Reliability of Ultrasonic

In-Service Inspection of 

Welds in Reactor Internals

of Boiling Water Reactors

Office of Nuclear Regulatory Research

NUREG/CR-7159

PNNL-14561
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Reliability of Ultrasonic

In-Service Inspection of 

Welds in Reactor Internals

of Boiling Water Reactors

Manuscript Completed: November 2012

Date Published: April 2013

Prepared by: 

G. J. Schuster, S. L. Crawford, A. A. Diaz,

P. G. Heasler, and S. R. Doctor

Pacific Northwest National Laboratory

P. O. Box 999

Richland, WA 99352

D. A. Jackson and W. E. Norris, NRC Project Managers

NRC Job Codes Y6604 and N6398

Office of Nuclear Regulatory Research

 

NUREG/CR-7159

PNNL-14561

iii

ABSTRACT

Instances of stress corrosion cracking in reactor pressure vessel internal components have 

been found, especially in the boiling water reactor (BWR) core shroud. Results from in-service 

inspection are an important aspect of integrity evaluations. One of the major goals of the work 

described in this report is to quantify the crack detection reliability and sizing error of ultrasonic 

inspection methods for reactor internals. A mockup is described, along with its application to 

the assessment on nondestructive evaluation reliability for in-service inspection of reactor 

internals. The mockup of a BWR core shroud includes cracks in some of the 40 welded 

assemblies selected to represent field conditions. The selected material and geometry include 

most conditions and alloys used in the core shroud and its support structure. This report 

provides an overview of the work being performed and focuses on the parametric study results 

relating ultrasonic response and its variance to inspection effectiveness. Results of a blind test 

of an in-service inspection vendor’s phased array technique are also provided in this report.

PAPERWORK REDUCTION ACT STATEMENT

This NUREG does not contain information collection requirements and, therefore, is not subject 

to the requirements of the Paperwork Reduction Act of 1995 (44 U.S.C. 3501 et seq.).

PUBLIC PROTECTION NOTIFICATION

The NRC may not conduct or sponsor, and a person is not required to respond to, a request for 

information or an information collection requirement unless the requesting document displays a 

currently valid OMB control number.

v

FOREWORD

The Nuclear Regulatory Commission (NRC) conducted research at Pacific Northwest National 

Laboratory (PNNL) to assess the density and distribution of flaws in reactor pressure vessel 

(RPV) welds that result from fabrication and repair processes. The research was initiated 

because analyses have shown that vessel behavior is sensitive to flaw location, type, size, 

orientation, and other flaw characteristics. Accurate estimates of flaw density and distribution 

are required as input to the computer codes that are used in performing structural integrity 

assessments. Research results were published in NUREG/CR-6945, Fabrication Flaw Density 

and Distribution in Repairs to Reactor Pressure Vessel and Piping Welds.

The subject report describes earlier work to quantify the crack detection reliability and sizing 

error of ultrasonic inspection methods for reactor internals. A parametric study was used to 

divide those inspections where performance was considered acceptable from those where 

inspection was considered difficult. This was accomplished by quantifying crack responses and 

attendant noise sources to show that reliable detection and accurate sizing were related to a 

high signal-to-systematic-noise ratio. A blind test of vendor performance was used to measure 

detection and sizing statistics for the difficult cases.

In publishing this report, the NRC’s purpose is to make the research results publicly available 

because the vendor’s phased array in-service inspection was conducted under blind test 

conditions, and this information will be a valuable source of information for future performance 

demonstration assessment. In addition, the results of the study provide insights related to 

reliable detection and accurate sizing.

vii

CONTENTS

ABSTRACT...............................................................................................................................iii

FOREWORD ..............................................................................................................................v

EXECUTIVE SUMMARY ..........................................................................................................xv

ACKNOWLEDGMENTS.........................................................................................................xvii

ABBREVIATIONS AND ACRONYMS .....................................................................................xix

1 INTRODUCTION..............................................................................................................1-1

2 WELDS IN BWR INTERNALS .........................................................................................2-1

2.1 BWR Internals Weld Locations..................................................................................2-1

2.2 Degradation and Inspections.....................................................................................2-6

3 IN-SERVICE INSPECTION OF REACTOR INTERNALS USING 

ULTRASONIC TESTING..................................................................................................3-1

3.1 Ultrasonic Modalities .................................................................................................3-1

3.2 45° Shear Waves ......................................................................................................3-2

3.3 High-Angle Longitudinal Waves ................................................................................3-4

3.4 Creeping Waves........................................................................................................3-4

3.5 Time-of-Flight Diffraction Technique..........................................................................3-5

3.6 Phased Arrays...........................................................................................................3-6

3.7 Deployment...............................................................................................................3-7

4 RELIABILITY METHODS.................................................................................................4-1

4.1 Blind Tests ................................................................................................................4-2

4.2 Round Robin Tests ...................................................................................................4-3

4.3 Parametric Studies....................................................................................................4-5

5 REACTOR INTERNALS CORE SHROUD MOCKUP DESIGN........................................5-1

5.1 Component Conditions..............................................................................................5-1

5.2 Reflector Types .........................................................................................................5-5

5.3 Acceptance Tests......................................................................................................5-8

5.4 Completed Mockup .................................................................................................5-14

6 PARAMETRIC STUDY.....................................................................................................6-1

6.1 Near-Side Access .....................................................................................................6-2

6.1.1 45° Shear Waves at 2.25 MHz, Base Metal Access.........................................6-2

6.1.2 75° Longitudinal Waves at 2.0 MHz, Base Metal Access.................................6-3

6.1.3 Crack Responses ............................................................................................6-4

6.1.4 Thermal Fatigue Cracks in Alloy 600 ...............................................................6-4

6.2 Echo Dynamics .........................................................................................................6-6
viii

6.3 Far-Side Through-Weld Metal Access.....................................................................6-10

6.4 Access to the Cracked Surface ...............................................................................6-12

6.5 Summary of Results................................................................................................6-16

7 BLIND TEST OF PHASED ARRAY PROCEDURE FOR FAR-SIDE ACCESS................7-1

7.1 Test Description ........................................................................................................7-1

7.2 Detection Performance..............................................................................................7-5

7.3 Depth Sizing Performance.........................................................................................7-5

7.4 Length Sizing Performance .......................................................................................7-8

7.5 Parametric Analysis of Blind Test Data....................................................................7-10

7.6 Summary.................................................................................................................7-12

8 CONCLUSIONS...............................................................................................................8-1

8.1 Parametric Study Conclusions ..................................................................................8-1

8.2 Blind Test of Phased Array Procedure ......................................................................8-2

9 RECOMMENDATIONS ....................................................................................................9-1

10 REFERENCES...............................................................................................................10-1
ix

FIGURES

1-1 Conceptual Diagram for Study of UT-ISI Reliability as Applied to Reactor 

Internals..........................................................................................................................1-2

2-1 Cutaway View of BWR-4.................................................................................................2-2

2-2 Cutaway View of Typical Core Shroud............................................................................2-3

2-3 Typical BWR Shroud Weld Layout..................................................................................2-4

2-4 Cutaway View of Jet Pump Assembly.............................................................................2-5

2-5 Typical Microstructure Content for a Boiling Water Reactor Core Shroud Weld. .............2-8

3-1 Commercial Transducers Typically Used for ISI of Austenitic Materials ..........................3-2

3-2 Near-Side Access (left) and Far-Side Access (right) Ultrasonic Examination 

Using Vertically Polarized 45° Shear Waves for Cracks That Are OppositeSurface Connected (from the transducer) .......................................................................3-3

3-3 Three Ultrasonic Signals (Corner Trap, Crack Face, and Crack Tip) from 

Opposite-Surface Connected Rough Cracks ..................................................................3-3

3-4 Near-Side (Base Metal Access) Shown in Top Image and Far-Side (Weld 

Metal Access) Shown in Bottom Image Ultrasonic Examination Using HighAngle Compression Waves for Scan-Surface Connected Cracks ...................................3-4

3-5 Creeping Wave Technique .............................................................................................3-5

3-6 Time-of-Flight Diffraction Technique ...............................................................................3-6

3-7 Core Shroud Examination Using the Siemens Power Corporation Lower 

Manipulator.....................................................................................................................3-8

4-1 Changes in POD versus Through-Wall Dimension of Flaws for Piping Weld 

Inspection in the Past Two Decades ...............................................................................4-1

4-2 Operating Characteristics Diagram .................................................................................4-3

4-3 Flaw Detection Frequency for Procedures in PISC III, Action 4.......................................4-4

4-4 Operating Characteristic Diagram for Techniques in PISC III, Action 4...........................4-4

4-5 Response versus Crack Tightness..................................................................................4-6

5-1 Welded Assembly Configuration for Mockup...................................................................5-3

5-2 First Welded Assemblies ................................................................................................5-3

5-3 Welding of Reactor Internal Mockup Assembly...............................................................5-4

5-4 Four Crack Initiation Sites...............................................................................................5-5

5-5 Example of Thermal Fatigue Crack Implant ....................................................................5-6

5-6 Thermal Fatigue Crack Notation .....................................................................................5-7

5-7 Near Side Access Responses from Notches, Side-Drilled Holes, and Thermal 

Fatigue Cracks Using 45° Shear Waves in a Submerged Arc Single-V Welded 

Assembly......................................................................................................................5-10
x

5-8 Responses Using 45° Shear for Base Metal and Weld Metal Access in a 

Submerged Arc Single-V Welded Assembly .................................................................5-10

5-9 Responses Using 60° Longitudinal for Base Metal and Weld Metal Access in a 

Submerged Arc Single-V Welded Assembly .................................................................5-11

5-10 Response Using Creeping Wave Probe SH-L-L Mode in a Submerged Arc 

Single-V Welded Assembly...........................................................................................5-12

5-11 Responses Using Creeping Wave Probe 75° Longitudinal Mode in a 

Submerged Arc Single-V Welded Assembly .................................................................5-13

6-1 Noise Sources Using 45° Shear Waves..........................................................................6-1

6-2 Responses of Notches to 45° Shear Wave Insonified Through Stainless Steel 

Base Metal......................................................................................................................6-2

6-3 Response of Notches to 75° Longitudinal Waves at 2.0 MHz Insonified 

Through Stainless Steel Base Metal ...............................................................................6-4

6-4 Thermal Fatigue Crack Responses to 45° Shear Waves at 2.25 MHz Insonified 

Through Stainless Steel Base Metal ...............................................................................6-5

6-5 Weld Solidification Cracks on the Near Side Using 45° Shear Waves in 

Stainless Steel................................................................................................................6-5

6-6 Response of Thermal Fatigue Cracks for Near-Side Access in Alloy 600 .......................6-6

6-7 Echo-Dynamic Curves for Three Notches in Stainless Steel...........................................6-7

6-8 Amplitude versus Duration of the Echo-Dynamic Curve for Notches in Stainless 

Steel ...............................................................................................................................6-7

6-9 Response of Notches in Stainless Steel to 45° Shear Waves at Three 

Frequencies....................................................................................................................6-8

6-10 Echo-Dynamic Curves for Rough Cracks in Alloy 600 to 45° Shear Waves. 

Duration of echo-dynamic curves for rough cracks shows some relationship 

with crack size. TFC 4 is approximately 75% through-wall. TFC 3 is 

approximately 50% through-wall. The echo-dynamic curve of the 10% notch is 

shown for reference. .......................................................................................................6-9

6-11 Thermal Fatigue Crack in Alloy 600 ..............................................................................6-10

6-12 Response of Notches to 45° Shear Waves at 2.25 MHz for Far-Side ThroughWeld Metal Access in Stainless Steel ...........................................................................6-11

6-13 Response of Notches to 45° Shear Waves for Far-Side Through-Weld Metal 

Access in Alloy 600.......................................................................................................6-11

6-14 Responses of Notches to Creeping Waves at 2 MHz for Far-Side ThroughWeld Metal Access in Stainless Steel ...........................................................................6-12

6-15 Dual-Element Probe, 70° 2-MHz, Detection Results for Notches Connected to 

Scanning Surface in Stainless Steel .............................................................................6-13

6-16 Dual-Element Probe, 70° 2-MHz, Detection Results for Thermal Fatigue 

Cracks Connected to the Scanning Surface in Stainless Steel......................................6-13
xi

6-17 Dual-Element Probe, 70° 2-MHz, Detection Results for Weld Solidification 

Cracks Connected to the Scanning Surface in Stainless Steel......................................6-14

6-18 Dual-Element Probe, 60° 2-MHz, Notch Tip Responses Through Base Metal 

for Notches Connected to the Scanning Surface in Stainless Steel ..............................6-15

6-19 Dual-Element Probe, 60° 2-MHz, Notch Tip Responses Through Weld Metal 

for Notches Connected to the Scanning Surface in Stainless Steel ..............................6-15

6-20 Dual-Element Probe, 60° Transmit-Receive Refracted Longitudinal 2-MHz, 

Thermal Fatigue Crack Tip Responses Through Base and Weld Metal for 

Cracks Connected to the Scanning Surface in Stainless Steel......................................6-16

6-21 Dual-Element Probe, 60° 2-MHz, Weld Solidification Crack Tip Responses 

Through Base and Weld Metal for Cracks Connected to the Scanning Surface 

in Stainless Steel ..........................................................................................................6-16

7-1 PNNL Mockup at EPRI NDE Center ...............................................................................7-2

7-2 Inspection Scanner and Presentation of Specimens in Blind Test...................................7-4

7-3 Blind Test Depth Sizing Performance for All Detected Cracks ........................................7-6

7-4 Blind Test Depth Sizing Performance Without Small Cracks...........................................7-7

7-5 Blind Test Dependence of Sizing Error on Tip Response................................................7-8

7-6 Blind Test Length Sizing .................................................................................................7-9

7-7 Blind Test Length Sizing Performance for All Cracks ......................................................7-9

7-8 Blind Test Length Sizing Performance Without Large Oversizing Errors of 

Greater Than 50 mm ....................................................................................................7-10

7-9 Typical Phased Array Image of Crack (units shown in millimeters) ...............................7-11

7-10 Relationship Between Response and Crack Size in Phased Array Blind Test 

on Stainless Steel in Reactor Internals Mockup ............................................................7-12

xiii

TABLES

2-1 Techniques Used in In-service Inspection of BWR Internals as Cited in 

Available Literature.........................................................................................................2-7

3-1 Relative Strengths and Weaknesses of Ultrasonic Modalities for Inspection of 

Austenitic Welds of Reactor Internals .............................................................................3-1

4-1 Estimate of Standard Deviation of Inspection Variables ..................................................4-5

5-1 Weld Profiles Selected for Study.....................................................................................5-2

5-2 First Five Plates Used in Design Tests for Mockup.........................................................5-8

5-3 Test Results Used for Mockup Design – Submerged Arc Single-V Welded 

Assembly......................................................................................................................5-13

5-4 Far-Side Access Responses from a 20% Notch for 75° Compression 

(Longitudinal) Waves for Various Weld Profiles ............................................................5-13

5-5 Notch Depths and Angles Selected for Mockup ............................................................5-15

5-6 Mockup Assemblies. Size of the cracks is withheld to permit mockup re-use in 

future blind tests. ..........................................................................................................5-16

6-1 Corner-Trap Signal-to-Noise Ratio for Access Conditions, Ultrasonic Modality, 

and Notches That Are Opposite-Surface Connected (from the Transducer) in 

Stainless Steel..............................................................................................................6-17

6-2 Detection Signal-to-Noise Ratio for Reflectors Connected to Scanning Surface

in Stainless Steel ..........................................................................................................6-17

6-3 Sizing Error for Notches That Are Opposite-Surface Connected (from the 

Transducer) in Stainless Steel. No sizing error estimate for thermal fatigue 

cracks or weld solidification cracks was available at the time of publication. For 

responses from crack tips, see Figure 6-4 and Figure 6-13. .........................................6-18

6-4 Sizing Error for Notches Connected to Scanning Surface in Stainless Steel. 

No sizing error estimate for thermal fatigue cracks or weld solidification cracks 

was available at the time of publication. For response from crack tips, see 

Figures 6-20 and 6-21...................................................................................................6-18

7-1 PNNL Stainless Steel Calibration Specimen Provided for Blind Test ..............................7-2

7-2 Stainless Steel Practice Specimens Provided for Blind Test ...........................................7-3

7-3 Detection Performance for Far-Side Access and Opposite Surface Connected 

Cracks in Blind Test of Stainless Steel Specimen...........................................................7-5

7-4 Blind Test Depth Sizing Error for Different Crack Types..................................................7-6

7-5 Blind Test Length Sizing Error for Crack by Type............................................................7-8

xv

EXECUTIVE SUMMARY

As a part of the Nuclear Regulatory Commission (NRC) Office of Nuclear Regulatory Research 

activities on evaluating the reliability of nondestructive examinations, the Pacific Northwest 

National Laboratory has measured the reliability of in-service inspection of boiling water reactor 

(BWR) internals components for stress corrosion cracking of the weld’s heat-affected zones. 

The primary objective of this study was to identify areas or conditions in which inspection 

performance was relatively low. A mockup of a BWR core shroud was used to quantify 

inspection performance. 

Inspection performance was quantified in this study by identifying the areas in which 

performance was high and then placing emphasis on the remaining difficult inspection cases. 

The identification of high-performance conditions was accomplished by use of a parametric 

study, which quantified crack responses and attendant noise sources to show that reliable 

detection and accurate sizing were related to a high signal-to-systematic-noise ratio. A blind 

test of vendor performance was used to measure detection and sizing statistics for the difficult 

cases. 

Results of the parametric study showed that access to the cracked surface (the crack initiation 

surface) strongly affected detection and sizing. For the inspection of weld heat-affected zones, 

reliable detection of cracking and accurate sizing could be accomplished when an ultrasonic 

transducer could be placed on the cracked surface. Furthermore, reliable detection and 

accurate sizing could be achieved for the near-side access condition to cracks that are 

connected to the surface opposite from the surface used for scanning. The parametric study 

showed that for far-side access (through the weld) to cracks connected to the opposite surface, 

detection and sizing performance was low and the most difficult of inspection conditions. 

A blind test of vendor performance for weld far-side access to heat-affected zone cracking also 

was conducted. The vendor used a phased array system to successfully detect 14 of 17 

cracked grading units while calling cracks in 3 of 33 blank grading units. This detection 

performance would meet the intent of the requirements of an ASME Code Section XI, Appendix 

VIII, performance demonstration test designed to Supplement 2, “Qualification Requirements for 

Wrought Austenitic Piping Welds,” because there were twice as many blank grading units as 

cracked grading units. The root mean square error for the blind depth-sizing test was 9.2 mm 

and would not pass. 

A re-analysis of the signals in the vendor’s phased array data revealed that no response was 

received from cracks less than 30% through-wall for far-side access. The absence of cornertrap responses explains, in part, the size dependence of crack detection. The origin of the 

signals in the phased array images was from the rough face of the cracks and from the crack 

tips. Sizing errors occurred because weak responses from crack tips were difficult to distinguish 

from strong responses from the crack face and from weld fabrication flaws. 

Techniques are described for imaging improvements, such as reduction of noise variance and 

use of hybrid imaging systems. Application of new technology to the difficult case of far-side 

inspection of austenitic welds is needed. The industry has worked extremely hard on the farside inspection issue for Appendix VIII Supplement 10 because field dissimilar metal welds limit 

most examinations to be conducted only from the far side. Some of the Supplement 10 

solutions may provide improvements to the BWR reactor internals inspections.

xvii

ACKNOWLEDGMENTS

The authors thank Deborah A. Jackson (now at NRC NMSS) and Wallace E. Norris of NRC 

RES for their technical and managerial guidance along with the continued support to being this 

work to successful closure. We also thank J. Muscara formerly of NRC RES (but now retired) 

for starting this technical work and for his initial technical guidance.

The authors thank S. H. Bush of PNNL for assistance with the configuration of light water 

reactor internals and A. F. Pardini of PNNL for assistance in the parametric study. The authors 

thank R. E. Bowey of PNNL for assistance in the blind test of the phased array system. We 

thank E. Prickett and K. E. Hass of PNNL for preparing and editing this manuscript. 

We thank the staff of the Electric Power Research Center NDE Center for hosting our activities 

during the blind test of the phased array system at their facility. The authors thank John Hayden 

of G.E. Nuclear Inspection Service for participating in a blind test of the PNNL reactor internal 

mockup using the G.E. phased array system. 

xix

ABBREVIATIONS AND ACRONYMS

ASME American Society of Mechanical Engineers 

BWR boiling water reactor 

BWROG Boiling Water Reactors Owners Group

BWRVIP Boiling Water Reactor Vessel and Internals Project 

EPRI Electric Power Research Institute 

FCP false call probability

FDF flaw detection frequency

IGSCC intergranular stress corrosion cracking

INPO Institute of Nuclear Power Operations

ISI in-service inspection

L longitudinal 

LPCI low pressure coolant injection

MRR mini-round robin

NDE nondestructive evaluation 

NDT nondestructive testing 

NRC U.S. Nuclear Regulatory Commission 

OC operating characteristics 

PDI Performance Demonstration Initiative

PIRR Piping Inspection Round Robin

PISC Programme for the Inspection of Steel Components 

PNNL Pacific Northwest National Laboratory 

POD probability of detection 

RMSE root mean square error 

RPV reactor pressure vessel 

SCC stress corrosion cracking (cracks)

SE safety evaluation

SG-CSS Subgroup on In-Service Inspection of Core Support and Internal 

Structures

SH shear 

TFC thermal fatigue crack 

TOFD time-of-flight diffraction 

UT ultrasonic testing 

VT visual testing

WSC weld solidification crack 

1-1

1 INTRODUCTION

As a part of the Nuclear Regulatory Commission (NRC) Office of Nuclear Regulatory Research 

program, “Evaluation of the Reliability of Non Destructive Examination Techniques,” and the 

follow-on program, “Reliability of Nondestructive Examination for Nuclear Power Plant Inservice 

Inspection,” the Pacific Northwest National Laboratory has assessed the reliability of in-service 

inspection of boiling water reactor (BWR) internals components for stress corrosion cracking of 

the weld’s heat-affected zones. This report provides estimates of flaw detection capability and 

sizing accuracy for the ultrasonic testing (UT) methods that have been employed. Also, the 

report provides an assessment of component accessibility and describes its effect on inspection 

capability. This report includes UT data from a realistic mockup, data assessment, and findings.

The ability of nondestructive evaluation (NDE) to reliably detect and accurately size stress 

corrosion cracks in stainless steel piping has been studied in research programs for more than 

20 years. Measurement of the performance of standard and advanced NDE practice has been 

made in round robins conducted by Pacific Northwest National Laboratory (PNNL) (Heasler 

et al. 1990; Heasler and Doctor 1996) and in the international Programme for the Inspection of 

Steel Components (Lemaitre et al. 1996). This report summarizes the results from these 

extensive databases to compare and contrast the inspection of piping to the inspection of 

reactor internals. 

The purpose of this research was to determine the detection and sizing capabilities of ultrasonic 

inspections as they apply to in-service inspection (ISI) of reactor internals. This evaluation of 

NDE reliability was to be applicable to the techniques practiced in the field. Special attention 

was given to the identification of areas in which performance may be relatively low. 

Figure 1-1 shows a conceptual diagram for the study of the UT-ISI reliability as it was applied to 

reactor internals. The activity at the center of the work was a parametric study of inspection 

performance. The goal of the parametric study was to identify areas in which performance was 

relatively low. For the cases of low performance, this study employed a mockup in a blind test 

to measure the performance of industry-deployed techniques that were thought to provide 

enhancements in the inspection. The performance in the best case has been combined with the 

estimates for the difficult cases to establish the NDE reliability of the ISI of reactor internals. 

This study also used failure history, especially crack types, and the physical properties of ISI 

methods to augment the understanding of reliability. At the bottom of Figure 1-1, the mockup of 

reactor internals provides the measurement environment for both the parametric study and the 

blind tests. The literature on NDE reliability measurement shows how a response model can be 

used in a parametric study to estimate detection and sizing performance. 

Section 2 contains a description of BWR internals and intergranular stress corrosion cracking 

that occurs in the heat-affected zones of the welds. The BWR internals are presented as a way 

of introducing the inspection problem. The welds in the BWR core shroud are described, and 

selected other components are illustrated. The physical properties of stress corrosion cracks 

(SCCs) are given, especially those important to ultrasonic inspection. 
1-2

Figure 1-1 Conceptual Diagram for Study of UT-ISI Reliability as Applied to Reactor 

Internals

Section 3 contains a description of the ultrasonic testing methods applied to the in-service 

inspection of BWR internals. The ultrasonic modalities covered are 45° shear waves, highangle compression waves, creeping waves, time of flight diffraction, and phased arrays. A brief 

description of mechanical deployment is provided for perspective on the application of NDE 

technologies. 

Section 4 contains a description of the NDE reliability literature. Research results from PNNL 

piping round robins and the Programme for the Inspection of Steel Components (PISC) are 

reviewed in Section 4. The use of the relative operating characteristics diagram is explained 

and an example of its use is given from the literature. The relevance of responses from 

degradation and from noise sources in inspection reliability also are reviewed in this section. 

Section 5 contains a description of the mockup built for this study. The geometrical conditions 

of the BWR core shroud that affect NDE reliability are given and included in the mockup design. 

The mockup was constructed for use in both a parametric study and a blind test. The results of 

the flaw acceptance tests are provided, and a list of the mockup assemblies is given. 

Section 6 describes the parametric study. Weld near-side access to SCC is shown to provide a 

highly reliable inspection. The echo-dynamics of corner trap signals is reviewed. Inspection 

reliability for weld far-side access to cracks is shown. Data from cracks connected to the 

scanning surface is also reported. 

Section 7 contains a description of a blind test for inspection performance through austenitic 

weld and the statistical results from it. Detection performance was measured using the grading 

unit approach specified for performance demonstration (American Society of Mechanical 

Engineers [ASME] Boiler and Pressure Vessel Code Section XI, Appendix VIII) (ASME 2004). 
1-3

Depth sizing and length sizing results are given with a parametric analysis of the blind test data 

for the case of far-side access (through the weld). 

Sections 8 and 9 present the conclusions and recommendations based on this work.

2-1

2 WELDS IN BWR INTERNALS

Components associated (both internally and externally) with the BWR reactor pressure vessel 

(RPV) vary widely in overall geometry, size, shape, and thickness. The configuration of BWR 

internals is documented in Shaw and McDonald (1993) and in BWRVIP-15 (1996). Figure 2-1 

(permission to use this copyrighted material is granted by the American Nuclear Society) depicts 

the arrangement of the reactor internals used in the General Electric BWR model 4. The 

location of the core shroud in the internals assembly can be seen in Figure 2-1.

There are three regions—the lower, central, and upper regions—that make up the volume 

enclosed by the BWR pressure vessel. The lower region, below the core plate, contains the 

lower plenum components described in BWRVIP-47 (1997) and the core shroud support 

structure. The central region, bounded by the top guide and the core plate, is the area 

containing the fuel. The upper region is bounded by the shroud head and the top guide. 

All three regions play an important part in the flow of energy from the reactor core to the turbine 

generators. Feedwater enters the vessel through the feedwater nozzle, is mixed with internal 

recirculating water, and flows into the annulus between the vessel wall and the core shroud. 

From the annulus, a portion of water leaves the vessel via the recirculating water outlet nozzle; 

the remainder enters the suction inlets of the jet pump assemblies. The external recirculation 

water is pumped through the recirculation loops and returns to the nozzle inlets of the jet 

pumps. All three streams join together in the pump diffuser and discharge into the plenum 

below the core. The water then flows up through the core where a portion of the liquid is 

evaporated by boiling into steam. This mixture of steam and water passes up through the 

steam separators and dryers. The separated water flows back down to the annulus for 

recirculation while the steam is sent to the turbine generator.

The core shroud provides vertical and lateral support for the top guide and core plate and 

provides a floodable volume for emergency cooling water. The core shroud is a stainless steel 

cylindrical assembly that supports the core plate, shroud head, and top guide both 

circumferentially and axially. The shroud head is manufactured of wrought austenitic stainless 

steel and closes off the core outlet, thereby forcing the steam to travel through the steam 

separators.

2.1 BWR Internals Weld Locations

Reactor internals are welded assemblies of wrought austenitic stainless steel, Alloy 600, and 

cast austenitic stainless steel. These welded assemblies include the core shroud, top guide, 

core plate, jet pump assembly, core spray internals, RPV attachments, shroud support 

assembly, liquid standby control, and lower plenum components. 

Figure 2-2 illustrates the typical BWR shroud layout with major circumferential welds. The weld 

locations for typical General Electric shroud designs for BWR models 3, 4, 5, and 6 are shown. 

The core shroud is a cylindrical, stainless steel assembly surrounding the core and provides a 

barrier for separating upward flow through the core from downward flow in the annulus.
2-2

Figure 2-1 Cutaway View of BWR-4 (Herrera and Stancavage 1988). Copyright 1988 by 

the American Nuclear Society (ANS), La Grange Park, Illinois; reprinted with 

permission.
2-3

Figure 2-2 Cutaway View of Typical Core Shroud. Source: Baker and Van Hoomissen

(1994), copyright © 1994 Electric Power Research Institute (EPRI), Inc.; 

reproduced with permission.

The core shroud is made up of several panels, two support rings (at the core plate and top 

guide elevations), and a third ring and steam dam at the shroud head. This entire assembly is 

held together by numerous vertical and circumferential welds (seams), as shown in Figure 2-3

welds (permission to use this copyrighted material is granted by the Electric Power Research 

Institute). In a typical BWR, there are seven or eight circumferential welds, and these welds are 

where cracking has been found. 
2-4

Figure 2-3 Typical BWR Shroud Weld Layout. Source: Baker and Van Hoomissen 

(1994), copyright © 1994 Electric Power Research Institute (EPRI, Inc.; 

reproduced with permission.

The locations of the welds associated with the top guide are illustrated in BWRVIP-26 (1996). 

The various wedges and hold-down devices for the top guide also are illustrated there. The top 

guide is bolted on a rim near the top of the core shroud and provides lateral support for the 

upper end of the fuel assemblies, neutron sources, and neutron monitoring instruments. Stress 

corrosion cracking has been found in the top guide assembly (BWRVIP-26 1996). 

The core plate assembly is a stainless steel circular plate, oriented horizontally, with axial 

stiffener plates beneath it. This plate provides support for the peripheral fuel bundles and is 

bolted to the support ledge in the bottom area of the core shroud. The various BWR core plate 

welds are illustrated clearly in BWRVIP-25 (1996). Stress corrosion cracking has been found in 

the heat-affected zone of welds in the core plate (BWRVIP-25 1996). 

The jet pumps play a critical role in the recirculation process of the BWR RPV. The flow is 

throttled by a flow control valve on the discharge side of the recirculation pump. A cutaway view 

of a typical jet pump assembly is shown in Figure 2-4. The position and orientation of the jet 

pump is shown relative to the shroud plate and the RPV recirculating inlet nozzle. 

More jet pump assembly details including welds, screws, shims, brackets, elbows, springs, and 

collars can be found in great detail with the potential failure locations in BWRVIP-41 (1997). 

In a BWR, a high-pressure core spray system is provided to make up water lost in any break or 

rupture of the primary system and to prevent the core from overheating. Water is sprayed 

directly onto the core using the core spray sparger. A low-pressure core spray system (with 

associated spray sparger) sprays water directly onto the fuel elements and is triggered during 

high containment pressure conditions or low reactor water level conditions. A clear view of 
2-5

Figure 2-4 Cutaway View of Jet Pump Assembly (USNRC Technical Training Center)
2-6

the typical core spray piping configuration depicting support brackets, the shroud, and shroud 

attachment is shown in BWRVIP-15 (1996). The core spray sparger is part of the BWR 

emergency cooling system. Cracking occurs in the core spray spargers, in particular the 

T-box/vent hole configuration. The inspection guidelines for the core spray internals can be 

found in BWRVIP-18 (1996). 

The configuration, degradation susceptibility factors, and potential failure location for vessel 

inside attachments is given in BWPVIP-48 (1998). Mechanical damage and SCC have been 

found in the attachments (BWRVIP-48 1998). 

The welds in the shroud support assembly for BWRs can be found in BWRVIP-15 (1996). The 

configuration of the shroud support assembly varies by BWR plant type, and the variations are 

described there. Ultrasonic inspections of the H8 and H9 welds have been conducted at more 

than 20 BWRs, and minor indications (those with no significant depth) were found (BWRVIP-38, 

1997).

The shroud support assembly has access holes that are used as manways during plant 

construction. These holes are sealed with cover plates made of Alloy 600 and are about 6.4-cm

(2.5-in.) thick. The covers are then welded in place with Alloy 82 or 182 weld metal. Alloy 182 

material is known to be susceptible to SCC (MacDonald 1994). 

The welds associated with the liquid standby control are given in BWRVIP-15 (1996). The 

inspection and flaw evaluation guidelines are presented in BWRVIP-27 (1997). 

The lower plenum components include the control rod guide tube, control rod housing, stub tube 

and orificed fuel support, in-core housing and support hardware, and the in-core flux monitor dry 

tube assembly. The configuration of these components, the failure history, and inspection 

guidelines are given in BWRVIP-47 (1997).

The BWRVIP reports addressed in this report at the time the work was being conducted were 

the ones considered most relevant to the scope of work. There have also been new revisions to 

the reports cited that have not been reviewed or considered because they were issued after this 

work was conducted. There are some other BWRVIP reports on inspection that may be of 

interest that include BWRVIP-42-A (2005) on LPCI, BWRVIP-49-A (2002) on instrumentation 

penetrations, BWRVIP-76 (1999) on core shrouds, BWRVIP-139-A (2009) on steam dryer, 

BWRVIP-180 (2007) on access hole covers, and BWRVIP-183 (2007) on top guide grid beam. 

The BWRVIP has continued to work and has created a total of 235 reports addressing BWR 

vessels and internals. These reports are listed on the EPRI website www.epri.com.

2.2 Degradation and Inspections

Topics for this section include utility experiences with SCC detection, the BWR Vessel and 

Internals Project (BWRVIP), and the ASME Code Section XI (2007) for the inspection of reactor 

internals. Vendor services for ISI of reactor internals are also covered. The owners of BWRs 

started the BWRVIP to coordinate efforts in managing the degradation (including SCC) of 

reactor vessels and internals. 
2-7

The BWRVIP is a program organized into four tasks: inspection, assessment, mitigation, and 

repair. This program has produced separate inspection guidelines for safety-related reactor 

internal components, including core shroud (BWRVIP-03), top guide (BWRVIP-26), core plate 

(BWRVIP-25), jet pump assembly (BWRVIP-41), core spray internals (BWRVIP-18), RPV 

attachments (BWRVIP-48), shroud support (BWRVIP-38), liquid standby control (BWRVIP-27), 

and lower plenum components (BWRVIP-47). The BWRVIP is intended to serve as the focal 

point of the regulatory interface for industry on BWR vessel and internals operability issues.

The inspection task of the BWRVIP is evaluating NDE inspection techniques including visual

testing (VT) and ultrasonic and eddy current methods. For accessible components, many field 

SCCs have been detected and length-sized by visual NDE methods, and an assessment of length 

sizing uncertainty was conducted using industry mockups. These mockups have been constructed for assessing and developing ultrasonic and eddy current testing of reactor internals. 

Internal components of BWRs that have experienced degradation are listed in Vessel Internals 

Inspection Summary (BWRVIP 1997). Table 2-1 shows the NDE techniques used in the inservice inspection of BWR internals. Inspections have detected SCC in the stainless steel 

shrouds, and cracking has been detected in other internal components that are composed of a 

variety of stainless steel alloys. The cracking occurs in the heat-affected zone of welds where 

grain boundary sensitization, residual stresses, and the corrosion chemistry produced by the 

radiation environment interact to cause SCC.

Table 2-1 Techniques Used in In-service Inspection of BWR Internals as Cited in 

Available Literature

Component Material NDE Technique Sources

Core shroud assembly Wrought austenitic stainless steel VT, UT [1], [2], [3], [4] 

Top guide assembly Wrought austenitic stainless steel VT [5] 

Core plate assembly Wrought austenitic stainless steel VT, UT [6] 

Jet pump assembly Wrought austenitic stainless steel VT, UT [7], [8], [9], [10], [11]

Core spray internals Wrought austenitic stainless steel VT [12] 

RPV attachments Alloy 182 VT [13] 

Shroud support 

assembly 

Alloy 600/182 VT, UT [14], [15] 

Liquid standby control Alloy 600/182 VT [16] 

Lower plenum 

components 

Wrought austenitic stainless steel, 

Alloy 600, cast austenitic 

stainless steel 

VT [17] 

[1] BWRVIP-03 (1995) [10] NRC (1980)

[2] Baker and Van Hoomissen (1994) [11] Baker et al. (1994)

[3] Bertz et al. (1994) [12] BWRVIP-18 (1996)

[4] Hayden (1998) [13] BWRVIP-48 (1998)

[5] BWRVIP-26 (1996) [14] BWRVIP-38 (1997)

[6] BWRVIP-25 (1996) [15] McDonald (1994)

[7] Hacker et al. (1998) [16] BWRVIP-27 (1997)

[8] BWRVIP-41 (1997) [17] BWRVIP-47 (1997)

[9] GE Nuclear Energy (1996)
2-8

Examples of stress corrosion cracks that were obtained by laboratory and field-induced SCC in 

piping welds can be seen in Good and Van Fleet (1986). The illustrations provided in Good and 

Van Fleet (1986) show the openness and features that can form in SCC. The cracks show 

branching and grain encirclement. These crack properties influence the reliability of UT-ISI. 

The branching and grain encirclement along the crack face scatter ultrasound. This scattering 

of ultrasound from such a rough crack face affects the ultrasonic images, as described in 

Sections 5 and 6.

Figure 2-5 depicts representative weld microstructure of a core shroud weld (permission to use 

this copyrighted material is granted by the Electric Power Research Institute). The 

microstructure of the weld metal significantly impedes the propagation of ultrasound. The 

results of the parametric study, given in Section 6, show the significance of weld microstructure 

in the detection and sizing of SCC in the heat-affected zone of welds in reactor internals. 

Figure 2-5 Typical Microstructure Content for a Boiling Water Reactor Core Shroud 

Weld. Source: BWRVIP-03 (1995), © 1995 Electric Power Research Institute, 

Inc., reproduced with permission.

Intergranular stress corrosion cracking in austenitic piping was a major issue for BWRs in the 

1980s. IGSCC had a significant impact relative to outages, occupational exposure, and 

capacity factor losses. In addition, primary piping systems were designed and are inspected in 

accordance with the ASME Code, which is incorporated by reference into the NRC regulations. 

Thus, measures were quickly implemented to address IGSCC in primary piping.

The Electric Power Research Institute (EPRI) and the BWROG recognized the potential 

susceptibility of reactor internals to IGSCC. IGSCC of reactor internals became a reality in the 

1993–1994 time frame when examinations at certain nuclear power plants detected cracks in 
2-9

core shrouds. However, unlike primary piping, most BWR internal components are not 

designed or inspected in accordance with the ASME Code (the ASME Code was developed to 

establish standards of safety related only to pressure boundary integrity). The BWRVIP, an 

association of utilities, was formed by BWR utility executives in mid-1994 to proactively address,

on a generic basis, issues related to reactor vessel and internals material. The BWRVIP 

established agreements with the NRC regarding commitments to the program because vessel 

internals were not adequately addressed by the ASME Code.

As the BWRVIP has matured, other objectives have been established, such as to identify or 

develop generic, cost-effective strategies; serve as a focal point for the regulatory interface with 

the industry; and share information among utilities. The development of generic solutions by the 

BWRVIP to address the degradation of vessel internals in conjunction with the regulatory 

commitments meant that the NRC did not have to expend considerable rulemaking resources to 

deal with internals degradation.

The BWRVIP addresses issues through the issuance of guideline reports. There is an 

inspection and evaluation guideline document for each internal component addressed by 

BWRVIP. These reports consolidate various procedures developed by the industry as well as 

information from NRC reviews and safety evaluations (SEs). The purpose of the guidelines is to 

ensure that a unified industry approach, which has been reviewed and accepted by the 

regulator, will be implemented for evaluating and maintaining the integrity of vessels and 

internals.

To date, over 150 guideline reports have been developed. A safety assessment was performed 

by the BWRVIP to identify which components needed to be addressed and to establish 

examination schedules. The safety assessment identified the components that are necessary 

for safe operation and shutdown of the reactor to maintain a coolable geometry, rod insertion 

times, and reactivity control, and to ensure instrumentation availability. It describes which 

locations are likely to experience degradation through IGSCC or other mechanisms and the 

safety consequences of failure for each location. It was determined that the following 

components needed to be addressed: reactor pressure vessel, core shroud, shroud support, 

core spray internals, jet pump assembly, top guide, core plate, lower plenum components, 

vessel inner diameter brackets, standby liquid control, low pressure coolant injection couplings, 

and instrument penetrations. The safety assessment also addressed issues such as reinspection schedules, flaw evaluation, and crack growth.

It was determined that an inspection focus group was needed to address best inspection 

practices and the development of new inspection methods, oversee inspection demonstrations, 

and deal with nondestructive examination uncertainties. Two other important groups were 

formed: the repair focus group to support the development of repair techniques and to define 

technical requirements for performing acceptable repairs; and the mitigation committee to 

establish and demonstrate methods for the mitigation of IGSCC.

To ensure compliance with NRC requirements, the inspection and guidelines reports addressing 

safety-related components are submitted to the NRC for review and approval. Utility BWRVIP 

programs must conform to the quality assurance requirements in Title 10 of the Code of Federal 

Regulations, Part 50, Appendix B, “Quality Assurance Criteria for Nuclear Power Plants and 
2-10

Fuel Reprocessing Plants.” Upon completion of the review of each report, the NRC issues a 

safety evaluation (SE). A SE provides the NRC’s conclusions relative to the BWRVIP 

assessment, as well as the limitations, if any, under which the approval is granted. Licensees 

participating in the BWRVIP and relying on a given report must commit to the program defined 

therein and complete the action items described in the SE. The NRC has determined that a 

commitment to a BWRVIP report and the SE provides reasonable assurance that the applicant 

will adequately manage degradation so that the intended functions of the component will be 

maintained consistent with the current licensing basis.

The BWRVIP program requires utilities to provide inspection results to the NRC. Any required 

repairs resulting from inspections must be qualified and implemented in accordance with 

BWRVIP guidelines. Utilities are required under this program to perform self-assessments to 

ensure compliance with program requirements. Independent assessments of individual nuclear 

power plant BWRVIP programs are conducted by the Institute of Nuclear Power Operations 

(INPO). INPO is a not-for-profit organization established by the nuclear power industry in 

December 1979 to promote the highest levels of safety and reliability in the operation of nuclear 

electric generating plants. This is achieved by establishing performance objectives, criteria, and 

guidelines for the nuclear power industry; conducting regular detailed evaluations of nuclear 

power plants; and providing assistance to help nuclear power plants continually improve their 

performance. Significant INPO findings are incorporated into the BWRVIP guidelines to 

address the issue(s) raised.
3-1

3 IN-SERVICE INSPECTION OF REACTOR INTERNALS 

USING ULTRASONIC TESTING

This section provides a description of the ultrasonic techniques that are applied for in-service 

inspection of BWR reactor internals. The ultrasonic modalities are discussed with their 

strengths and weaknesses. The use of 45° shear wave probes is described, and an explanation 

of sizing using crack tip diffraction is given. High-angle (60°–70° from surface normal) 

compression (longitudinal) waves are useful when inspecting through weld metal. Creeping 

wave probes are described and some signal interpretation is given. The time-of-flight diffraction 

(TOFD) technique propagates ultrasound through the weld metal in a pitch-catch arrangement 

and can provide accurate sizing. Phased-array techniques are being applied with increasing 

frequency in nuclear applications. 

3.1 Ultrasonic Modalities

The most commonly used ultrasonic in-service inspection modalities are listed in Table 3-1 with 

their respective strengths and weaknesses. Quantitative response data are provided in Section 

6 for some of the modalities. For TOFD, an extended explanation is provided in this section. 

The results of a blind test of a phased array procedure are given in Section 7 of this report. 

Table 3-1 Relative Strengths and Weaknesses of Ultrasonic Modalities for Inspection of 

Austenitic Welds of Reactor Internals

Modality Characteristic Strengths Characteristic Weaknesses

45° shear wave Near-side detection and sizing Insensitive for far side cracks

60° to 70° compression 

wave 

Cracks connected to scanning 

surface and diffraction sources on 

far side 

Insensitive to corner trap for farside cracks

60° to 70° shear wave Corner trap for far side cracks Mode converted signals produce 

image artifacts 

Creeping wave Sensitive to surface-connected 

discontinuities 

Multiple modalities require extra 

interpretation 

Time-of-flight diffraction Accurate sizing Transmission through weld metal 

is required 

Phased array Multipath acoustic illumination Relatively low sensitivity or 

dynamic range 

Figure 3-1 shows some typical commercial transducers used for inspecting austenitic materials. 

The transducer on the left is a near-surface, dual-element, 4.0-MHz, 45° refracted longitudinal 

wave probe with a 25-mm focal depth. Second from left is a near-surface, dual-element, 

4.0-MHz, 60° longitudinal wave probe with a 20-mm focal depth. Third from the left is a farsurface, dual-element, 2.0-MHz creeping wave probe with a 50-mm focal depth. Fourth is a 
3-2

far-surface, dual-element, 2.0-MHz, 60° refracted longitudinal wave probe with a 75-mm focal 

depth. On the right is a far-surface, single-element, 2.25-MHz, 45° shear wave probe.

Figure 3-1 Commercial Transducers Typically Used for ISI of Austenitic Materials

3.2 45° Shear Waves

Ultrasonic examination for heat-affected zone cracking using vertically polarized 45° shear 

waves is represented in Figure 3-2, which shows the annulus side of the core shroud on the 

right and the fuel side on the left. Most inspections of the core shroud will be made from the 

annulus side. Cracks form on either side of the weld and can originate from the annulus or from 

the fuel side. The weld metal is shown as a number of individual weld passes with radially 

oriented grains. The weld grains, coarse compared to the parent material, are a significant 

scattering source of ultrasonic energy. The scattering depends strongly on grain size and 

orientation and on the wavelength of the ultrasound. The propagation of 2.25-MHz vertically 

polarized shear waves is impeded significantly by the weld grains, making far-side inspection 

unreliable compared to the inspection reliability for cracks located on the near side.

Sizing with a 45° shear wave probe is accomplished using ultrasonic energy diffracted from the 

crack tip, as shown in Figure 3-3. As illustrated, the transducer is moving toward the crack in 

the three sketches. The top sketch shows the center ray of the transducer striking the corner of 

the crack with the back wall of the test piece. For 45° shear waves, this corner trap signal will 

be bright, generally more than 20 dB brighter than other signals from the crack. The middle 

sketch shows the center ray of the transducer striking the face of the rough crack. Sound will be 

diffracted from the face of the rough crack and recorded by the transducer. The bottom sketch 

shows the center ray of the transducer striking the tip of the crack. Sound will be diffracted from 

this tip, and it can be used to provide an estimate of crack size.
3-3

Figure 3-2 Near-Side Access (left) and Far-Side Access (right) Ultrasonic Examination 

Using Vertically Polarized 45° Shear Waves for Cracks That Are OppositeSurface Connected (from the transducer)

Figure 3-3 Three Ultrasonic Signals (Corner Trap, Crack Face, and Crack Tip) from 

Opposite-Surface Connected Rough Cracks
3-4

3.3 High-Angle Longitudinal Waves

High-angle (60° to 70° from surface normal) compression (longitudinal) wave examination is 

shown in Figure 3-4. The inspection of a K weld profile, typical for a ring-to-cylinder weld in the 

core shroud, is shown. The crack is located on the ring side of the weld. Inspections are shown 

for near-side access on the top and far-side access at the bottom. The crack is connected to 

the same surface that is used for conducting the inspection. Data will show that the crack faces 

return acoustic energy to the transducer. Little or no corner trap reflection is measured in this 

case. 

Figure 3-4 Near-Side (Base Metal Access) Shown in Top Image and Far-Side (Weld 

Metal Access) Shown in Bottom Image Ultrasonic Examination Using HighAngle Compression Waves for Scan-Surface Connected Cracks

3.4 Creeping Waves

The creeping wave technique is a variation of the high-angle longitudinal (L) wave that 

introduces multiple ultrasonic modalities into the component. Figure 3-5 shows the multiple 

modalities transmitted by a creeping wave probe. The transducer and angled elements that 

introduce 75° longitudinal waves and 33° shear waves into the test material are depicted. For 

cracks that are opposite-surface connected, three insonifications must be considered. As the 

creeping wave probe is translated toward the crack, the first insonification will be from the 

75° main longitudinal wave. As the transducer approaches the crack, the secondary 

75° longitudinal waves will insonify it. Finally, the 33° shear waves insonify the crack, and a 

corner trap reflection is recorded. Thus, a complex but unique response is produced. This 
3-5

techniques requires good access to allow all three modalities to insonify the entire inspection 

zone.

Figure 3-5 Creeping Wave Technique

3.5 Time-of-Flight Diffraction Technique

In the TOFD technique, a pair of transducers is placed astride a weld, and flaws throughout the 

volume of the weld and heat-affected zone are insonified as the probe pair is traversed along 

the weld axis. Flaws of all orientations and types are detected and displayed in real time 

(except where no tip signal is present). The limiting factors are access to both sides of the weld 

and propagation of ultrasound through weld metal. 

The TOFD technique differs from that of the conventional pulse-echo ultrasonic inspection in 

two major ways: 

• TOFD has an improved ability to reliably locate and size flaws in unfavorable orientations. 

• TOFD measurements of depth and through-wall extent rely only on the ability to detect and 

measure signal time of arrival.

Typical pulse-echo techniques are sensitive to parameters such as probe position, beam 

spread, incident and skew angles, and flaw roughness and orientation. Because TOFD is a 

forward-scattering technique, it can be much less sensitive to these variables. For these 

reasons, TOFD can be used for accurately locating and sizing flaws and for monitoring flaw 

growth. Work performed by AEA Technology shows the sizing accuracy that TOFD provides 

(Lilley 1994). However, interpretation can be challenging since only tip signals are detected. 

For example, in cases where two small flaws are stacked above one another, will they be 

correctly interpreted as two small flaws or one large flaw?

The approach is based on getting two responses: (1) obstruction of the lateral wave or back 

wall echo for surface-connected cracks and (2) tip-forward scattering. Figure 3-6 shows the 

transmitting and receiving transducer pair used in TOFD. The technique is shown producing 

four responses in the receiving transducer. The first to arrive is the lateral wave. Such a lateral 

wave is always present in the inspection data when no cracks are connected to the scanning 

surface. The cracks tips (upper and lower) diffract acoustic energy sensed by the receiver. 
3-6

These diffraction signals are used to detect, locate, and size cracks in reactor internals. Finally, 

a back-wall echo should be available for the case in which no cracks are connected to the 

opposite surface.

Figure 3-6 Time-of-Flight Diffraction Technique

All responses must propagate through the weld metal. Sufficient scanning surface must be 

available for the motion of both the transmitter and the receiver. For reactor internals, fillet 

welds and other obstructions often limit the inspection.

3.6 Phased Arrays

The phased array technique refers to a process in which ultrasonic data are collected by 

continuous and incremental variation of the ultrasonic beam angle, typically in azimuthal 

direction, while scanning the object under investigation. This technique potentially offers 

advantages over conventional techniques employing search units with fixed beam angles 

because a considerably greater amount of information can be obtained about the shape and 

orientation of reflectors in the test material. 

Each phased array search unit consists of an array of individual ultrasonic transmitting and 

receiving elements. These elements are activated separately in a defined, pre-selected time 

delay pattern to obtain the desired beam angle originating in a plane perpendicular to the 

separation of the individual crystal elements. A linear time delay between the stimulation of the 

different elements results in an inclined sound field, while a variation of the linear time delay 

distribution results in a variation of the angle of refraction. Using a curved time delay pattern 

similar to the shape of an optical lens, a focused sound field at a certain distance from the 

elements is generated. Changing the radius of the optical lens will change the focus of the 

sound field; therefore, a controlled variation of the curved time delay pattern allows a variance of 

the focal length of the sound field within the near-field distance for the transducer site. The 

combined time delay pattern enables the phased array system to electronically steer the sound 

field. Different search unit designs allow for electronic variation of either the incidence angle or 

the lateral angle. The same time-delay pattern utilized for transmitting is used in the receiving 

mode.
3-7

The phased array system can operate using either shear or compressing (longitudinal) waves. 

A phased-array system is capable of employing a number of scanning techniques and is 

controlled remotely using a variety of manipulator systems.

3.7 Deployment

The inspection problems inherent in ISI of BWR internals arise from many factors, including 

component geometry, effectiveness of inspection technique, inspection hardware (scanner 

mechanisms), material type and thickness, surface finish, and limitations with regard to space 

and accessibility. BWR plants have experienced significant inspection challenges and identified 

cracking in access hole covers, steam dryers, jet pump beams, instrument line supports, and 

the core shrouds. Because owners are responsible for addressing their specific inspection 

issues, they prepare and implement internals inspection programs. To select the most effective 

inspection methodology, a variety of factors need to be considered, including materials, 

susceptibility to cracking, applied stresses, radiation fields, accessibility, and available 

inspection techniques. Planning for internals inspection requires careful and detailed technical 

evaluation.

Manipulation (mechanical) systems have been developed for field inspections of internals 

components. The WesDyne Inc. system for automated inspection of the vertical and horizontal 

welds in a BWR core shroud is described in Davis and Huntington (1998). Figure 3-7 shows a 

core shroud examination using the Siemens Power Corporation lower manipulator (Fisher and 

Tagliamonte 1994; permission to use this copyrighted material is granted by the Electric Power 

Research Institute). The inspection is conducted from the outside surface of the core shroud. 

The scanning device uses the vessel wall for support. Horizontal telescoping of the search unit 

is provided to increase coverage and compensate for mechanical interference from the jet 

pumps.
3-8

Figure 3-7 Core Shroud Examination Using the Siemens Power Corporation Lower 

Manipulator. Source: Fisher and Tagliamonte (1994), Electric Power 

Research Institute, Inc.; reproduced with permission granted by EPRI.
4-1

4 RELIABILITY METHODS

The ability of nondestructive testing to reliably detect and accurately size stress corrosion 

cracks in stainless steel piping has been studied in research programs for about 25 years. 

Measurement of the performance of standard and advanced NDE practice has been made in 

PNNL round robins and in the international Programme for the Inspection of Steel Components 

(PISC III). This chapter summarizes the results from these studies. 

Figure 4-1 shows probability of detection (POD) versus through-wall extent of flaws in various 

studies, over the past two decades, using logistic regression. The lowest performance level is 

shown for the Piping Inspection Round Robin (PIRR) (Heasler and Doctor 1996) study that took 

place in 1981. This study is the benchmark of performance in the early 1980s, meeting the 1970s 

ASME Code Section XI prescriptive inspection requirements. It was just after the PIRR that the 

issue of inspection reliability became high priority for austenitic welds. The mini-round robin 

(MRR) (Heasler et al. 1990) results in 1985, following industry-developed training and initial

performance demonstration testing, showed that smaller flaws could be more reliably detected 

with a moderate increase in performance for intermediate flaws. The PISC-AST (Lemaitre 1994) 

results of 1990 show a significant improvement over the PIRR and, except for very small flaws, 

show an improvement over the MRR results. There are two results for the Performance 

Demonstration Initiative (PDI) data (Doctor and Becker 2002), which reflect the performance 

demonstration testing that is in the ASME Code Section XI, Appendix VIII. The PDI data are

plotted for all inspectors who successfully passed the performance demonstration requirements 

and a second curve for all inspectors who attempted the performance demonstration test (passed 

and failed). It can be seen that both of these PDI-based curves show improvements over the 

preceding three round robin studies.

Figure 4-1 Changes in POD versus Through-Wall Dimension of Flaws for Piping Weld 

Inspection in the Past Two Decades (Doctor and Becker 2002)
4-2

4.1 Blind Tests

Performance in blind tests can provide an estimate of two relevant crack detection statistics of 

an inspection procedure: POD and false call probability (FCP). The POD is the probability that 

a crack in a specific length of weld material will be detected by the inspection. The FCP is the 

probability that an inspection will characterize a similar unit of blank material (defect-free) as 

cracked. The tests typically consist of a fixed number of blank and cracked grading units that 

are presented in random order. 

The size of the blind test is determined by its primary objective—to identify procedures/

equipment/personnel that can correctly characterize fixed units of a welded component. It can 

be shown that a test with 10 cracked grading units and 20 blank ones is a powerful test for 

screening inspection capability (Heasler et al. 1986; Heasler et al. 1990; Heasler and Doctor 

1996). If the pass/fail criterion is detecting 8 of the 10 cracks and having not more than 3 false 

calls, then an inspection with an assumed POD = 90% and an FCP = 10% has about a 90% 

chance of passing the test. An inspection with an assumed POD = 50% and a FCP of 30% has 

only about a 1% probability of passing the test. It is important that the cracked and blank 

grading units have a representative amount of part noise and interference signals. Part noise is 

back-scattered ultrasonic signal from the metal’s microstructure and surface geometry (both

inside and outside the part). Interference signals are an important source of systematic noise.

In the field, the unit of material that must be characterized is a weld. Clearly, it is impractical for 

a blind test to use this basic unit. Furthermore, weld units of different sizes cannot be used 

easily in this analysis because comparable statistics can be calculated from only blank and 

cracked weld units of the same size. To avoid confusion and allow for simpler test results, it is 

best to use the same unit of material, called the grading unit, to estimate FCP and POD. 

The comparison of POD and FCP performance of inspection can be plotted on what is called an 

operating characteristics (OC) diagram. These OC diagrams frequently are employed to 

evaluate all sorts of diagnostic systems, particularly in the medical field (Swets and Pickett 

1982; Swets 1983). Points in the OC diagram can represent the performance of individual 

procedure/equipment/personnel. Figure 4-2 shows an example of a typical OC diagram. The 

performance of each procedure/equipment/personnel should fall above the diagonal line in the 

plot. The diagonal line represents the performance that could be achieved with random 

guessing and therefore represents the worst performance that a procedure might achieve. The 

best performance in the diagram is represented by the point (FCP = 0, POD = 1), which 

indicates that a procedure/equipment/personnel discriminates perfectly between cracked and 

blank grading units.

The OC diagram shows that it is not sufficient to examine POD alone. For example, the 

performance of Inspection A in Figure 4-2 is obviously better than Inspection B, if we consider 

only POD. But the performance of Procedure A lies on the diagonal, which shows that 

Inspection A’s high POD (and high false call rate) was achieved by classifying blank material 

as flawed. Inspection B, on the other hand, has demonstrated the ability to more effectively 

distinguish between cracked and blank material.
4-3

Figure 4-2 Operating Characteristics Diagram (Heasler et al. 1986)

In the above discussion, we have assumed that fixed decision criteria were used. In fact, an 

inspector can change the decision criteria, and the performance can vary along what is called 

the relative operating characteristic (ROC) curve shown in Figure 4-2.

4.2 Round Robin Tests

The PISC III conducted studies of ultrasonic inspection of stainless steel piping welds. The 

objective was to identify high-performance inspection techniques and procedures and to inform 

codes and standards organizations of the results obtained (Lemaitre et al. 1996). The study 

investigated the influence of geometrical conditions, such as counterbore, on detection 

performance. Different metallurgical conditions were investigated as well as flaw type. An 

ASME Code Section XI, Appendix VIII type of performance demonstration test was simulated.

Figure 4-3 shows the detection performance for procedures in the inspection of stainless steel 

welds in PISC III. Half of the teams detected more the 75% of the flaws. All of the flaws above 

7 mm in depth had a flaw detection frequency (FDF) greater than or equal to 0.9.

The best techniques used both compression and shear waves. Shear wave techniques were 

more sensitive than compression wave techniques and enabled detection of more difficult flaws. 

Shear wave techniques generated more indications from geometry. Operator skill in combining 

information from shear and compression wave probes was the key to achieving good detection 

and low false calls. A number of teams had a high number of false calls that exceeded the 

acceptance criteria.
4-4

Figure 4-3 Flaw Detection Frequency for Procedures in PISC III, Action 4 (Lemaitre 

et al. 1996)

Figure 4-4 provides the OC diagram for techniques in the inspection of stainless steel welds in 

PISC III. Large scatter in results by teams that used similar procedures (techniques) show that 

they should be applied by skilled and experienced operators. Training is needed on realistic 

specimens. Access to both sides of the weld is needed. Scanning in two different directions, 

perpendicular to the weld, improved detection performance.

Figure 4-4 Operating Characteristic Diagram for Techniques in PISC III, Action 4 

(Lemaitre et al. 1996)
4-5

The five teams that did well on all categories used an automated procedure and worked at the 

noise level of the test specimens. Good depth-sizing techniques used crack-tip diffraction. 

Amplitude drop methods did not work reliably.

4.3 Parametric Studies

Previous work conducted on parametric performance of ultrasonic testing of piping welds is 

summarized in this section. In parametric studies conducted by PNNL, the estimates of 

responses and their variances are made in stainless steel piping (Becker et al. 1981). The data 

from the parametric study are included in this section to show that 6 dB of response variance 

can be expected in UT-ISI. 

Estimates for various sources of response variance are listed in Table 4-1 along with estimates 

made by Silk (1978) and Forli (1979a, b). The standard deviations for the overall inspection 

process are similar, although they are derived from different sources. The data reported by Silk 

resulted from a compilation of approximately 16 different experiments on fatigue cracks in 

aluminum and steel. The data by Forli are the result of a round robin test on 70 m of mild steel 

butt welds, 10 to 26 mm thick. The study reported by Forli employed 12 inspection teams and 

60 cracks, which were predominantly fabrication flaws reflecting lack of root penetration and 

lack of fusion.

Table 4-1 Estimate of Standard Deviation of Inspection Variables (Becker et al. 1981)

Becker et al. (1981), ±dB Silk (1978), ±dB Forli (1979a,b), ±dB

σ3 Within inspection 2 2 4.3

σ2 Between inspections 2 - 2.7

Coupling - 2 -

σ1 Crack-to-crack 5.4 6.1 3.3

Crack orientation 3.6 3.5 -

Crack roughness 1.4 3.0 -

Crack transparency 3.7 4.0 -

σT Total inspection 6.1 6.7 6.1

The within-inspection error is probably the most well-documented of all the variables. The 

PNNL data are based on approximately 500 measurements under laboratory conditions, while 

Forli’s 586 measurements represent field conditions. Forli’s data were collected at 10%, 20%, 

and 50% of a calibration response, which may account for the higher within-inspection 

variability. PNNL’s estimate of 2 dB, for between-inspector standard deviation, is an 

engineering judgment based on the differences expected due to search unit and instrument 

selection. Silk did not estimate between-inspector errors; however, the variable is accounted for 

in the total estimate. Silk also provides an estimate of ±2 dB for coupling errors that are not 

evaluated separately by the other two studies. 
4-6

The total error due to the flaw character is made up of the contributions due to defect 

orientation, roughness, and transparency. Only the total flaw contribution, 3.3 dB, is available 

from the data by Forli. Because the Forli data are based on fabrication flaws, the major 

contributors are most likely orientation and roughness, with transparency having little influence. 

The estimates from the PNNL study are slightly less conservative than those presented by Silk, 

particularly in regard to flaw roughness effects. This difference exists partly because a portion 

of Silk’s data was measured at 5 MHz instead of the 2.25 MHz used in the PNNL study. 

Increased frequency increases the scattering effects due to roughness. 

Figure 4-5 shows the effect of a 75% compressive stress on the measured signal response. 

Although a 75% yield compressive stress may appear very conservative, it should be noted that 

the data are for dry fatigue cracks. Liquid-filled cracks, SCCs, and tight thermal fatigue cracks 

may produce larger losses in signal amplitude with no external stress applied. The 

experimental data are shown in Figure 4-5, along with the response curve for ideal reflectors of 

aspect ratio 0.2 and the fitted curve. The mean of the measured flaws is 7 dB below the ideal 

response curve, with a standard deviation of 4 dB. The 2σ error bars for this measurement also 

are shown in Figure 4-5.

Figure 4-5 Response versus Crack Tightness (Becker et al. 1981)
5-1

5 REACTOR INTERNALS CORE SHROUD MOCKUP DESIGN

In this section, the reactor internals mockup is presented with the data used to design it. A 

description of BWR reactor internals and the core shroud weld configuration was provided in 

Section 2. Sections 3 and 4 provided an explanation of the ultrasonic modalities applied to 

reactor internals and the measurement of reliability. Here, we describe the development of 

some terms for an analysis of possible mockup conditions, our tests of the mockup design to 

support both a parametric study of NDE reliability and a blind test of vendor performance, and 

the assemblies that were built. Illustrations are presented that describe the use of ultrasonic 

testing on core shroud conditions and how these conditions are represented in the mockup. 

The focus of this study is the quantification of inspection reliability of the vertical and horizontal 

core shroud welds. Because BWR internals have different designs, the access restrictions and 

therefore ultrasonic coverage will vary from plant to plant. The mockup described here is 

representative in that it provides a range of access conditions to a crack or degraded material 

that was found to have a significant effect on NDE reliability. This section provides an in-depth 

discussion of the component conditions studied, types of simulated flaws studied, the 

acceptance test employed, and a complete description of the mockup parameters. Quantitative 

performance of the ultrasonic techniques using the mockup is given in Section 6. The statistics 

that were acquired in a blind test of an ISI vendor’s phased array system are given in Section 7.

5.1 Component Conditions

The inspection areas of interest include the heat-affected zone of the horizontal and vertical 

welds in the core shroud. Inspection from the outside of the core shroud would normally be 

preferred because the inside is accessible only through the top guide. But the inspection 

modality from the outside must look though the weld material, and the weld microstructure is 

known to make this inspection unreliable when vertically polarized shear waves are used. A 

normal beam inspection from the outside might detect the crack, but is complicated by the core 

shroud geometry. Angle beam inspections from the support ring surface would need to use 

steep inspection angles (30° or less) and inspect through more than 15 cm of stainless steel. 

Access conditions may limit the completeness of an outside-only inspection. The space 

between the core shroud and the vessel wall is not large. The distance between the two is 

typically 45 cm. The region between welds H4 and H9 contains the jet pumps. The geometry of 

the support rings can lead to insonification difficulty. The presence of weld crowns can prevent 

proper scan patterns. 

The core shroud and support assembly is made from two types of base metal—wrought 

austenitic stainless steel and Alloy 600 (Inconel). The wrought austenitic stainless steel portion 

is above weld H7 (see Figure 2-2, where the weld numbers are identified). The nominal 

thickness of this portion of the core shroud is 40 mm to 50 mm. But it is also composed of 

75-mm-thick support rings. The portion of the core shroud that is 40 mm to 50 mm thick is 

referred to as the shroud cylinder. The Alloy 600 shroud support assembly is below weld H7. 

The nominal thickness of the Alloy 600 is 75 mm. It is also composed of a thicker shroud 

support pedestal. 
5-2

Table 5-1 lists the weld profiles used in BWR core shrouds. The weld profiles vary according to 

location in the core shroud, as described in Table 5-1. The amount of weld metal in the 

ultrasound path to a flaw or degradation site can influence the inspection reliability. For this 

reason, all of the weld profiles listed in Table 5-1 are represented in the mockup.

Table 5-1 Weld Profiles Selected for Study

Weld Profile Typical Core Shroud Weld

Single V Vertical welds

K Ring to cylinder welds

Double V Cylinder to cylinder welds

Double J Vertical welds

The configuration of the welded assemblies for the mockup is given in Figure 5-1. The size of a 

welded assembly for use in the mockup is 36 cm across the weld, 25 cm along the weld, and 

5 cm thick. The thickness was chosen to be representative of the BWR core shroud. The 

distance along the weld is enough to allow for two grading units in a blind test. The distance 

across the weld is enough to permit high-angle insonification of the weld’s heat-affected zones. 

The approximate weight of such an assembly is 36 kg, light enough to facilitate manual handling 

by PNNL NDE staff.

Four weld profiles were selected for representation in the mockup. Figure 5-1 provides a sketch 

of the weld profiles and the descriptive name given to that profile, such as K weld. The weld 

metal in wrought-to-wrought stainless steel welded assemblies has a coarse-grained 

microstructure and strongly scatters ultrasound. The amount of the weld metal and the throughwall profile of the weld are important variables in the parametric study reported here. 

It is interesting to note the features that are missing in the assemblies shown in the Figure 5-1. 

No counterbore or weld root conditions are present; those conditions generally add difficulty to 

the NDE reliability of the UT-ISI. The absence of these piping conditions will simplify the 

analysis of reliability, as described in Section 6. Typical weld crown conditions that occur in 

installed BWR core shrouds were included in the mockup.

Figure 5-2 shows the first five welded assemblies built to test and assess the mockup design. 

These assemblies—with various weld profiles, side-drilled holes, notches, and other reflectors—

were used to evaluate the usefulness of the assemblies. The welds were fabricated in the 

vendor’s controlled environment using a convenient down-hand welding position. Figure 5-3 

shows the welding of one of the reactor internals mockup assemblies. The results of initial 

testing performed on these assemblies are reported in Section 5.3.
5-3

Figure 5-1 Welded Assembly Configuration for Mockup

Figure 5-2 First Welded Assemblies
5-4

Figure 5-3 Welding of Reactor Internal Mockup Assembly (photo courtesy of FlawTech, 

Inc., Concord, North Carolina; reprinted with permission)

Four crack initiation sites can be expected on welds in the core shroud. Figure 5-4 shows a 

K-weld profile with a crack initiation from the fuel side of the top guide support ring of a BWR 

core shroud. In many BWR core shrouds, this weld is labeled the H3 weld. In-service 

inspection data show that cracks can initiate from the fuel or annulus side of the core shroud. 

The ring or cylinder side of the weld’s heat-affected zone can crack. Four cracking sites should 

be considered for heat-affected zone cracking of the welds in the core shroud. In this case, the 

crack originates on the fuel side of the core shroud at the edge of the weld crown, the inside ring 

cracking site. Again, cracks are most likely to follow the angled weld profile. Weld crowns are 

variable, and some are not ground flush, which can limit the inspection surface.
5-5

Figure 5-4 Four Crack Initiation Sites

5.2 Reflector Types

The mockup was designed to provide reflectors representative of field conditions and to meet 

the intent of the requirements of ASME Code Section XI, Appendix VIII performance 

demonstration test. At least 10 cracks are required over a range of depths with various 

conditions such as weld crown or limited scanning. Grading units are established with a 

minimum of 75 mm of weld, and blank grading units are provided as required. The types of 

flaws are to be representative of field flaws such as SCC.

Stress corrosion cracks are very difficult to grow to simulate the NDE responses of field-grown 

SCC. As a result, other crack types are used to mimic SCC for the purpose of evaluating NDE 

reliability. Two types of flaws were selected for the mockup—thermal fatigue cracks (TFCs) and 

weld solidification cracks (WSCs). These two crack types have very different ultrasonic 

response properties. The TFCs produce strong reflections similar to SCC that has become 

transgranular or fully open. The WSCs are reported to be more like tight SCC (Watson and 

Edwards 1996). The WSCs are made by filling an area that has been notched to the flaw 

dimensions (in depth and length) with a contaminated gas tungsten arc weld. The WSC forms 

in the welded notch. 

Rough crack faces can be expected in the case of SCC. Figure 5-5 shows that thermal fatigue 

crack implants can provide such roughness. The shape and size of the crack can be measured 

before and after the implant in the mockup assemblies.

Fuel Side

Ring Cracking Site

Fuel Side

Cylinder Cracking

Site

Annulus Side

Ring Cracking Site

Annulus Side

Cylinder Cracking Site
5-6

Figure 5-5 Example of Thermal Fatigue Crack Implant (photo courtesy of FlawTech, 

Inc., Concord, North Carolina; reprinted with permission)

Artificial TFCs are employed in welded stainless steel assemblies for tests of austenitic piping 

as specified in ASME Code Section XI, Appendix VIII. Figure 5-6 provides the notation and 

design of TFC implants. The mockup used this type of crack (and weld solidification cracks) to 

represent SCC in the core shroud. The top and bottom illustrations in Figure 5-6 are typical of 

piping in which the crack is formed after the weld preparation surfaces are machined onto the 

two pieces of stainless steel to be joined by welding but before welding starts. A tension bar is 

welded to one of the base metal pieces, and the TFC is initiated using a process of heating and 

cooling the desired crack location while the tension bar mechanically cycles the material. The 

crack forms, and the tension bar separates from the parent material with a small coupon of base 

metal attached. This coupon is removed from the tension bar and seal-welded back onto the 

base metal in preparation for welding. The welding is done, joining the two base metal pieces 

with the tight thermal fatigue crack created in base metal to create a representative condition for 

near-side inspection access. The idea is to create a crack that is oriented along the heataffected zone of the weld and provide a sound path to the flaw face that is entirely within base 

metal.

The second illustration from the top in Figure 5-6 shows the work needed to create a TFC that is 

designed for inspection through the weld, far-side access. Here the weld is completed first, and 

then the crack is created in the heat-affected zone of the weld. To do this, an excavation that 

faces the weld is formed, and the tension bar is attached by welding. The thermal and 

mechanical cycling separates the tension bar from the test specimen with a piece of the test 

specimen attached to the tension bar as in the first case. Then the coupon is removed from the 

tension bar and seal-welded back into the test piece. The final step is to fill the excavation with 

weld metal to create the far-side inspection condition. This approach creates a crack that 

follows the orientation of the heat-affected zone and provides a sound path through the 

austenitic weld to the flaw face. 

The third illustration in Figure 5-6 is a test of the acoustic properties of the implant process. The 

crack is formed with a tension bar, but the coupon is not replaced and seal-welded to form a 

crack. The area where the crack should be is repaired by welding. This approach creates a site 

for testing for detectable conditions associated with the coupon implant process. 
5-7

Figure 5-6 Thermal Fatigue Crack Notation

Machined reflectors in the PNNL mockup provided reference reflectors for calibration of the UT 

modalities that are used in the inspection of the core shroud. A number of notches were 

included for use in the laboratory phase of the study. These machined reflectors were used to 

establish the upper bound on the UT response and to quantify response dependence on 

reflector properties such as size and tilt. Notches were used in an evaluation of material 

properties such as attenuation and scattering. A series of vertical notches, from 5% to 75% 

through-wall, and notches at positive and negative angles relative to vertical at 30% throughwall, were built for the parametric study.

The mockup was designed to permit evaluation of UT reliability for inspection through base 

metal and weld metal. All cracks and notches were installed in the heat-affected zone of the 

weld. Because the TFCs are implanted with weld metal on one side, the mockup contained 

separate flaws for assessing base metal and weld metal inspection effectiveness.

Weld CL

Facing Base Metal

TFC

TFC

Weld CL

Weld Repair Condition

Weld CL

TFC

Weld CL

Flaw Implant

Coupon

TFC

Facing Weld Metal

Facing Base Metal

TFC

Flaw Implant

Coupon

Excavation

for Implant

Create a TFC

Remove Coupon

Fill with Weld

Metal

Flaw Implant Coupon

TFC
5-8

5.3 Acceptance Tests

It was important to measure some of the responses for ultrasonic modes being used by industry 

before completing the mockup design. The data from the mockup were sufficient to estimate 

flaw response as a function of flaw through-wall extent for most of the UT inspection modalities 

described in Section 3. The responses were estimated separately for corner-trap and tipdiffracted signals. The responses and their variances were estimated separately for all 

inspection modalities used to inspect through the weld. 

As described in this section, there are a number of cases in which the most convenient 

inspection is through the weld metal. The reliability of these inspections is known to be in doubt 

and, for this reason, it was estimated separately. 

Sometimes detection could be accomplished using diffracted signals. This detection method 

was selected when no corner-trap method was available. Diffracted signals are generally 20 dB 

lower in amplitude than corner-trap reflections, so the use of diffracted signals was evaluated 

separately in terms of their signal-to-noise ratio. 

The grain structure of stainless steel limited the UT response, and higher frequencies were 

often strongly attenuated, especially in thicker material. Variability in grain structure, in different 

samples of type 304 stainless steel, had an impact on ultrasonic signal strengths.

Table 5-2 shows the specimens used to validate the mockup design, a necessary first step 

before building the complete mockup for use in a parametric study and a blind test estimating

ISI performance. Two types of welding were used—shielded metal arc and submerged arc 

welding. Four weld profiles were used—single and double V, J, and K. Each of the first five 

plates shown in Table 5-2 was inspected and UT images created. A signal-to-noise level for 

each of the notches and TFCs was calculated from the UT images and formed the basis for 

evaluation of the four weld types (single V, double V, J, and K).

Table 5-2 First Five Plates Used in Design Tests for Mockup

Plate Name Weld Type Calibration Cracks Notches

CS-02-01 Shielded metal arc, 

Single V

3 side drilled holes 10% TFC 10% and 20%

CS-02-02 Submerged arc, 

Single V

3 side drilled holes 10% TFC 10% and 20%

CS-02-03 Shielded metal arc, 

Double V

3 side drilled holes None 10% and 20%

CS-02-04 Shielded metal arc, 

J weld

3 side drilled holes None 10% and 20%

CS-02-05 Shielded metal arc, 

K weld

3 side drilled holes None 10% and 20%
5-9

A signal-to-noise ratio of 6 dB or greater was required for a flaw to be recorded. This is based 

on prior round robin test experience in which detection results were found to be unreliable when 

the signal-to-noise ratio was less than 6 dB. In the base metal inspection, the creeping wave 

mode detected all of the flaws with a signal-to-noise ratio of 16 dB or greater. Turning to the 

difficult inspection of weld metal, there was no detection of the 10% notch in either the double-V

or the single-V weld. The 10% notch in the K weld, as viewed from the angled-side, also was 

undetected. All three of these welds produced a difficult-to-inspect condition. 

A 40% through-wall weld repair condition was fabricated. First, a TFC that followed the 30° 

angle of the weld fusion line was created. This 6-mm thick, 40% through-wall coupon was 

removed and the double-V weld and repair made with the shielded metal arc welding process. 

This weld repair was imaged with the 2-MHz dual, 10 × 34 mm, 50-mm focus, 75° longitudinal 

probe, from the four possible access directions with results as follows.

For inspections from the scan surface on the same side of the assembly as the repair with weld 

metal access, the weld repair was not acoustically detected. For inspection from the repaired 

surface with base material access, the weld signals in the repair region were higher in amplitude 

than outside the weld repair area. The peak signals from the weld repair weld signal areas were 

−8.2 and −7.1 dB, respectively. The base metal peak weld signals were −9.5 dB. Thus we see 

a difference of up to 2.4 dB in the weld repair region. This difference is small enough that it 

would be called a no-detect condition. Images from the opposite surface inspection show no 

detection of the weld repair from either the base metal or weld metal access.

Figure 5-7 shows the responses from notches, side-drilled holes, and thermal fatigue cracks 

using 45° shear waves. Ultrasonic inspections of the submerged arc single-V welded plate 

were performed. This plate contains one 10% deep and one 20% deep notch, a 10% deep 

TFC, and three side-drilled holes at one-quarter, one-half, and three-quarters of the wall 

thickness depth. Data in Figure 5-7 show that the base metal inspection is straightforward. The 

strength of this mode is that a strong corner-trap signal for flaw detection and a weaker tip 

signal for flaw depth sizing are found in base metal inspection. The confusion from multiple 

wave modalities is eliminated, as the compression (longitudinal) wave is not present. This 

wave, however, did not penetrate the weld metal very well. 

The responses in Figure 5-7 are given relative to the noise level. The root mean square noise 

level is shown in decibels as a function of time from the transducer excitation. Shoe noise is the 

largest contributor to noise level until approximately 30 microseconds has passed. Part noise, 

from the stainless steel, is shown at about −77 dB. An elevated noise level at the back surface 

of the component is shown.

Figure 5-8 provides the responses using 45° shear waves for base metal and weld metal 

access. Base metal results (hollow symbols) show that all defects were detected. This includes 

the notch tip signals that are 25 dB below the corner-trap signals (the TFC data are not plotted 

but fall approximately 5 dB lower than notch data). The weld metal inspection detected only the 

one-quarter wall thickness deep side-drilled hole and both notch corner trap signals. These 

signals are significantly lower than their base metal counterparts (−15 dB and −35 dB, 

respectively). 
5-10

Figure 5-7 Near Side Access (Through Base Metal) Responses from Notches, SideDrilled Holes, and Thermal Fatigue Cracks Using 45° Shear Waves in a 

Submerged Arc Single-V Welded Assembly.

Figure 5-8 Responses Using 45° Shear for Base Metal and Weld Metal Access in a 

Submerged Arc Single-V Welded Assembly

Data from the 60° longitudinal, 2-MHz, 75-mm focus probe showed that the base metal 

inspection found the three side-drilled holes (SDHs), the flaw corner-trap, and tip signals. From 

the weld metal inspection only the one-quarter wall thickness and the three-quarters wall 

thickness SDH signals were found. Results are shown in Figure 5-9.

-110

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

0 10 20 30 40 50 60

Time (usec)

Amplitude (dB)

SDHs Notch Corner Traps TFC corner trap Notch Tip Signals TFC Tip Signals

Part Noise Surface Scratches

Shoe Noise
5-11

Figure 5-9 Responses Using 60° Longitudinal for Base Metal and Weld Metal Access in 

a Submerged Arc Single-V Welded Assembly

Of the probes evaluated in this laboratory study, the probe having the best through-weld 

inspection effectiveness was the creeping wave probe. At the design angle of 75°, a 

longitudinal wave is produced along with a 33° shear wave, and a creeping wave (with 

longitudinal velocity) that travels just below the surface is also generated. Four modes of 

inspection are possible with this probe as was shown in Figure 3-5. The first mode is from the 

near-surface creeping wave that will detect very deep cracks or crack openings (base of cracks) 

that are scan-surface-connected. The second mode is the far-surface creeping wave generated 

by transverse (head) waves leaking from the near-surface creeping waves, which convert to a 

creeping wave at the far surface. The third mode is from the 33° shear wave mode converting 

at the far surface to a longitudinal wave then reflecting off a crack face and returning to the 

transducer as a longitudinal wave (noted as the SH-L-L wave). The fourth mode is the 75° 

longitudinal wave mode. 

The results from the SH-L-L mode are shown in Figure 5-10. Both base metal and weld metal 

access conditions show detection of the two notches and one TFC. The response levels differ 

in that the base metal 20% notch is approximately 16 dB higher than the 10% TFC or 10% 

notch. In contrast, the weld metal shows only a 5 dB to 9 dB difference with the 10% TFC 

actually appearing stronger than the 10% notch. However, in both weld metal and base metal 

inspections, the 20% notch response is larger than the 10% flaw responses, as expected.
5-12

Figure 5-10 Response Using Creeping Wave Probe SH-L-L Mode in a Submerged Arc 

Single-V Welded Assembly

Figure 5-11 shows the responses for the 75° compression (longitudinal) mode of a creeping 

wave probe. From the manual results (confirmed also by automated data), the 10% TFC from 

the weld access side is the strongest signal. It is 5 dB stronger than the base metal access 

condition and occurs earlier in time. The 20% notch produces the same response from both 

base metal and weld metal access conditions, but the weld metal response is delayed in time 

due to velocity differences in the weld material or beam steering. Similar results are found for 

the 10% notch.

Table 5-3 gives the test results used for guiding the mockup design. Responses are measured 

from corner traps, side-drilled holes, and crack tips. Four ultrasonic modes are used—45° 

shear, 60° longitudinal, creeping wave (SH-L-L mode), and 75° longitudinal. Two access 

conditions were tested—cracks on the near side of the weld and cracks on the far side of the 

weld. Table 5-4 lists the far-side access responses from a 20% notch for 75° longitudinal probe 

for the various weld profiles. The double-V weld profile had the lowest response of all the weld 

profiles for far-side access using high-angle longitudinal waves. From this data, the double-V

weld was selected as the difficult configuration for the mockup. Two K-weld TFCs are also in 

the mockup and will allow a comparison of TFCs in the K and double-V welds.

In summary, the inspection through base metal is straightforward. Flaw tip signals were found 

with the 45° shear, 45° longitudinal, 60° longitudinal, and 75° longitudinal probes, thus providing 

sizing information. Signal-to-noise levels of approximately 8 decibel or better were achieved. 

Inspections through the weld metal, however, proved to be more difficult. Detection of the 

shallow flaws is accomplished most reliably with the 75° longitudinal probe. All three reflectors 

(10% and 20% notches, and 15% TFC) were detected with each of the three probe modes—

longitudinal wave, shear-L-L wave, and ID creeping wave.
5-13

Figure 5-11 Responses Using Creeping Wave Probe 75° Longitudinal Mode in a 

Submerged Arc Single-V Welded Assembly

Table 5-3 Test Results Used for Mockup Design – Submerged Arc Single-V Welded 

Assembly

Ultrasonic Mode Corner Trap Side Drilled Holes Tip Signal

45° shear, near-side −26 dB −40 dB −53 dB

45° shear, far-side −62 dB −55 dB No detect

60° longitudinal, near-side −78 dB −75 dB −82 dB

60° longitudinal, far-side No detect −82 dB No detect

Creeping wave, SH-L-L, near-side −62 dB

Creeping wave, SH-L-L, far-side −70 dB

75° longitudinal, near-side −55 dB

75° longitudinal, far-side −68 dB

Table 5-4 Far-Side Access Responses from a 20% Notch for 75° Compression 

(Longitudinal) Waves for Various Weld Profiles

Weld Profile 75° L Response, Far-Side Access

Single V, SMAW −70 dB

Single V, SAW −68 dB

Double V −76 dB

J −75 dB

K −72 dB
5-14

5.4 Completed Mockup

The mockup consists of small welded plates designed to represent conditions important to 

ultrasonic inspection of the core shroud and the shroud support assembly. These small welded 

plates represent the vertical and horizontal shroud weld cracking sites. Notches are included in 

the mockup for an evaluation of the effects of through-wall extent and tilt. Thermal fatigue 

cracks and weld solidification cracks have been placed in the heat-affected zones of the weld in 

sufficient number so that the mockup can be used to simulate a blind test that meets the intent 

of ASME Code Section XI, Appendix VIII. The mockup can be divided into “grading units” with 

twice the amount of blank material as cracked material. The assemblies permit inspection from 

both sides (fuel and annulus sides) and weld crowns include both “as-welded” and ground-flush 

conditions. 

The mockup contains both 304 stainless steel and Alloy 600 (Inconel). The Alloy 600 material 

costs, machining costs, and welding costs are two or three times higher than for type 304 

stainless steel, so the mockup uses relatively little Alloy 600. The Alloy 600 portion of the 

mockup is intended for comparing and contrasting the ultrasonic signal qualities with more 

extensive measurements in 304 stainless steel. The mockup consists of 40 welded assemblies 

of 304 stainless steel and 10 welded assemblies of Alloy 600. 

The mockup contains representative weld profiles because they are likely to affect the UT 

response. The weld profile changes the grain alignment of the weld metal because the adjacent 

base metal serves as the principal heat sink. The amount of heat generated in the welding 

process is greater for submerged metal arc welding than for manual welding. The heat 

deposition rate and the size of the weld bead affect the attenuation and scattering properties of 

the weld metal. 

The mockup permits evaluation of UT reliability for inspection through base metal and weld 

metal. Thermal fatigue cracks are installed in the heat-affected zone of the weld. Because the 

thermal fatigue cracks are implanted with weld metal on one side, the recommended mockup 

contains separate flaws for base metal and weld metal inspection. 

The mockup was designed to permit inspection on both sides of the assemblies. In an 

inspection of the core shroud, cracks can be connected to the scanning surface or connected to 

surface opposite to the scanning surface. The mockup also permits simulation of inspection 

from the inside and outside of the core shroud. 

Machined reflectors are provided for calibration of the UT modalities used in core shroud 

inspections. A number of notches are included for use in the laboratory phase of this study. 

These machined reflectors will be used to establish the upper bound on the UT response and to 

quantify the response’s dependence on reflector properties such as size and tilt. The notches 

will be used also in an evaluation of material properties such as attenuation and scattering. 

Table 5-5 shows the notch depths and angles included in the mockup.
5-15

Table 5-5 Notch Depths and Angles Selected for Mockup

Depth (%) Angle (degrees)

−30 −25 −20 −15 −10 −5 0 5 15 25

5 x

10 x

20 x

30 x x x x x x x x x x

40 x

75 x

The mockup was designed within the spirit of the ASME Code Section XI, Appendix VIII. At 

least ten cracks are provided over a range of depths with various conditions such as weld crown 

or limited scanning. At least four of the cracks are 10% to 30% through-wall, at least four of the 

cracks are 31% to 60% through-wall, and at least two of the cracks are 61% to 100% throughwall. The cracks are randomly located in grading units. Grading units are described in Section 

XI, Appendix XIII of the ASME Code. 

Grading units were established with a minimum of 75 mm of weld, and blank grading units were 

provided as required. The grading units were all the same size with one or two grading units 

located on each side of the weld for every assembly. Grading units were located randomly 

along the weld, and cracks were located within the grading units according to the ASME Code. 

Enough flaw-free material was provided so that 20 blank grading units were contained in the 

mockup. 

The mockup consisted of both stainless steel plates and Alloy 600 plates, 32 welded assemblies 

of 304 stainless steel and 5 welded assemblies of Alloy 600. A complete list of mockup 

assemblies appears in Table 5-6. 
5-16

Table 5-6 Mockup Assemblies. Size of the cracks is withheld to permit mockup re-use 

in future blind tests.

Material, 

Thickness Weld Surface Reflectors

304 SS, 50 mm SMAW, single V Flush TFC, 10 and 20% notches, 3 SDHs 

304 SS, 50 mm SAW, single V Flush TFC, 10 and 20% notches, 3 SDHs 

304 SS, 50 mm SMAW, double V Flush 10 and 20% notches, 3 SDHs 

304 SS, 50 mm SMAW, J Flush 10 and 20% notches, 3 SDHs 

304 SS, 50 mm SMAW, K Flush 10 and 20% notches, 3 SDHs on flat side, 10 and 20% 

notches on angled side 

304 SS, 50 mm SMAW, double V Flush Base metal facing TFC, weld repair, weld metal facing 

TFC 

304 SS, 50 mm SMAW, double V Flush 2 TFCs 

304 SS, 50 mm SMAW, double V Flush 2 TFCs 

304 SS, 50 mm SMAW, double V Flush Blank 

304 SS, 50 mm SMAW, double V Flush Blank 

304 SS, 50 mm SMAW, double V Flush Blank 

304 SS, 50 mm SMAW, double V Flush Blank 

304 SS, 50 mm SMAW, double V Flush Blank 

304 SS, 50 mm SMAW, double V As welded Blank 

304 SS, 50 mm SMAW, double V Flush Blank weld, 10% end milled notch 

304 SS, 50 mm SMAW, double V Flush 30% and 40% notches 

304 SS, 50 mm SMAW, double V Flush 5% and 75% notches 

304 SS, 50 mm SMAW, double V As welded 30% notch 

304 SS, 50 mm SMAW, double V Flush 30% notches at −5, −10, 15° 

304 SS, 50 mm SMAW, double V Flush 30% notches at −20, −25, −30° 

304 SS, 50 mm SMAW, double V Flush 30% notches at 5, 15 25° 

304 SS, 50 mm SMAW, double V Flush TFC 

304 SS, 50 mm SMAW, double V Flush 2 TFCs 

304 SS, 50 mm SMAW, double V As welded TFC 

304 SS, 50 mm SMAW, double V As welded TFC 

Alloy 600, 75 mm SMAW, double V Flush TFC 

Alloy 600, 75 mm SMAW, double V Flush TFC 

Alloy 600, 75 mm SMAW, double V Flush TFC 

Alloy 600, 75 mm SMAW, double V Flush Blank weld, 10% end milled notch 

304 SS, 50 mm SMAW, double V Flush 50% notch 

304 SS, 50 mm SMAW, double V Flush TFC 

304 SS, 50 mm SMAW, double V As welded TFC 

304 SS, 50 mm SMAW, K Flush 2 TFCs 

Alloy 600, 75 mm SMAW, double V Flush TFC 

304 SS, 50 mm SMAW, double V Flush WSC 

304 SS, 50 mm SMAW, double V Flush WSC 

304 SS, 50 mm SMAW, double V Flush WSC 

SS: stainless steel; SMAW: shielded metal arc weld; SAW: submerged arc weld; TFC: thermal fatigue 

crack; WSC: weld solidification crack; SDH: side-drilled hole 
6-1

6 PARAMETRIC STUDY

An analysis of signal-to-noise ratio is important for all physical measurements, especially where 

performance is relatively low. The effects of surface finish, crack-to-crack variability, and other 

factors that cause the responses to vary should be qualified to predict reliability and an analysis of 

variance is one method to achieve this. 

Figure 6-1 provides an example of inspection noise sources with variance. Three noise sources 

are shown—base metal part noise, shoe noise, and electronic noise. The responses are shown 

using a logarithmic scale and are calibrated using a standard reflector—a 10% end milled notch in 

a 50-mm thick stainless steel specimen. Time in the part is shown in microseconds on the 

abscissa, and this scale can be converted to part path by multiplying by the speed of sound in the 

material and dividing by two. 

Electronic noise is shown at a constant level of approximately −100 dB, five orders of magnitude 

below the response from the reference reflector. The vertical bars on the data points show one 

standard deviation, approximately ±2 dB, with the responses from electronic noise. Electronic 

noise can be reduced by improving the electronics or by temporal averaging. 

Shoe noise is caused by acoustic echoes within the standoff material, which for contact probes is 

typically a plastic wedge. Shoe noise, the principal noise source early in time, decays to the level 

of the electronic noise late in time. Shoe noise is reduced by separating the receiver from the 

transmitter in the standoff material and in optimizing the plastic wedge design to reduce acoustic 

reflections in the wedge. 

Figure 6-1 Noise Sources Using 45° Shear Waves

-120

-100

-80

-60

-40

-20

0

20 30 40 50 60 70 80 90

time (usec)

Response relative to cal notch (dB)

part noise (03) shoe noise electronic noise
6-2

Part noise is caused by the microstructure of the base metal, type 304 stainless steel. This is the 

principal noise source for inspections of stainless steel components. Part noise levels vary but in 

the example drop by −40 dB after 70 microseconds, three orders of magnitude below the 

calibration reflector. Part noise can be reduced by using spatial averaging techniques.

6.1 Near-Side Access

Measurement of detection performance in the parametric study is useful for predicting 

performance of an inspection in the field and predicting the results of a screening test. 

Performance is measured by the ability to correctly distinguish cracked from blank material. The 

calibration reference reflector that was used for these tests was a 10% through-wall notch.

6.1.1 45° Shear Waves at 2.25 MHz, Base Metal Access

Figure 6-2 shows the response-time chart of a highly effective detection test, the response of 

notches insonified through base metal at 2.25-MHz, 45° shear waves. The high performance is 

evidenced by the separation of the corner-trap and tip signal from the non-flaw interference and 

noise sources. These data were obtained with the notches on the surface opposite to the 

inspection surface. Figure 6-2 shows the part noise with standard deviation, interfering signals 

from surface scratches, interfering signal from weld noise, corner trap signals, and tip signals. 

The nomenclature used in this figure is that a notch that has a through-wall depth of 75% will be 

identified as N75.

Figure 6-2 Responses of Notches to 45° Shear Wave Insonified Through Stainless Steel 

Base Metal

N5

N75

N10CAL

N40 N30

N10

N20

N75 N40

N30

N20

N10

N10CAL

N5

-70

-60

-50

-40

-30

-20

-10

0

10

20

20 30 40 50 60 70 80

Time (usec)

Amplitude Relative to Cal Notch (dB)

Part Noise Corner Tip Weld Noise Scratches
6-3

The part noise is shown as a time series of mean and standard deviation of 16 A-scans taken on 

stainless steel mockup plate No. 03. This noise level is present in the base material and is 

produced by backscattered ultrasonic energy from the base metal microstructure. 

Data for surface scratches are also shown in Figure 6-2, and they produce a response of about 

20 dB higher than the minimal part noise. Time of flight to the scratches is not different from the 

corner trap signals from the notches. Signals from weld microstructure (weld noise) produce 

response and time behavior similar to scratches. 

Corner-trap responses from the notches are +40 dB to +60 dB above the noise floor and +6 dB to 

+26 dB above the scratches and weld noise. The amplitude variability of 20 dB in the notches 

cannot be explained by notch size, as noted in Section 6.2. Tip signals are also shown in Figure 

6-2. Good amplitude-time separation from the other signals is shown except for the tip signal 

from the 5% notch.

6.1.2 75° Longitudinal Waves at 2.0 MHz, Base Metal Access

Figure 6-3 shows the results of the 75° longitudinal wave at 2.0 MHz on the notches in the 

mockup specimens. These data were obtained with the notches on the surface opposite to the 

inspection surface. Figure 6-3 shows the part noise with standard deviation, corner trap signals, 

and tip signals. 

The part noise is shown as a time series of mean and standard deviation of 16 A-scans taken on 

stainless steel mockup plate #23. This noise level is present in the base material and is produced 

by backscattered ultrasonic energy from the base metal microstructure. The noise level is 

elevated with respect to the calibration notch (−25 dB) compared to the 45° shear-wave (−55 dB). 

Note: the scattering has not increased relative to the shear-waves; instead, the calibration-notch 

response is diminished by mode conversion losses.

Corner trap responses from the notches are −6 dB to +10 dB relative to the calibration notch. The 

amplitude variability of 16 dB in the response from the notches does not follow the same pattern 

as the 45° shear-wave data. Considerable time variability is also evident. 

Tip signals are also shown in Figure 6-3. Good amplitude-time dependence is shown except for 

the tip signal from the 10% notch. Most of the tip signals are at 25 dB to 35 dB of the noise level 

because they are not diminished by mode conversion losses.
6-4

Figure 6-3 Response of Notches to 75° Longitudinal Waves at 2.0 MHz Insonified 

Through Stainless Steel Base Metal

6.1.3 Crack Responses

The responses from the TFCs and WSCs were measured when insonified through base-metal 

using 45° shear waves. Figure 6-4 shows the TFC responses are −6 dB of the notches (see 

Figure 6-2). The signals from the WSCs could be found with careful measurement and a-priori 

knowledge of crack location. But the signals from the WSCs could not be distinguished from all of 

the interference signals. Figure 6-5 shows the WSC responses for the 30% through-wall crack 

are −20 dB of the response from a 30% through-wall TFC when using 45° shear waves.

6.1.4 Thermal Fatigue Cracks in Alloy 600

Figure 6-6 shows the response-time chart for thermal fatigue cracks in Alloy 600 using 45°, 

2.25-MHz SV-waves for base metal access. The response from the cracks varied from −8 dB to 

−22 dB of the calibration notch. Some interference from the weld was present at –21 dB. Surface 

scratches produced interference at a relatively low level of –30 dB. Part noise is shown with 

standard deviation. The responses from the tip signals of the thermal fatigue cracks are shown to 

be clearly separate from the part noise.

N50

N40

N20 N30

N75

N10

N5

N50

N10

N20

N30

N75 N40

-70

-60

-50

-40

-30

-20

-10

0

10

20

10 30 50 70 90

Time (usec)

Amplitude Relative to Cal Notch 

(dB)

Part Noise Corner TIP
6-5

Figure 6-4 Thermal Fatigue Crack Responses to 45° Shear Waves at 2.25 MHz Insonified 

Through Stainless Steel Base Metal

Figure 6-5 Weld Solidification Cracks on the Near Side Using 45° Shear Waves in 

Stainless Steel

TFC10

TFC40

TFC30

TFC75

TFC50

TFC75

TFC50 TFC30

TFC40 TFC10

-70

-60

-50

-40

-30

-20

-10

0

10

20

20 30 40 50 60 70 80

Time (usec)

Amplitude Relative to Cal Notch (dB)

Part Noise - Plate 24 Corner TIP Surface Scratches
6-6

Figure 6-6 Response of Thermal Fatigue Cracks for Near-Side Access in Alloy 600

6.2 Echo Dynamics

The echo-dynamic curves were recorded for the response of notches to 45° shear waves in 

stainless steel. The analysis described here is applicable to other cases, but the specific results 

will vary with part thickness, notch size, and transducer characteristics. Figure 6-7 shows the 

echo-dynamic curves for three of the notches in the mockup. These curves were measured to the 

surrounding noise level. Note that the shapes of the curves do not depend on through-wall extent 

from a 5% (N5) to a 75% (N75) through-wall notch.

Figure 6-8 shows that the duration of the echo-dynamic curve for notches in stainless steel has an 

approximately linear relationship with amplitude (in decibels). No relationship exists between 

notch size and the echo-dynamic curve because, for the transducer selected, the amplitude of the 

response determines all of the properties of the curves, and amplitude does not depend on notch 

size.

Figure 6-9 shows the amplitude response of the notches as a function of their through-wall extent. 

The data shown in Figure 6-9 were taken with 45° shear, 9.5-mm-diameter transducers. The 

notches were on the opposite surface as in Figure 6-2. These data were taken to assist in 

explaining the unexpected variability in response from the notches. The notches were made part 

of the mockup design because they should produce simple, easily explained responses. In 

Figure 6-9, the 1.5-MHz transducer produced the expected response. The 10% notch filled the 

transducer beam, and the larger notches produced a response equivalent to the 10% notch. The 

5% notch did not fill the transducer beam and consequently gave a reduced response.

-80

-60

-40

-20

0

0 50 100 150

Time (us)

Average Noise Level (dB)

Corner Tip Weld noise scratches Part noise

N10

TFC1

TFC10

TFC30

TFC40

TFC30 TFC40 TFC75

TFC75
6-7

Figure 6-7 Echo-Dynamic Curves for Three Notches in Stainless Steel

Figure 6-8 Amplitude versus Duration of the Echo-Dynamic Curve for Notches in 

Stainless Steel

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

5

10

50 55 60 65 70 75 80 85

Time (usec)

Response (dB)

N75 N50 N5

8

9

10

11

12

13

14

15

16

17

18

-15 -10 -5 0 5 10

Amplitude (dB)

Duration (usec)

N5 N75

N20

N10

N50

N30
6-8

Figure 6-9 Response of Notches in Stainless Steel to 45° Shear Waves at Three 

Frequencies

The 2.25-MHz data are taken from Figure 6-2. The 10% and 20% notches are 4 dB to 6 dB 

above the response from the calibration notch (10%) when they should be 0 dB. The 75% notch 

is −5 dB of the calibration notch. The 3.5-MHz data show that the response variations are 

systematically amplified at the increased frequency.

A combination of effects is probably producing the variations. The principal effects that were 

considered are 

• surface finish – The specimen that contains the 10% and 20% notches has a machinedsurface finish. The other specimens have an “as-rolled” finish. This is considered the 

principal cause of the increased response in the 10% and 20% notches. 

• base metal variability – The part noise in the specimens varies by 6 dB with the specimen that 

contains the 10% and 20% notches showing the least scattering and the specimen that 

contains the 5% and 75% notches showing the most. This is considered the principal cause 

of the decreased response in the 75% notch. 

• notch tilt and roughness error – This is not considered to be the principal cause. 

• notch length – The notches were specified with a 2-to-1 aspect ratio (length-to-depth). It is 

possible, but unlikely, that the notch length affects the coherency of the energy that is returned 

to the transducer.

The echo-dynamic curves were recorded for the response of thermal fatigue cracks to 45° shear 

waves. Figure 6-10 shows the echo-dynamic curves for two thermal fatigue cracks and the 

calibration notch in the Alloy 600 specimens. These curves were measured to the surrounding 

noise level. The TFCs are listed in order of increasing through-wall extent, with TFC 3 the 

smallest and TFC 4 the largest. The duration of these echo-dynamic curves showed some 

dependence on crack size. The difference in smoothness between notches and cracks accounts 

for this difference in dependence of the duration of the echo-dynamic curve on reflector size. 

-20

-15

-10

-5

0

5

10

15

20

25

30

35

0 10 20 30 40 50 60 70 80

Percent Through-Wall

Amplitude Relative to Cal Notch 

(dB)

1.5MHz 2.25MHz 3.5MHz
6-9

Figure 6-10 Echo-Dynamic Curves for Rough Cracks in Alloy 600 to 45° Shear Waves. 

Duration of echo-dynamic curves for rough cracks shows some relationship 

with crack size. TFC 4 is approximately 75% through-wall. TFC 3 is 

approximately 50% through-wall. The echo-dynamic curve of the 10% notch 

is shown for reference. 

Figure 6-11 shows a scanned image of a thermal fatigue crack in one of the Alloy 600 specimens. 

Ultrasonic responses are received from the face of the crack. These stacked responses generate 

the increased duration of the echo-dynamic curve with increasing crack size.

-45

-35

-25

-15

-5

5

20 30 40 50 60 70 80 90 100 110

Time (us)

Amplitude Relative to Cal Notch (dB)

TFC #4 10% Notch TFC #3
6-10

Figure 6-11 Thermal Fatigue Crack in Alloy 600

6.3 Far-Side Through-Weld Metal Access

Far-side access is the second of four access conditions considered in the parametric study. Here 

the cracks are connected to the surface that is opposite to the scanning surface, and the 

ultrasonic sound field passes through the weld metal to the cracks. 

Figure 6-12 shows the response-time chart from a low-detection performance case, the corner 

trap response of notches to 2.25 MHz, 45° shear waves through weld-metal. The signals from the 

notches could be found with careful measurement and a priori knowledge of notch location. 

However, the signals from all of the notches could not be distinguished from all of the weld-noise 

interference. The TFC and WFC corner-trap responses could not be measured through weld 

metal. This is consistent with the 20-dB reduction in response found for notches when inspecting 

through weld metal as opposed to base metal.
6-11

Figure 6-12 Response of Notches to 45° Shear Waves at 2.25 MHz for Far-Side ThroughWeld Metal Access in Stainless Steel

Figure 6-13 Response of Notches to 45° Shear Waves for Far-Side Through-Weld Metal 

Access in Alloy 600

Figure 6-13 shows the response-time chart for notches in Alloy 600 using 45°, 2.25-MHz shear 

waves for weld metal access. The response from the corner trap signals from the notches varied 

from −18 dB to −35 dB of the calibration notch. Interference from weld noise is similar to that from 

stainless steel as shown in Figure 6-12, in that it covers much of the response range for the 

corner-trap signals. The responses from the tip signals of the notches are shown to be clearly 

detectable at a reduced level with respect to the case of base metal access. 

N5

N20 N10

N40 N30

N75

-70

-60

-50

-40

-30

-20

-10

0

10

20

20 30 40 50 60 70 80

Time (usec)

Amplitude Relative to Cal (dB)

Part Noise Plate 03 Corner Weld Noise
6-12

Figure 6-14 shows the responses for weld metal access to notches using a creeping wave probe 

in stainless steel. The corner-trap signals could be detected at about 6 dB above the weld noise. 

The tip signals were found with a greater signal-to-noise ratio than the corner traps because of the 

absence of fabrication flaws in the interior of the mockup specimens.

Figure 6-14 Responses of Notches to Creeping Waves at 2 MHz for Far-Side ThroughWeld Metal Access in Stainless Steel

6.4 Access to the Cracked Surface

In the core shroud welds, cracks form on either side of the component. This section describes the 

detection performance for the case in which the ultrasonic transducer can be placed on the 

surface from which the crack originates. Results for inspection through base metal and weld 

metal are given.

Figure 6-15 shows the detection performance for a 70°, 2-MHz dual-element probe in the case of 

access to the cracked surface in stainless steel. The data shown are for notches inspected 

through the base metal and through the weld metal. The responses were approximately the same 

for weld metal and base metal access. The responses from the notches did not vary much with 

the through-wall size of the notch.

Figures 6.16 and 6.17 show the responses for TFCs and WSCs, respectively. The responses 

from the cracks do not differ from the responses from the notches.

N40

N30

N20 N50

N10 N75

N75 N40

N50

N30

N20

N10

-70

-60

-50

-40

-30

-20

-10

0

10

20

10 30 50 70 90

Time (usec)

Amplitude Relative to Cal Notch (dB)

Part Noise Plate 23 Corner TIP Weld Noise
6-13

Figure 6-15 Dual-Element Probe, 70° 2-MHz, Detection Results for Notches Connected to

Scanning Surface in Stainless Steel

Figure 6-16 Dual-Element Probe, 70° 2-MHz, Detection Results for Thermal Fatigue Cracks 

Connected to the Scanning Surface in Stainless Steel 

-70

-60

-50

-40

-30

-20

-10

0

10

20

0 10 20 30 40 50 60 70 80

Time (usec)

Amplitude Relative to Cal Notch (dB) 

Notches, near Notches, far Weld Noise Scratches Part Noise - Plate 7C046

-70

-60

-50

-40

-30

-20

-10

0

10

20

0 10 20 30 40 50 60 70 80

Time (usec)

Amplitude Relative to Cal Notch (dB) 

TFC, near TFC, far Weld Noise Scratches Part Noise - Plate 7C047
6-14

Figure 6-17 Dual-Element Probe, 70° 2-MHz, Detection Results for Weld Solidification 

Cracks Connected to the Scanning Surface in Stainless Steel

Tip signals were measured for the case of access to the cracked surface using a 60°, 2-MHz 

dual-element probe. The responses from notch tips are shown for inspection through base metal 

in Figure 6-18 and through weld metal in Figure 6-19. The responses for inspection through weld 

metal are 5 dB lower than through base metal but still provide a good signal-to-noise ratio for the 

defects. Some of the notch tip signals were not detected. 

Figures 6-20 and 6-21 show the responses for tip diffraction from TFCs and WSCs. The 

responses from the crack tips are similar to the responses from the notch tips. Sizing error is 

described in the next section. 

-70

-60

-50

-40

-30

-20

-10

0

10

20

0 10 20 30 40 50 60 70 80

Time (usec)

Amplitude Relative to Cal Notch (dB)

WSC-Base Metal WSC - Weld Metal Part noise
6-15

Figure 6-18 Dual-Element Probe, 60° 2-MHz, Notch Tip Responses Through Base Metal for 

Notches Connected to the Scanning Surface in Stainless Steel

Figure 6-19 Dual-Element Probe, 60° 2-MHz, Notch Tip Responses Through Weld Metal for 

Notches Connected to the Scanning Surface in Stainless Steel

-70

-60

-50

-40

-30

-20

-10

0

10

20

0 10 20 30 40 50 60 70 80

Time (usec)

Amplitude Relative to Cal Notch (dB) 

Part Noise Tips - manual Weld Noise

N10 N20

N30

N40

N50

N75

N40

N50

N30

N20

-70

-60

-50

-40

-30

-20

-10

0

10

20

0 10 20 30 40 50 60 70 80

Time (usec)

Amplitude Relative to Cal Notch (dB)

Part Noise Tips - manual Weld noise
6-16

Figure 6-20 Dual-Element Probe, 60° Transmit-Receive Refracted Longitudinal 2-MHz, 

Thermal Fatigue Crack Tip Responses Through Base and Weld Metal for 

Cracks Connected to the Scanning Surface in Stainless Steel

Figure 6-21 Dual-Element Probe, 60° 2-MHz, Weld Solidification Crack Tip Responses 

Through Base and Weld Metal for Cracks Connected to the Scanning Surface 

in Stainless Steel

6.5 Summary of Results

This section summarizes the detection and sizing performance provided in the time-response 

charts from the parametric study. Detection results use the signal-to-noise ratio for the minimum 

size reflector. Sizing results are given in root mean square sizing error.

TFC50

TFC75

TFC30

TFC50

TFC75

-80

-70

-60

-50

-40

-30

-20

-10

0

10

20

0 10 20 30 40 50 60 70 80

Time (usec)

Amplitude Relative to Cal Notch(dB)

TIPs - Base Metal TIPs - Weld Metal Weld noise Part noise

-80

-70

-60

-50

-40

-30

-20

-10

0

10

0 10 20 30 40 50 60 70 80

Time (usec)

Amplitude Relative to Cal Notch (dB)

Tips - Base Metal Tips - Weld Metal Weld noise Part noise

WSC 30

WSC 75
6-17

Table 6-1 summarizes the detection results for notches that are not connected to the scanning 

surface. Signal-to-noise ratio for corner-trap responses are listed for detection through base 

metal and weld metal. The creeping wave probe did not detect the 5% notch when inspecting 

through base metal. For weld metal access, the creeping wave probe did not detect notches less 

than 30% through-wall.

Table 6-2 summarizes the detection results for reflectors connected to the scanning surface. All 

reflectors were detected with a signal-to-noise ratio of 4 to 1. 

Sizing results for the case of notches connected to the surface opposite to the ultrasonic 

transducer are listed in Table 6-3. The 45° 2.25-MHz, shear wave probe had a root mean square 

error (RMSE) of less than 1 mm when inspecting through base metal. For the case of sizing 

notches that are connected to the scanning surface, Table 6-4 shows that the RMSE was less 

than 1 mm for base metal and weld metal access. 

The data show that in the case of a high ratio of signal to systematic noise, a parametric study 

can be used to quantify and assess NDE reliability. Such was possible in the case of reactor 

internals because of the lack of weld root, counterbore, and geometrical reflectors that made 

piping inspections difficult. Inspection through austenitic weld metal for cracks that are oppositesurface connected was shown to have low signal to systematic noise performance and the next 

section shows that a blind test is needed to quantify performance.

Table 6-1 Corner-Trap Signal-to-Noise Ratio for Access Conditions, Ultrasonic Modality, 

and Notches That Are Opposite-Surface Connected (from the Transducer) in 

Stainless Steel

Modality Access

Notch TFC WSC

SNR Size SNR Size SNR Size

45° Shear 

(2.25 MHz)

Near 2 5% 2 10% <1 -

Far <1 - <1 - <1 -

Creeping Wave

(2.0 MHz)

Near 2 5% 2 10% <1 -

Far 2 30% <1 - <1 -

Table 6-2 Detection Signal-to-Noise Ratio for Reflectors Connected to Scanning Surface 

in Stainless Steel

Modality Access

Notch TFC WSC

SNR Size SNR Size SNR Size

70° Compression

(2.0 MHz)

Near 4 5% 4 10% 4 10%

Far 4 5% 4 10% 4 10%
6-18

Table 6-3 Sizing Error for Notches That Are Opposite-Surface Connected (from the 

Transducer) in Stainless Steel. No sizing error estimate for thermal fatigue 

cracks or weld solidification cracks was available at the time of publication. 

For responses from crack tips, see Figure 6-4 and Figure 6-13.

Modality Access

RMSE Notches

(%) (mm)

45° Shear

(2.25 MHz)

Near 1.3 0.6

Far no data no data

Creeping Wave

(2.0 MHz)

Near 6.5 3.3

Far 9.7 4.9

Table 6-4 Sizing Error for Notches Connected to Scanning Surface in Stainless Steel. No 

sizing error estimate for thermal fatigue cracks or weld solidification cracks 

was available at the time of publication. For response from crack tips, see 

Figures 6-20 and 6-21.

Modality Access

RMSE Notches

(%) (mm)

60° Compression

(2.0 MHz)

Near 1.2 0.6

Far 0.5 0.3
7-1

7 BLIND TEST OF PHASED ARRAY PROCEDURE FOR 

FAR-SIDE ACCESS

At the EPRI Reactor Pressure Vessel Inspection Conference held in Santa Fe, New Mexico, on 

June 22–24, 1998, an ISI vendor and PNNL agreed to conduct a blind test evaluation of the ISI 

vendor’s phased array inspection procedure that was developed for the BWR core shroud. The 

vendor conducted inspections of the PNNL core shroud mockup at the EPRI NDE Center in 

North Carolina on July 27–31, 1998. PNNL shipped the mockup specimens to the EPRI NDE 

Center and had two PNNL staff members present during the week of the testing. The principal 

objective was to measure the inspection performance for the most difficult case of inspection 

through stainless steel weld metal for cracks connected to the surface opposite from the 

scanning surface. The secondary objective was to measure the parametric performance of the 

phased array technique—especially the signal and noise levels. 

PNNL laboratory measurements showed that inspecting through weld metal to access cracks 

connected to the opposite surface is difficult. Consequently, a blind test was conducted of 

approximately 381 linear centimeters (150 inches) of weld with the cracks on the far side of the 

weld. Practice specimens were made available to the inspection staff prior to conducting the 

blind test. The vendor recorded the detection results as start and stop coordinates, and PNNL 

applied these data to the grading units that were established prior to testing. For each 

detection, the vendor provided the depth and response of the crack’s tip signal. 

7.1 Test Description

PNNL shipped 20 mockup assemblies to the EPRI NDE Center. The size and weight of the 

specimens permitted the manual handling and presentation during the test. Figure 7-1 shows 

the mockup shipment box sent to the EPRI NDE Center. The specimens were prepared for the 

blind test by taping all of the grading units to obscure the true state. Tape was also placed over 

the identifying part numbers provided by the vendor of the specimens to obscure their 

permanent identity. The blind test was limited to the weld far-side condition. PNNL staff 

members performed the handling and presentation of the mockup specimens.

Calibration scans were performed for all transducers used in the inspections of the PNNL 

mockup. Instrument settings were controlled so that, except for receiver gain, they were left 

unchanged after being established during calibration. Table 7-1 describes the stainless steel 

calibration specimen provided to the blind test. The mockup specimen, labeled C1, was 

provided to the vendor before the blind test was conducted. A single calibration reflector, a 10% 

end-milled notch, was provided. The calibration reflector was used to test the phased array 

probe and establish system settings. Instrument settings, such as phased array focal laws, 

were left unchanged after calibration. The blind test required that the only system parameter 

that could be changed was the overall receiver gain. The receiver gain could be changed to 

add sensitivity for the inspection of the cracks in the blind test. Practice specimens were 

provided to determine the needed additional sensitivity.
7-2

Figure 7-1 PNNL Mockup at EPRI NDE Center

Table 7-1 PNNL Stainless Steel Calibration Specimen Provided for Blind Test

Specimen Name Description

C1 10% end milled notch
7-3

Practice specimens were inspected with the phased array system. Scans of blank specimens 

with and without weld crown were made to establish the inspection sensitivity for blank material. 

A specimen with a thermal fatigue crack, including the true-state data for the crack, was made 

available. Measurements of the response from the corner trap and crack tip were successfully 

recorded. A specimen with a weld solidification crack was made available, and measurements 

of corner trap and crack tip signals were successfully made. Table 7-2 describes the practice 

specimens provided to the blind test.

Table 7-2 Stainless Steel Practice Specimens Provided for Blind Test

Specimen Name Description

P1 Blank without weld crown

P2 Blank with weld crown

P3 TFC vertical, TFC tilted

P4 WSC

Note: The thermal fatigue crack and weld solidification crack 

were approximately 30% through-wall. Exact sizes are withheld 

so that the cracked specimens can be reused in blind tests.

These practice specimens were inspected after calibration and before the start of the blind test. 

Two blank specimens were provided to establish the inspection sensitivity for the baseline 

condition. The blank practice specimen, P1, was presented, and no detectable indications were 

found. The weld crown on the blank practice specimen, P2, did not produce detectable 

indications or other difficulties for the phased array technique. A specimen, P3, with two 

thermal fatigue cracks was presented for far-side inspection (through the weld), and the truestate parameters, location, depth, and length, for the cracks were provided. Measurements of 

location, depth, and length for the two TFCs in P3 were easily achieved. A practice specimen, 

P4, was presented with a WSC, and the measurements compared favorably to the true-state 

information.

A blind test of performance for the inspection of cracks connected to the opposite surface was 

made. Inspection of 20 assemblies containing cracks adjacent to the weld was conducted with 

emphasis on far-side access. The assemblies were presented in a predetermined order. 

Responses from detectable TFCs and WSCs were recorded by the vendor procedure. The start 

and end of detectable cracks were recorded, based on the vendor procedure, for length sizing. 

Depth size of detectable cracks was estimated. Figure 7-2 shows the vendor’s inspection 

scanner and the presentation of specimens in the blind test. The inspections with conventional 

UT (as shown) and phased array ultrasonic probes were conducted using a pipe scanner and 

not the in-vessel manipulator for inspection of reactor internals. For this study, we were 

interested in assessing the NDE inspection technology and not the in-vessel manipulator. The 

conventional probe is shown in the photograph to illustrate the scanning method. However, no 

evaluation of a conventional UT technique was performed in the blind test. The blind test was 

conducted solely to provide the detection and sizing performance of the phased array 

technique.
7-4

Figure 7-2 Inspection Scanner and Presentation of Specimens in Blind Test

The blind test had varying amounts of similarity to actual field conditions. Factors that were 

similar to field conditions were presence of weld crown, a range of surface roughness 

conditions, coupling, transducer, and instrument settings. The factors that were different from 

field conditions were the use of artificially implanted flaws, the use of a pipe scanner instead of 

the actual core shroud scanner, and the orientation of the mockup specimens (lying down) 

versus a core shroud cylinder. 

The core shroud scanner was not used in the blind test. However, because this is a large 

device, the positioning error, length sizing error, and repeatability are expected to be different 

from that of a pipe scanner such as the one used in the blind test. Depth sizing error should not 

be affected by the scanner employed because the transducer is in contact with the component 

in both cases.
7-5

7.2 Detection Performance

Fifty grading units made of type 304 stainless steel were presented in the blind test. Of these 

grading units, 17 were cracked and 33 were blank. Fourteen of the grading units were correctly 

characterized as cracked. Three cracks were reported in the 33 blank grading units. These 

detection results are summarized in Table 7-3.

Table 7-3 Detection Performance for Far-Side Access and Opposite Surface Connected 

Cracks in Blind Test of Stainless Steel Specimen

Detections (%) False Calls (%)

82 9

Crack type did not influence detection performance. WSCs and TFCs were detected with equal 

frequency. Crack tilt did not contribute either. All cracks that were larger than 15 mm (30%) in 

through-wall extent were detected. Of the three missed cracks, two were more than 5 mm 

(10%) in through-wall extent and one was more than 25 mm (25%) in through-wall extent. The 

largest missed crack, more than 25 mm, was a TFC in a specimen with weld crown. For the 

smaller cracks, one was a TFC and the other a WSC in specimens with weld ground flush. 

Three false calls were made, and all of them were in grading units with the weld ground flush. A 

10% through-wall crack was called in a grading unit with a weld repair. A 15% through-wall 

crack was called in simple blank weld metal. A false call was made when an indication in blank 

material was characterized as a 50% through-wall flaw. Two fabrication flaws, in blank and 

cracked grading units, were reported, but because their size was reported as small, without 

through-wall extent, they were not counted as false calls. This false call rate is quite low when 

compared to the results for far-side access in PNNL’s piping inspection round robin, where 

probability of detection and false call probability were shown to be approximately equal—

detection performance not different from those achieved by random guessing (Heasler and 

Doctor 1996).

7.3 Depth Sizing Performance

Tip-diffracted signals were used to depth size all of the indications in the blind test. Table 7-4 

shows the RMSE for the depth size measurements for different crack conditions. For all 

detected cracks, the RMSE is 9.2 mm of depth. Figure 7-3 shows the depth sizing performance 

trend line for all cracks. The data show that the technique tends to oversize small cracks and 

undersize large ones. 
7-6

Table 7-4 Blind Test Depth Sizing Error for Different Crack Types

Crack Type RMSE (mm)

All cracks 9.2

Without small cracks (<10 mm) 8.3

TFCs only 10.9

WSCs only 1.6

Without angled cracks 7.7

Without straight cracks 8.4

Weld crown flush 8.2

Figure 7-3 Blind Test Depth Sizing Performance for All Detected Cracks

Because there was considerable difficulty detecting and sizing the smaller cracks, the RMSE 

depth sizing error is recalculated without them. Cracks less than 10 mm (20%) in through-wall 

extent are considered to be in the small category. The RMSE, at 8.3 mm, did not change much 

compared to the RMSE for all cracks but Figure 7-4 shows considerable change in the trend 

line. There is still some tendency to undersize the large cracks but the large effect of the 

oversizing of small cracks has been removed from the linear trend analysis. The sizing 

performance on larger cracks shown in this figure is better than Figure 7-3 but there is still some 

tendency to undersize large cracks.

Actual Performance

RMSE: 9.2 mm

0

5

10

15

20

25

30

35

40

45

50

0 5 10 15 20 25 30 35 40 45 50

True-state depth-size (mm)

UT measured depth-size (mm)

Ideal Performance
7-7

Figure 7-4 Blind Test Depth Sizing Performance Without Small Cracks

Tip diffracted signals were used to depth size all of the indications in the blind test. Table 7-4 

shows the RMSE for the depth size measurements for different crack conditions. Of the 

conditions listed in the table, only the RMSE for WSC cracks is significantly different, acceptably 

low at 1.6 mm.

The RMSE for depth sizing is better for WSCs than for TFCs when inspecting through austenitic 

weld metal. The TFCs contributed most to depth sizing error, with an RMSE of 10.9 mm. In an 

effort to explain this large sizing error, three subsets of the TFC cracks were removed one at a 

time from the set of all cracks: angled cracks, straight cracks, and cracks with weld crown. 

Table 7-4 shows that these RMSEs do not differ significantly from the RMSE for all cracks,

indicating that none of these single factors contributed most of the sizing error.

Figure 7-5 shows the sizing error dependence on tip response. This scatter diagram shows that 

the TFC tip signals that contributed most to the sizing error were the ones with the strongest 

amplitude response. There are a number of explanations for this error: the weld can produce 

strong responses that cannot be easily distinguished from crack tips, and the anisotropic weld 

metal refracts the ultrasonic beam, giving a false position for the crack tip. This second 

explanation requires that the beam steering produces errors of more than 12 mm. Satellite 

reflectors associated with the TFCs are not the most likely explanation for the depth sizing 

errors because the amount of weld metal above the TFC should be less than 3 mm.

Actual Performace

RMSE: 8.3

0

5

10

15

20

25

30

35

40

45

50

0 5 10 15 20 25 30 35 40 45 50

True-state depth-size (mm)

UT measured depth-size (mm)

Ideal Performance
7-8

Figure 7-5 Blind Test Dependence of Sizing Error on Tip Response

7.4 Length Sizing Performance

Table 7-5 gives the RMSE of 72 mm for length sizing of the 14 cracks detected in the blind test. 

The length sizing performance for the WSCs was better than that for the TFCs, at an RMSE of 

5.8 mm compared to 85 mm. 

There were no large undersizing errors, and except for the few large oversizing errors, the crack 

length was accurately measured. When the oversizing errors are excluded, the RMSE for 

length sizing is reduced to 10 mm. The length sizing performance was degraded by a number 

of measurements that exceeded the true length by more than 100 mm, as shown in Figure 7-6. 

The trend lines with and without the large oversizing errors are given in Figures 7-7 and 7-8, 

respectively.

Table 7-5 Blind Test Length Sizing Error for Crack by Type

Crack Type RMSE (mm)

All cracks 72.1

TFCs only 85.3

WSCs only 5.8

Without oversizing

(errors < 50 mm)

10.1

-12

-10

-8

-6

-4

-2

0

2

-25 -20 -15 -10 -5 0 5 10 15 20 25

Depth Sizing Error (mm)

Response (dB)

TFCs WSCs
7-9

Figure 7-6 Blind Test Length Sizing

Figure 7-7 Blind Test Length Sizing Performance for All Cracks

-50

0

50

100

150

200

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Crack number

Error (mm)

Actual Performance:

RMSE: 72.1mm

0

20

40

60

80

100

120

140

160

180

200

0 20 40 60 80 100 120 140 160 180 200

True-state length (mm)

UT measured length (mm)

Ideal Performance
7-10

Figure 7-8 Blind Test Length Sizing Performance Without Large Oversizing Errors of 

Greater Than 50 mm

7.5 Parametric Analysis of Blind Test Data

The mockup’s use of a combination of TFCs and WSCs was designed to bracket the range of 

ultrasonic responses from fully open to very tight SCC. The WSCs are known to give a low 

response, when inspected through base metal, and as such are considered to be more like tight 

SCC. The blind test results show that the opposite is the case when inspecting through 

austenitic weld metal—the WSCs are easier to detect and characterize than the TFCs. 

Satellite reflectors were reported by vendor staff in one of the mockup specimens that contained 

a TFC. Satellite reflectors originate from the flaw implant process and are not meant to include 

weld noise or part noise but rather only the unintended energy returned from the flaw implant. 

Vendor staff were concerned about satellite reflectors on only one of the TFC specimens. The 

TFC specimens do contain a small amount of austenitic weld metal above the crack, and this is 

a concern because this condition is generally not present in the field unless there are welding 

flaws or a weld repair at that location. The size of the welded area above the crack is designed 

to be less than 3 mm. Because the depth sizing error was dominated by errors greater than 

7.6 mm and because there was both over- and undersizing, it is reasonable to assume the 

satellite reflectors did not make a significant contribution to depth sizing error.

Figure 7-9 shows the typical signals from a crack using a phased array system. The azimuthal 

scan was done electronically (without probe motion) from 30° to 80° with respect to surface 

normal. The back surface of the part is indicated by the horizontal line at 51 mm (2 in.). The 

crack signals are above this line. None of the cracks, inspected through the weld by the phased 

Actual Performance

RMSE: 10.1 mm

0

50

100

150

200

250

0 50 100 150 200 250

True-state length (mm)

UT measured length (mm)

Ideal Performance
7-11

array system, produced a corner trap signal. Signals that crossed the back surface line were 

determined to be image artifacts. The crack signals in the image are shown between the 

horizontal 49.87-mm and 18.7-mm lines and the vertical lines at 35.8 mm and 50.9 mm.

Figure 7-9 Typical Phased Array Image of Crack (units shown in millimeters)

Figure 7-10 shows the relationship between response and crack size in the phased array blind 

test on the reactor internals mockup. The responses from thermal fatigue cracks and weld 

solidification cracks are charted by crack size. A logistic curve was fit to the response as a 

function of the crack size, R(s), defined as 

R(s) = (1 + exp(β1 – β2s))-1

The fit values are β1 = 6.0 and β2 = 0.20 where s, the crack size, is given in percentage throughwall. The reactor internals mockup specimens were 51 mm thick. The RMSE of the logistic 

function fit to the response data is 9.5% of maximum.

Figure 7-10 shows four cracks with a 10% through-wall size. No response could be found in the 

inspection data from these four cracks, and these are charted with zero response. One cracked 

grading unit with a 10% TFC was called cracked in the blind test, but the reflector was a midwall fabrication flaw and not the 10% TFC.
7-12

Figure 7-10 Relationship Between Response and Crack Size in Phased Array Blind Test 

on Stainless Steel in Reactor Internals Mockup

Figure 7-10 shows three 30% through-wall TFCs at 40% of the response range and one 30% 

WSC at 75% of response range. One of the 30% TFCs was missed in the blind test, but the 

data review showed that the signal was present. The vendor procedure required that the signal 

be present in two scans of the phased array, and the missed 30% through-wall crack produced 

a response in only one scan line. The minimum detected response for the blind test was 40% of 

maximum. This gives the phased array technique used in the blind test an 8-dB dynamic range 

in detectable responses. 

7.6 Summary

The blind test showed that the vendor phased array inspection procedure could detect and size 

cracks through austenitic weld metal. The detection performance would meet the intent of a 

detection performance demonstration test designed to ASME Code Section XI, Appendix VIII 

Supplement 2, “Qualification Requirements for Wrought Austenitic Piping Welds.” The depth 

sizing RMSE was 9.2 mm for all cracks. The length sizing RMSE was 72 mm for all the 

detected cracks. The TFCs contributed most of the error to both length and depth sizing 

measurements. 

The depth and length sizing performance would not pass any of the requirements in 

Appendix VIII. However, the requirements in Appendix VIII are based on the need to detect and 

characterize small flaws in the components covered by the specific supplements. In the case of 

the core shroud, the critical flaw size is very large, so it is expected that less stringent 

requirements might need to be met. Consequently, to put the sizing performance into 

perspective, further work on assessing requirements for pass/fail criteria is in order. 

The parametric study showed that the measurement of responses, using a logarithmic scale, 

relative to a standard reflector can provide important insights to the sensitivity of the technique 

to part noise, weld noise, surface scratches, crack tips, and corner-trap responses. The blind 

test showed that the usefulness of ISI data is increased by such calibration.

0

20

40

60

80

100

120

0 20 40 60 80 100 120

Size (% through-wall)

Response (% of range)

TFCs WSCs Logistic

(4)

(2) (2)

(3)
8-1

8 CONCLUSIONS

The combined use of a parametric study with a blind test can quantify the reliability of the 

ultrasonic inspection of reactor internals. A mockup, described in Section 5, can be designed 

for both. The data collected in a parametric study can quantify the performance of ultrasonic inservice inspection techniques and identify areas where performance is relatively low. A blind 

test can be conducted to further assess the low performance in difficult cases, such as weld farside access to heat-affected zone cracking on the surface opposite from the one being scanned. 

The operating characteristic diagram shows that it is not sufficient to examine probability of 

detection without also measuring false call probability. Similarly, it is not enough to measure 

crack response without considering systematic noise from, for example, weld grains. The 

Programme for the Inspection of Steel Components (Lemaitre 1994) showed that the operating 

characteristic diagram for procedures in the inspection of stainless steel welds produced large 

scatter in results by teams that used similar procedures. They concluded that training and 

performance demonstration testing are needed on realistic specimens. Access to both sides of 

the weld is needed. Scanning in two different directions, perpendicular to the weld, improved 

detection performance. 

Estimates of response variance are important when quantifying signal-to-noise ratios for cracks 

and degradation. A PNNL parametric study (Becker et al. 1981) has shown that the standard 

deviation of crack responses is similar, 6 dB, even when it is derived from different sources. 

Crack tightness is included in this estimate. 

Inspection performance was quantified in this study by identifying the areas in which 

performance is high and then placing emphasis of subsequent study on the difficult inspection 

cases. The first part of the study was accomplished by conducting a parametric study, which 

quantified crack responses and attendant noise sources to show reliable detection and accurate 

sizing. The second part of the study used a blind test of vendor performance to show detection 

and sizing statistics for the difficult cases. 

8.1 Parametric Study Conclusions

The parametric study of UT-ISI of reactor internals quantified the performance of industry-type 

techniques and identified the areas in which reliability is relatively low. To accomplish this, the

parametric study established the amplitude and pattern of noise from different sources, 

determined the effects of weld and crack type on signal strength, and established the accuracy 

of sizing methods. 

Results of the parametric study showed that access to the cracked surface strongly affected 

detection and sizing. In the case of inspection of weld heat-affected zones for cracking, reliable 

detection and accurate sizing could be accomplished when the ultrasonic transducer could be 

placed on the cracked surface. Furthermore, reliable detection and accurate sizing could be 

achieved for the near-side access condition to cracks that originate from the surface opposite to 

the scanning surface. The parametric study showed that for weld far-side access to cracks, 

detection and sizing performance was low. 
8-2

Inspection through austenitic weld metal was a case in which performance was relatively low. 

Vertically polarized ultrasonic shear waves often failed to detect side-drilled holes through 

austenitic weld metal. Ultrasonic compression (longitudinal) waves sometimes detected sidedrilled holes through the weld metal, but performance was greatly reduced from the base-metal 

case. The performance of creeping-wave probes, when inspecting through austenitic weld 

metal, was better than that for the other ultrasonic modalities evaluated in this study. 

An analysis of the differences in signal strengths between weld solidification cracks and thermal 

fatigue cracks showed similar and dissimilar features. Similar UT responses were received from 

both crack types when the cracks were connected to the scanning surface. For the inspection 

of cracks connected to the surface opposite the scanning surface, the responses were 18 dB 

weaker for the WSC with base metal access. For inspection through weld metal, some WSC 

responses were not detectable. Southwest Research Institute research showed that its WSCs 

are 60% lower in response than real SCC (Watson and Edwards 1996). Using this estimate, 

our data showed that TFCs were higher in response than real SCC. 

For the TFCs, the cracking process uses a repair weld to complete the flaw implant. It is 

important to know if the repair weld betrays the presence of the flaw. Our data showed that the 

implant repair weld does not produce a detectable response when probes typically used by 

industry are used. As a consequence, we do not believe the TFC weld repair to be a significant 

concern.

Weld type—submerged vs. shielded metal arc—is less important than weld profile, with shielded 

metal arc weld being slightly more difficult to penetrate with ultrasound. Core shroud welds are 

made by submerged metal arc, but because shielded metal arc welds are easier to make and 

more difficult to inspect, the shielded metal arc process was used as a conservative substitute. 

8.2 Blind Test of Phased Array Procedure

A blind test was shown to be needed for measuring UT-ISI performance in those cases where 

the signal-to-noise levels are relatively low—weld far-side access. Fifty grading units of type 

304 stainless steel were presented in the blind test. Of the 50 grading units, 17 were cracked

and 33 were blank. Fourteen of the grading units were correctly characterized as cracked. 

Three cracks were reported in the 33 blank grading units. This detection performance would 

pass ASME Code Section XI, Appendix VIII Supplement 2, “Qualification Requirements for 

Wrought Austenitic Piping Welds.” The root mean square depth sizing error for the blind test 

was 9.2 mm and would not pass. 

A re-analysis of the signals in the vendor’s phased array data revealed that no response was 

received from cracks less than 30% through-wall for far-side access. The absence of cornertrap responses explains, in part, the size dependence of crack detection. The signals in the 

phased array images originated from the rough face of the cracks and from the crack tips. 

Sizing errors originated from difficulty distinguishing the weak responses from the crack tips 

from the strong responses produced by the crack face and those signals caused by weld 

fabrication flaws. 
8-3

Crack type did not influence detection performance. WSCs and TFCs were detected with equal 

frequency. Crack tilt did not contribute, either. All cracks larger than 15 mm (30%) in throughwall extent were detected. 

Three false calls were made, and all of them were in grading units with the weld ground flush. A 

10% through-wall crack was called in a grading unit with a weld repair. A 15% through-wall 

crack was called in a simple blank weldment. A false call was made when an indication in blank 

material was characterized as a 50% through-wall welding (fabrication) flaw. This false call rate 

is quite low when compared to the results from PNNL’s piping inspection round robin for far-side 

access of austenitic welds where probability of detection and false call probability were shown to 

be approximately equal (detection performance not measurably different from guessing).

A combination of TFCs and WSCs in the mockup was designed to bracket the range of 

ultrasonic responses from fully open to very tight SCC. The WSCs are known to give a low 

response when inspected through base metal and, as such, are considered to be more like tight 

SCC. The blind test results show that the opposite is the case when inspecting through 

austenitic weld metal: the WSCs are easier to detect and characterize than the TFCs. 

The data show three 30% through-wall TFCs at 40% of the response range and one 30% WSC 

at 75% of the response range. One of the 30% TFCs was missed in the blind test, but the data 

review showed that the signal was present. The minimum detected response for the blind test 

was 40% of maximum. This gives the phased array technique used in the blind test an 8-dB 

dynamic range in detectable responses.

9-1

9 RECOMMENDATIONS

In this section, recommendations are made where enhancements can be achieved: imaging 

improvements, such as reduction of noise variance and hybrid imaging systems, and application 

of the findings of this work to the difficult case of the far-side inspection of austenitic piping 

welds. Signal-to-noise improvements can be made by reducing systematic noise and its 

variance from microstructure and surface geometry. Weld microstructure imaging is interesting 

for the early detection of degradation. Application of systematic noise analysis to piping welds 

is a significant challenge. 

Image quality can be improved through signal-to-noise improvements. The response and 

variance from weld microstructure can be reduced through spatial averaging. Small indications 

from weld noise can be suppressed by spatial-frequency filtering. Calibrated responses are 

useful for understanding variances and their effect on decision making. 

Decision thresholds have been an important part of data analysis (Berens 1988). Application of 

decision thresholds on calibrated ultrasonic responses for flaw detection has been difficult, and 

current practice is to analyze inspection data to within some multiple of the noise level, typically 

+6 dB of that level. Still, the significance of calibrated ultrasonic response is great when 

expressed on a logarithmic scale. As discussed in Section 4, a standard deviation of 6 dB can 

be expected in calibrated responses from SCC in welded components. 

Imaging of weld microstructure could be investigated. New image reconstruction algorithms can 

improve characterization of the shape and orientation of reflectors in welded assemblies. 

Sensitive image techniques that image weld grains can be useful for addressing smooth cracks 

and tight cracks. 

Imaging systems can fuse multiple data streams into images of weld condition. A hybrid 

system, based on phased arrays and creeping waves, might be interesting for data fusion 

imaging through weld metal. 

The successful part of this work on reactor internals should be extended to austenitic piping 

welds. The ultrasonic in-service inspection of wrought austenitic, cast austenitic, and dissimilar 

metal welds is more difficult than the inspection of the welds in the BWR core shroud. First, the 

inside and outside surfaces of pipes complicate ultrasonic inspections (Morris and Becker 

1982). Second, the counterbore and weld root on the inside of the pipes produce ultrasonic 

reflections that are difficult to distinguish from degradation. Imaging of the weld microstructure 

and the shape and orientation of geometrical conditions could be developed. Optimizing the 

detection algorithms for application to ultrasonic inspection of piping weldments could be 

investigated.

10-1

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Reactor Internals of Boiling Water Reactors