NRC / EPRI Steam Generator Task Force MeetingDetection of SG Tube Degradation Using Single or...

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


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


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


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


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


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


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


General Discussion Items


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


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


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


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


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


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


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).


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


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


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


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


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)


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


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


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


Guidance for Auto Analysis


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


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


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


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



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



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


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


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


Tube-to-Tube Wear Amplitude vs Depth


Tube-to-Tube Wear – Amplitude vs Depth

• NRC requested to see the data plotted as amplitude vs

depth

Bobbin ETSS 13091.1


Probe Variability


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


Probe Variability – Experiment Results

• Comparison of four tester/probe combinations (550/130 mix channel)

• 690TT 3/4” tubing

• EPRI foreign object wear samples


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


Summary of Recently Published EPRI

Reports


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


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


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


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


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


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


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.


• 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


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


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


Standing Issues


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


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


Interim Guidance

• No interim guidance since last meeting


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


Operating Experience


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


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


NRC Discussions/Items of Interest

Together…Shaping the Future of Electricity

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