nrc / epri steam generator task force meetingdetection of sg tube degradation using single or...
TRANSCRIPT
NRC / EPRI Steam Generator Task
Force Meeting
February 19, 2014
2 © 2014 Electric Power Research Institute, Inc. All rights reserved.
Agenda
8:30 am Introductions NRC and Industry
Opening Remarks NRC and Industry
EPRI SGMP Steam Generator Task Force Update (Industry)
• Open Technical Issues
• Noise Monitoring
• General Discussion Items
• Divider Plate Crack Propagation
• Tube-to-Tubesheet Weld Cracking
• Guidance for Auto Analysis
• Tube-to-Tube Wear Amplitude vs Depth
• Probe Variability Experiment Results
• Summary of Recent EPRI SGMP Reports
10:00 am Break
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Agenda
10:20 am Standing Issues (Industry)
• Status of Industry Guidelines
• NEI 03-08 Deviations
• Operating Experience
11:20 am NRC Discussion/Items of Interest (NRC)
• Steam Generator Requirements in ASME Code
• TSTF-510 Implementation
• New Issues
11:45 am Address Public Questions/Comments (NRC)
12:00 pm Adjourn
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Acronyms
• ARC – Alternate Repair Criteria
• ASME – American Society of Mechanical Engineers
• AVB – Anti Vibration Bar
• CDS – Computer Data Screening
• EPRI – Electric Power Research Institute
• ESW – Electroslag Welding
• ETSS – Examination Technique Specification Sheet
• FDB – Flow Distribution Baffle
• FM – Weld Consumable – Filler Metal
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Acronyms
• GTAW – Gas Tungsten Arc Welding
• IG – Interim Guidance
• IGA – Intergranular Attack
• IGSCC – Intergranular Stress Corrosion Cracking
• LLMC – Long Life Magnetic Coil
• LTMA – Low Temperature Mill Annealed
• MA – Mill Annealed
• MULC – Magnetic U-Bend Long Cone
• NDD – No Detectable Degradation
• NDE – Non Destructive Examination
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Acronyms
• NEI – Nuclear Energy Institute
• NRC – Nuclear Regulatory Commission
• ODSCC – Outside Diameter Stress Corrosion Cracking
• OTSG – Once Through Steam Generator
• POD – Probability of Detection
• PSI – Preservice Inspection
• PWR – Pressurized Water Reactor
• PWSCC – Primary Water Stress Corrosion Cracking
• RPC – Rotating Probe Coil
• RSG – Replacement Steam Generator
• SEM – Scanning Electron Microscope
• SG – Steam Generator
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Acronyms
• SGMP – Steam Generator Management Program
• SGTF – Steam Generator Task Force
• SMAW – Shielded Metal Arc Welding
• TSP – Tube Support Plate
• TSTF – Technical Specification Task Force
• TT – Thermally Treated
• TTS – Top of Tubesheet
• TTW – Tube-to-Tube Wear
• TW – Through wall
• U.S. – United States
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Open Technical Issues - SGTF
• Noise Monitoring
– Recommendations have been provided to the Examination
Guidelines Revision 8 Committee and the Integrity
Assessment Revision 4 Committee
• New appendix has been drafted for the Examination
Guidelines for noise monitoring
• Final drafts for industry review by summer 2014
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General Discussion Items
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Divider Plate Crack Propagation
Scenarios 1 and 2 assume cracks initiate in the divider plate
assembly and propagate to the low allow steel
Technical report, “Steam Generator Management Program: Flaw
Tolerance Evaluation of the Steam Generator Channel Head,”
3002000411, published April 2013
Conclusion: The structural integrity of the SG channel head would
not be compromised by a crack in the divider plate
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Scenario 3 assumes cracks initiate in the divider plate assembly
and propagate to the tube-to-tubesheet weld
Scenarios 4 and 5 assume cracks initiate in the tube-to-tubesheet
weld or the cladding
Tube-to-Tubesheet Weld Cracking
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Tube-to-Tubesheet Weld Cracking
• Industry has accepted a 24% chromium content as a conservative threshold for PWSCC initiation
– Chromium levels down to 20% have excellent resistance to initiation based on testing and operational experience
• Objective of this project was to determine the chromium content in an autogenous weld between Alloy 690 tubing and 82 or 182 weld material
– Construct a tube-to-tubesheet mockup to investigate
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Field Material Review
• Review of chromium content in tube and tubesheet cladding material
• SGMP Alloy 690TT steam generator tubing specification recommends a
minimum chromium concentration of 28.5 weight percent
• 191 heats of Alloy 690 tubing material were reviewed and the minimum
concentration is 29 weight percent
– All three tubing suppliers were represented
• 17 heats of Alloy 82 were reviewed and the minimum chromium is 19.6
• 27 heats of Alloy 182 were reviewed and the minimum chromium is 13.6
Statistic
Alloy 690 (product analysis)
FM 82 FM 182
Mean 29.3 20.7 14.4
Maximum 29.7 21.5 16.5
Minimum 29.0 19.6 13.6
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Fabrication of the Mockup
%Cr is on the upper end for cladding but we are measuring dilution not
absolute composition
Material used for tube-to-tubesheet mockup
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Fabrication of the Mockup
82 - Electroslag Welding (ESW)
Strip cladding
182 Shielded Metal Arc Welding
(SMAW) process
SGMP developed prototypical
tube-to-tubesheet mockups to
analyze chromium levels in the
tube-to-tubesheet welds
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Analysis of the Chromium Content
• The mockup was designed as a 3 X 4 tube-to-tubesheet matrix having 82 cladding on one side and 182 cladding on the other side
• An Alloy 690 tube was inserted into each drilled cylinder, positioned flush with each end, and welded autogenously using the Liburdi Dimetric P-Head autogenous GTAW welder (see next slide).
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Analysis of the Chromium Content
• Plugs were removed from the tubesheet, cross sectioned, and weld dilution was evaluated
• It had been estimated that the weld dilution would be 50% from the tube material and 50% from the cladding material since the weld is autogenous GTAW
40 cross-sections
were available for 82
welds and 34 for 182
welds to include in
the tube-to-tubesheet
weld dilution
distributions
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Comparison of Mockups
• SGMP has a mockup from a
replacement steam generator
fabricator with Alloy 82 clad and 690
tube material
– Used to compare geometry of the fabricated mockup welds and to compare dilution
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Comparison of Mockups
Mockup from RSG Fabricator SGMP Mockup
Similarity between the etched weld cross-section in the SGMP mockup to the one
fabricated for this project
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Analysis of Chromium Content
• The EPRI SEM system was used to mark and measure areas of
features on sample cross-section macrostructures
– Measurements were used to compute the weld dilution
– Dilution measurements were analyzed to a “normal distribution”
Normal distribution for the Dilution Ratio
for 690 tubes welded in 82 cladding
Normal distribution for the Dilution Ratio for
690 tubes welded in 182 cladding
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Analysis of Chromium Content
• The combined dataset was used to determine the mean dilution factor
– Appropriate estimation of reality for Dilution Equation (below)
0
2
4
6
8
10
12
14
16
18
0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8
Fre
qu
en
cy
Dilution Ratio
µ=0.52
σ=0.08
%Crweld = %Crtube (0.52) + %Crclad(0.48)
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Conclusions
• An autogenous weld deposit for 690 tubing material and 82
cladding has sufficient chromium content to be resistant to
initiation or propagation of PWSCC (exceeds 24% Cr)
– These welds constitute the majority of the tubesheet
• The weld deposit for 690 tubing material and 182 cladding
has less chromium, but the chromium is high enough to be
resistant to initiation and slow propagation of PWSCC
Statistic % Cr in Weld
(82 clad) % Cr in Weld
(182 clad)
Mean 25.2 22.1
Maximum 25.8 23.2
Minimum 24.5 21.8
Predicted Cr Compositions
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Stress Analysis of the Tubesheet
• Finite element analysis of a Westinghouse Model 51 SG channel
head was used to determine the stresses during normal, upset,
and accident conditions
• Tensile loads are shown to be present in the divider plate region
– The dominant mechanical bending loads are in the plane that
drives a horizontal crack and would not turn the crack
vertically.
– Assuming a crack initiates, it would propagate horizontally
• The Alloy 182 cladding in the center of the tubesheet is the most
susceptible to PWSCC
– This area of concern is in compression
• Results and conclusions were independently verified by third
party
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Conclusions
• Stresses in the channel head region of the SG are not sufficient to propagate a crack through fatigue into the channel head such that it would compromise structural integrity of the channel head
• Chromium levels in the tube-to-tubesheet welds are sufficient to resist PWSCC initiation
• Orientation of the dominant stresses associated with the tubesheet to divider plate regions strongly favor horizontal crack propagation in the stub runner or stub runner to tubesheet weld
• The center of the tubesheet is in compression thus the driving force for PWSCC initiation/propagation is not present
• Final report will be published in 2014 to conclude the divider plate and tube-to-tubesheet weld projects
• Industry will submit final report to NRC and request a generic safety evaluation to allow utilities to amend aging management plans regarding inspections of the channel head and tube-to-tubesheet welds
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Guidance for Auto Analysis
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Typical Two Party Data Analysis Process for
Detection of SG Tube Degradation (SG Exam
Guidelines Revision 7)
Data Acquisition
Primary Data
Analysis
Resolution Data
Analysis Team
Secondary
Data Analysis
Identify Differences
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Example: Single Party Data Analysis Process for
Detection of SG Tube Degradation Using Multiple
Analyzers and Single Technique
Note: Single party data analysis for detection is not currently allowed by Revision 7 of the EPRI SG Examination Guidelines
Data Acquisition
Single Party
Data Analysis using
Multiple Analyzers
Results
Reported
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Example: Single Party Data Analysis Process for
Detection of SG Tube Degradation Using Single or
Multiple Analyzers and Multiple Techniques
Note: Single party data analysis for detection is not currently allowed by Revision 7 of the EPRI SG Examination Guidelines
Bobbin Probe
Data Acquisition
Single Party
Data Analysis
Bobbin Probe Results
Reported
Array Probe
Data Acquisition
Single Party
Data Analysis
Array Probe
Results Reported
29 © 2014 Electric Power Research Institute, Inc. All rights reserved.
EPRI Data Analysis Performance Assessment
Project
• Background
– Low-noise steam generator tubing provides an excellent
opportunity for utilities to use automated systems for
eddy current data analysis
– New guidance permitting single-party automated
analysis is being considered for Rev. 8 of the SG
examination guidelines
– Since corrosion of Alloy 690 SG tubes has not been
reported, data from Alloy 600MA SG tubes was used in
this project
30 © 2014 Electric Power Research Institute, Inc. All rights reserved.
EPRI Data Analysis Performance Assessment
Project
• Objective
– To provide results from a study on the performance of a
single-party automatic data analysis system compared to
the required two-party independent analysis process.
• Benefit
– Provide information for the SG Examination Guidelines
Rev 8 Committee that can be used as a technical basis
for development of guidance for single-party automatic
data analysis
31 © 2014 Electric Power Research Institute, Inc. All rights reserved.
EPRI Data Analysis Performance Assessment
Project
• Work Scope
– Obtain eddy current data acquired from an older
replaced steam generator having Alloy 600MA tubing
– Determine “Ground Truth” results
– Compare ground truth analysis results with analysis
results for:
• Single-party auto
• Primary manual
• Secondary CDS, with manual edits
• Resolution (based on 2-party analysis process)
– Compare performance of the various analysis processes
32 © 2014 Electric Power Research Institute, Inc. All rights reserved.
Planned Auto Analysis Project Work Scope –
Bobbin Data
• Determine data analysis performance for bobbin data at
selected tube locations:
– Hot Leg TSP
– Cold Leg TSP
– Freespan
– AVB
• Determine data analysis performance for bobbin data
based on significance of degradation:
– ≥ 40% TW, ≥ 1 volt
– < 40% TW, < 1 volt
• Planned completion date: Spring 2014
33 © 2014 Electric Power Research Institute, Inc. All rights reserved.
Planned Auto Analysis Project Work Scope –
Rotating Probe Data
• Determine data analysis performance for RPC data at
selected tube locations:
– Hot Leg TSP
– Cold Leg TSP
– TTS
• Determine data analysis performance for RPC data based
on significance of degradation
• Planned completion date: Spring 2014
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Tube-to-Tube Wear Amplitude vs Depth
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Tube-to-Tube Wear – Amplitude vs Depth
• NRC requested to see the data plotted as amplitude vs
depth
Bobbin ETSS 13091.1
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Probe Variability
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Probe Variability
• Industry investigated the potential for generic implications after a plant experienced unexpected voltage variations
– Bobbin ARC
• The pulled tube database for bobbin ARC includes data from both probe types
– Bobbin ETSS(s) used by utilities to size volumetric indications
• The SGMP investigated the ETSSs and found no issue with regard to using different bobbin probes and evaluating their equivalencies
• Exam Guidelines contain guidance to address the issue
– Bobbin leakage screens in the In Situ Pressure Test Guidelines
• The bobbin voltage screens are for volumetric indications only
• The In Situ Pressure Test Guidelines provide bobbin voltage leakage screening values that are developed from destructive exam data sets and are conservatively established.
• Industry performed a one-to-one comparison of the two probe types to determine the effect on detection and sizing of actual flaws
38 © 2014 Electric Power Research Institute, Inc. All rights reserved.
Probe Variability – Experiment Results
• Comparison of four tester/probe combinations (550/130 mix channel)
• 690TT 3/4” tubing
• EPRI foreign object wear samples
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Probe Variability – Experiment Results
• Plant data that initiated this experiment identified a minor
difference in voltage between two probe/tester
combinations
– Average difference of about 0.013 volts for low voltage
crack indications (<2 volt indications)
• These experimental results are consistent with the plant
data and identify the same small difference over a range of
flaw sizes
• These minor differences do not dictate any further guidance
• Industry considers this issue closed
40 © 2014 Electric Power Research Institute, Inc. All rights reserved.
Summary of Recently Published EPRI
Reports
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Assessment of Steam Generator Tube Plugs
Objectives
• Develop comprehensive generic tube plug defect
management plan (guidance) encompassing all
Westinghouse designed tube plugs currently installed in
steam generators using Failure Modes and Effects Analysis
methods.
• Include all tube plugs, of any material, shop installed or in-
service installed
• Foreign and domestic (U.S.) plug hardware included
42 © 2014 Electric Power Research Institute, Inc. All rights reserved.
Assessment of Steam Generator Tube
Plugs
Objectives
• Determine critical parameters for performing tube plug
assessments
– Provide guidance for what utilities must know about their
tube plugs (design, material, tube installed or not,
installation non-conformances, etc) and what inspections
are required (visual, eddy current, etc) to adequately
assess tube plugs
43 © 2014 Electric Power Research Institute, Inc. All rights reserved.
Assessment of Steam Generator Tube
Plugs
Benefits
– Provide information for utilities to include in Degradation
Assessments, Condition Monitoring and Operational
Assessments for tube plugs of all materials
Schedule
– Complete
– Technical report published August 2013, Deliverable No.
3002000636
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Effect of Noise on Sizing
Objectives
• SGMP has developed procedures and tools to assist utility engineers in understanding the probability of detection (POD) in steam generator eddy current data in the presence of noise
• This project focused on the ability to accurately size crack-like indications
Tasks
• Compile baseline datasets from ODSCC and PWSCC performance demonstrations
• Inject controlled levels of tube noise (0.5 to 2 volts) using Westinghouse Data Union Software
• Eddy current analyst sizes the dataset
• Assess the results to determine the effect of different levels of noise on the accuracy of sizing the crack indications
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Effect of Noise on Sizing
Results
• For both axial ODSCC and circumferential PWSCC the sizing
error decreases with increasing signal to noise ratio
• For the majority of the datasets, smaller ratios result in the larger
uncertainties with a tendency towards overestimation.
• A comparison between analyst uncertainty and uncertainties
associated with noisy data shows no significant difference
Status
• Project is complete
• Technical report published November 2013, Deliverable No.
3002000637
• This information will be used in the next revision of the Integrity
Assessment Guidelines
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Production of Steam Generator Tubing
Flaws by Laboratory Autoclave Exposures
• Objective: Fabricate stress corrosion cracking flaws in steam generator tubing for use for inspection technique qualification.
• Approach: Expose steam generator tubing to doped steam, high temperature caustic, and room temperature acidic environments under different stress states.
• Results: Crack samples were fabricated that had eddy current characteristics similar to pulled tube samples of the same through-wall depth.
• Future work may include eddy current measurements, destructive examinations, additional flaw fabrication, or investigation of alternate flaw fabrication techniques.
EPRI Report 3002000482 published September 2013
47 © 2014 Electric Power Research Institute, Inc. All rights reserved.
Production of Steam Generator Tubing
Flaws by Laboratory Autoclave Exposures
Environment Doped steam Caustic solution Acidic solution
Temperature 400°C 315°C 25°C
Pressure 20.7 MPa + 70 kPa H2 14.5 MPa 14.0 MPa
Chemicals 100 ppm Na2SO4, NaF,
NaNO3 and 200 ppm NaCl
1M NaOH +
10,000ppm Pb (as
PbO)
1M Na2S4O6
Most susceptible
materials LTMA LTMA Sensitized
Typical type of flaw pure IGSCC pure IGSCC IGSCC + IGA
Note: Expected failure time is strongly dependent on the applied and residual stress
on the specimen surface exposed to the corrosive environment.
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• Future Work
• Produce cracks using thermal fatigue
– Initial flaws had large signal amplitudes
– Technique has been refined to produce more
representative flaws on the inside and outside diameter
surfaces
Production of Steam Generator Tubing Flaws by Thermal Fatigue
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Production of Steam Generator Tubing Flaws by Focused Ion Beam • Future Work
• Focused Ion Beam machining to produce flaws
• Atomic-scale machining of
synthetic “cracks”
• Initial flaws have been NDD
39 x 2.3 x 7.9 mils deep
50 © 2014 Electric Power Research Institute, Inc. All rights reserved.
Computed Tomography to Replace
Destructive Testing
• Future
• Computed Tomography
imaging could be used
to determine flaw
dimensions non-
destructively
• With proper
qualification, computed
tomography could
replace the destructive
metallurgical process,
thereby preserving the
grading unit for future
use
51 © 2014 Electric Power Research Institute, Inc. All rights reserved.
Standing Issues
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SGMP Industry Document Status
and Revision Schedule
Guideline Title Current
Rev #
Report # Last
Pub
Date
Imple-
mentation
Date(s)
Interim
Guidance
Review
Date
Comments
SG Integrity
Assessment
Guidelines
3 1019038 Nov
2009
9/1/10 SGMP-IG-
10-01
SGMP-IG-
12-01
Rev 4 in
progress
EPRI SG In Situ
Pressure Test
Guidelines
4 1025132 Oct
2012
10/10/13 None 2014
PWR SG
Examination
Guidelines
7 1013706 Oct
2007
9/1/08 SGMP-IG-
08-04
SGMP-IG-
12-01
Rev 8 in
progress
PWR SG Primary-
to-Secondary
Leakage
Guidelines
4 1022832 Sept.
2011
4/11/2012
7/11/2012
None 2014
53 © 2014 Electric Power Research Institute, Inc. All rights reserved.
SGMP Industry Document Status
and Revision Schedule
Guideline Title Current
Rev #
Report # Last
Pub
Date
Imple-
mentation
Date(s)
Interim
Guidance
Review
Date
Comments
PWR Primary
Water Chemistry
Guidelines
6 1014986 Dec
2007
6/17/08
9/17/08
SGMP-IG-
09-01
SGMP-IG-
11-02
Rev 7 in
progress
PWR Secondary
Water Chemistry
Guidelines
7 1016555 Feb
2009
8/20/09
11/20/09
SGMP-IG-
13-01
2014 2013 Review
Committee
determined no
revision
necessary
Steam Generator
Management
Program
Administrative
Procedures
3 1022343 Dec
2010
9/1/11
12/31/11
None N/A
Steam Generator
Degradation
Specific Flaw
Handbook
1 1019037 Dec
2009
N/A None N/A Rev 2 in
progress
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Interim Guidance
• No interim guidance since last meeting
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NEI 03-08 Deviations
• Three long-term deviations
– Two Steam Generator Examination Guidelines, R7
– Single party auto analysis
– Steam Generator Integrity Assessment Guidelines, R3
– Use of site-specific sizing indices
• One short term deviations
– Steam Generator Examination Guidelines, R7
– PSI prior to hydro
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Operating Experience
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US Operating Experience – Alloy 600TT Tubing
• Majority of the 600TT fleet have H* approved
• Cracking continues to be identified in Alloy 600TT tubing
• SGMP is investigating the need for additional guidance for
inspection sample plans for Alloy 600TT tubing
Location
ODSCC PWSCC
Axial Circ Axial Circ
U-Bend X
TSP/FDB X
TTS/Exp Trans X X X X
Tubesheet X
Tube End X X
Ding* X
* Some indications at small voltage dings below reporting threshold
Latest Data from each US Utility from the Steam Generator Degradation
Database (Degradation Mechanism Table) as of January 2014
58 © 2014 Electric Power Research Institute, Inc. All rights reserved.
US 690TT Operating Units Affected by Wear
MECHANISM
NUMBER OF DOMESTIC UNITS
AFFECTED
Foreign Object Wear 17
U-Bend Support Wear 25
Support Structure Wear 32
Tube-to-Tube Wear 4
Latest Data from each US Utility from the Steam Generator Degradation Database
(Degradation Mechanism Table) as of January 2014
• Majority of the 690TT fleet have minor wear
• All operating replacement OTSGs have significant numbers of wear indications
• Last planned replacement included improvements to address lessons learned
• Only 4 of the 37 units with recirculating SGs have significant numbers of wear
indications
• The majority of the wear indications in the 690TT fleet are below 20%TW
• Four units have had wear indications that required in situ pressure testing
• 3 for foreign object wear, 1 for tube-to-support wear
• Performance criterion met – no leakage
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NRC Discussions/Items of Interest
Together…Shaping the Future of Electricity