). If slippage is discovered, the implications of the discovery and corrective action shall be provided.

Serial No.22-274 Docket No. 50-423 Page 2 of 2 If you have any questions or require additional information, please contact Mr.

Dean E. Rowe at (860) 444-5292.

Sincerely, Michael J. O'Connor Site Vice President - Millstone Attachments:

1) Millstone Power Station Unit 3, End of Cycle 21 Steam Generator Tube Inspection Report

2) Acronyms Commitments made in this letter: None cc: U. S. Nuclear Regulatory Commission Region I 475 Allendale Road, Suite 102 King of Prussia, PA 19406-1415 R. V. Guzman Senior Project Manager - Millstone Power Station U.S. Nuclear Regulatory Commission One White Flint North, Mail Stop 08-C 2 11555 Rockville Pike Rockville, MD 20852-2738 NRC Senior Resident Inspector Millstone Power Station

Serial No. 22-27 4 Docket No. 50-423 Attachment 1 Millstone Power Station Unit 3 End of Cycle 21 Steam Generator Tube Inspection Report MILLSTONE POWER STATION UNIT 3 DOMINION ENERGY NUCLEAR CONNECTICUT, INC. (DENC)

Serial No. 22-27 4 Docket No. 50-423 Attachment 1, Page 1 of 25 End of Cycle 21 Steam Generator Tube Inspection Report Transmittal of this report satisfies Millstone Power Station Unit 3 (MPS3) Technical Specification (TS) 6.9.1.7 which specifies that a report shall be submitted within 180 days after the initial entry into MODE 4 following completion of an inspection performed in accordance with TS 6.8.4.g, Steam Generator (SG) Program. During MPS3's spring 2022 refueling outage (3R21 ), steam generator inspections were completed in accordance with TS 6.8.4.g. Initial entry into Mode 4 occurred on May 18, 2022; therefore, this report is required to be submitted to the NRG by November 14, 2022.

This attachment provides a summary of the MPS3 SG inspection results and specific responses to each of the TS 6.9.1.7 reporting requirements. Attachment 2 contains a list of acronyms.

Introduction MPS3 is a four loop Westinghouse pressurized water reactor with Westinghouse Model F SGs. Each SG was fabricated with 5626 U-bend thermally treated lnconel 600 tubes. The tubing is nominally 0.688 inches outside diameter with a 0.040-inch nominal wall thickness.

During SG fabrication, the tubes were hydraulically expanded over the full depth of the 21.23-inch thick tubesheet. The tubesheet was drilled on a square pitch with 0.98 inch spacing. There are 59 rows and 122 columns in each SG. The radius of row 1 Li-bends is 2.20 inches. Li-bends in rows 1 through 10 were stress relieved after being formed.

The secondary side tube support structures include eight 405 stainless steel support plates and six upper support bars (anti-vibration bars) made of chrome plated lnconel 600. The first or lower support plate is a partial plate with drilled tube holes, commonly referred to as a flow distribution baffle.Figure 9 contains a schematic depicting the general arrangement of the steam generator support configuration without dimensions.

SG-A and SG-C were last inspected during the Fall 2017 refueling outage (EOC18) and had operated for 297 .1 EFPM prior to that outage. SG-B and SG-D were last inspected during the Spring 2019 refueling outage (EOC19) and had operated for 313.9 EFPM preceding that outage. At the time of this inspection, the Unit 3 SGs have accrued approximately 346.1 Effective Full Power Months (EFPM) of operation as of the End of Cycle (EOG) 21 (April 2022). Consequently, SGs A and C had operated for 49 EFPM since the last inspection of those SGs and SGs B and D had operated for 32.2 EFPM since those SGs were last inspected. An inspection for SGs A and C was planned for the Fall 2020 outage, but it was deferred to Spring 2022 in accordance with License Amendment No. 277.

The MPS3 SGs operate with a hot-leg temperature of 617°F and have experienced no detectable primary to secondary leakage.

Each of the MPS Unit 3 SGs was screened to identify any low row indications of improper heat treatment. None were identified. All four SGs were also screened for long row indications of improper heat treatment (-2 sigma tubes) and associated high residual stress.

This evaluation identified 67, 30, 39, and 23 tubes in SGs A, B, C and D, respectively, which

Serial No.22-274 Docket No. 50-423 Attachment 1, Page 2 of 25 may have been improperly heat treated. These tubes were examined full length with array probes and closely scrutinized during the analysis process.

There were no deviations taken from Mandatory and/or Needed (Shall) requirements important to tube integrity from the EPRI Guidelines referenced by NEI 97-06 during the examination or the cycles preceding the EOC21 examination.

TS 6.9.1.7 Reporting Requirements This section provides responses to each of the reporting requirements specified in MPS3 TS 6.9.1. 7. Bold wording represents TS verbiage. The required information is provided immediately following the restatement of each reporting requirement.

A report shall be submitted within 180 days after the initial entry into MODE 4 following completion of an inspection performed in accordance with TS 6.8.4.g, Steam Generator (SG} Program. The report shall include:

a. The scope of inspections performed on each SG, One hundred percent of the operational tubes in all four SGs were inspected full length using eddy current examination techniques. The majority of the tubing length was examined with bobbin probes. The Li-bends of rows 1 and 2 (955 in-service tubes) were examined with a +Point' probe technique in addition to the bobbin probe examination of the straight legs of the tubes. An additional augmented sample of 1,736 tube locations was examined with a +Point' probe. The augmented sample inspections were performed in areas of special interest including hot leg expansion transitions, tube overexpansion locations, dents/dings, as well as locations where the bobbin probe response was ambiguous.

As a result of concerns for possible degradation at dent/ding locations, the special interest scope of dents/dings with a +Point' probe was increased significantly. The specific dent/ding scope performed during the EOC21 inspection included 100% of all dents/dings 2:

2 Volts located in the hot leg straight section and 100% of all dents/dings 2:5 Volts in the U-bend and cold-leg sections of the tubes.

It should be noted that both terms Dent and Ding refer to a plastic deformation of the tube that results in a reduction in the tube diameter. The two different terms were used to differentiate between the location of the signals. Historically (early generation designs) the term dent referred to local tube diameter reductions due to corrosion products from carbon steel (typically, drilled carbon steel tube support plates). The term ding referred to local tube diameter reductions due to mechanical means (manufacturing, vibration, incidents during maintenance activities, or impact from foreign objects). Since the eddy current signals from both dents and dings are similar, the location of the indication was used to differentiate which term was used (dent for indications at supports and ding for all free span indications).

Serial No.22-274 Docket No. 50-423 Attachment 1, Page 3 of 25 At MPS Unit 3, the referenced dent signals do not represent the same phenomena as classical denting on older generation units caused by drilled carbon steel support plate corrosion damage. Since the MPS Unit 3 are not similar in design (i.e., quatrefoil stainless steel tube support plate design vs. drilled hole carbon steel tube support plate design) these same "denting" issues do not directly apply to the MPS Unit 3. Tube support plate areas are not susceptible to denting caused by corrosion of the tube support plates. However, the historical nomenclature assigned to these signals has existed in the database since the steam generators were installed and has remained unchanged since that time.

No scope expansions were required; however, the base scope was augmented with additional rotating probe (including magnetically bias probes) to resolve ambiguous indications consistent with the special interest criteria.

An additional augmented sample of 25,345 tube locations was inspected with an array coil probe. The array coil probe sample included 100% of the hot leg top-of-tubesheet (TTS) locations (22,070 tubes), and approximately 13% of the cold leg TTS locations (2,949 tubes). The extent of the TTS examinations was from the first support structure detected above the secondary face of the tubesheet to 15.2 inches below the secondary face of the tubesheet; except for the tubes categorized as high residual stress tubes. For the high residual stress tubes, the extent included the entire full length of the tubes.

A License Amendment Request (LAR) to implement a Permanent Alternate Repair Criteria (PARC) for tube degradation near the tube end was approved for use prior to the 3R15 outage. The PARC excludes the lower portion of the tubes (more than 15.2 inches below the top of the tubesheet on both the hot and cold leg sides) from augmented inspection (i.e.,

array or rotating probe) requirements. Furthermore, tubes with degradation (other than tube slippage) more than 15.2 inches below the top of the tubesheet do not require plugging.

Thus, that portion of the tube adjacent to the tube ends, (where linear indications were identified during 3R12), was not required to be inspected with array or +Point' probes during 3R21.

During 3R21, each primary channel head in all four SGs was visually examined prior to the installation of eddy current probe manipulators. This examination revealed no evidence of degradation, no evidence of plug leakage, and no foreign objects. The proper tube number, plug type, and plug position were verified for all previously installed plugs.

Westinghouse issued a Nuclear Safety Advisory Letter, NSAL-12-1 regarding SG channel head degradation. The recommended action is to perform a visual scan of both hot and cold legs of the inside surface of the channel head. Key areas of inspection include the channel head cladding, the divider plate-to-channel head weld and the visible portion of the weld at the top of the channel head bowl drain tube. The purpose of the inspection is to detect any gross defects such as indications in welds, missing weld filler material, a breach of the weld metal, unusual discoloration of the weld metal, dings or gouges, etc.

A video camera visual examination in accordance with the requirements of NSAL-12-1 was conducted on all fours SGs during 3R21. The examination identified no evidence of degradation.

Serial No.22-274 Docket No. 50-423 Attachment 1, Page 4 of 25 During 3R21, secondary side activities were performed in SGs A, B, C, and D and included the following:

• High pressure sludge lancing removed a total of 159 lbs. The quantities of sludge removed in each SG is as follows: SG-A (44 lbs.), SG-B (33 lbs.), SG-C (45 lbs.), and SG-D (37 lbs.).

• Post-sludge lancing visual examination of the TTS annulus and no-tube lane to assess as-left material condition and cleanliness, and to identify and remove any retrievable foreign objects.

• Visual investigation of accessible locations having eddy current indications potentially related to foreign objects, and if present, removal of those retrievable foreign objects.

o Five foreign objects were removed individually from the SGs (see Table 1).

o An abundance of objects was also removed by sludge lancing. These objects include scale, small pieces of Flexitallic gasket, and hard deposits.

o Two locations of newly detected foreign object wear were observed during 3R21. The presence of the foreign objects causing the wear was not observed.

o One lodged legacy foreign object was visually confirmed and left in place Table 1: 3R21 Foreign Object Tracking FOTS SG Item Location Description 3R21 Action 3R21 Results Irregularly ECT not required since shaped metallic located in periphery object found in annulus Object removed A 14 Annulus annulus during from SG by FOSAR 3R21 post lance inspection Perform FOSAR Lancing Strainer Pieces of scale and A 16 TTS N/A Parts at 3R21 Flexitallic gasket Array was NOD (no R56-C65 Metallic piece of Perform array on wear) on R57-C65 scale discovered affected/bounding tubes.

affected/bounding tubes B 8 R56-C64 by SSI in outer R57-C64 periphery tubes at 3R21 Perform FOSAR Object removed

@TSH from SG by FOSAR Irregularly Array was NOD (no shaped metallic Perform array on wear) on R59-C66 affected/bounding tubes affected/bounding tubes.

object discovered B 9 R59-C67 by SSI in outer

@TSC periphery tubes Perform FOSAR Object removed at 3R21 from SG by FOSAR Lancing Strainer Pieces of scale and B 10 TTS N/A Parts at 3R21 Flexitallic gasket

Serial No. 22-27 4 Docket No. 50-423 Attachment 1, Page 5 of 25 FOTS SG Item Location Description 3R21 Action 3R21 Results Inspect affected and Metal block bounding tubes (2-tube bounding) with ECT to check Array was NDD (no wear) previously R1-C4 for movement of object on bounding tubes.

wedged between C 1 R1-C5 and/or active wear. Part configuration was blowdown pipe

@TSC +3" confirmed to be fixed at and tube. Visually inspect to 3R21 R1-C4 plugged. reconfirm location and fixity of object.

Lancing Strainer Pieces of scale and C 4 TTS N/A Parts at 3R21 Flexitallic gasket

+Point confirmed array Array PLP caused Inspect PLP with +Point PLP by Irregularly R55-C69 shaped metallic Perform array/+Point on No wear detected on D 18 R56-C69 piece of scale affected/bounding tubes affected/bounding tubes

@TSC discovered by SSI in outer periphery Perform SSI if +Point tubes at 3R21 Object removed confirms from SG by FOSAR

+Point confirmed array Inspect PLP with +Point PLP Array PLP R57-C65 caused by soft, Perform array/+Point on No wear detected on D 22 @TSC non-metallic affected/bounding tubes affected/bounding tubes

+0.38" objects in periphery tubes Perform SSI if +Point Object removed confirms from SG by FOSAR Lancing Strainer Pieces of scale and D 23 TTS N/A Flexitallic gasket Parts at 3R21

• During 3R21, visual inspections of the feedring, J-nozzles, and feedring supports were completed in all four SGs. External inspections focused on weld integrity, structural orientation, and any unexpected flow patterns. All external surfaces appeared structurally sound and in proper working order with no apparent degradation observed. J-nozzle overspray was observed on some of the riser barrels. The overspray appeared as a discoloration of the riser barrel surface with no significant material loss noted. Internal inspections were performed on all the J-nozzles in each SG by visually inspecting the interface between the feedring and the J-nozzle. All nozzles appeared structurally sound with no significant FAG observed.

• During 3R21, specific components associated with the feedring (tees, reducers, elbows, piping, etc.) in SGs B and D were ultrasonically (UT) examined. The UT inspection results revealed no signs of significant wall loss or degradation.

The results of all secondary-side visual examinations performed were satisfactory, with no degradation detected.

Serial No. 22-27 4 Docket No. 50-423 Attachment 1, Page 6 of 25

b. Degradation mechanisms found, The existing degradation mechanisms found during 3R21 included AVB wear, TSP wear, non-support structure volumetric degradation (legacy foreign object wear where the object has been removed, fabrication and maintenance related wear). Corrosion-related degradation, located above the H* distance, was also detected for the first time in the MP3 steam generators.

c. Nondestructive examination techniques utilized for each degradation mechanism, Table 2 identifies the examination techniques used for each identified degradation mechanism.

Table 2: Examination Techniques for Degradation Mechanisms Detected Degradation Mechanism Technique Used AVB Wear Bobbin Detection and Sizing Tube Support Plate Wear Bobbin Detection and +Point Sizinq Volumetric Including Foreign Object Wear Bobbin/Array Detection and +Point Sizinq Top-of-Tubesheet ODSCC Arrav/+Point Detection and +Point Sizinq

d. Location, orientation (if linear), and measured sizes (if available) of service induced indications, A total of 88 Non-AVB Volumetric Wear indications were recorded during the examination, with 57 of those recording wall losses of < 20%. The 31 indications that recorded wall losses of ~20% are listed in Table 3. There was a total of 736 AVB wear indications reported in all SGs with 589 of those sized at< 20% through wall. The 147 indications with wall losses of ~20% are listed in Table 4.

Table 3: Summary of Non-AVB Wear Volumetric Degradation ~0%

Max Circ Depth Axial Length Length SG Row Col Volts Location (3/4TW) (in) (in) Cause SGA 2 109 0.25 06 -0.59 23 0.32 0.29 TSP Wear SGA 3 112 0.29 06C -0.99 25 0.32 0.33 TSP Wear Foreign SGA 7 3 0.22 TSC +0.13 25 0.32 0.39 Object Wear SGA 15 68 0.26 07C -0.75 23 0.32 0.33 TSP Wear SGA 15 74 0.44 07(-0.57 36 0.27 0.25 TSP Wear SGA 23 76 0.25 03C +0.32 22 0.16 0.42 TSP Wear Foreign SGA 24 7 0.22 04H +3.63 25 0.16 0.33 Object Wear Foreign SGA 28 112 0.41 01H +0.51 37 0.35 0.37 Object Wear

Serial No.22-274 Docket No. 50-423 Attachment 1, Page 7 of 25 Max Circ Depth Axial Length Length SG Row Col Volts Location (%TW) (in) (in) Cause SGA 35 71 0.26 08(-0.93 21 0.24 0.33 TSP Wear Foreign SGA 47 24 0.27 01C +0.91 28 0.27 0.35 Object Wear Foreign 0.2 01C+0.93 23 0.27 0.33 Object Wear SGA 47 25 Foreign 0.3 01C +1.15 30 0.27 0.33 Object Wear Foreign SGA 59 60 0.21 08H -1.65 24 0.27 0.33 Object Wear Foreign SGB 1 119 0.18 TSC + 4.03 21 0.27 0.37 Object SGB 44 98 0.45 02H + 15.76 29 0.24 0.33 Fabrication Foreign SGC 8 61 0.23 TSH +0.45 25 0.27 0.33 Object Wear SGC 17 52 0.59 04H -0.35 33 0.24 0.31 TSP Wear SGC 17 70 1.21 06H -0.56 45 0.27 0.29 TSP Wear SGC 35 97 0.21 05H -.067 20 0.27 0.37 TSP Wear SGC 48 88 0.46 07C -0.88 27 0.27 0.42 TSP Wear Foreign SGC 52 77 0.26 TSC +0.08 28 0.27 0.29 Object Wear Foreign SGC 52 79 0.31 TSC +0.08 31 0.32 0.39 Object Wear Foreign SGC 54 64 0.21 TSH +0.13 24 0.16 0.42 Object Wear SGC 56 65 0.25 06H +13.36 24 0.32 0.29 Fabrication Foreign SGC 56 69 0.25 TSH +0.11 27 0.21 0.33 Object Wear SGD 17 24 0.68 07(-064 36 0.29 0.33 TSP Wear SGD 35 73 0.39 04H - 0.43 26 0.27 0.31 TSP Wear Foreign SGD 46 99 0.21 01H + 0.59 24 0.21 0.33 Object Foreign SGD 52 42 0.22 01H + 0.56 25 0.32 0.27 Object Foreign SGD 52 91 0.33 01H + 2.48 32 0.32 0.29 Object Foreign SGD 58 51 0.21 01C + 0.59 24 0.27 0.35 Object

Serial No. 22-27 4 Docket No. 50-423 Attachment 1, Page 8 of 25 Table 4: Millstone 3 EOC21 Inspection Summary - AVB Wear Indications ~20% TW Amplitude Wear Depth SG Row Col AVB No.

(Volts) (3/4TW)

A 26 115 AVl 1.2 27 A 28 115 AVl 1.49 33 A 30 9 AV5 1.46 30 A 30 113 AV5 0.95 24 A 34 46 AV5 1.32 29 A 34 46 AV6 1.63 32 A 34 48 AV3 1.16 27 A 34 73 AV4 1.67 33 A 34 73 AV5 1.79 35 A 34 109 AV4 0.9 23 A 35 59 AV2 1.25 28 A 35 60 AV4 1.4 30 A 35 60 AV5 1.13 26 A 35 71 AV4 1.26 28 A 35 77 AV3 0.69 20 A 37 45 AV2 1.88 35 A 37 45 AV3 1.06 25 A 37 69 AV5 0.77 20 A 37 69 AV6 0.91 23 A 37 90 AV3 0.94 24 A 37 91 AV5 1.06 25 A 37 106 AV4 0.77 20 A 38 52 AV3 1.02 25 A 39 57 AV2 1.46 31 A 39 57 AV3 0.82 22 A 39 57 AV4 0.96 24 A 39 60 AV4 0.97 24 A 39 71 AV5 0.83 21 A 40 45 AV4 1.29 28 A 40 71 AV3 0.87 22 A 40 71 AV4 1.12 26 A 40 94 AV5 0.53 20

Serial No. 22-27 4 Docket No. 50-423 Attachment 1, Page 9 of 25 Amplitude Wear Depth SG Row Col AVB No.

(Volts) (%TW)

A 41 61 AV4 0.97 24 A 41 102 AV4 2.01 37 A 42 33 AV3 0.75 20 A 42 53 AV4 1.12 26 A 42 53 AV5 2.18 37 A 42 77 AV4 1.05 26 A 42 93 AV4 1.05 25 A 42 93 AV5 0.96 24 A 42 98 AV3 1.15 27 A 42 98 AV4 1.37 30 A 42 101 AV3 1.01 25 A 42 101 AV4 1.5 31 A 42 101 AV5 0.75 20 A 42 102 AV3 2.21 38 A 42 102 AV4 2.49 40 A 42 102 AV5 1.02 25 A 43 87 AV4 0.77 21 A 43 87 AV5 0.9 23 A 43 102 AV4 0.78 21 A 44 64 AV2 1.29 29 A 44 64 AV3 0.72 20 A 44 74 AV5 1.29 28 A 44 74 AV6 0.78 21 A 44 98 AV2 0.88 23 A 45 98 AV4 0.78 21 A 45 99 AV4 0.72 20 A 46 99 AV4 0.79 21 A 46 99 AV5 0.77 21 A 49 96 AV6 0.84 23 A 50 44 AV5 1.82 34 A 50 50 AV4 0.74 20 A 50 76 AV2 1.11 27 A 50 76 AV3 0.99 25 A 50 76 AV4 1.62 33

Serial No.22-274 Docket No. 50-423 Attachment 1, Page 10 of 25 Amplitude Wear Depth SG Row Col - AVB No.

(3/4TW)

(Volts)

A 50 82 AV3 1.4 31 A 50 82 AV4 0.81 22 A 50 87 AV2 0.98 24 A 50 87 AV3 0.75 20 A 51 65 AV5 0.86 23 A 52 66 AV4 1.17 27 A 53 81 AV3 1.38 29 B 33 109 AV3 0.79 20 B 34 109 AV3 0.85 22 B 38 104 AV4 1.26 27 B 41 34 AV4 1.73 33 B 41 34 AV5 1.65 32 B 41 50 AV4 1.02 24 B 41 69 AV5 1.72 31 B 42 21 AV5 1.3 26 B 42 98 AV2 1.63 32 B 42 98 AV3 2.43 39 B 43 100 AV3 1.2 25 B 43 100 AV4 1.36 28 B 50 88 AV3 0.92 20 B 50 88 AV4 1.07 22 B 54 36 AV5 1.78 34 C 25 116 AV6 0.73 21 C 37 15 AV2 0.89 24 C 37 15 AV5 1.09 28 C 39 17 AV2 0.72 21 C 39 17 AV4 0.83 23 C 39 17 AV5 0.69 21 C 41 42 AV3 1.12 27 C 41 54 AV3 0.7 20 C 41 54 AV5 0.93 25 C 41 62 AV2 0.99 26 C 41 62 AV3 0.71 22

Serial No. 22-27 4 Docket No. 50-423 Attachment 1, Page 11 of 25 Amplitude Wear Depth SG Row Col AVB No.

(Volts) (%TW)

C 41 62 AV3 1.43 32 C 41 62 AV4 0.77 22 C 41 62 AV4 1.58 34 C 41 62 AV5 1.64 34 C 41 65 AV4 0.63 20 C 41 65 AV5 0.83 24 C 42 20 AV3 0.93 24 C 42 20 AV4 1.09 26 C 42 20 AV5 0.97 25 C 42 23 AV3 0.77 21 C 42 23 AV4 1.45 31 C 42 23 AV5 1.22 28 C 49 96 AV5 0.91 25 C 49 96 AV6 1.07 27 C 50 93 AV6 1.29 27 C 56 41 AV5 1.03 25 D 24 117 AV1 0.76 21 D 28 114 AV2 1.2 28 D 30 114 AV6 0.73 21 D 37 106 AV3 0.71 20 D 37 106 AV4 1.59 33 D 37 106 AV5 0.9 23 D 40 99 AV4 1.01 26 D 40 99 AV5 1.5 32 D 40 102 AV4 0.9 24 D 40 103 AV2 0.75 21 D 40 103 AV3 1.07 27 D 40 103 AV4 1.91 36 D 40 103 AV5 1.24 29 D 41 26 AV5 1.48 31 D 41 26 AV6 1.54 32 D 41 39 AV3 0.81 22 D 41 39 AV4 0.85 22

Serial No. 22-27 4 Docket No. 50-423 Attachment 1, Page 12 of 25 Amplitude Wear Depth SG Row Col AVB No.

(Volts) (%TW)

D 41 39 AV5 1.2 28 D 41 78 AV3 0.85 23 D 41 78 AV4 0.73 21 D 49 62 AV1 0.92 23 D 49 62 AV2 0.87 22 D 49 62 AV3 1.52 31 D 49 66 AV2 1.6 33 D 49 67 AV2 0.95 25 D 49 67 AV3 0.82 22 D 49 95 AV2 0.71 20 D 49 95 AV3 0.73 20 D 49 95 AV4 0.67 20 D 52 67 AV4 0.81 22 D 54 49 AV3 0.83 23 D 55 84 AV5 0.91 24 During 3R21, corrosion-related degradation, above the H* distance, was detected for the first time in the MP3 steam generators. This single indication was detected on the hot-leg side of SGA in tube 10-63 at TSH +0.32". The indication was detected with the array probe (as part of the 100% TTS examination performed in each SG). The indication was confirmed using the +Point' probe and characterized as circumferentially oriented ODSCC degradation. The indication measured 10%TW with a circumferential length of 0.42 inches.

During 3R21, tube 10-63 in SGA was stabilized (using the shorter TTS stabilizer) and plugged on both ends.

Table 5: Summary of 3R21 Corrosion Related Degradation Location SGA, Tube 10-63 at TSH + 0.32" Orientation Circumferential ODSCC Amplitude (300 kHz +Point'} 0.36 Volts Maximum NOE Depth 10%TW Circumferential Extent 59 Degrees, 0.42 inches Percent Degraded Area (PDA} 0.59

e. Number of tubes plugged during the inspection outage for each degradation mechanism, Based on inspection results, four (4) tubes were plugged during the 3R21 outage.

Serial No.22-274 Docket No. 50-423 Attachment 1, Page 13 of 25 Table 6: Summary of 3R21 Plugging SIG Row Col Hot-Leg Cold-Leg Degradation Mechanism SG-A 10 63 Plugged/Stabilized Plugged ODSCC SG-A 42 102 Plugged Plugged/Stabilized AVB Wear SG-B 42 98 Plugged/Stabilized Plugged AVB Wear SG-C 17 70 Plugged Plugged TSP Wear

f. The number and percentage of tubes plugged to date and the effective plugging percentage in each steam generator.

Table 9 provides the total number of tubes plugged to date and the effective plugging percentage in each SG.

Table 7: Number Tubes Plugged to Date SGA SG B SGC SGD Prior to 3R21 51 25 22 91 During 3R21 2 1 1 0 Total After 3R21 53 26 23 91 Percentage 0.942 0.462 0.409 1.617 Overall Percentage 0.857 Since no sleeving has been performed in the MPS3 steam generators, the effective plugging percentage is the same as the actual plugging percentage.

g. The results of condition monitoring, including the results of tube pulls and in-situ

testing, No tubes were pulled, and no in-situ pressure tests were performed. The condition monitoring assessment concluded that the structural integrity, operational leakage, and accident induced leakage performance criteria were not exceeded during the operating interval preceding 3R21.

To perform the CM assessment for AVB wear, a CM limit curve was developed using the EPRI Flaw Handbook Calculator. Since the circumferential extent of AVB wear is <135°, it is appropriate to use the EPRI Flaw Handbook "Part-Through wall Axial Volumetric Degradation" flaw model to evaluate the CM limit. The Flaw Handbook Calculator uses Monte Carlo methods to model the various uncertainties. The CM limit includes material property uncertainties, model uncertainties, and NOE depth sizing uncertainties to establish a curve of allowable NOE measured depths (i.e., the CM limit curve).

Using the input parameters summarized in the DA, the 95/50 CM limit curve of Figure 1 was developed. This limit curve is applicable to the AVB wear indications reported at MP3 during 3R21.

Serial No. 22-27 4 Docket No. 50-423 Attachment 1, Page 14 of 25 On Figure 1, the depths of all AVB wear reported during 3R21 are plotted with the assumption that the axial extent of the wear was 2.5 inches. This is a very conservative assumption representative of the upper limit of near-parallel contact between the AVBs and tubes. It should also be noted that there is no significant reduction in the depth limit curve from 2.5 inches to 3 inches and beyond. Since all the reported AVB wear depths (including the maximum, 40% TW), lie below the CM Limit Curve, the CM requirements for structural integrity are satisfied for AVB wear.

To perform the CM assessment for TSP and FOB wear, a CM limit curve was developed using the EPRI Flaw Handbook Calculator. Monte Carlo methods were used to incorporate material property uncertainties, model uncertainties and NOE depth sizing uncertainties to establish a curve of allowable NOE measured depths. Because of eddy current coil "look-ahead," the +Point' probe overestimates the length of degradation. Therefore, the lengths as-measured are upper-bound estimates of the actual flaw length, and no additional allowance for length sizing uncertainty is included in the CM curve.

Using the input parameters summarized in the DA, together with the EPRI Flaw Handbook

"<135° Part-Through-wall Axial Volumetric Degradation" model, the 95/50 CM limit curve of Figure 2 was developed. This limit curve is applicable to the TSP wear indications (including any baffle plate wear) reported at MP3 during 3R21.

On Figure 2, the depths of all TSP and FOB wear reported in each of the four SGs during 3R21 are plotted using the +Point' measured axial lengths and depths. Since all the reported TSP/FOB wear depths lie below the CM Limit Curve, the CM requirements for structural integrity are satisfied for TSP and FOB wear at 3R21.

Serial No. 22-27 4 Docket No. 50-423 Attachment 1, Page 15 of 25 Figure 1: CM Limit Curve for AVB Wear CM Lim i t Curve for AVB Wea r ETSS 96041. 3 R:l 70

.. CMLimit SGA

. SG O

~GC G)

• SG O s,

,JJ 3J i;i 2,

I~

() () _\ l ~~ 7.!, * .!,

Figure 2: CM Limit Curve for TSP and FDB Wear CM Limit Curve for TSP/FDB W ea r ETSS 96910.1 100.0

_.J, CM Limit 90.0

• 1SGA

• SG B 80.0 SGC

• SG D 70.0 60.0 50.0 40.0 30.0 20.0 I I I

• 10.0 I -

t 0 .000 0 .200 0 .400 LOCO 1.200 1.400 0 .600 0 .800

Serial No.22-274 Docket No. 50-423 Attachment 1, Page 16 of 25 Volumetric degradation caused by foreign object wear and fabrication processes is evaluated in this section. All these indications (54 total) are identified in Figure 3 through Figure 5 along with the ETSSs that were used to measure each indication. A unique CM limit curve was developed to capture the sizing uncertainty for each unique ETSS used . Each CM limit curve in this section is based on use of the EPRI Flaw Handbook "<135° Part-Through-wall Axial Volumetric Degradation" model.

• 4 of the volumetric flaws (fabrication indication) were sized using ETSS 21998.1. Since the flaws lie below the 95/50 CM limit curve illustrated in Figure 3, the CM requirements for structural integrity are satisfied.

• 48 of the volumetric flaws were sized as foreign object wear using ETSS 27901.1. Since the flaws lie below the 95/50 CM limit curve illustrated in Figure 4, the CM requirements for structural integrity are satisfied.

• 2 of the volumetric flaws were sized as foreign object wear using ETSS 27902.1. Since the flaws lie below the 95/50 CM limit curve illustrated in Figure 5, the CM requirements for structural integrity are satisfied.

Figure 3: CM Limit Curve for Volumetric Flaw Using ETSS 21998.1 CM Li m it Cu rve for Vo l Ind ica ti ons ETSS 21998. 1 100 i'" J ~

CMLimit 5G B 80 5GC I 70 I

I 60 I so 30 20 I

I*

o~---~---~-------------------~

0.2 0.4 0.6 0.8 1.2 1.4

Serial No. 22-27 4 Docket No. 50-423 Attachment 1, Page 17 of 25 Figure 4: CM Limit Curve for Foreign Object Wear Flaws Using ETSS 27901.1 CM Limi t Curve for Foreign Obj ect Wear ETSS 27901.1 100

-+CMUm;t j

90

, SGA

• SG B 80 70

I::

I 60 50

• o

• I f

-- 1 I

I

~1 30

• • I l 20 I . .I 1=

10  : ** I* :

1.

0 0 0.2 o.* 0.6 0.8 1.2 1.*

Figure 5: CM Limit Curve for Foreign Object Wear Flaw Sized with ETSS 27902.1 CM Lim it Cu rve for Foreign Object Wear ETSS 27902.1

......,___ CM Limi t SGA I

, I SGC 60 50

-1 I 40 -

i 30 20 l I 0.2 0.4 0.6 0.8 1.2 1.4

Serial No.22-274 Docket No. 50-423 Attachment 1, Page 18 of 25 A single circumferential ODSCC indication was identified, during the 100% TSH array probe examination, on the hot-leg in SGA, tube 10-63 at TSH +0.32". The indication was confirmed using the +Point' probe and characterized as circumferential ODSCC. A review of the prior inspection data was inconclusive in determining if the flaw was present in history, as the indication's response was influenced by adjacent hard sludge deposits.

During 3R21, since the array probe signal exhibited rotation towards the flaw-plane, the indication was reported as circumferential ODSCC.

The indication's +Point' voltage measured 0.36 Volts. Using ETSS 21410.1, the indication measured 10%TW with a circumferential extent of 59 degrees (0 .42 inches). A very conservative estimate of the indication's actual PDA of 0.59 was taken to be 2.0 for evaluation purposes. Applying the ETSS uncertainty parameters to determine the upper 95/50 PDA value yields the following:

ETSS 21410.1 PDA uncertainty parameters

• Slope = 1.05

• Intercept = -0.011

• StdErr = 13.43 Upper 95/50 = (EOC21 PDA) *(Slope)+ (Intercept)+ (1.645) *(StdErr)

Upper 95/50 = (2.0) *(1.05) + (-0.011) + (1.645) *(13.43)

Upper 95/50 = 24.2 PDA Since the upper 95/50 PDA (24.2 PDA) does not exceed the conservative EOC structural limit of 77 PDA calculated using the EPRI Flaw Handbook Calculator, it is concluded that the circumferential ODSCC indication satisfied the SIPC.

To assess this indication against the AILPC, the upper 95/50 maximum through wall depth must be compared to the pop-through depth. The ETSS 21410.1 depth sizing uncertainty is first applied to the determine the upper 95/50 depth as follows:

ETSS 21410.1 Depth uncertainty parameters

• Slope= 1.0

• Intercept = 0.0

• StdErr = 13.8 Upper 95/50 = (EOC21 Depth) *(Slope)+ (Intercept)+ (1.645) *(StdErr)

Upper 95/50 = (10) *(1.0) + (0.0) + (1.645) *(13.8)

Upper 95/50 = 32. 7 %TW The pop-through depth of a circumferential crack of up to 300 degrees is greater than 90% TW. Therefore, it is concluded that the circumferential ODSCC indication would not have leaked during a limiting accident, thus satisfying the AILPC.

Serial No. 22-27 4 Docket No. 50-423 Attachment 1, Page 19 of 25 In summary, both condition monitoring structural and leakage integrity have been demonstrated for the circumferential ODSCC indication located in SGA, tube 10-63, at outage 3R21.

It is a requirement of the IAGL that current inspection results be used to validate prior operational assessment (OA) inputs and assumptions. This ensures that any adjustments that may be necessary going forward are incorporated into the current outage OA. A review of the most recent OA for each steam generator (3R20 OA Deferral for SGs A/C and 3R19 OA for SGs B/D) was performed and the key inputs and assumptions were compared with the results from the current outage (3R21 ). This review, summarized in Table 8, shows that the as-found conditions were bounded by the projections from the previous OA for all cases with exception of the SG-B upper 95/50 AVB wear growth rate. Specifically, the SG-B upper 95/50 AVB wear growth rate (at 3R21) measured 1.23 %TW/EFPY compared to the 3R19 projected growth rate of 1.1 %TW/EFPY.

This comparison provides the basis for concluding that the prior OA assumptions were appropriate with exception of the higher than predicted SG-B upper 95/50 AVB wear growth.

To correct this issue, the largest upper 95/50 AVB wear growth rate (from all four SGs) should have been used instead of the largest growth rate from the two SGs examined at 3R19 (SGs B/D). Going forward (as performed in this OA), the growth rates in all four SGs were evaluated, and the SG exhibiting the largest upper 95/50 AVB wear growth rate was used to project structural integrity, for AVB wear, in all SGs.

Table 8: Summary of Prior QA Validation Assumed or calculated in the Prior inspection Observed during the 2020 OA Deferral (SG A/C) or projection 3R21 Outage the 3R19 OA (SG BID) satisfied 95/50 growth rate of 1.519 SG NC: 95/50 = 2.2 %TW/EFPY Yes 3/4TW/EFPY AVB Wear 95/50 growth rate of 1.23 SG BID: 95/50 = 1.1 %TW/EFPY No 3/4TW/EFPY SG NC: Maximum growth rate of Maximum growth rate of 2.2 Yes 5.5 %TW/EFPY 3/4TW/EFPY TSP/FDB Wear SG B/D: Maximum growth rate of Maximum growth rate of 3.7 Yes 5.0 %TW/EFPY 3/4TW/EFPY Minimum observed margin between SG NC: Minimum projected margin 3R18 depth and 61%TW structural between EOC depth and the 95/50 Yes depth (axial length 2.5 inch) is structural limit is 4.6 %TW TSP/FDB 16%TW Wear Minimum observed margin between SG B/D: Minimum projected margin 3R19 depth and 61%TW structural between EOC depth and the 95/50 Yes depth (axial length 2.5 inch) is structural limit is 6.5 %TW 25%TW

Serial No.22-274 Docket No. 50-423 Attachment 1, Page 20 of 25 No growth over the operating period from 3R18 to 3R21 (SG A/C) or No growth observed in non-support from 3R 19 to 3R21 (SG B/D) for volumetric degradation previously Non- repeat degradation left in-service. left in service.

Support Yes Volumetric If growth occurs or new degradation A few flaws were newly reported, Degradation develops, it will not exceed but none exceeded structural structural performance criteria prior performance criteria.

to 3R21.

Operational No measurable leakage in any SG Projected: 0 gpm Yes Leakage since the last ISi SG A/C: 4.2 EFPY from 3R18 to 4.081 EFPY Yes 3R21 EFPY SG B/D: 3.0 EFPY from 3R19 to 2.682 EFPY Yes 3R21 3(NOPD) 3 * (1340) = 4020 psid Bounding 3960 psid Yes The Operational Assessment must demonstrate that the three performance criteria will not be exceeded prior to the next scheduled examination in each steam generator. Consistent with the Dominion Fleet Administrative Procedure for performing CMOAs, this operational assessment addresses degradation mechanisms known to exist in the MP3 steam generators: {AVB wear, TSP/FOB wear, volumetric degradation (from foreign object wear or fabrication/maintenance damage) and TTS circumferential ODSCC}, together with the impact of these mechanisms over the upcoming operating interval. Since all four SGs were examined during the 3R21 outage (spring 2022), the next scheduled tube inspection (in all SGs) is planned for 3R22; resulting in a single fuel cycle operating interval of assumed 1.5 EFPY, a conservative measure considering that 1.4 EFPY per cycle is typically bounding for MP3.

Table 9: OA for Structure Wear Degradation Mechanism Maximum Depth(%) Structural Limit Depth(%)

Predicted at 3R22 AVB Wear 51 61 TSP/FDB Wear 66.5 69.6

Serial No.22-274 Docket No. 50-423 Attachment 1, Page 21 of 25 Figure 6: 3R22 Worst Case Burst Pressure - Circumferential ODSCC @TTS 0.20 ~ - - - - -- - - - - - - -- - - - - -- ~ - - -- --

I I

I I

I I

---* 3R22 Min BP I I

I 0.15 I

~ I I

I

c I

.c I'

~

I n.

a, 0. 10 I

.<!: I 1;j I

i Lower 95/50: 8434 psi  :

E I I

, I '

u

,:I '

0.05 1 3XNOPD; 4020 psi -------/---- Probability: 0.05 I

I I , / ,1

, I

,;~/ :

I I

I I

I

______ ,,~ ~

0.00 + - - ---1-- - - ~ -- --.-- --  ;,.=::.c....__ ~ - L . . - - ~ - - ~ - - - 1 3,000 4,000 5,000 6,000 7,000 8,000 9,000 10,000 11 ,000 Projected 3R22 Worst Case Burst Pressure (psi)

Table 10: Probabilities of Burst and Leakage at (3*NOPD)

Circumferential ODSCC @TTS Population POB POL Full Bundle < 0.002% 0.074%

SG Program Maximum 5% 5%

Allowable

Serial No. 22-27 4 Docket No. 50-423 Attachment 1, Page 22 of 25 Figure 7: 3R22 Worst Case Burst Pressure - Axial ODSCC @TSPs 0.25 ~ - - - - - - - - - - - - - - - - - - - - - - -~ - - - - - ,

,I I

I I

I I

---

3R22 Min BP /

0.20 I I

I I

I

cCV I I

I

..Q 0.15 I I

I

~ I I

0.

., I I

~ ,'

I CV

i 0.10 u

E Lower 95/50: 6476 psi 1 I ,'

I ,'

3XNOPD: 4020 psi _________!,/ ~--- p b b'l't o 05 0.05 l ,,*j- ro a 11 y: .

I ,.-- I I --~ I

~---- I I --~--*- I 0.00 +-----------._,,-,_-"ll'

, .=--=---'--- -----~ - - - ~ -----*-~---~-------1 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000 Projected JR22 Worst case Burst Pressure (psi)

Table 11: Probabilities of Burst and Leakage at (3*NOPD)

Axial ODSCC @TSPs Population POB at 3Af> POL Full Bundle 0.238% 0.12%

SG Program Maximum 5% 5%

Allowable

Serial No. 22-27 4 Docket No. 50-423 Attachment 1, Page 23 of 25 Figure 8: 3R22 Worst Case Burst Pressure -Axial ODSCC @Dents/Dings 0.20 - r - - - - - - - - - - - - - - - - - - - - - - - - - - -.-, I I

I I

I I

I

---

3R22 Min BP I I

0.15 I

I I

I

~

I I

c ,,

I (0

.c a.. ,,

I 11> 0.10 Lower 95/50: 9851 psi 1 /

.2:

jg 'I ,:

I E

I

'I ,{

u I ,'

0.05 1 3XNOPD: 4020 psi Probability: 0.05 ------/-----

I / I I

I / I I ,, I I ,, I I ~,,' I I _____ ,_, I I

0.00 +------- - ~ - - - ~- - - - ---=='---~---.&.~---1 3,000 4,000 5,000 6,000 7,000 8,000 9,000 10,000 11,000 Projected 3R22 Worst Case Burst Pressure (psi)

Table 12: Probabilities of Burst and Leakage at (3*NOPD)

Axial ODSCC @Dents/Dings Population POB at 3LlP POL Full Bundle < 0.002% 0.0%

SG Program Maximum 5% 5%

Allowable

h. The primary to secondary LEAKAGE rate observed in each SG (if it is not practical to assign the LEAKAGE to an individual SG, the entire primary to secondary LEAKAGE should be conservatively assumed to be from one SG) during the cycle preceding the inspection which is the subject of the report, No primary to secondary SG leakage was reported during Cycle 21.

i. The calculated accident induced leakage rate from the portion of the tubes below 15.2 inches from the top of the tubesheet for the most limiting accident in the most limiting SG. In addition, if the calculated accident induced leakage rate from the most limiting accident is less than 2.49 times the maximum operational primary to secondary leakage rate, the report should describe how it was determined;

Serial No.22-274 Docket No. 50-423 Attachment 1, Page 24 of 25 For the purposes of the condition monitoring assessment, and in accordance with the permanent alternate repair criteria, the accident leakage attributed to degradation within the tubesheet below the H* dimension must be estimated by applying a factor of 2.49 to the operational leakage. There was no recordable operational leakage during Cycle 21; therefore, the leakage from this degradation during a limiting accident would have been zero (i.e., 2.49 X 0).

j. The results of monitoring for tube axial displacement (slippage). If slippage is discovered, the implications of the discovery and corrective action shall be provided.

Tube slippage monitoring was performed on all four SGs using the bobbin coil data during 3R21. There was no detection of slippage during the 3R21 examination.

Serial No.22-274 Docket No. 50-423 Attachment 1, Page 25 of 25 Figure - 9 Model F General Support Configuration AV6 1

Positive direction 08H - F4======;=:~=========* ...-- - * - Location of OSC SlgnConvlintion Change ot sign cotwentfon (OSC + 2,00"')

for reporting 0711 - ~=:z:s=:11111\111:111111:iii--===ii=..---llllllllflt - 07C indications f

Positive direction ohlgl'I conventfoo 04H *- for reporting tndkatlons

~ "' .

03H

- - 03C 02H

- W=:::::=~==;::===::=====-====;:,1- - 02C on, TSH

--  !=::;=======~=-===-==-=:a..;M - 01C TEH - -

- TSC TEC J,CI.

Serial No.22-274 Docket No. 50-423 Attachment 2 Acronyms MILLSTONE POWER STATION UNIT 3 DOMINION ENERGY NUCLEAR CONNECTICUT, INC. (DNC)

Serial No.22-274 Docket No. 50-423 Attachment 2, Page 1 of 1 Acronyms AVB Anti-Vibration Bar OVR Above Tubesheet Over Expansion BET Bottom of the Expansion Transition OXP Over Expansion BLG Bulge PID Positive Identification C Column PLG Tube is plugged CL Cold Leg PLP Possible Loose Part DDH Ding or Dent Signal - Reviewed in PTE Partial Tubesheet Expansion History PWR Pressurized Water Reactor DDI Distorted Dent or Ding Indication PWSCC Primary Water Stress Corrosion DDS Ding or Dent Signal - Non- Cracking Confirming w/RPC R Row DNG Ding RAD Retest Analyst Discretion DNT Dent Indication RBD Retest - Bad Data ECT Eddy Current Test RIC Retest - Incomplete EFPY Effective Full Power Years RRT Retest - Restricted Tube EPRI Electric Power Research Institute S/N Signal-to-Noise Ratio ETSS Examination Technique SAI Single Axial Indication Specification Sheet SCC Stress Corrosion Cracking F/L Full Length SCI Single Circumferential Indication FAC Flow Accelerated Corrosion SG Steam Generator FDB Flow Distribution Baffle SLG Sludge FO Foreign Object SSI Secondary Side Inspection FOTS Foreign Object Tracking System SVI Single Volumetric Indication HL Hot Leg TEC Tube End Cold Leg IGA Intergranular Attack TEH Tube End Hot Leg INF Indication Not Found TFH Tangential Flaw-Like Signal - Reviewed INR Indication Not Reportable in History LPI Loose Part Indication TFS Tangential Flaw-Like Signal - Non-LPR Loose Part Removed Confirming w/RPC LPS Loose Part Signal TSC Top of Tubesheet Cold Leg MRPC Motorized Rotating Pancake Coil TSH Top of Tubesheet Hot Leg NDD No Detectable Degradation TSP Tube Support Plate NDE Nondestructive Examination TTS Top of Tubesheet NDF No Degradation Found TWD Through-Wall Depth NEI Nuclear Energy Institute  % TW Percent Through-Wall NQH Non-quantifiable Indication - VOL Volumetric Indication Reviewed in History NQI Non-quantifiable Indication OA Operational Assessment ODSCC Outer Diameter Stress Corrosion Cracking