NUREG/CR-6471, Vol. 1

PNNL-11143

Characterization of Flaws in

U.S. Reactor Pressure Vessels

Density and Distribution of Flaw

Indications in PVRUF

Prepared by

G.J. Schuster, S.R. Doctor, P.G. Heasler

Pacific Northwest National Laboratory

Prepared for

U.S. Nuclear Regulatory Commission
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NUREGICR-6471, Vol. 1

PNNL-11143

Characterization of Flaws in

U.S. Reactor Pressure Vessels

Density and Distribution of Flaw

Indications in PVRUF

Manuscript Completed: September 1998

Date Published: November 1998

Prepared by

G.J. Schuster, S.R. Doctor, P.G. Heasler

Pacific Northwest National Laboratory

Richland, WA 99352

D.A. Jackson, NRC Project Manager

Prepared for

Division of Engineering Technology

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

NRC Job Code L1099 and W6275
NUP.EG/CR-6471, Vol. 1 has been

reproduced from the best available copy.

For sale by the U.S. Government Printing Office

Superintendent of Documents, Mail Stop: SSOP, Washington, DC 20402-9328

ISBN 0-16-049841-4
Abstract

Characterization of Flaws in US. Reactor Pressure Vessels is a multi-volume report. Volume 1, this document, gives the

results of a non-destructive examination conducted at the Oak Ridge National Laboratory's Pressure Vessel Research User

Facility (PVRUF) on a vessel fabricated for a canceled nuclear power plant. Volume 2, in preparation, will document the

results of Pacific Northwest National Laboratory's (PNNL's) destructive validation of the flaw rates in the PVRUF

evaluation.

A nondestructive evaluation was made of the fabrication flaws in an unused U.S. nuclear reactor pressure vessel. The

examination was conducted at the Oak Ridge National Laboratory's Pressure Vessel Research User Facility (PVRUF) on a

vessel fabricated for a canceled nuclear power plant of a pressurized water reactor (PWR) design. The inspections were

made using the fleldable, real-time Synthetic Aperture Focusing Technique for Ultrasonic Testing (SAFT-UT) system

developed by PNNL under sponsorship of the U.S. Nuclear Regulatory Commission. Twenty linear meters of weldment

were inspected by SAFT-UT, including the entire circumferential beitline weld of the vessel. Ten different inspection modes

were used, including five different modes specifically selected for the inspection of the inner 25 mim (1.0 in.) of the vessel

wall.

There were 2500 detectable indications in the SAFT-UT inspections of the PVRUF vessel. The largest number of these, 982,

were found at the clad-to-base metal interface, but 978 of these were less than 2 mm (0.08 in.) in size. In the near surface

zone, the weld metal contained 98 detectable planar indications. The density of indications was four times higher in the

weldment than in the base metal. The distribution of the empirical data provided enough information to apply a parametric

model of the cumulative flaw rate to six different subsets of the data, and to obtain reasonable confidence bounds on the

results. Recommendations are given for validating the indication rates by selective destructive analysis, to provide the

necessary high quality flaw statistics for use in fracture mechanics calculations, such as those used in pressurized thermal

shock (PTS) analysis.

iii iii NUREG/CR-6471

Contents

Page

A b s ratract ...... ................... ... .................................................................................... ........ ii

Executive Summary............................................................................................................. xv

Acknowledgments............................................................................................................... xvii

Glossary and Abbreviations..................................................................................................... xix

1 Introduction................................................................................................................. 1.1

1. 1 Reasons for Estimating Fabrication Flaw Rates ................................................................... 1.1

1.2 Principal Objectives.................................................................................................. 1.2

1.3 Secondary Objectives ................................................................................................ 1.2

1.4 Flaws of Concern to Reactor Pressure Vessel Integrity............................................................ 1.2

2 Description of Reactor Pressure Vessel Material ......................................................................... 2.1

2.1 Differences Between PWRs and BWI; s............................................................. .............. 2.1

2.2 Fabrication Methods................................................................................................. 2.1

2.3 Materials.............................................................................................................. 2.2

2.4 Weld Procedures ..................................................................................................... 2.2

2.5 Description of PVRUF............................................................................................... 2.3

2.6 Categorization of RPVs ............................................................................................. 2.3

3 SAFT-UT Measurements of RPV Material............................................................................... 3.1

3.1 SAFT-UT Equipment Configuration at PVRUF ................................................................... 3.1

3.2 Measurement Plan for PVRUF...................................................................................... 3.1

3.3 Calibration of SAFT-UT Inspections ............................................................................... 3.2

3.4 Scanning Procedure and Vessel Coordinate System ............................................................... 3.3

3.5 Amount of Material Inspected ...................................................................................... 3.4

4 Analysis Methods .......................................................................................................... 4.1

4.1 Detection of Indications at PVRUF................................................................................. 4.1

4.2 Clustering of Detections............................................................................................. 4.4

4.3 Sizing and Characterizing Indications at PVRUF.................................................................. 4.4

5 Results of the Analysis of SAFT-UT Inspections at PVRUF Vessel.................................................... 5.1

5.1 Analysis of SAFT-UT Images ...................................................................................... 5.1

5.2 Indications in the Near-Surface Zone............................................................................... 5.1

5.2.1 Indications in the Near-Surface, Weldment................................................................. 5.1

5.2.2 Indications in the Near-Surface, Heat Affected Zone ...................................................... 5.2

5.2.3 Indications in the Near-Surface, Base Metal................................................................ 5.2

5.2.4 Clad and Clad-to-Base Metal Interface ..................................................................... 5.2

v v ~NJRiEG/CR-64

1
Contents

Page

5.3 Indications Outside the Near Surface Zone .......................................................................... 5.4

5.3.1 Indications in the Weidment .................................................................................... 5.4

5.3.2 Indications in the Heat Affected Zone.......................................................................... 5.5

5.3.3 Indications in the Base Metal.................................................................................... 5.5

6 Distribution of Indications of Flaws in SAFT-UT Data ................................................................... 6.1

6.1 Joint Frequency Distribution of Indications in the Near Surface Zone............................................. 6.1

6.2 Joint Frequency Distribution of Indications Outside the Near Surface Zone...................................... 6.2

7 Estimates of SAFT-UT Performance ........................................................................................ 7.1

7.1 Probability of Detecting a Flaw ....................................................................................... 7.1

7.2 Sizing Error .............................................................................................................. 7.1

7.3 False Call Probability................................................................................................... 7.1

8 Flaw Rate Estimates........................................................................................................... 8.1

8.1 Use of SAFT-UT Performance Data .................................................................................. 8.1

8.2 Method of Estimation: Maximum Likelihood .............................. ........................................ 8.2

8.3 Maximum Likelihood Using the Weibull Distribution .............................................................. 8.2

8.4 Data Requirements for the Estimation Method....................................................................... 8.3

8.5 Near Surface Zone Flaw Rate Estimates.............................................................................. 8.3

8.6 Outside the Near Surface Zone Flaw Rate Estimates ................................................................ 8.5

8.7 Comparison to Other Published Flaw Rate Estimates ............................................................... 8.6

9 Discussion of Flaw Rate Estimates .......................................................................................... 9.1

9.1 SAFT-UT Flaw Indications and Inspections During Fabrication................................................... 9.1

9.2 PVRUF Flaw Rates and the Marshall Distribution .................................................................. 9.1

10 Validation Plan................................................................................................................ 10.1

10.1 Removal of Material from PVRUF for Destructive Testing........................................................ 10.1

10.2 Selection of Indications for Destructive Testing..................................................................... 10.1

11I Reliability Requirements for 151 Systems................................................................................... 11.1

11.1 Quantitative Model of Inservice Inspection.......................................................................... 11.1

11.2 Scenarios for Evaluating Different Aspects of Inspection Capability.............................................. 11.2

11.2. 1 Perfect Inspection Scenario .................................................................................... 11.3

11.2.2 Imperfect Detection Scenario.................................................................................. 11.3

11.2.3 Imperfectly Calibrated POD Scenario ........................................................................ 11.3

11.2.4 Imperfect Sizing Scenario...................................................................................... 11.4

11.2.5 Imperfectly Calibrated Sizing Scenario ...................................................................... 11.4

11.3 Flaw Distribution Function ............................................................................................ 11.4

NUREG/CR-647 1 vi
Contents

Page

11.4 Results from Scenario Evaluations ................................................................................... 11.5

11.4.1 Base Case Scenario Results.................................................................................... 11.5

11.4.2 Imperfect Detection Scenario Results......................................................................... 11.5

11.4.3 Imperfectly Calibrated POD Scenario Results ............................................................... 11.6

11.4.4 Imperfect Sizing Scenario Results ............................................................................ 11.6

11.4.5 Imperfectly Calibrated Sizing Scenario Results.............................................................. 11.7

11.5 Recommended Requirements for IS ....................................... :.......................................... 11.7

12 Conclusions.................................................................................................................... 12.1

13 Recommendations.............................................................................................................. 13.1

14 References .................................................................................................................... 14.1

Appendix A - SAFT-UT Images of the Most Significant Flaws............................................................... A.1I

Appendix B - Inspections of the Midland Vessel..............;............................................... I.................. B.1I

vii vii NUREG/CR-647 1

Figures

Page

1.1 Significant flaws for vessel fracture from pressurized thermal shock events ........................................ 1.6

1.2 Significant flaws for vessel fracture from low temperature over pressurization events .............................. 1.7

1.3 Effects of surface length on fracture evaluation ....................................................................... 1.8

1.4 Effects of subsurface flaw location and orientation on fracture evaluation .......................................... 1.9

1.5 Effect of cladding on fracture evaluation............................................................................... 1.10

1.6 PTS flaws of equivalent severity........................................................................................ 1.11

2.1 Typical PWVR with internals ............................................................................................ 2.5

2.2 General schematic for PVRUF vessel showing weld identification numbers and configuration .................... 2.6

2.3 Photograph of the PVRUF vessel from the outside ................................................................... 2.7

2.4 Photograph of the PVRUF vessel from the inside..................................................................... 2.7

3.1 Block diagram of SAFT-UT field system as configured at PVRUF.................................................. 3.5

3.2 SAFT-UT data acquisition and display system as configured for PVRUF........................................... 3.5

3.3 Instrument platform and scanner pendulum showing center pivot.................................................... 3.6

3.4 Insonification for inspection mode 1, near surface normal beam ..................................................... 3.6

3.5 Insonification for inspection mode 2, near surface 700 L wave at 00 skew .......................................... 3.7

3.6 Insonification for inspection mode 3, near surface 700 L wave at 90' skew......................................... 3.7

3.7 Insonification for inspection mode 4, near surface 700 L wave at 1800 skew ........................................ 3.7

3.8 Insonification for inspection mode 5, near surface 700 L wave at 270' skew ....................................... 3.8

3.9 Insonification for inspection mode 6, full volume 450 S wave at 00 skew ........................................... 3.8

3.10 Insonification for inspection mode 7, full volume 450 S wave at 90' skew.......................................... 3.8

3.11 Insonification for inspection mode 8, fuill volume 450 S wave at 1800 skew......................................... 3.9

3.12 Insonification for inspection mode 9, full volume 450 S wave at 2700 skew......................................... 3.9

3.13 Insonification for inspection mode 10, full volume normal beam.................................................... 3.9

ix ix NUREG/CR-647 1
Contents

Page

3.14 SAFT-UT scanner attached to the calibration block for calibration of inspection modes 6 through 9,

450 S wave inspections .................................................................................................... 3.10

3.15 SAFT-UT scanner attached to the calibration block for calibration of inspection mode 10, normal beam......... 3.10

3.16 Signal strength from 1/4 thickness side drilled hole in calibration block for inspection mode 1 .................... 3.11

3.17 Signal strength from 1/4 thickness side drilled hole in calibration block for inspection modes 2 through 5 ..... 3.11

3.18 Time variable gain curve for modes 6 through 9, 45' S wave inspections ............................................ 3.12

3.19 Signal strength after TVG amplification from 1/4 thickness side drilled hole in calibration block for

inspection modes 6 through 9.............................................................................................. 3.12

3.20 Signal strength after TVG amplification from 1/2 thickness side drilled hole in calibration block for

inspection modes 6 through 9 ............................................................................................. 3.13

3.21 Signal strength after TVG amplification from 3/4 thickness side drilled hole in calibration block for

inspection modes 6 through 9................................................................... ;.......................... 3.13

3.22 Time variable gain curve for mode 10, full volume normal beam ..................................................... 3.14

3.23 Signal strength after TVG amplification from 1/4 thickness side drilled hole in calibration block for

inspection mode 10......................................................................................................... 3.14

3.24 Signal strength after TVG amplification from 1/4 thickness side drilled hole in calibration block for

inspection mode 10...................................................;...................................................... 3.15

3.25 Signal strength after TVG amplification from 3/4 thickness side drilled hole in calibration block for

*inspection mode 10......................................................................................................... 3.15

4.1 Near surface detection record.............................................................................................. 4.5

4.2 Full volume detection record.............................................................................................. 4.6

4.3 Embedded, compact time-of-flight shape.................................................... ;............................ 4.7

4.4 Embedded, broadened time-of-flight shape.............................................................................. 4.8

4.5 Tip signal pattern showing two wave packets in the proper orientation for tip signals from a vertically

oriented planar reflector.................................................................................................... 4.9

4.6 Surface wave pattern showing two wave packets in the proper orientation for re-radiation from a

round trip of a volumetric flaw ............................................................................................ 4.10

4.7 Coin shape near the clad-to-base-metal interface ........................................................................ 4.11

4.8 Normal beam shape below the clad-to-base-metal interface ............................................... I............. 4.12

NUJREG/CR-647 1x
Contents

Page

4.9 Shape of indications in the clad-to-base-metal interface ................................................................ 4.13

4.10 Detection of a cloud-like shape............................................................................................ 4.14

4.11 Distance matrix for the clustering of detections from different inspection modes .................................... 4.15

4.12 Multimodal detection sheet for one portion of the weidment........................................................... 4.16

4.13 Characterization work sheet for one indication .......................................................................... 4.17

5.1 Mode 1 detection of planar indication #1 in the near surface zone..................................................... 5.8

8.1 Typical probability of detection curve.................................................................................... 8.8

8.2 Confidence bounds on model parameters for sample size of 3 ......................................................... 8.8

8.3 Cumulative flow rate function with 95% confidence bounds for a sample size of 3 ................................. 8.9

8.4 Confidence bounds on model parameters for sample size of 15........................................................ 8.9

8.5 Cumulative flaw rate function with 95% confidence bounds for a sample size of 15 ................................ 8.10

8.6 Parameter estimates with confidence bounds for simple Weibull fit to the SAFT-UT data of 9 planar

indications in the near surface weldment................................................................................. 8.10

8.7 Estimate of cumulative flaw rate function with 95% confidence interval for simple Weibull fit to the

SAFT-UT data of 9 planar indications in the near surface weldment.................................................. 8.11

8.8 Parameter estimates with confidence bounds for simple Weibull fit to the SAFT-UT data of 21 planar

indications in the near surface base metal ................................................................................ 8.11

8.9 Estimate of cumulative flaw rate function with 95% confidence interval for simple Weibull fit to the

SAFT-UT data of 21 planar indications in the near surface base metal................................................ 8.12

8.10 Parameter estimates with confidence bounds for simple Weibull fit to the SAFT-UT data of 26 planar

indications in the weldment below the near surface ..................................................................... 8.12

8.11 Estimate of cumulative flaw rate function with 95% confidence interval for simple Weibull fit to the

SAFT-UT data of 26 planar indications in the weldment below the near surface .................................... 8.13

8.12 Parameter estimates with confidence bounds for simple Weibull fit to the SAFT-UT data of 14 planar

indications in the heat affected zone below the near surface............................................................ 8.13

8.13 Estimate of cumulative flaw rate fuinction with 95% confidence interval for simple Weibull fit to the

SAFT-UT data of 14 planar indications in the heat affected zone below the near surface ........................... 8.14

8.14 Parameter estimates with confidence bounds for simple Weibull fit to the SAFT-UT data of 7 volumetric

indications in the base metal below the near surface.................................................................... 8.14

xi xi NUREG/CR-647 1
Contents

Page

8.15 Estimate of cumulative flaw rate function with 95% confidence interval for simple Weibull fit to the

SAFT-UT data of 7 volumetric indications in the base metal below the near surface ............................... 8.15

8.16 Parameter estimates with confidence bounds for simple Weibull fit to the SAFT-UT data of 31 planar

indications in the base metal below the near surface.................................................................. 8.15

8.17 Estimate of cumulative flaw rate function with 95% confidence interval for simple Weibull fit to the

SAFT-UT data of 31 planar indications in the base metal below the near surface .................................. 8.16

11.1 Impact of pressure vessel flaws on structural integrity................................................................ 11.8

11.2 Imperfectly calibrated POD scenario ......................................................................... -*-'*11.8

11.3 Flaw distribution functions X1, X2 X3, X4, X5, and X6 from PVRUF .................................................. 11.9

11.4 Flaw distribution functions: solid curves represent true functions, dashed curves represent found functions

for X I, X2, X3 , X4, ?,5, and 1%6. , = 1 mmn is used........................................................................ 11.10

NUJREGICR-647 1 xii
Tables

Page

1.1 Nomenclature for estimating cumulative flaw rates..................................................................... 1.2

2.1 Summary of base material volumes and areas in the PVRUF vessel .................................................. 2.4

2.2 Summary of weld volumes in the PVRUF vessel....................................................................... 2.4

3.1 SAFT-UT inspection plan for PVRUF................................................................................... 3.2

3.2 Step sizes and sampling rates for SAFT-UT inspections of the beltline weld in PVRUF............................. 3.3

3.3 Amount of material inspected by SAFT-UT at PVRUF................................................................ 3.4

4.1 Detection rule statements................................................................................................. 4.3

5.1 Planar indications in the weidment of the near-surface................................................................. 5.2

5.2 Volumetric indications in the weldment of the near-surface............................................................ 5.2

5.3 Planar indications in the heat-affected zone of the near-surface ....................................................... 5.3

5.4 Volumetric indications in the heat-affected zone of the near-surface.................................................. 5.3

5.5 Planar indications in the base metal of the inner surface zone.......................................................... 5.3

5.6 Volumetric indications in the base metal of the inner surface zone.................................................... 5.4

5.7 Planar indications in the cladding and in the clad-to-base metal interface............................................. 5.4

5.8 Volumetric indications in the cladding and in the clad-to-base metal interface ....................................... 5.5

5.9 Planar indications in the weldment ...................................................................................... 5.5

5.10 Volumetric indications in the weldment................................................................................. 5.6

5.11 Planar indications in the heat-affected zone ............................................................................. 5.6

5.12 Planar indications in the base metal .................................................................................... 57

5.13 Volumetric indications in the base metal ............................................................................... 57

6.1 PVRUF: Number of near surface indications by category ............................................................. 6.2

6.2 PVRUF: Number of indications outside the near-surface zones by category......................................... 6.3

7.1 POD for advanced procedures from fits to PISC-II data................................................................ 7.2

xiii xiii NUREG/CR-647 1
Contents

Page

7.2 Estimated POD for SAFT-UT inspections of the Midland blocks....................................................... 7.2

7.3 True-state table for through-wall extent.................................................................................... 7.2

8.1 Maximum likelihood fits run to determine sample size .................................................................. 8.4

8.2 Parameter confidence bound widths for different sample sizes.......................................................... 8.4

8.3 PVRUF flaw rate function parameters...................................................................................... 8.6

8.4 Published flaw rate function parameters ................................................................................... 8.6

8.5 PVRUF estimates of cumulative indication rates from Weibull fits to the indication frequencies.................... 8.7

8.6 Published estimates of cumulative flaw rates for weldment using Weibull fit ......................................... 8.7

10.1 Location of the largest indications along the weld, near surface zone .................................................. 10.2

10.2 Location of the largest indication along the weld outside the near surface zone....................................... 10.2

11.1 Flaw distributions from PVRUF ........................................................................................... 11.5

11.2 RMSE for base case ......................................................................................................... 11.5

11.3 RMSE (imperfect detection)/RMSE (base case)....S.....I.......I.......................................................... 11.6

11.4 RMSE (imperfectly calibrated POD)/RMSE (imperfect detection) ..................................................... 11.6

11.5 RMSE (imperfect sizing)/RMSE (base case) .............................................................................. 11.6

11.6 RMSE (imperfect calibrated sizing)/RMSE (imperfect sizing) ......................................................... 11.7

NUREG/CR-647 1xi xiv
Executive Summary

This report contains the results of a nondestructive examination of an unused U.S. nuclear reactor vessel for material

fabrication flaws. This is Volume 1 of a multi-volume set where the second volume contains the results of a destructive

validation of the indications reported here. Volume 2 contains descriptions of the removal of material from the PVRUF

vessel, the conduct of confirmatory NDE techniques and metallographic analysis, and the confirmation of flaw rates for the

vessel.

In this volume, we provide a description of the examination that was conducted at the Oak Ridge National Laboratory's

Pressure Vessel Research User Facility (PVRUF) on a vessel fabricated for a canceled nuclear power plant. Under the

sponsorship of the U.S. Nuclear Regulatory Commission, PNNL conducted a sequence of inspections using the Synthetic

Aperture Focusing Technique for Ultrasonic Testing (SAFT-UT) for the purpose of detecting and characterizing zones

containing any fabrication (preservice) flaws.

Twenty linear meters of weldment were inspected by SAFT-UT, including the entire beltline weld of the vessel. Ten

different inspection modes were used, including 5 inspections specifically selected to examine the inner 25 mm (1.0 in.) of

the vessel wall. The 10 inspection modes produced complementary information on the size, type, location, and density of the

indications of flaws in the vessel. The added value of each modality was demonstrated in detection and characterization of

the indications as described in this report. The calibration and vessel scanning procedures were routinely and successfully

applied during the 18 months of data collection at PVRUF. The image quality of the SAFT-UT inspections was very high

because of the fine sample spacing specified in the measurement plan and the careful execution of the calibration and

scanning procedure.

Among the principal findings of this report are the 2500 detectable indications in the SAFT-UT inspections of the PVRUF

vessel. Only these inspection results are included here. Where sizing results are reported, the SAFT-UT sizing rules were

used to conservatively size indication zones to insure that all potentially large flaws would be included in the validation plan.

Confirmations obtained by destructive tests, construction radiographs, or complementary NDE techniques are not included in

this report.

At the start of the SAFT-UT inspections of the PVRUF vessel, PNNL expected that 80% of the indications would be small,

that is, on the order of 2 mmn (0.08 in.) in through-wall size. This was based on experience gained from the inspections of the

Midland vessel. The data presented in this report show that 97% of the indications were less than 2 mm in through-wall size

in the near surface zone, that is, within 25 mm of the inner surface of the PVRUF vessel. For the remainder of the vessel,

that is, the portion outside the near surface zone, the data show that 80% of the indications were smaller than 4 mm (0. 16 in.)

in through-wall size. The rate of false detections in the analysis is expected to be low, because the detection rules were

developed from the high correlation of SAFT-UT indications with flaws validated by destructive tests of material removed

from the Midland vessel (see Appendix B).

The distribution of the empirical data provided enough information to apply a parametric model of the cumulative flaw rate

to six different subsets of the data, and obtain reasonable confidence bounds on the results. It was possible to extract distinct

distributions for the weldment and the base metal in the near surface zone. A comparison of these two distributions shows

significantly higher indication rate in the weldment compared to the base metal in the near surface zone. It was also possible

to extract different distributions for the weldment, heat affected zone, and base metal in the portion of the vessel outside the

near surface zone (i.e., beyond 25 mm from the inner surface). These distributions also show a significantly higher indication

rate in the heat-affected zone and weldment than in the base metal.

For the near surface zone, the comparison of indication distributions for the weldment and base metal reveals a greater

estimated density of indications in the weldment. For all planar flaws, the density ratio is 4 to I for the weldment compared

to the base metal in the near surface zone. This ratio increases to 28 to 1 when only the indications larger than 6 mm are considered. This is a reasonable result, because planar flaws include cracks and lack of fusion in the weld, and the base metal is

xv xv NUREGICR-647 1
Executive Summary

thought to contain mostly volumetric flaws with little through-wall extent. It is expected that validation of the SAFT-UT

indications by means of destructive testing will provide valuable metallographic data on the nature of flaws in the two

distinct populations.

For the portion of the vessel outside the near surface zone, comparisons of the indication distributions for the weidment, heataffected zone, and base metal shows greater estimated densities of indications in the heat-affected zone and weldment. For

all planar flaws, the density ratio is 4 to 1 for the weldment compared to the base metal. For all planar flaws, the density ratio

is 8 to 1 for the heat-affected zone compared to the base metal. It was expected that the mid-wall portion of the base metal

would contain a population of volumetric flaws with little through-wall extent. A determination of the nature of the larger

SAFT-UT indications in the outside the near-surface portion of the vessel wall can significantly add to our understanding of

the distribution of fabrication flaws in reactor pressure vessels.

This report contains estimated flaw rate functions for six different subsets of the SAFT-UT indications, along with confidence bounds. A principle conclusion of this project is that the flaw densities predicted by the SAFT-UT inspections are

significantly greater in all six cases than those predicted by a Marshall distribution of flaws (Marshall, 1982). This difference

is explained by the higher sensitivity of the inspections reported here compared to those employed during the Marshall Study.

The Marshall distribution was developed in the 1970s and was based on the best inspection results of that time.

The inspection effectiveness of reactor pressure vessels for the mid-i 1970s has been quantified by the Plate Inspection

Steering Committee Phase I (PISCI) studies. The inspection effectiveness of RPVs based on use of high sensitivity

inspections has been quantified by the performance of advanced techniques in the Programme for the Inspection of Steel

Components Phase 2 (PISCII).

In this report, recommendations are given for validating the indication rates by selective destructive analysis to provide the

high quality flaw statistics necessary for probabilistic fracture mechanics analysis, such as pressurized thermal shock (PTS)

analysis. The inspection data provided enough indications to estimate the six different indication rates in the reactor pressure

vessel (RPV). These indication rates, after validation, can be used as the flaw rates for the PVRUF RPV and for other vessels

that were fabricated using similar .procedures.

Volume 2, in preparation, will Provide the results of the destructive validation: the confirmed flaw density and distribution.

Although these results have not yet been published, the principle findings of the destructive validation are: the 2500

indications in the SAFT-UT data are flaws; the flaws are mostly less than 4 mm (0. 16 in.) in size (as predicted); the fusion

lines of the structural weld with the base metal contain an elevated concentration of vertical planar discontinuities; flaws

greater than 8 mm (0.32 in.) in size are associated with repairs; and the flaws are complex (a combination of cracks, lack of

fusion, slag, and voids). What is chiefly not supported by the destructive validation test is the applicability of one of the

sizing rules. The sizing rule in question states that two isolated, vertically oriented ultrasonic echoes can be explained by the

presence of one crack and the echoes indicate the top and bottom of that crack. The application of this rule is confounded by

the fact that the concentration of small flaws on the fusion line creates signals that are not easily distinguished from crack tips

of a large crack. Specifically, some of the larger SAFT-UT indications reported in these NDE results will be characterized as

two small flaws in the destructive examinations reported in Volume 2.

NUJREG/CR-647 1xv xvi
Acknowledgments

The authors would like to acknowledge the contribution of Mr. Robert Bowey in the fabrication of SAFT-UT data acquisition

equipment for use at PVRUF and the contributions of Mr. Steven Minister and Ms. Mariana Campbell in the analysis of the

SAFT-UT data. The authors would like to thank the cooperative staff at the Oak Ridge National Laboratory, in particular

Dr. William Pennel, Mr. Fred Jackson, Dr. Daniel McGuire, and Mr. Paul Burns for providing support during the SAFT-UT

inspections of the PVRUF vessel. Mr. Jeff Prior, Mr. Robert (Moe) Schoefield, and Mr. Luke Sunwoo did an outstanding job

of operating the SAFT-UT system at ORNL and providing very accurately documented data for PNNL to use.

The authors wish to thank Cathy Stephens and Marsha Brehm for assistance in preparing this manuscript.

The authors thank the U.S. Nuclear Regulatory Commission, Office of Nuclear Regulatory Research for supporting this work

and in particular the NRC program managers, Ms. Deborah A. Jackson, Mr. Craig A. Hrabal, Dr. Joseph Muscara, and

Mr. Allen L. Hiser, Jr.

xvii xvii NUREG/CR-647 I

Executive Summary

Glossary and Abbreviations

BWR. Boiling Water Reactor. A nuclear reactor in which the coolant is water, maintained at such a pressure as to

allow it to boil and form steam.

Butt weld. The structural welds in RPVs (piping, etc.). This includes the RPVs circumferential and axial welds

but does not include the cladding weidments.

Base metal. The metal that composes the RVP plates or forged rings. RPV plates (e.g., alloy A533B) and forged

rings (e.g., A508) are assembled into an RPV by butt-welding.

Cumulative flaw rate. The density of flaws greater than a specified size.

False call. The characterization of a blank unit of material as flawed or cracked.

Flaw. An imperfection or unintended discontinuity in a material. A void, porosity, inclusion, lack of fusion or

crack that is physically distinct from the metallic microstructure.

Flaw density. The number of flaws per unit length, area, or volume.

Flaw depth size. See through-wall extent.

Flaw distribution. The number of flaws measured in separate categories.

Flaw rate. The flaw density expressed as a function of flaw through-wall extent.

Fusion line. One of two lines, on the cross-section of the weld, that form the boundary between the weld metal

and the base metal.

Heat-affected zone (I-IAZ). A portion of the base metal (adjacent to the weld) whose microstructure is altered by

heat deposited during welding.

Indication (of a flaw). The response or evidence of a flaw from the application of NDE. For ultrasonic NDE, a

coherent packet of (ultrasonic) energy that is characterized as originating from a flaw.

Inclusion. A foreign solid, (e.g., slag, scale, oxide, or non-metallic substance) entrapped in the base metal or weld

metal.

LTOP. Low temperature over-pressurization.

Lack of fusion . Lack of metallic bond between weld passes or between a weld pass and the base metal.

LWR. Light water reactor. Either of two nuclear fission reactor designs (see BWR and PWR) that heat water as a

means of power production.

xix xix NUREG/CR-6471
Executive Summary

Marshall Distribution. A cumulative flaw rate in the weld metal of RPVs. See Marshall 1982.

Midland vessel. The Consumers Power Unit 2 reactor vessel. See Babcock & Wilcox 1989.

Near-surface zone. The first 25mn (1.0 inch) of RPV material from the cladding's wetted surface.

Outside the near-surface zone. The remainder of vessel wall when the near-surface zone is excluded.

PVRUF vessel. The Pressure Vessel Research User's Facility vessel, at Oak Ridge National Laboratory, was a

PWR vessel from a cancelled U.S. plant. See Pennel 1989.

Planar flaw. A flat two-dimensional flaw in a plane other than parallel to the surface of the component. In this

study, it includes a crack or lack of fusion that is oriented vertically in the vessel.

POD. Probability of Detection. the expected value for the fraction of flawed or cracked units of material that will

be found to be flawed or cracked by an inspection system.

Porosity. A group of voids located in close proximity to each other.

PWR. Pressurized Water Reactor. A nuclear reactor in which the coolant is water, maintained at such a pressure

as to keep it from boiling.

PTS. Pressurized Thermal Shock.

RP V. Reactor Pressure Vessel.

Ring down. An ultrasonic term that refers to the period of time, following excitation of the transmit transducer,

when acoustic interference is present at the receiver.

SAFT-UT. Synthetic Aperture Focusing Technique for Ultrasonic Testing. See Doctor 1995.

Size. See through-wall extent.

Through-wall extent. The maximum dimension, normal to the surface of the component, of the rectangle

circumscribing the flaw.

Void. A volume of gas entrapped in the vessel material.

Volumetric flaw. A three-dimensional flaw such as a void, porosity, or inclusion without vertical orientation in

the vessel.

Weld profile. The shape of the weldment when sectioned across the weld.

NULREG/CR-647 1 xx
Executive Summary

Wiebull Distribution. A distribution that has been used extensively to deal with such problems as reliability and

life testing. The continuous random variable "s" has a Weibull Distribution, with parameters a and f3, if its

density function is given by

f(s) = aIpso e-asI1 s>o

= 0, elsewhere

where oc> 0 and 3 > 0.

xxi xxi NUREG/CR-647 1

1 Introduction

The U.S. Nuclear Regulatory Commission initiated a program at the Pacific Northwest National Laboratory

(PNNL) with the major objective of estimating the rate of

occurrence of fabrication flaws in U.S. light-water reactor

pressure vessels (RPVs). In this study, RPV material was

examined using a state-of-the-art ultrasonic inspection

system, the Synthetic Aperture Focusing Technique for

Ultrasonic Testing (SAFT-UT), to detect and characterize

flaws created during fabrication.

This chapter discusses those flaw characteristics that are

predicted by fracture mechanics calculations to be important for vessel integrity. Design and fabrication information on RPVs is presented in Chapter 2, especially on the

subclass of vessels used in pressurized water reactors

(PWYRs), along with the specifications for the vessel

inspected in this program. Chapter 3 specifies the measurements performed by SAFT-UT and includes a

description of the SAFT-UT equipment and the calibration procedures. The analysis of the SAFT-UT inspections is described in Chapter 4, and example indications

are presented to explain the process of detecting and

characterizing the ultrasonic images produced by SAFTUT. Chapter 5 discusses the most significant indications

in the inspection and documents their important features.

The distributions of the indications in those categories

important for vessel integrity are presented in Chapter 6.

Chapter 7 describes the tests measuring the performance

of SAFT-UT as it pertains to probability of detection and

sizing error, which are necessary inputs for calculating

flaw rates. Chapter 8 presents the methodology for fitting

a parametric rate function to the distribution of indications

in the SAFT-UT measurements. Chapter 9 describes the

validation plan for the characterized indications in the

SAFT-UT data. Chapter 10 explains the ISI reliability

requirements needed to predict flaw rates in vessels. The

conclusions and recommendations that were derived from

this program are given in Chapters 11I and 12. Further

reference is provided in Chapter 13. The SAFT-UT

images of the most significant flaws can be found in

Appendix A.

Previous work has been included here. The analysis-.

before-test document was integrated into this report and

the details of the inspections of the material removed

from the Midland vessel are included as Appendix B.

Volume 2 of this report, in preparation, will document the

results of the destructive validation of the SAFT-UT indications. As of this writing, the destructive validation is

largely completed and the principle findings are: the

2500 SAFT-UT indications are flaws; the flaws are

mostly small (as predicted), and the fusion line between

the weld and the base metal contains an elevated concentration of small vertical planar discontinuities; flaws

greater than 8mm in size are associated with repairs; and

the flaws are complex (a combination of cracks, lack of

fusion, slag, and voids).

What is chiefly not supported by the destructive analysis

in the analysis of SAFT-UT indications presented here is

the applicability of one of the sizing rules. The elevated

concentration of small discontinuities on the weld fusion

line created signals that are difficult to distinguish from

crack tips. Consequently, the reader should expect the

sizes of some of the flaws to be amended in Volume 2.

1.1 Reasons for Estimating

Fabrication Flaw Rates

Estimates for flaw rates are an important input to structural assessments by probabilistic fracture mechanics,

such as those relating to the Pressurized Thermal Shock

(PTS) scenario. The PTS issue can be an important factor

in questions pertaining to plant life extension for some

plants. Computer codes such as the VISA code

(Simonen, et al. 1986a) require accurate estimates of the

flaw rates in the reactor vessel to determine the likelihood

of a vessel failure during a PTS event. The majority of

past work in probabilistic fracture mechanics (PFM)

considered cracks to be expressed in terms of a single

crack size parameter (size in the depth dimension). A

two-dimensional crack is much more realistic but considerably more complex. Some PFM codes are capable of

treating a two-dimensional crack, and are based on the

assumption that a two-dimensional crack is a semielliptical surface crack.

These codes can provide the capability of considering

more realistic and specific flaw rate information, but

because of the lack of empirical data on fabrication flaw

distributions, conservative assumptions are made about

the initial flaw size distribution, aspect ratios, and

1.1 1.1 NUREG/CR-647 1
Introduction

through-wall locations. Studies (Simonen et. al,. 1986a,

Simonen and Khaleel 1995) have shown that the probability of vessel failure is sensitive to: the location of the

flaw in the vessel (i.e., near surface versus interior of the

vessel wall); the flaw type (e.g., cracks, lack of fusion,

porosity, inclusions, etc.); and the flaw aspect ratio (i.e.,

flaw length as well as depth). Therefore, it would be

useful to have flaw rate estimates that are based on

empirical data.

Future work, in NRC JCN W6275, will measure flaw

density and distribution in material removed from the

Shoreham, River Bend Unit 2, and Hope Creek Unit 2

vessels. These distributions, together with information on

vessel fabrication techniques will be used to produce generalized flaw density and size distributions for application

to the entire population of vessels of all classes. An existing statistical and weld process model, as developed by

Chapman (1993) in the U.K., is being evaluated for its

ability to predict vessel-specific flaw densities and size

distributions for use with and comparison to in-service

inspection (151) results from operating reactors. The data

from this project will be used to benchmark and calibrate

Chapman's predictive model. A calibrated predictive

model, such as the one developed by Chapman, is

expected to provide, when used with ISI data, a means of

extrapolating the flaw rates from this project to the entire

population of vessels in the U.S.

1.2 Principal Objectives

This program concentrated on estimating six different

flaw rates in one reactor pressure vessel, the Pressure

Vessel Research User Facility (PVRUF) vessel located at

Oak Ridge National Laboratory. The PVRUF vessel is a

PWR vessel that was fabricated by Combustion Engineering but never put into service. The pressure vessel manufacturing was started in 1978 and completed in 1982. It is

constructed from A533B alloy steel.

At the start of this study, six different flaw rates were

proposed that were all combinations of the two flaw types

of interest (planar and volumetric) and three different

locations (near surface, weldment, and base metal) as

illustrated in Table 1. 1. The three locations referred to in

the table are defined as follows: "Near Surface" designates the region within 25 mm (1 in.) of the inner surface

of the vessel wall. "Weldment" includes all welded material not within 25 mm of the inner surface and also the

heat-affected zone of the base metal. "Base Metal" refers

t o all base metals not within 25 mm, of the inner surface

and outside the heat-affected zone. The heat-affected

Table 1.1 Nomenclature for estimating

cumulative flaw rates

Flaw type

Location Planar Volumetric Units

Near Surface A5P(s) A,,(s) # flaws/m'

Weldment Awp,(s) A,.(s) # flaws/rn 3

Base Metal App(s) Ap9,(s) # flaws/in 3

s =flaw depth (cm)

zone is the zone within the base metal that undergoes

structural changes but does not melt during welding.

The cumulative flaw rate function, Akj(D) describes the

average number of flaws of type j greater than size D that

occur in a unit of material in location k. For example,

A~(I 15) describes how many planar flaws with a depth

greater than 15 mm would occur in a square meter of

material within 25 mm of the inner surface. By definition, the negative of the derivative of the cumulative function represents the desired flaw rate function. This estimation problem is often discussed in terms of both the

flaw rate and the cumulative rate. It is important to recognize that once one is determined, the other can be

computed.

1.3 Secondary Objectives

The SAFT inspection produced flaw length information,

so it is possible to consider flaw rate functions that

depend on flaw length as well as on flaw depth. However, reliable estimation of such a two-dimensional function requires more data than needed to estimate the six

flaw rates proposed. Nevertheless, estimates are made of

both length and depth from the produced inspection data.

1.4 Flaws of Concern to Reactor

Pressure Vessel Integrity

Characteristics of the fabrication flaws in reactor pressure

vessels are necessary inputs for fracture calculations to

assess reactor pressure vessel integrity. Fracture mechanics calculations are typically based on idealized flaws

with worst-case characteristics. It is implied that "fracture mechanics flaws" often represent or approximate the

significant flaws in the real vessel, which may have irregular and random features.

The majority of SAFT-UT indications in vessel inspections come from several categories of naturally occurring

NUREG/CR-647 112 1.2
Introduction

flaws. These flaws do not, in general, correspond to the

hypothetical and worst-case flaws that are usually

assumed for purposes of fracture mechanics calculations.

This analysis of SAFT-UT inspections focused on detecting and characterizing indications that approximate the

idealized flaws of fracture mechanics models. Some

categories of natural flaws are as follows.

Base metal flaws, which are present in the original plate

or forging prior to vessel assembly and welding. Typical

flaws of this type would be inclusions and laminations,

which are most often encountered in the mid-wall region

of the vessel cross-section, and with orientations parallel

to the surfaces of the finished vessel.

Underclad cracks, which are produced by the heat input

and stresses associated with the welding process used to

apply'the cladding to the inner surface of the vessel.

These cracks will be in the base metal, initiating from the

clad-to-base metal interface. Typical cracks are shallow

in depth. The susceptibility of RPVs to such cracking

depends on the particular grade of vessel steel and the

welding process used to clad the vessel. If cracks are

present in a given vessel, then there are likely to be a

large number of such cracks.

Cracks in the cladding itself, which can be produced by

stress corrosion cracking. These cracks will be shallow,

but if conditions are right, they could occur in large numbers for a given vessel.

Welding flaws, which can occur either within the weld

metal itself, or within the heat-affected-zone of the base

metal. These flaws could occur anywhere within the

thickness of the welded cross-section. Because root

passes for welds are often made using special procedures,

a bimodal distribution for such flaws can be postulated.

The flaw distributions used in fracture mechanics calculations for RPVs tend to focus on welding and heataffected zone flaws. In these calculations, the flaws are

typically treated in a worst-case fashion, by assuming that

the flaws are at the inner surface of the vessel.

In this study, we are interested in reporting flaw sizes and

flaw locations that have the greatest impact on vessel

integrity. Vessel failure modalities are described here

because they determine which features of the measured

flaw population are significant.

The pressurized thermal shock (PTS) and low-temperature over pressurization (LTOP) transients are two of the

most important scenarios for reactor pressure vessel

failure. Both of these events have been addressed in

fracture mechanics calculations, and both contribute

significantly to RPV failure probability. The PTS event

produces high cooling within the inner vessel wall with

associated high tensile thermal stresses and low fracture

toughness in this region. Thus, small flaws near the clad

inner surface are of primary concern.

The LTOP event has quite different characteristics. There

are essentially no thermal stresses, and the stresses from

internal pressure are relatively uniform through the wall

of the vessel. During LTOP events, the entire vessel wall

is also rather uniformly at low temperatures; in some

cases these temperatures approach the lower shelf portion

of the material's toughness-versus-temperature curve.

Therefore, with relatively uniform distributions of both

tensile stresses and fracture toughness, a flaw anywhere

within the vessel wall could impact vessel integrity.

However, buried flaws remain much less significant than

surface flaws.

Results of fracture mechanics calculations predict the

probability that the event of interest will cause vessel

failure, but also for purposes of this discussion indicate

the sizes and locations of flaws that are most likely to

cause such failures. Two sets of calculations are discussed here, which address the bounding events of PTS

and LTOP. The PTS case corresponds to a sudden cooling of a vessel starting from its initial state at operating

temperature, whereas the LTOP events consists of a rapid

increase in pressure for a vessel starting from a cold shutdown condition. PTS events present a severe challenge to

vessel integrity but only to the inner region of the vessel

wall. The stress levels for LTOP events are at more

modest values, but the entire vessel wall is challenged at

low temperatures, where the vessel material has relatively

low resistance to brittle fracture.

Both the PTS and LTOP transients are more than postulated events, because both have actually occurred on several occasions at operating reactors. However, on all such

occasions, the vessels have survived undamaged, because

critical conditions of flaw size and material embrittlement

did not exist at the time of the event for the particular

vessels of concern.

PTS calculations have been performed in a previous study

using the VISA-Il computer code (Simonen et al. 1986b).

Figure 1. 1 shows histograms plotted from the results of

these VISA-II calculations. The plots show the depths

and locations of the flaws that were the root cause of the

simulated vessel failure. The flaw location is defined as

the distance from the inner tip of the flaw to the inner

surface of the vessel. The Octavia flaw distribution

1.3 1.3 NUREGICR-647 I
Introduction

(Vesely, 1978) was assumed for these calculations, as

well as a large flaw length relative to flaw depth. Flaws

were assumed to be distributed uniformly or randomly

through the vessel wall.

For PTS events, the calculations indicate that flaws situated outside the inner 30% of the vessel wall contribute

little to RPV failure probability. Also, flaw depths greater

than about 25% of the vessel thickness contribute little to

failure probability, because the thermal stresses do not

extend that far into the vessel. On the other hand, small

flaws of depth less than 10% of the vessel wall make a

substantial contribution. For PTS conditions, flaws as

small as 6 mm (0.25 in.) can make a sizable contribution

to the overall vessel failure probability.

LTOP calculations have been performed in a previous

study using the VISA-11 computer code (Simonen et al.

1986b) and assuming a severe pressure excursion at low

temperature (as might occur during reactor startup). The

assumed conditions for the selected LTOP example had a

relatively high initial value of ductile brittle transition

temperature and a low level of radiation damage. The

vessel material was essentially on the lower shelf of the

fracture toughness versus temperature curve during the

LTOP event. The objective was to define a limiting case

with relatively little variation of fracture toughness

through the vessel wall. This case, although not representative of most LTOP situations, serve as a contracting

bounding case relative to the conditions of PTS events.

Figure 1.2 shows histograms plotted from the VISA-11

results, presenting statistics on the depths and locations of

the flaws that caused the simulated vessel failures for the

LTOP event. Unlike the PTS scenario, flaw depths of less

than 10% of the vessel wall thickness are relatively unimportant for LTOP failures. Rather, it is the somewhat

larger flaws in the range of 10% to 3 0% of wall thickness

that contribute the most toward vessel failure under LTOP

conditions. Flaws greater than 30% of wall thickness

contribute less to the probability of vessel failure, because

they are much less likely to occur.

For the example LTOP event addressed here, the calculations show that flaws situated within the outer part of the

vessel wall should be of concern, in addition to flaws near

the inner surface. Figure 1.2 indicates that about half the

calculated vessel failures are due not to surface flaws, but

to flaws within the interior of the vessel wall. The other

half of the calculated failures are due to flaws near surfaces of the vessel (inner and outer 10% of the wall thickness). In this regard, a given flaw within the inner 10% of

the wall is about four times as likely to cause a vessel

failure as the same flaw located elsewhere in the vessel

wall. The results also show a similar but less severe

sensitivity to flaws within .the outer 10% of the vessel

wall.

Deterministic calculations of stress intensity factors for

PTS conditions have been reported by Simonen, Johnson,

and Simonen (1985), and these results supplement the

insights gained from probabilistic fracture mechanics calculations. Of particular interest in this discussion are

flaws near the inner surface of the vessel, and the localized stress states that exist near the interface between the

cladding and the base metal.

Figures 1.3 to 1.5 shows plots of calculated stress intensity factors Ki for various flaw configurations. For purposes of predicting the potential for brittle fracture, the

factors have been normalized with respect to the governing fracture toughness at the tip of the crack. All calculations accounted for the stress state induced by the

differential thermal expansion of the cladding relative to

the base metal. The irradiated fracture toughness of the

cladding was assumed to be the same as that for the base

metal. Calculations were for the Rancho Seco transient,

and for the same vessel parameters as described above for

the probabilistic calculations. Figure 1.3 shows the relationship between flaw length and flaw depth on calculated

stress intensity.

Figure 1.4 indicates that cracks parallel to the vessel surface have no impact on vessel integrity. In fact, these

results show negative stress intensity factors, meaning

that cracks will tend to close rather than open. It should

be noted that these results address the embrittled region of

the vessel beltline, and may not apply to other parts of the

vessel with more complex states of stress.

There are significant driving forces that propagate small

cracks that are entirely within the cladding, cracks that

extend only slightly into the base metal, or base metal

cracks at the interface with the cladding, Figure 1.5.

Vessel integrity is quite sensitive to such cracks, and

improved information on both the number and sizes of

cracks in cladding is therefore desirable. Nondestructive

Evaluation (NDE) examinations should include detailed

examinations of the cladding, and the base metal region

adjacent to the cladding. It should be noted that

underc lad cracks for Case 2 of Figure 1.5 can grow into

the clad metal, which (unlike Case 3) is assumed to have

the same low level of toughness as the base metal..

Figure 1.6 shows various flaws (shown to scale on a

cross-section of a vessel wall) that has the same significance relative to vessel fracture in a PTS event. Clearly, a

NUREG/CR-647 I1. 1.4
Introduction

long shallow crack extending through the cladding, shown

at left, has a disproportionate impact on vessel integrity,

although it may be of relatively small size. By comparison, a much larger subsurface crack, shown at right, is no

more severe than the shallow crack through the cladding.

Other evaluations of PTS have often observed an insensitivity to crack size, once the threshold crack size is

exceeded. The comparisons depicted in Figure 1.6 support this observation.

Summing up, it is evident that a high priority should be

given to detecting and characterizing flaws near the inner

surface of the vessel (by size, shape, orientation, etc.).

Within this region, flaws as small as 6 mm (0.25 in.) in

depth can be significant, including flaws within the cladding. In addition, flaws within the remainder of the vessel

wall should be detected and characterized. Somewhat

larger flaws in this region, approximately 20 mm and

greater, contribute to vessel failure in probabilistic fracture mechanics (PFM) calculations of LTOP events.

1.5 1.5 NUREGICR-647 1
Introduction

0

(Il

0

a,

U

0

a60

50

40

30

20

10

0

60

s0

40

30

20

10

j33

47

20

I I* I I I I

S

U.

C.

0 10 20 30 40 50 60 70 80 90 lOC

Flaw Location - From Inner Surface, a/t(%)

(a) Contribution of Various Flaw Locations to

Vessel Fracture Probability

44

31

L2

0

Flaw Size, % of Wall

(b) Contribution of Various Flaw Sizes to Vessel

Fracture Probability

Figure 1.1 Significant flaws for vessel fracture from pressurized thermal shock events (Simonen 1984)

NUREG/CR-647 11. 1.6
Introduction

0

0

(L

60

5o

40

30

20

10

0

0 10 20 30 40 50 60 70 80

Flaw Location - From Inner Surface, a/t(%)

(a) Contribution of Various Flaw Locations to

Vessel Fracture Probability

go 100

80

U.

"6

60

40

20

62

22

-6 8

0

0 10 20 30 40 50 ~0

Flaw Size. % of Wall

70 80 90 100

(b) Contribution of Various Flaw Sizes to Vessel

Fracture Probability

Figure 1.2 Significant flaws for vessel fracture from low temperature over pressurization events (Simonen 1984)

1.7 1.7 NULREGICR-647 I
Introduction

FLAW DEPTH, mm

0 10 20 30 40 50 60

0 0.05 0.10 0.15 0.20 0.25

FLAW DEPTH, a/w

Figure 1.3 Effects of surface length on fracture evaluation (Simonen 1984)

0.30

NUREG/CR-647 11. 1.8
Introduction

2 .C

-0.5

d5 a=8

0 0.05 0.10 0.15 0.20 0.25 0.30

FLAW LOCATION, d/W.

Figure 1.4 Effects of subsurface flaw location and orientation on fracture evaluation (Simonen 1984)

1.9 1.9 NUREG/CR-647 1
Introduction

FLAW DEPTH, mm

30 40

UNUREG/CR-647 1

0 0.05 0.10 0.15 0.20 0.25 0.30

FLAW DEPTH, a/w

Figure 1.5 Effect of cladding on fracture evaluation (Simonen 1984)

1.10
introduction

Vessel OD

Vessel ID

Figure 1.6 PTS flaws'of equivalent severity

1.11 1.11 NrJREG/CR-647 1

2 Description of Reactor Pressure Vessel Material

The reactor vessel is the most critical component in the

light water reactor primary pressure boundary, since a loss

of integrity will uncover the reactor core. Vessel integrity

is of particular concern in, PWRs because of the high

coolant pressures, the high neutron doses, the limited makeup rate for lost coolant, and the event scenarios that

challenge the integrity of the RPV.

For BWrRs, the recent BWR Owners Group (BWROG)

program on vessels and internals has performed analysis for

various vessel failure scenarios. This work has found that

the failure probabilities are much lower for the circumferential welds than for the axial or longitudinal welds. BWR

vessel integrity is being driven by the structural integrity of

axial or longitudinal shell course welds and the associated

fabrication flaws in these welds (EPRI, 1995).

This program concentrated on measuring the fabrication

flaws in two PWR reactor pressure vessels: the PVRUF

vessel, and blocks removed from the Midland vessel (see

Appendix B). The PVRUF vessel was fabricated by

Combustion Engineering (CE) but was never put into

service, Manufacture of the vessel was completed in 1982

and it is constructed from A533B alloy steel. The shell

courses were fabricated from rolled and welded plate.

About 30 similar reactor vessels in service in the U.S. were

fabricated by CE out of A533B material.

The Midland vessel was fabricated by Babcock and Wilcox. The shell courses of the Midland vessel were fabricated out of forged rings by piercing a large ingot of A508

steel and rolling it until a ring of the proper shape was

obtained. There are approximately 12 operating PWRs in

service in the U.S. that were manufactured by B&W out of

A508 material.

The estimates obtained in this study may be applied to the

general population of reactors, but corrections must be

made, depending on how similar the specific reactor is to

the sub-population we have investigated. One way of

accomplishing this is through coordination with work in

progress on other NRC programs. Future work, in NRC

JCN W6275, will gather information on vessel fabrication

techniques for use in producing generalized flaw density

and size distributions for application to the entire population of vessels or to classes of vessels. An existing statistical model, as developed by Chapman (1993) in the U.K.,

is being evaluated for its ability to predict vessel-specific

flaw densities and size distributions for use with and compa rison to in-service inspection (151) results from operating

reactors. The data from this project will be used to benchmark and calibrate Chapman's predictive model. A calibrated predictive model such as the one developed by

Chapman is expected to provide, when used with ISI data, a

means of extrapolating the flaw rates from this project to

the entire population of vessels in the U.S.

2.1 Differences Between PWRs and

BWRs

Figure 2.1 illustrates a typical pressurized water reactor

(PWR) pressure vessel and intemnals. A Il000-MWe class

vessel is about 4.32 mn (170 in.) in diameter and 13.7 mn

(45 ft.) high; the wall is about 228.6 mm (9 in.) thick.

The PWrR system uses external steam generation and separation, so the PWvR vessel is much shorter than the BWR

vessel. The PWR system does not use internal jet pumps;

hence, the PWR vessel has a smaller diameter than the

BWR RPV. The PWR operates at a higher pressure, typically 15.5 MPa (2250 psi), and a temperature of 288-3 16'C

(550-600'F). Accordingly, the PWR wall thickness is

significantly greater than that of the BWR, to accommodate

the higher operating pressure.

Vessel integrity is of particular concern in PWRs because

of the high coolant pressures and high neutron doses. The

problem of radiation embrittlement is reduced in BWRs

because of their larger diameters, their lower pressures,

more sources of water to cover the core, and the greater

amount of water shielding the shell from the reactor core.

However, radiation damage in BWR vessels could become

a limiting factor, depending on material chemistries and

accumulated fluence. Most BWR vessels have an

end-of-life fluence (E > 1 MeV) of about 5 x 101" n/cm2

compared to a typical end-of-life fluence for a PWR vessel

of about l x 1019 n/cm2.

2.2 Fabrication Methods

Similar techniques have been used to fabricate BWR and

PWR RPVs. Some PWR vessels have been constructed

entirely from forged rings, flanges, and nozzles. The Midland vessel was fabricated in this manner. Alternatively,

rolled and welded plate have been used to fabricate the

2.1 2.1 NUREG/CR-647 1
Description of Reactor Pressure Vessels

shell courses, and forgings for the flanges and nozzles. The

PVRUF vessel was fabricated in this way. All BWR

vessels have used the latter construction, with shell courses

of rolled and welded plate.

For the rolled and welded construction practice, the first

step in the fabrication sequence is to hot-form plate into

1200 segments. Three 1200 segments are then welded

together into a shell course. The shell courses are typically

clad with austenitic stainless steel weidment using either

the multiple wire or strip cladding submerged arc welding

process.

Three shell courses typically make up the cylindrical portion of a PWR. The upper shell course is usually thicker

than the intermediate and lower shell courses. The additional thickness is required to reduce the stresses associated

with the nozzle penetrations. Nozzle forgings are then

welded into large diameter holes in the nozzle shell course.

The vessel's upper flange is welded to the top of the nozzle

shell course, and the intermediate shell course is welded to

the bottom of the nozzle shell course, forming one of two

vessel subassemblies. Following welding, a manually

applied cladding layer is deposited on the zone around the

weld in order to link the cladding already existing on the

two shell courses.

The second vessel subassembly consists of the lower shell

course and the bottom head. The final steps are to weld the

upper and lower subassemblies together and stress relieve

the assembly. Fabrication of the top head is conducted in a

similar manner.

A forged vessel is constructed in a similar fashion, except

that the shell courses are one-piece ring forgings. This construction technique avoids longitudinal weldments and

generally requires between three and five forged rings to

construct a reactor vessel.

2.3 Materials

Reactor vessel steel plate specifications have evolved since

the beginning of the commercial nuclear power industry.

The original steels selected, A212 and A302B, were in

widespread use in the construction of fossil-fueled power

plant components. All of the plates are low-alloy ferritic

steels. A2 12 was used only in the very early plants, which

are now decommissioned. A much more widely used

material, A302B, is a carbon-manganese-molybdenum. steel

that was used in the quenched and tempered condition.

With the increasing size of RPVs, greater hardenability was

needed. The addition of nickel to the A302B composition

provided the necessary increased hardenability to achieve

the desired mechanical properties. This steel was initially

known as A302B Modified. Later, it became the present

grade A533B Class 1, which is the most widely used material for construction of RPVs.

In 1973, ASME Code and ASTM developed limits that

were placed on the percentage of copper and phosphorus

permissible for use in the beltline region of RPVs, where

the neutron flux is high. The influence of weld chemistry

on embrittlement were first addressed in Regulatory Guide

1.99, Revision 1 (U.S. NRC, 1979) and later updated in

Revision 2 (U.S. NRC, 1988). The reduction of copper and

phosphorus served to minimize the sensitivity of the steel

to radiation embrittlement. The steel of choice was A533

for the beltline region. The current ASME Code and

ASTM requirements for beltline materials in the reactor

vessel for a new plant specify that the content of residual

elements such as copper, phosphorus, sulfur, and vanadium

should be controlled to low levels. However, in selecting

the optimum amount of nickel allowed, its, deleterious

effect on radiation embrittlement should be balanced

against its beneficial metallurgical effects and its tendency

to lower the initial reference temperature for the unirradiated vessel material.

Pressure vessel forging materials have also evolved 'with

time. The material used most extensively for RPV flanges,

nozzles, and rings has been A508 Class 2 steel. Production

problems were encountered with this material, in that

underclad cracks occurred with certain cladding procedures. Application of strip cladding tended to produce

small cracks on the order of several millimeters in depth in

some of the heat-affected zones. It was eventually discovered that the presence of chromium in this forged material was the root cause of the cracking. Such cracking has

never been observed in the A533B Class I plate material or

the submerged arc weld metal. To eliminate underclad

cracking, A508 Class 3 material is now used in place of

A508 Class 2.

2.4 Weld Procedures

Full-thickness welding is required to assemble the shell

courses, the nozzle forgings, the flange forgings, the top

and bottom heads, and any internal or external support

pads. The most frequently used technique is the automatic

submerged arc welding procedure. The materials consumned in this welding process are a manganesemolybdenum-nickel filler wire and a granulated flux that

NUREG/CR-647 12. 2.2
Description of Reactor Pressure Vessel Material

minimizes atmospheric contamination and provides ingredients to form a slag to remove oxides during the welding

process. The type of flux material is important because the

mechanical properties of the weldment can differ, depending on what flux is used. Welds made by Babcock &

Wilcox employed Linde 80 flux which typically gives

lower values of upper-shelf Charpy V-notch properties than

other fluxes. Combustion Engineering (CE) and Chicago

Bridge & Iron used Linde 0091, 1092, and 124 fluxes;

these three fluxes produce similar mechanical properties.

Narrow-gap submerged arc welding is a variant of this

welding technique used primarily for circumferential seam

welds. The benefit of this variation is reduced weld metal

volume and fabrication time. However, their upper shelf

energy values are statistically and physically distinct on the

basis of flux type (ABB-CE, 96)

Manual welding was used frequently for complex configurations, for repairs of base material, or for areas of weld

buildup. A shielded metal arc (SMA) welding procedure

was used in such instances. The SMA electrode is a wire

coated with a bonded flux, typically an E8018 electrode.

The electroslag welding technique was used for fullthickness welds in some of the earlier BWIR pressure vessels. This technique provides high deposition rates.

Because of their coarse-grained cast microstructure,

electroslag welds must be austenitized, quenched, and tempered in a manner similar to the treatment for the base

metal.

All interior surfaces of the PWR vessel were clad with

austenitic stainless steel to inhibit general corrosion and the

buildup of radioactive crud. BW~R vessels were clad below

the steam-water interface. Three cladding processes were

used. Automatic submerged arc welding with an SMA

electrode was used when possible due to its high deposition

rate. The process used either multiple wires or strip electrodes of Type 308 or 309 stainless steel. In areas where an

automatic process was not possible, shielded metal arc or

gas tungsten arc welding were utilized.

2.5 Description of PVRUF

The PVRUF pressure vessel was assembled by Combustion

Engineering for a nuclear power plant that was never

completed and its general configuration is illustrated in

Figure 2.2. The pressure vessel has a diameter of.4.39 mn

(173 in.), a height of approximately 13.34 mn (525 in.), and

is made out of A533B material. The wall thickness of the

pressure vessel varies from one region to the next, but

around the beltline it is 22 cm (8.6 in.).

Table 2.1 shows the amount of base material in the PVRUF

vessel. This table gives the volume and surface area for

base material by vessel section (see Figure 2.2 for the

relative location of each vessel section). Table 2.2 shows

the amount of weldment in the PVRUF. This table gives

the linear amount of weld in each section of PVRUF.

Figure 2.2 identifies the welds by number. Figure 2.3 is a

photograph of the PVRUF from the outside as it lay on its

side during the inspections by SAFT-UT. Figure 2.4 is a

photograph of the inside of the PVRUF vessel showing the

instrument platform used for the SAFT-UT measurements.

2.6 Categorization of RPVs

Information is needed on the RPV fabrication characteristics in order to develop a basis for extrapolating flaw

distribution estimates obtained from SAFT-UT inspections

of the PVRUF RPV to the general population of RPVs in.

service. Useful information includes vessel history,

methods of fabrication, welding processes, heat treatments,

repairs during fabrication, plate and weld material types,

and the preservice inspection results.

The Electric Power Research Institute (EPRI) has assembled a material chemistry database for RPVs. The database

is intended to aid in the resolution of RPV radiation

embrittlement/life extension issues, and does not contain

information on RPV fabrication methods and procedures.

However, it does contain design information on operating

RPVs. The plant name, utility company name, vessel supplier, date of commercial operation, edition of the Code

followed during vessel fabrication, shell materials, vessel

ID, shell thickness, and cladding thickness are given in the

database.

In response to generic letter 92-01, the Nuclear Regulatory

Commission (NRC) has gathered a substantial amount of

fabrication, chemical property, and fracture toughness

information on RPV materials, plates, and welds. A

Reactor Vessel Integrity Data Base was prepared under

sponsorship of the NRC Office of Nuclear Reactor Regulation. This database was also assembled to address RPV

radiation embrittlement/life extension issues, and is very

useful for categorizing RPVs.

2.3 2.3 NUREG/CR-647 1
Description of Reactor Pressure Vessels

Table 2.1 Summary of base material volumes and areas in the PVRUF vessel

Assembly Dimensions Volume (in') Area (inr)-

Upper Head 2. 18 m Ri, 2.3 7 m Ro, 18.8 cm wall clad = 6 mm 11.95 57.98

Upper Shell

Shell 3.22 mm OD long, 4.34 m ID 9.80 71.79

Inlet Nozzle 4 Noz, 1.21 m OD, .71 m ID, 1.05 m long 4.48 15.57

Outlet Nozzle 4 Noz, 1.21 m OD, .75 m ID, .98 m long 2.50 11.75

Total Upper Shell: 28.68 157.09

Inter. Shell 4.37 m ID, 4.81 m OD, 2.67 m long 8.50 36.63

Lower Shell 4.37 m ID, 4.81 m OD, 2.67 m long 8.50 36.63

Bottom Head 2.24 mn R, 13.4 cm wall 8.54 59.41

Total 54.22 289.75

Table 2.2 Summary of weld volumes in the PVRUF vessel

Weld no. description Length (in) X-Sect. (Mn) Volume (Mn)

Weld- I Dome to torus 10.74 0.0055 0.05 85

Weld-2 Torus to flange 12.47 0.0043 0.0543

Weld-3 Flange to upper shell 13.62 0.0082 0.1115

Weld-4 Upper shell to inter, shell 13.73 0.0067 0.0925

Weld-S Inter. shell to lower shell 13.73 0.0075 0.1025

Weld-6 Lower shell to bottom head torus 13.73 0.0040 0.0547

Weld-7 Bottom head torus to bottom head dome 10.73 0.0040 0.0427

Weld-8 Upper shell axial welds (3X) 9.66 0.0075 0.0725

Weld-9 Inter. shell axial welds (3X) 8.00 0.0057 0.0454

Weld-lO Lower shell axial welds (3X) 8.00 0.0057 0.0454

Weld-IlI Nozzle welds:

Inlet nozzles 18.67 0.6932 0.1901

Outlet nozzles 16.13 0.5966 0.1636

Totals 149.24 1.0339

NUREG/CR-647 124 2.4
Description of Reactor Pressure Vessel Material

U! ling lug

Upper suppof plate

Ineanwah sumon ledbge

Comnbainot

Supwon coatunn

1.108 nore.

Thewnsai Uafled

ReIactr vassM

A.ccs pant

RPlah supponl

sonom Suwpon casting,

OCNININ

U-'

Conuin mad samua tube

H Iold-down ann~g

*Alignment pin

*Coningol maguide tube

*Conain "'~ adO S~SW

Conual mad cluster (wi~ttaraw"I

()miles n=me

"flfe radial SUPport

Lower coam MANa

Care suppor cohnmns

bntnauntenlaon thlmAlg guides

Figure 2.1 Typical PWR with internals (Luk 1993)

2.5 2.5 NUREG/CR-647 I
Description of Reactor Pressure Vessels

Upper

- Head

j Weici-3 Flanee to upper shell

Upper

Shell

4%

Weld-7 Bottom head torus

to bottom head dom~e

Figure 2.2 General schematic for PVRUF vessel showing weld identification numbers and configuration

NUREG/CR-647 12. 2.6
D'-scription of Reactor Pressure Vessel Material

Figure 2.3 Photograph of the PVRUF vessel from the outside

W11 Mw~5I~

Figure 2.4 Photograph of the PVRUF vessel from the inside

2.7 2.7 NUREGICR-647 1

3 SAFT-UT Measurements of RPV Material

The SAFT-UT field system is an excellent choice for

evaluating fabrication flaws in nuclear pressure vessel

material due to its validated high probability of detection

of small flaws and its proven sizing accuracy for flaws in

thick-section steel. The high performance of this system

in this application is due to its focal properties.

In SAFT-UT, data is collected over a large area, using a

small transducer with a diverging sound field. This technique, synthetic aperture focusing, has an advantage over

physical focusing techniques in that the resulting image is

full-volume focused over the entire inspection area.

Traditional physical focusing techniques provide focused

images only over a limited zone at the depth of focus of

the lens. In SAFT-UT, digital signal processing of the

data reproduces the focal properties of a large focused

transducer.

Within a regional aperture, the coherent summation for

each image point involves shifting a locus of A-scans by

predicted time delays, and summing the shifted A-scans

(digitized ultrasonic transients). Each picture element

(pixel) is thus based on a spatial average of a number of

points to produce the final value for display. Spatial

averaging produces a second benefit of SAFT, an enhanced signal-to-noise ratio, suitable for detection and

characterization of small reflectors in heavy section steel.

For a complete description of the SAFT-UT system see

Doctor et al., 1995.

3.1 SAFT-UT Equipment

Configuration at PYRUF

In the development and evaluation of SAFT-UT technology, an important goal has been to improve the reliability

of the inspection of reactor pressure vessels. Based on the

experience with the Midland RPV blocks (see Appendix B), the SAFT-UT system was upgraded to better cope

with the large volume of data that results from this kind of

inspection, and to provide faster and more accurate scans.

A redesigned SAFT-UT system was assembled for the

PVRUF inspections, and a block diagram is shown in

Figure 3. 1. A Compaq 486/25 personal computer performs the data acquisition and a Sun Microsystems

graphics workstation performs the SAFT processing and

data display. A photograph of the SAFT-UT data acquisition and display system at PVRUF is shown in Figure 3.2. Figure 3.3 shows the instrument platform inside

the vessel, with the cross-vessel scanner track support

assembly that was built to enable fast and convenient

repositioning of the scanner track.

3.2 Measurement Plan for PVRUF

The inspection plan for the PVRUF vessel was based on

previous SAFT-UT results on material forom the Midland

vessel, the condition of the PVRUF vessel (especially the

clad surface roughness), and the necessity for off-line

analysis of the SAFT-UT data. Previous use of SAFT-UT

indicated that small flaws could be expected, and that 10

different ultrasonic inspection modes would be needed to

characterize the ultrasonic indications. The surface conditions of the PVRUF vessel were typical of U.S. RPVs

(based on visual inspection by PNNL staff of the cladding

on other RPVs and on knowledge of U.S. clad fabrication

processes). The roughness of the clad surface limited the

scanning speed. Data analysis was performed at PNNL.

One of the principal findings of the SAFT-UT inspection

of material removed from the Midland reactor pressure

vessel was that most of the flaws (80%) were less than

2 mm (0.08 in.) in through-wall extent. That was one of

the reasons for operating the SAFT-UT system with high

spatial sampling rates. Because the SAFT-UT system's

lateral resolution is sufficient to show flaw features on the

order of one wavelength, the step sizes were set to one

half of one wavelength in both the X and Y scanner directions. This had the effect of producing smooth images of

the flaws, with the best capacity to separate small flaws

that may be close together.

Ultrasonic inspection of the PVRUF vessel was conducted. Table 3.1 specifies the ten inspection modes.

Five of the modes were used for inspection of the inner

25 mm (1.0 in.) of the vessel wall. A 4-MHz, dualelement, normal-beam transducer was used in inspection

mode #1 to provide sensitivity to volumetric flaws

(porosity and slag inclusions) near the inner surface of the

vessel. Figure 3.4 shows the insonification pattern in

mode 1. A 2-MHz, dual-element, 700 angle-beam

3.1 3.1 NUREGICR-647 1
SAFT-UT Measurements

Table 3.1 SAFT-UT inspection plan for PVRUF

Inspection Beam (skew) Frequency, Y length, X length, File size,

no./Itype direction MHz cm (in.) cm (in.) MB

Near-surface zone inspections

1 / Normal beam NA 4.0 23(9) 23 (9) 22

2 / 700, L-wave +X 2.0 23 (9) 23 (9) 8

3 / 70', L-wave +Y 2.01 23 (9) 23 (9) 8

4 / 7Q0 , L-wave -x 2.0 23 (9) 23 (9) 8

5 / 70', L-wave -Y 2.0 23 (9) 23 (9) 8

Inspection of the base-metal weld

6 /45', S-wave +X 1.5 23 (9) 28(11) 80

7 / 450, S-wave +Y 1.5 23 (9) 23 (9) 75

8 /450, S-wave -x 1.5 23(9) 28(11) 80

9 / 450, S-wave -Y 1.5 23 (9) 23 (9) 75

10 /Normal beam NA 5.0 23(9) 12.7(5) 157

transducer was used in modes 2 through 5 to provide

sensitivity to planar flaws (cracks or lack of fusion) that

may be oriented along or across the weld. Figures 3.5

through 3.8 show the insonification patterns using the 7Q0

L wave transducer.

In a similar fashion, additional five inspection modes

were used to detect and characterize indications in the

vessel material located more than 25 mm (1.0 in.) below

the inner vessel surface. A 1.5-MHz, 450 angle beam

shear transducer was used in inspection modes 6 through

9 to provide sensitivity to planar flaws that may be

oriented along or across the weld. Figures 3.9 through

3.12 show the insonification patterns for the 450 S-wave

inspection modes. A 5-MHz normal-beam transducer was

used in inspection mode 10 to provide sensitivity to volumetric flaws. Figure 3.13 shows the insonification

domain for the mode 10 inspection.

Table 3.1 also shows the scanning details and file sizes

for the different SAFT-UT inspection modes. The five

inspections of the upper 25 mm of the vessel wall had

23 x 23 cm (9 x 9 in.) scanning apertures. The five inspections of the base metal weld had apertures and file sizes

listed in the table. Table 3.2 shows the step sizes and

sampling rates for the PVRUF inspections.

Staff of the Oak Ridge National Laboratory (ORNL) were

trained in the proper operation of the SAFT-UT system

and collected all the data, with relatively little PNNL

oversight. The inspections started with the beltline weld.

This is the circumferential weld between the intermediate

and lower shell courses of the vessel, labeled weld #5 in

Figure 2.2. After the beltline weld, the circumferential

weld between the intermediate and upper shell course

(weld #4) was inspected. The SAFT-UT system worked

very well, but after inspecting 20.3 m (800 linear in.) of

weld, 50 GBytes of data, and 18 months of operation the

SAFT-UT system needed repair in order to do further

inspection. We would have liked to conduct more inspections, but our data indicated that a sufficient number of

indications had been detected; so the data acquisition task

was ended.

3.3 Calibration of SAFT-UT

Inspections

Calibration of the SAFT-UT system was performed when

the system was turned on and when the transducers were

changed. For calibration, the SAFT-UT scanner was

moved to the calibration blocks that were made available

for the calibration procedure. SAFT-UT inspection

modes I through 5 used PNNL calibration block #I -2A51B-i1 and inspection modes 6 through 10 used PVRUF

block #196-103. Figure 3.14 shows a photograph of the

scanner attached to the calibration block for calibration of

modes 6 through 9. The three side drilled holes for 450 S

wave calibration are visible in the photograph. Figure 3.15 shows a photograph of the scanner attached to

the calibration block for calibration of mode 10.

For ease of operation, a computerized calibration procedure was provided to the inspection team. The computerized procedure first prompts the operator for the transducer type.

NUREG/CR-647 I3. 3.2
SAFT-UT Measurements

Table 3.2 Step sizes and sampling rates for SAFT-UT inspections of the beitline weld in PVRUF

No.

1

2

3

5

6

7

8

9

10

Y length,

cm (in.)

23 (9)

23 (9)

23 (9)

23 (9)

23 (9)

23 (9)

23 (9)

23 (9)

23 (9)

23 (9)

Y step,

cm (in.)

0.08 (0.03)

0.15 (0.06)

0.15 (0.06)

0.15 (0.06)

0.15 (0.06)

0.10 (0.04)

0.10 (0.04)

0.10 (0.04)

0.10 (0.04)

0.06 (0.025)

X length,

cm (in.)

23 (9)

23(9)

23(9)

23 (9)

23 (9)

28(11)

23 (9)

28(11)

23 (9)

13(5)

X step,

cm (in.)

0.08 (0.03)

0.15 (0.06)

0.15 (0.06)

0.15 (0.06)

0.15 (0.06)

0.10 (0.04)

0.10 (0.04)

0.10 (0.04)

0.10 (0.04)

0.06 (0.025)

Sample

rate MHz

25

12

12

12

12

6

6

6

6

25

Normal Beam, full volume:

KB-A, #M2 1240, 0.25" diameter, 5 MHz.

-or- KB-A, #M2 1241, 0.25" diameter, 5 MHz.

Normal Beam, near surface:

Sigma, #2001-89001, dual element, 4 MHz.

Angle Beam, near surface:

RTD 70, L wave, #84-23, dual element, 2 MHz.

Angle Beam, full volume:

KB-A, # 17275, 450, S wave, .375" dia., 1.5MHz

-orKA-A, #15 124, 450, S wave, .375" dia., 1.5MHz

After the transducer type is selected, the computerized

procedure retrieves the established gain settings for the

data acquisition electronics and starts the transducer

excitations on the side drilled holes in the calibration

blocks. The signal level for each of the side drilled holes

was checked and documented for the selected transducer.

The documentation included a photograph of the signal

level on an oscilloscope.

Figure 3.16 shows the signal strength from the 1/4 tnickness side drilled hole in the PNNL calibration block for

the near surface normal beam transducer used in SAFTUT inspection mode 1. The signal strength of the transducer signal from this reflector was reproduced to calibrate each transducer type at the start of a set of inspections. Figure 3.17 shows the signal strength from the 1/4

thickness side drilled hole in the PNNL calibration block

for the near surface angle beam transducer used in

inspection modes 2 through 5.

Figure 3.18 shows. the electronic distance-amplitude correction (DAC) that was applied to the signal from the full

volume angle beam transducer. This correction was

applied with a time varying gain (TVG) amplifier that

produced the gain slopes shown in the figure. This

electronic DAC curve was maintained for all inspections

with the full volume angle beam transducer. Figures 3.19

through 3.21 show the signal strengths from the calibration block after the time variable gain amplifier applied

the electronic DAC.

Figure 3.22 shows the electronic distance-amplitude correction (DAC) that was applied to the signal from the full

volume normal beam transducer. Figures 3.23 through

3.25 show the signal strengths from the calibration block

after the time variable gain amplifier applied the electronic DAC.

3.4 Scanning Procedure and Vessel

Coordinate System

After the calibration of the transducer, the computerized

data acquisition procedure prompted for the inspection

mode (1 through 10) and for the starting coordinate of the

scanner on the weld's circumnferencve. The file name for

the inspection data was constructed from the inspection

mode and the starting circumferential coordinate. For

example, "5sl 360" identified the file as an inspection of

weld 5, using mode 1, and starting at circumferential

PNNL coordinate 9.144 mn (360 in.).

PNNL's inspection coordinate system for the beltline

weld was measured along the weld's inside diameter,

starting with 0.0 inches at the bottom dead center as the

3.3 3.3 NUREG/CR-647 1
SAFT-UT Measurements

vessel lay on its side. The weld coordinate increased

clockwise with the observer facing the back (or bottom)

of the vessel. The weld center line had been marked with

punch marks on the clad surface by the manufacturer of

the vessel. PNNL marked the weld circumference with

triple punch marks every 203 mm (8.0 in.) starting at the

bottom of the vessel. The coordinate across the weld was

measured as a positive or negative distance from 0.0

inches at the weld center line, with positive being closer

to the vessel entrance (flange side).

PNNL's inspection coordinate system for the circumferential weld between the intermediate and upper shell

courses was not the same as for the beltline weld, because

of the inside diameter change from the intermediate to

upper shells. The scanner track was moved to the

entrance (flange) side of the weld in order to accommodate the smaller inside diameter of the upper shell course,

and the coordinate system was changed as follows. The

weld coordinate increased counter clockwise with the

observer facing the back (or bottom) of the vessel. The

weld center line was punch marked in the same way as

the beitline weld. The coordinate across the weld was

measured as a positive or negative distance from 0.0

inches at the weld center line, with positive being closer

to the vessel entrance (flange).

3.5 Amount of Material Inspected

SAFT-UT was used to inspect all of the beltline weld and

approximately half of the weld of the intermediate to

upper shell course for a total of 20 meters (800 inches) of

inspected weld in 10 inspection modes. Table 3.3 lists the

volumes and surface areas for the SAFT-UT inspection at

PVRUF, based on the nominal cladding thickness of

6 mm and the weld cross sections from the construction

drawings.

Table 3.3 Amount of material inspected by SAFT-UT at PVRUF

Clad

Clad-to-,Base Metal Interi

Weld

Heat-affected Zone

Base Metal

Near surface zone

0.027 cubic meters

face 4.6 sq. meters (surface area)

0.0082 cubic meters

*0.005 cubic meters

0.075 cubic meters

Deeper zone

0. 19 cubic meters

0.051 cubic meters

0.90 cubic meters for angle beam

0.28 cubic meters for normal beam

Weld

Heat-affected Zone

Base Metal

N-UREG/CR-647 I3. 3.4
SAFT-UT Measurements

Pipe Scanner

remote pulser/preamp

write once

laser disk

write once

laser disk

Figure 3.1 Block diagram of SAFT-UT field system as configured at PVRUF

Figure 3.2 SAFT-UT data acquisition and display system as configured for PVRUF

3.5 3.5 NUREG/CR-647 1
SAFT-UT Measurements

Figure 3.3 Instrument platform and scanner pendulum showing center pivot

Only Insects the Upper 25mm

Figure 3.4 Insonification for inspection mode 1, near surface normal beam

NUREG/CR-647 13. 3.6
SAFT-UT Measurements

Only Inspects Upper 25 mm Across the Weld

Figure 3.5 Insonification for inspection mode 2, near surface 701L wave at 00 skew

Only Inspects Upper 25 mm

Figure 3.6 Insonification for inspection mode 3, near surface 7011L wave at 900 skew (along the weld)

Only Inspects Upper 25 mm

Figure 3.7 Insonification for inspection mode 4, near surface 701L wave at 1800 skew (across the weld)

3.7 3.7 NUREG/CR-647 1
SAFT-UT Measurements

Only Inspecft Upper 25,mm

Figure 3.8 Insonification for inspection mode 5, near surface 70*L wave at 2700 skew (along the weld)

Upper 12 mm Not Inspected

Figure 3.9 Insonification for inspection mode 6, full volume 45*S wave at 00 skew

Upper 12 MM Not Inspeted

Figure 3.10 Insonification for inspection mode 7, full volume 450S wave at 900 skew

NUREG/CR-647 138 3.8
SAFT-UT Measurements

Upper 12 DMm Not Inspected

Figure 3.11 Insonification for inspection mode 8, full volume 45 0

S wave at 1800 skew

Upper 12 Mm Not Inspected

Figure 3.12 Insonification for inspection mode 9, full volume 450

S wave at 2700 skew

Upper 12 mm~ Not Inspected

Figure 3.13 Insonification for inspection mode 10, full volume normal beam (full volume, normal beam)

3.9 3.9 NUREG/CR-647 I
SAFT-UT Measurements

Figure 3.14 SAFT-UT scanner attached to the calibration block for calibration of inspection

modes 6 through 9, 450S wave inspections

Figure 3.15 SAFT-UT scanner attached to the calibration block for calibration of inspection mode 10, normal beam

NUREGICR-6471 313.10
SAFT-UT Measurements

a

0.5

0.4

0.3-

0.2-

0.1

0-

-0.1

-0.2-

-0.3

-0.4

-0.5

0 5 11 110 15 20 25

Time (usec)

Figure 3.16 Signal strength from 1/4 thickness side drilled hole in calibration block for inspection mode 1

09

0.5

0.4 -

0.3 -

0.2 -

0.1

0-

-0.1

-0.2 -

-0.3-

-0.4

-0.5

0 5 10 15 20 25

Time (usec)

Figure 3.17 Signal strength from 1/4 thickness side drilled hole in calibration block for inspection modes 2 through 5

3.11 3.11 NUREG/CR-647 1
SAFT-UT Measurements

rate ... :6.25 (Mhz)

length.: 1412 (225.6 Usec)

delag..: 5.524 (Usec)

Rl) range .. 1

B) offset ...

C) coupling.: (

0) video ....

E) reps .....1

.000 Wvots p-jj)

.570 Wvots)

RC: coupled)

ff (RF)

F) timee.: 14.888

6) slopell: 4

H) delay .: 64.963

1) timnel.: 39.913

J) slopel: 2

K)1 time2.: 29.893

1) slope2: 1

0.400

0.200

50.000

-0.200

-0.400

mijc rose cond s

Figure 3.18 Time variable gain curve (TYG) for modes 6 through 9, 450S wave inspections

1 -

0.8 -

0.6 -

0.4 1~ K i W 11M___1

0

0.2 -

0-

-0.2 -

-0.4 -

-0.6 -

-0.8 -

-1 -

0 50 100 150 200

Time (usec)

Figure 3.19 Signal strength after TVG amplification from 1/4 thickness side drilled hole in

calibration block for inspection modes 6 through 9

NUREG/CR-6471 313.12
SAFT-UT Measurements

I1-

0

0.8 -

0.6 -

0.4 -

0.2 -

0-

-0.2 -

-0.4 -

-0.6 -

~i ýN I v

-0.8 -

-1 -

0 50 .100 150 200

Tkm, (usec)

Figure 3.20 Signal strength after TVG amplification from 1/2 thickness side drilled hole in

calibration block for inspection modes 6 through 9

1 -

0.8.

0

0.6

0.4]

0.2-

0-

-0.2 -

-0.4 -

-0.6 -

L .

------------------------------- I

1ý Dr v

-0.8

-1

0 so 100 150 200

Thme (usec)

Figure 3.21 Signal strength after TVG amplification from 3/4 thickness side drilled hole in

calibration block for inspection modes 6 through 9

3.13 3.13NUREG/CR-647 1
SAFT-UT Measurements

rate ... :25 (Mhz)

length.: 2128 (85.12Usec)

delay..: 0.800(Usec)

R) range ... :1.350 (volts p-p)

8) offset ... 9 .520 (volts)

C) coupling.: (RC coupled)

0) video....:off (RFJ

E) reps ....:I

F) timeB.: 4.880

6) slopell: 0

H) delay.: 16.867

1) time 1.: 14.863

J) slopel: le

K) time2.: 24.883

L) slope2: 1

0.500

50.000

.0.500

microseconds

Figure 3.22 Time variable gain curve for mode 10, full volume normal beam

2-

1.5.

0+I

...... ...............

.2 -

0 20 40 60 80 100

Time (usec)

Figure 3.23 Signal strength after TVG amplification from 1/4 thickness side drilled hole in

calibration block for inspection mode 10

NUREG/CR-6471 313.14
SAFT-UT Measurements

2

1.5

0.5

0

-0.5

-2

-~~IF0 20 40 60 80 100

Tint (usoc)

Figure 3.24 Signal strength after TVG amplification from 1/4 thickness side drilled. hole in

calibration block for inspection mode 10

2

1.5

0.5

0 0

-0.5

-2

0 20 40 60 80 100

Thne (usec)

Figure 3.25 Signal strength after TVG amplification from 3/4 thickness side drilled hole in

calibration block for inspection mode 10

3.15 3.15 NUREG/CR-647 1

4 Analysis Methods

In this chapter we describe the data presentation software

and the analysis process that was uniformly applied to the

SAFT-UT inspections of the PVRUF vessel. The inspection data was imaged on a graphics workstation, and a set

of rules was applied to the images to detect and characterize the flaw indications. The rule statements are

defined in this chapter, with examples of their application.

4.1 Detection of Indications at PVRUF

Rules were developed and used to detect about 2500 flaw

indications in the SAFT-UT data from the PVRUF vessel.

The first step in the detection process is the recognition of

objects in an image of the ultrasonic volumetric data. An

object in an ultrasonic image is defined to be a collection

of adj.acent pixels (picture elements) that form a

recognizable shape.

The brightness and shape of an object are the two most

important properties used in recognition that an indication

may be a flaw. The location of the object, its proximity to

other objects, and the surrounding random noise are also

used in the recognition rules.

The joining of objects that are closely spaced (by applying proximity rules) into an indication of a flaw, and the

separation of objects into indications of multiple flaws is

the second most important step in the detection process.

Small flaws on the order of 1 to 2 mm (0.04 to 0.08 in.) in

size are expected in the SAFT-UT data, and such closely

spaced small flaws should be resolved into separate

indications. On the other hand, large flaws are known to

produce an indication in ultrasonic data that is composed

of separate closely spaced ultrasonic objects, and rules are

required to correctly classify them as single large flaws.

After the identification of an indication as one or more

objects in the volumetric data, the location of the center of

the indication is recorded. The center locations are used

in the characterization step to fuse detections in the different inspection modes into a single characterized indication. Ultimately, the location of the characterized indication is an important input in determining its significance

to vessel integrity. The appropriate method of estimating

the center of the indication depends on the kind of

object(s) making up the indication. The rules that define

this process are described in the following.

Estimates of the through-wall extent (depth size) of the

indications are recorded in the detection process. This

measurement is made because only the most significant

flaws in the SAFT-UT data need to be fully characterized.

The measurement of through-wall extent is used to rank

o .rder the indications for full characterization.

The signal-to-noise ratio is sometimes useful in detection.

The signal level and the noise level are recorded in the

detection process according to a set of rules. The signal

level or the signal-to-noise ratio may become important

later on in the characterization of PVRUF data. For

example, a destructive validation may indicate that the

ratio is a valuable discriminant of flaw type.

Spreadsheets were used to document the results of the

detection of indications in the SAFT-UT data from the

PVRUF vessel. The detection procedure examines each

file for detectable indications. The inner 25 mm (1.0 in.)

of the vessel is examined first, and then the remaining

deeper portion (25 mm to 225 mm) is analyzed. The

results are entered in separate spreadsheets because the

procedures and rules are different for the two cases.

Figure 4.1 is a typical spreadsheet format of a near surface detection record. The entries are organized by row

and each entry (row) has a number, filename, position (X,

Y, and Z), estimates of through-wall extent (DZ) signal

level (S), noise level (N); and comment. For the near surface detection procedure, 10 files are analyzed for detectable indications (one file for each of the 10 inspection

modes). Each of the 10 files has at least one entry in the

spreadsheet. If the inspection file is missing or does not

contain any detectable flaws, then the filename is entered

with a comment of "no file" or "no calls." The position

of each detectable indication is recorded in the spreadsheet to within ± 0.25 mm according to a procedure

described below. The estimate of though-wall extent is

made as a part of the detection process because this is one

of the most significant characteristics, and provides a way

of rank ordering the detections for full characterization.

The signal and noise level fields may be used to select

indications of different signal strengths for validation in

the destructive test plan. The comment field is used to

record the method of estimating through-wall extent,

other variables of the detection process that do not have a

dedicated field in the spreadsheet, or observations about a

4.1 4.1 NUREG/CR-647 I
Analysis Methods

particular indication that should be documented and

considered in the evaluation process.

Figure 4.2 shows a full volume detection record and provides the second example of a detection spreadsheet. This

one uses the five inspections of the deeper portion

(25 mm through 225 mm) of the vessel. Except for the

reduced number of inspections compared to the near

surface spreadsheet, the fields and entries of both spreadsheets are the same. The procedure of detecting the deeper indications is different and described below.

The first step in the detection of flaw indications is object

recognition in the computer generated images. The object

recognition rules are listed in Table 4. 1. The table lists a

rule name, the rule statement, and a reference to an

example SAFT-UT image.

In practice, the images resulting from ultrasonic inspections are typically cluttered with noise. This is especially

true for small fabrication flaws in thick section steel,

because of the high gain used to amplify the weak signals

from small flaws deep in the steel. It should be noted

here that the measurement plan called for the detection of

small flaws in order to estimate the fabrication flaw distribution. Therefore, the test protocol was set up and run

at the highest sensitivity possible without saturating the

signals from the flaws. Consequently, the images are

noisier than would normally be obtained from an inspection designed to ASME code requirements.

Figure 4.3 shows an example of an unbroadened time of

flight (TOF) shape produced by a small embedded reflector. This isolated shape is found in the transducer's side

view projection. The wave packet has the 3/2 wavelength

width characteristic of the transducer's broadband

response. This pattern is generated by the movement of

the transducer's angled beam pattern across the small

embedded flaw. The SAFT-UT inspections of the Midland block (see Appendix B) confirmed that most of these

shapes are detectable as indications of embedded flaws,

but, in a tight wave packet such as this one, there is little

information about the size of the reflector.

These tight wave packets can be produced by the top of a

volumetric flaw (slag or porosity) or by the top or bottom

of a planar flaw (crack or lack of fusion in the weld). For

each recognized object of this kind, an entry is made in a

detection spreadsheet. The through-wall extent of these

indications is estimated to be less than one half wavelength of the ultrasound in the material. For detectable

echoes with isolated wave packets below the clad-to-basemetal interface and above 10 mm (0.39 in.) in 70'L

inspections, the through-wall extent is estimated at

1.5 mm (0.06 in.) (1/2 wavelength).

Figure 4.4 shows an example of an embedded, broadened

TOF Shape. In this case, the wave packet is broadened

from the impact-limited case in Figure 4.3, and the broadening can be used to estimate the size (through-wall

extent) of the indication. The through-wall extent of the

indication is estimated to be the width of the wave packet

less three halves of one wavelength. These are small

flaws, and a conservative approach in this analysis is to

consider the width to be an indicator of the top and bottom of a small planar flaw. The top of a planar flaw will

have, for the transducer used, a 3/2 wavelength response;

so the size is estimated to be the wave packet width minus

the transducer response.

Figure 4.5 shows an example of a tip signal pattern where

two wave packets are in the proper orientation for tip signals from a vertically oriented planar flaw. When ultrasound is incident on the top or bottom of a vertically

oriented discontinuity, the sound is diffracted and a weak

signal can sometimes be detected from both ends of the

discontinuity. Thus, pairs of TOF shapes can be generated from the top and bottom (tip signals) of planar flaws

separated vertically by the size of the flaw (through-wall

extent), so the pair is detected as one indication.

Figure 4.6 shows an example of two wave packets in the

proper orientation for creeping wave re-radiation from a

round trip of a volumetric flaw. Volumetric flaws, such

as slag inclusions or porosity, can produce a pair of TOF

shapes separated along the insonification direction by the

time of flight of one round trip of the flaw's

circumference.

Figure 4.7 shows an example of a coin (semi-circular or

elliptical) shape near the inner surface of the vessel.

Underclad cracks will have a large aspect ratio, distinguishing them from near-surface inclusions, and often

producing a coin shape in the transducer end view of

angle beam inspections.

Figure 4.8 show a normal beam echo below the clad-tobase-metal interface. These shapes are detectable as

indications of a flaw when the signal-to-noise ratio is

greater than 6 dB. The application of this rule does not

greatly affect the size distribution of flaws, because normal beam indications do not produce a measurement of

NUREG/CR-647 I4. 4.2
Analysis Methods

Table 4.1 Detection rule statements

Rule name Rule statement Example

Unbroadened TOF An isolated unbroadened TOE shape is detectable as a flaw. Through-wall Figure 4.3

extent is estimated to be less than one half wavelength.

Broadened TOE An isolated broadened TOF shape is detectable as a flaw. Through-wall extent Figure 4.4

is estimated to be the width of the wave packet less three-halves of the

wavelength.

Tip Pattern A pair of vertically aligned TOF shapes is detectable as the top and bottom of Figure 4.5

one flaw. Through-wall extent is estimated from the vertical component of

the distance between the two TOF shapes.

Reradiation A pair of TOF shapes that are aligned along the sound path are detectable as Figure 4.6

one flaw. Through-wall extent is estimated from the time-of-flight of one

round trip of the flaw circumference, assuming a cylindrical shape for the flaw.

Coin Shape A coin shape is detectable as an underclad flaw. Through-wall extent is Figure 4.7

estimated based on analyst judgement.

Normal Beam An indication in a normal beam inspection is detectable as a flaw when the Figure 4.8

signal-to-noise ratio exceeds 6 dB. Through-wall extent estimates are not

made from normal beam inspections.

Clad-to-Base Metal Indications in the clad-to-base metal interface are detectable as flaws when Figure 4.9

their amplitude exceeds 30 percent of full scale. Though-wall extent estimates

are usually less than 2 mm for these indications.

Cloud-Like Tight clusters of TOE shapes are detectable as one indication. Through-wall Figure 4. 10

extent is sometimes estimated from the loss-of-signal in several modes.

through-wall extent. Detections in normal beam inspections are used to locate and characterize an indication as

volumetric, or to separate volumetric flaws that may be

close to one another.

The physical basis for the normal beam rule derives from

the SAFT-UT inspections of the Midland blocks. The

echoes from flaws in normal beam inspections are distinguishable from noise sources, based on image structure

and clustering patterns. The destructive analysis of Midland block 1-8 showed that the echoes with signal-tonoise ratios down to 6dB in the SAFT-UT data correlated

well with the actual positions of the flaws.

Figure 4.9 represents a detection of an object located in

the clad-to-base-metal interface. These shapes are detectable as indications of a flaw when their amplitudes are

greater than 30 percent of fall scale. The application of

this rule requires that the calibration of the probes be

maintained. The spreadsheet contains the amplitude for

the detected indications, so that adjustments can be made

in the detections after destructive validation.

The physical basis for this rule also derives from the

SAFT-UT inspections of the Midland blocks. The echoes

from flaws at the clad-to-base-metal interface using RTD

700 L probes do not produce a shape that distinguishes

them from echoes due to interface roughness. The

destructive analysis of Midland block 1-8 showed that the

echoes with amplitudes greater than 30 percent of full

scale in the SAFT-UT data correlated with the positions

of the destructed flaws.

Figure 4. 10 shows an example of a cloud-like indication.

These indications are recognized from the tight cluster of

TOE shapes. The through-wall extent of a cloud-like

indication is sometimes estimated from the loss of signal

in several modes.

4.3 4.3 NUREG/CR-647 1
Analysis Methods

4.2 Clustering of Detections

A significant portion of the indications detected in~the

first pass through the SAFT-UT inspections will be multiple detections of the same indication. In principle, a

flaw in the vessel could be detected 10 different times,

once in each of the 10 inspe 'ction modes. The method for

clustering the detections from the individual inspections

into a reduced detection lisi, where multiple detections of

the same indication are identified and recorded, is discussed below.

Figure 4.11 shows the distance matrix for the 14 indications detected in the first pass through the 10 inspection

modes in one portion of the PVRUF weldment. The distance matrix is used to identify multiple detections of the

same indication. It tabulates the calculated Euclidian distances of the indications from each other. The matrix

shows that indications 2 and 1.1 are close (distance=

2 mmn [0.08 in.]) and that 3 and 12 are also close

(distance = 4.2 mmn [0.17 in.]). This information is

passed to the multimodal detection sheet described below.'

Figure 4.12 shows the multimodal- detection sheet for the

14 indications shown in the! distance matrix of Figure 4.11. Zone 2 in the multimodal detection sheet shows

that indications 2 and I11 are now considered to be two

detections of the same indication. Zone 3 shows indications 3 and 12 are considered to be two detections of the

same indication. The 14 indications are thus reduced to

12.

4.3 Sizing and Characterizing

Indications at PVRUF

SAFT-UT sizing rules were created to conservatively size

indication zones to ensure that all potentially larger flaws

would be included in the validation plan.

After the number of original detections is reduced by

identifying the multiple detections of the same indication,

a characterization is performed on the largest of the indications from the reduced list. Figure 4.13 shows the characterization worksheet for one indication. There were

three detections of this indication in the first pass in

modes 1, 2, and 4; where mode 1 is the near surface

normal beam inspection, and modes 2 and 4 are near surface 700 L wave inspections. The first step in the characterization is to review the other modes to determine if a

confirmation of the indication can be found by increasing

the detection sensitivity. In the example, a confirmation

was found in mode 5 and a comment was made identifying the entry as a confirmation. The characterization

worksheet contains the information from the detection

sheets plus additional information from the characterization steps.

In the characterization of the largest indications, the

length and width of the indication is estimated from the

loss of signal (LOS) in each of the modes where an

estimate can be made. The spreadsheet uses the location

of the indication to calculate whether the indication is in

the weldment, base metal, heat affected zone, or

cladding-using a formula derived from the weld cross

section.

' The spreadsheet also records whether the indication is

volumietric or planar. An indication is always considered

planar if tip signals are identified. In most cases, a

'normal beam detection is evidence of a volumetric

indication.

NUREG/CR-647 144 4.4
U'

Al B IC I0 E IF IG IH III J IK I H

2 9.Filnam X 2 Y 2 Z DZ N OMMENTS, Cs'

r4 T 5s0 8 2.2 ___5.62 ___0.44 179 80 _________________________

5 2 5s0 8 -2.15 ___ .2 ___0.46 208 65 _________________________

6 3 5s0 8 -1.95 ___2.37 ___0.43 205 80 _________________________

7 45sl 8 There are 3 echoes in the CB1 with signal levies of 100 counts or more.

8 5 5SO 8 There iIs echoes below the CBI with signal levels of 80 or more.

9 6 5s2 8 3.54 _ 6.08 ___0.21 0.06 108 15 Since this call Is in the C81. DZ is the minimum. 0.27

10 7 5s2 8 4.2 ___6.02 ___0.24 0.08 105 15 SInmce this call Is In the CBI, DZ Is the minimum. 0.27

11 8 5s2 8 4.02 __0.44 _ 0.24 0.06 103 15 Since this call Is In the CBI, DZ is the minimum. 0.27

1.2 9 5s2 8 -1.38 6.14 ___0.61 0.07 15 5 OH=.25,DZ=DH-3/2(Iambda)

13 10 5s2 8 -0.421__ 2.84 _ 0.67 0.06 12 4 DH-.21,DZ=DH-3/2(lambda). DZ=Minimum. ___

14 11 6s2 8 1.-86 1.52 __0.56 0.06 25 8 DH=.24,DZ=DH-312(lambda), DZ=Minlmum. ___

15 12 5s3 8 -0.3 0.99 0.24 0.06 120 22 Since this call is In the C13I, DZ Is the minimum. 0.27

16 13 5s3 8 0.72 - 7.59 ___0.24 0.06 101 30 Since this call Is In the C131. DZ is the minimum. 0.27

17_ 145s- No file _ _

18 15 5s5 8 -1.08 _ -0.28 - 0.24 117 45 Since this call Is In the C131. DZ is the minimum. 0.27

1916 5s5 8 -1.08 -0.76_ _ 0.21 109 45 ISlnce this callIs In the CBI, DZis the minimum. 0.27

20 17 5s5 8 -0.66 -0.88 _ 0.24 101 45 Since this call is In the C131, DZ Is the minimum. 0.27

21 18 5s6 8 No calls (shapes).

.22 19 7 a -s -- - No calls (shaps).

1231201 5s8 18 12.3 1 4.64 __ 1 04 57 25 DHZ.12, DZ=D-3/2(tambda). DZ=Milrmum.___

1241211 5s9 18. 1No calls (shapes).

Figure 4.1 Near surface detection record. The filename columns follow the protocol described in Section 3.2. The column labeled

"X" is given in inches from the weld centerline. "Y" is the distance along the weld, also in inches. "Z" is the distance in inches

from the clad surface. "IDZ" is the through-wall extent of the irradiation in inches. The column "IS" is the signal level in digitizer

units (0-255). "IN" is the noise level, also in digitizer units.

9

0

0%

.15

0
I I;

0%

I 101 U CIS 103.52 3.2 0.08 21 DMO-W1

2 536 1041 -024 I101.92 2.97 0.46 20 4 d.

-J ftS 104 -0.8 _ 970 3.2 0.07 Is 5 D14-0.9. DZ.OtI-.12

4 5S8 1041 0.6O 95.44 9.06 0.04 24 5 014-0.1. DZ.OM-.12 O A pr"kt

5 SiB 1041 4.48 969 8.14 OA 15 5 04-017, DZ=0I4.12

Su 104i-0.= 102.96 2.94 0.36 .14 4 Ti ftMPds mu bnederflOm uwual. bis two !2shfidic6lns

7T US 1041 40.16 W672 3.2 0.06 13 4 D~O.OZ-Oi-.12

F8 585 1041 0.AS _ 97.28 4.17 0.04 5 1 7 094-0.15 MDZOII.2

9 WU 1041 0.62 __102.46 2.06 0.04 17 2 0H-15. DZH..12

10 155041 -0.34 _ 102.8 -2.46 0.3 15 3 echo.2

11 588 104 4.4 C_10.2 2.91 005 21i 4 041 ZO-1

*T5X8 1041 -0.26 - 96.6M 3.06 0C05 20 5 04-0.17 DZ.041-.12

0.6 995..14 4 4.94 0.06 16 6 0"0.DZVHt-.12

__11041 .218 1_ 3A - 4A03 ,0.121 15i 7 094-0.7, DZ-D14-.12

FigureU 4.2FUN volume detection record. The fi'le name columns follow the protocol described in Section 3.2. The column "X"

gives the distance in inches from the weld centerline. "Y" is the distance along the weld In Inches. "Z" Is the distance In inches

from the clad surface. "DZ" Is the through-wall extent of the irradiation In inches. The column "S" is the signal level in

digitizer units (0-255). "N" Is the noise level also in digitizer units.
Analysis Methods

Figure 4.3 Embedded, compact time-of-flight shape

4.7 4.7 NUREG/CR-647 1
Analysis Methods

Figure 4.4 Embedded, broadened time-of-flight shape

NUREG/CR-647 14. 4.8
Analysis Methods

Figure 4.5 Tip signal pattern showing two wave packets in the proper orientation for tip

signals from a vertically oriented planar reflector

4.9 4.9 NUREG/CR-647 1
Analysis Methods

Figure 4.6 Surface wave pattern showing two wave packets in the proper orientation for

re-radiation from a round trip of a volumetric flaw

NURLEG/CR-6471 414.10
Analysis Methods

Figure 4.7 Coin shape near the clad-to-base-metal interface

4.11 4.11 NUREG/CR-647 1
Analysis Methods

Figure 4.8 Normal beam shape below the clad-to-base-metal interface

NUREG/CR-6471 414.12
Analysis Methods

Figure 4.9 Shape of indications in the clad-to-base-metal interface

4.13 4.13 NtJRG/CR-6471
Analysis Methods

Figure 4.10 Detection of a cloud-like shape

NUREG/CR-6471I41 4.14
__ 1 2 3 4 5 6 7 8 9 10 11 12 13 14

1 0.00 0.86 .3.24 7.12 6.31 0.45 2.41 3.27 1.27 0.84 0.89 3.29 2.56 5.05

-2 0.86 0.00 2.45 6.92 6.20 0.52 1.62 2.64. 1.04 0.70 0.08 2.48 2.29 4.29

3 3.24 2.45 0.00 5.93 5.44 2.97 0.84 1.03 2.97 3.02 2.46 0.17 2.28 2.09

4 7.12 6.92 5.93 0.00 2.19 7.20 6.10 4.98 7.84 7.60 6.97 6.07 14.67 5.31

5 6.31 6.20 5.44 2.19 0.00 6.47 5.53 4.43 6.95 6.84 6.27 5.61 3.95 5.51

6 0.45 0.52 2.97 7.20 6.47 0.00 2.14 3.13 1.03 0.48 0.52 3.00 2.60 4.78

7 2.41 1.62 0.84 6.10 ,5.53 2.14 0.00 1.26 2.23 2.21 1.63 0.89 1.90 2.77

8 3.27 2.64 1.03 4.98 4.43 3.13 1.26 0.00 3.35 3.31 2.68 1.19 1.50 2.20

9 1.27 1.04 2.97 7.84 6.95 1.03 2.23 3.35 0.00 0.66 1.02 2.97 3.17 4.95

10 0.84 0.70 3.02 7.60 6.84 0.48 2.21 3.31 0.66 0.00 0.66 3.02 2.97 4.87

11 0.89 0.08 2.46 6.97 6.27 0.52 1.63 2.68 1.02 0.66 0.00 2.48 2.36 4.29

12 3.29 2.48 0.17 6.07 5.61 3.00 0.89 1.19 2.97 3.02 2.48 0.00 2.43 2.07

13 2.56 2.29 2.28 4.67 3.95 2.60 1.90 1.50 3.17 2.97 2.36 2.43 0.00 3.48

14 5.05 4.29 ,2.09 15.31 15.51 4.78 ý2.77 2.20 4.95 4.87 4.29 12.07 3.48 0.00

Figure 4.11 Distance matrix for the clustering of detections from different inspection modes. The table shows the distances

between 14 distinct indication in inches. This matrix shows the indications 2 and 11 are close (distance = 0.08").

ON

k1
U)

U)

0

0-U)

ON

ZONE X X2Yy Y2 2 ECHES

1 016 103.52 3.2 1

2 0.34 -0.24..101.92 2.91 2.917 11

3 -0.26 -0.08 96.96 97.04 3.06 3.2 312

5 4.48 96.96 8.14 5

6 -0.32 102.96 2.94 -6

7 -0.16 98.72 3.2 7--

8 0.56 97.28 4.17 8

9 0.62 102.48 2.06 9

10--- -0.34 102.88 2.46 10

11 4026 96.96 3.06 *- - - -- - - - - *12

12 -2.18 93.84 4.03 1 - --..

13 0 0 0 15 161718 19 20

14 ----- 0 -----------0 0 ..---..-- I15 1 617 181920

1s 0 0 0 15 16 17 18 19 20

16 --------- ----------------------0-------------- ---- -- ----- ---- ---- - - - - - -

17 0 0 0 15 16 17112

29 0 __ 0 0 __ 15 16 17 18 19 20 --------------- ----------- ------------ _ - -- -------- - -------- ---- - -- - i

Figure 4.12 Multimodal detection sheet for one portion of the weidment. This sheet shows the indications reduced to account for

multimodal detection. Zone 2 in this matrix shows that indications 2 and 11 are not considered to be two detections of the same

indication. Coordinates of the indications in this table are given in inches.
T. 1ina. OX Y:'-,T DY. 2: DZ: S. N 'METHOD: MAERA I CQMMENTS : .. LOS LOS: m~dB- ___

1 Ws 352.2 0.05 0.18 303 0.12 0.42 120 50 n/a weld v Original detection

.2 5sl 352A12 0.18. 0.2 350 0.14 0.39 131 16 n/a weld v Oriinal detecton

3 5s2 352.d12 - - - - - - - - - - ____ __No confirmataion

4 553 352.c12 -- - _ __ No confirmatin

-5 5s4 1352Ac2 0.35 1350.5 0.51 0.69 Y3 14 6 top-bffom weld v Original detection

6 5s5 135U.12 0.3 0.5 350.1 0.59 0.09 14 7 packet weld v Confirmatin

7 5s5 352.2 ___No confirmation

8 5W 352.2 - - -- - -No confirmation

9 5s8 352.2 No confimation

10 59 352.2 V No confirmation

Figure 4.13 Characterization work sheet for one indication. There were 3 detections of this indication in the final pass in

modes 1, 2, and 5. A confirmation of this indication was found in mode 5. The position and size of the indication

is given in inches. The signal and noise levels are given in digitizer units (0-255).

0%

-J

U,

U,

0

5 Results of the, Analysis of SAFT-UT Inspections at PVRIJF Vessel

In this chapter, the analysis of SAFT-UT images is

discussed, and the important features of the most significant indications are described. The indications are

detected and characterized by interpreting the images

provided by the SAFT-UT system. The characteristics of

the larger indications are provided in tabular form and

references are given to the complete set of SAFT-UT

images in Appendix A. Where sizing results are given,

SAFT-UT rules were used to conservatively size indication zones to insure that all potentially larger flaws would

be included in the validation plan.

5.1 Analysis of SAFT-UT Images

SAFT-UT images are presented in three engineering

views: side view, end view, and C-scan view. The image

in Figure 5.1 shows the echo from planar indication #1 in

the weldment of the inner-surface zone in mode 1: near

surface normal beam. The figure is made up of the three

engineering views and a view of the file header information. The display at the top left of the figure is the B-scan

end view. The display at the top right is the C-scan view.

The display at the bottom right is the B-scan side view.

Finally, the display at the bottom left is the header

information from this data file: 5s1_352.c 12.

The display of the B-scan end view is made up of three

parts. The first is the image of the indication. Tick marks

along the left side and bottom of the image can be used to

estimate the material coordinates of the flaw. The gray

scale bar at the right shows the amplitude scale of the

data--in this view varying from 131 to 0, as indicated.

The final part of the end view is the axis label portion,

containing five lines of information. The first line identifies the view as the B-Scan end view. By definition, the

B-Scan end view in SAFT-UT is a view of Y vs. Z, where

Y is the length axis as measured along the circumference

of the vessel, and Z corresponds to the depth axis as

measured from the vessel's inner surface toward the outer

surface. The next three lines give the material coordinates

of the data volume. Here, the Y-axis is the horizontal axis

and starts at 350.11 inches at the left of the image and

ends at 349.84 inches at the right of the image (metric

units are not available on the SAFT-UT images at this

time and the analysis described in this section will be

inches to be consistent with the axis on the SAFT-UT

images). The Z axis is the vertical axis and starts at

0.268 in. at the top of the view and ends at 0.564 in. at the

bottom of the view. The fifth and last line gives the

distance between tick marks on the left and bottom edges

of the image, 0.05 in. in this case. The X-axis is perpendicular to the display plane in the B-Scan end view.

The indication shown in Figure 5.1 is detectable, based on

its signal strength of 131/255. The signal amplitude of

131 can be found at the bottom of the gray scale. The

location of the indication is X = 0.20' (from the B-Scan

side view), Y = 348.95" (from the B-Scan end view), and

Z = 0.40" (ftrom the B-Scan side view).

5.2 Indications in the Near-Surface

Zone

The near-surface zone of the vessel is defined to be the

first 25-mm of vessel thickness starting at the clad surface. This zone includes the cladding itself, which are

nominally 6 mm thick, and the first 19-mmn of ferric steel

immediately outside the cladding.

The near-surface zone is further sub-divided into four

regions. The near-surface weldment is defined to be the

ferritic steel weld metal below the cladding. The near

surface heat-affected zone is defined to be the first 6-mmn

of ferritic steel outside the near-surface weldment. The

near-surface base metal is all ferritic steel outside the

heat-affected zone. The cladding and clad-to-base-metal

interface is the fourth region.

5.2.1 Indications in the Near-Suirface,

Weidment

This section reports thei characterization of the eleven

indications that were equal to or greater than 2 mm in size

in the near-surface weldment. Tables 5.1 and 5.2 list the

characterization data for nine planar indications and two

volumetric ones. Through-wall extents of up to 8 mmn

were estimated. The tables give the location, type, and

size of the indications. Appendix A describes the characterization process and identifies the analysis rules that

were applied. Also included in Appendix A are the

images of the indications.

5.1 5.1 NUREG/CR-647 1
Results of the Analysis

Table 5.1 Planar indications in the weidment of the near-surface

Location Location

Through-wall extent Length Width in depth from weld

size size size Coin SNR

ID Mode (mm) Basis (mm) (mm) Z (mm) X (mm) shape value Figure

1 Det: 1, 4, 8 different depths 13 13 10 8 good 131/16 A.1I

10 3 WP (mode 4) 18

Conf: 5

2 Det: 5 6 different depths NA 7 12-18 8 none 75/15 A.2

Conf: 3 2.5 WP (mode 5)

3 Det: 4 3 WP( mode 4) 10 NA 21 -8 none 13/6 A.3

4 Det: 4 3 different depths 14 NA 16-19 -4 to -8 some 25/12 A.4

Conf: 8 2.3 WP (mode 4)

5 Det: 2, 4 2.3 WP (modes 2,4) 10 NA 11 -6 to -14 some 63/12 A.5

12

6 Det: 4 2 WP(mode 4) 9 NA 16 -3 none 18/6 A.6

7 Det: 2 2 WP(mode 2) 11 NA 15 -6 none 52/12 A.7

8 Det: 7 2 WP( mode 7) NA 10 23 11 none 69/28 A.8

9 Det: 4 2 WP(mode 4) 11 NA 24 12 none 16/3 A.9

Det =detection mode; NA = not applicable; Pat = pattern; Conf = confirmation mode; Loc = location; Ind = Indication; WP =wave packet;

SNR = signal-to-noise ratio; Length = along the weld; width = across the weld.

Table 5.2 Volumetric indications in the weldment of the near-surface

Location Location

Through-wall extent Length Width in depth from seld

Detection size size size Coin SNR

ID mode (mm) Basis (mm) (mm) Z (mm) X (mm) shape value Figure

1 Det: 7, 8 6 different depths 10 10 18 to 24 -6 none 97/30 A.10

Conf: 3,4,5,9 2 WP (modes 7, 8)

2 Det: 9 6 different depths 13 14 20 to 26 9 good 117/35 A.1I1

Conf:2, 5,6,7,8 3 WP (mode 9)

5.2.2 Indications in the Near-Surface, Heat

Affected Zone

Tables 5.3 and 5.4 list the characterizations of the three

indications that were greater than 2 mm (0.08 in.) in size

in the near-surface, heat affected zone. One of the indications was planar and two were volumetric. Through-wall

extents of up to 7 mm (0.28 in.) were estimated.

5.2.3 Indications in the Near-Surface, Base

Metal

Tables 5.5 and 5.6 report the characterizations of the 31

largest indications in the near surface base metal.

Twenty-one of the indications were characterized as

planar and ten as volumetric. Volumetric indication

number nine is omitted because it was merged with

another indication. Through-wall extents of up to 8 mm

(0. 16 in.) were estimated.

5.2.4 Clad and Clad-to-Base Metal Interface

Tables 5.7 and 5.8 report the characterizations in the clad

and clad-to-base metal interface. Table 5.7 lists four indications characterized as planar with through-wall extents

of 1.5 mm (0.06 in.), as examples of the smaller indications in the SAFTTUT inspection of the cladding. Six of

the indications were characterized as volumetric with

through-wall extent estimated up to 3 mm (0. 12 in.).

NUREG/CR-647 152 5.2
Results of the Analysis

Table 5.3 Planar indications in the heat-affected zone of the near-surface

Location Location

Through-wall extent Length Width in depth from weld

size size size Coin SNR

ID Mode (mm) Basis (mm) (mm) Z (mm) X (mm) shape value Figure

I Det: 2, 3, 5 3 WP (modes 2, 3, 5) 14 10 13-14 11-18 good 88/25 A. 12

Table 5.4 Volumetric indications ia the heat-affected zone of the near-surface

Location Location

Through-wall extent Length Width in depth from weld

Detection size size size Coin SNR

ID mode (mm) Basis (mm) (mm) Z (mm) X (mm) shape value Figure

1 Det: 6 7 cloud-like shape 7 13 20 6 to 17 none 118/25 A.13

Conf:5 5 WP (mode 6)

2 Det: 2 3 different depths 13 8 11ltol14 -14 to -25 none 71/20 A.14

Conf. 1 2.5 WP (mode 2)

Table 5.5 Planar indications in the base metal of the inner surface zone

Location Location

Through-wall extent Length Width in depth from weld

Detection size size size Coin SNR

ID mode (mm) Basis (mm) (mm) Z (mm) X (mm) shape value Figure

1 Det: 8 8 cloud-like shape 7 NA 18 122 none 165/70 A.15

2.5 WP (mode 8)

2 Det: 8 7.4 cloud-like shape 13 NA 20 112 none 190/35 A. 16

3.5 WP (mode 8)

3 Det: 2 4 WP (mode 2) 12 NA 15 122 none 22/9 A. 17

4 Det: 2 4 WP (mode 2) 9 NA 11 -46 none 25/5 A. 18

5 Det: 4 3.6 WP (mode 4) 15 NA 12 -49 good 19/9 A. 19

6 Det: 3, 5 3.5 WP (modes 3, 5) 12 20 13 to 16 -29 good 29/10 A.20

Conf: 1 3 different depths

7 Det: 4 3 WP (mode 4) 9 NA 13 32 none 22/7 A.21

8 Det: 3 2.5 WP (mode 3) NA 11 15 100 none 14/7 A.22

9 Det: 3 2.5 WP (mode 3) NA 7 17 -102 none 27/7 A.23

10 Det: 2 2.5 WP (mode 2) 7 NA 13 -18 none 24/10 A.24

11I Det: 4 2.5 WP (mode 4) 15 NA 15 20 none 15/6 A.25

12 Det: 8 2.5 WP (mode 8) 11 NA 19 113 some 216/45 A.26

13 Det: 8 2.3 WP (mode 8) 10 NA 14 142 none 154/50 A.27

1.6 ring pattern

14 Det: 8 2.3 WP (mode 8) 10 NA 20 43 none 143/45 A.28

15 Det: 2 2.3 WP (mode 2) 14 NA 17 -30 none 16/7 A.29

16 Det: 3 2.3 WP (mode 3) NA 7 18 -104 none 12/6 A.30

17 Det: 8 2.3 WP (mode 8) 8 NA 20 46 none 130/50 A.31

18 Det: 7 2 WP (mode 7) NA 11 22 39 none 85/30 A.32

19 Det: 7 2 WP (mode 7) NA 16 25 106 none 91/40 A.33

20 Det: 3 2 WP (mode 3) NA 8 12 81 none 24/8 A.34

21 Det: 2, 4 1.5 WP (modes 2,4) 8 NA 8 -24 good 143/25 A.35

5.3 5.3 NUREG/CR-6471
Results of the Analysis

Table 5.6 Volumetric indications in the base metal of the inner surface zone

Location Location

Through-wall extent Length Width in depth from weld

size size size Coin SNR

ID Mode (mm) Basis (mm) (mm) Z (mm) X (mm) shape value Figure

1 Det: 9 6 cloud-like pat. 6 7 19 to 20 28 none 88/40 A.36

Conf. 1

2 Det: 2 3 WP (mode 2) 13 7 18 to 20 79 none 8/3 A.37

Conf: 3 2 different depths.

3 Det: 7 .2.5 ring around pat. NA 12 20 71 none 100/25 A.38

4 Det: 5 2.3 WP (mode 5) 12 12 16 -64 none 16/4 A.39

Conf: 2

5 Det: 4 2 WP (mode 4) . 9 5 11-12 26 none 99/20 A.40

Conf: 10 1 different depths

6 Det: 1, 7 2 different depths 5 10 17 to 19 34 none 119/30 A.41

1 WP (ix1 ,ides 1, 7)

7 Det: 8 1.6 ring around pat. 8 NA 19 123 none 195/45 A.42

8 Det: 8 1.6 ring around pat. 12 NA 14 145 some 148/40 A.43

10 Det: 1, 2 1.5 WP (modes1, 2) 11 11 10 tol11 79 good 212/25 A.44

Conf: 3,4 1 different depths

I1I Det: 1, 2 1.5 ind. w/o TOF 10 7 8-9 24 none 102/25 A.45

I different depths

Table 5.7 Planar indications in the cladding and in the clad-to-base metal interface

Location Location

Through-wall extent Length Width in depth from weld

size size size Coin SNR

ID Mode (mm) Basis (mm) (mm) Z (mm) X (mm) shape value Figure

1 Det: 2 1.5 ind. w/o TOF 14 NA 6 NA none 130/30 A.46

2 Det: 3, 5 1.5 ind. w/o TOF NA 7 6-7 NA none 170/30 A.47

I different depths

3 Det: 2 1.5 ind. w/o TOF 8 NA 6 9 none 97/30 A.48

4 Det: 3 1.5 ind. w/o TOF NA 16 6 14 good 83/15 A.49

5.3 Indications Outside the Near

Surface Zone

The zone outside the near surface zone is defined to be

that portion of the vessel wall thickness starting at 25 mm

(1.0 in.) of depth from the clad surface and extending to

the outside surface of the vessel. This outer zone is

further subdivided into three regions: weldment, heataffected zone, and base metal. The heat-affected zone is

defined to be the first 6 mmn (0.24 in.) of base metal

adjacent to the weldmnent.

5.3.1 Indications in the Weidment

This section reports the characterization of the largest

indications in the weldmnent below the near surface zone.

There were 28 fully characterized indications greater than

or equal to 4mmn (0. 16 in.) in the zone. Of these 28

indications, 26 were characterized as planar and 2 as

volumetric. Tables 5.9 and 5. 10 list the 16 largest planar

indications and the 2 volumetric ones, respectively.

NUREG/CR-647 154 5.4
Results of the Analysis

Table 5.8 Volumetric indications in the cladding and in the clad-to-base metal interface

Location Location

Through-wall extent Length Width in depth from weld_

size size size Coin SNR

ID Mode (mm) Basis (mm) (mm) Z (mm) X (mm) shape value Figure

I Det: 2 Conf:3 3 WP (mode 2) 12 10 7-8 -36 some 162/20 A.50

I different depths

2 Det: 2,-5 3 different depths 11 12 5-8 15 none 92/30 A.51

Conf: 1,5 2.3 WP (modes 2, 5)

3 Det: 3 2 different depths 6 18 6-8 18 good 83/25 A.52

Conf: 1,5 1.5 coin shape

4 Det: 1, 4 2 different depths 16 8 7-9 98 none 2 10/20 A.53

Conf:3 1.5 ind. w/o TOF

5 Det: 2, 5 1.5 ind. w/o TOF 12 11 6 21 some 112/40 A.54

Conf:3

6 Det: 1, 2 1.5 ind. w/o TOF NA NA 7-8 94 none 255/20 A.55

Conf:3 1 different depths

Table 5.9 Planar indications in the weldment

Location Location

Through-wall extent Length Width in depth from weld

size size size SNR

ID Mode (mm) Basis (mm) (mm) Z (mm) X (mm) value Figure

1 Det: 6 14 tip signal pat. 18 NA 64 -8 36/5 A.56

2 Det: 10 13 tip signal pat. 25 9 115 -8 56/11 A.57a-b

Conf: 6

3 Det: 6 11 tip signal pat. 18 NA 75 -7 21/3 A.58a-b

Conf: 8 1 different depths

4 Det: 6 11 tip signal pat. 18 NA 73 -7 25/5 A.59

5 Det: 6 9 tip signal pat. 16 NA 81 -6 37/6 A.60

6 Det: 6 9 tip signal pat. 30 NA 211 23 27/11 A.61

7 Det: 6, 8 9 tip signal pat. 16 NA 63-68 -8 1515 A.62a-b

8 Det: 6, 8 7 WP (modes 6, 8) 24 NA 213-218 6 31/13 A.63a-b

5 different depths

9 Det: 6 7 WP (mode 6) 30 NA 213 24 121/15 A.64

10 Det: 6 6 tip signal pat. 19 NA 117 -7 60/12 A.65

I1I Det: 6 6 WP (mode 6) 17 NA 213 16 90/45 A.66

12 Det: 6 6 WP (mode 6) 25 NA 213-215 16 109/40 A.67a-b

Conf: 8 2 different depths

13 Det: 6 6 WP (mode 6) 15 NA 209 10 43/10 A.68

14 Det: 6 6 WP (mode 6) 27 NA 212 9 115/40 A.69

15 Det: 6 4 cloud-like pat. 42 NA 212 10 53/13 A.70

16 Det: 6 4 cloud-like pat. 25 NA 203 -8 44/10 A.71

5.3.2 Indications in the Heat Affected Zone 5.3.3 Indications in the Base Metal

There were 14 characterized indications greater than or

equal to 4 mm (0. 16 in.) in the heat affected zone. All 14

were characterized as planar. Table 5. 11 lists the

characterizations.

In the base metal below the near surface zone, there were

39 characterized indications greater than or equal to

4 mm. Of these, 31 were characterized as planar and 8 as

volumetric. Tables 5.12 and 5.13 report the characterizations of the 16 largest planar indications and the 8 volumetric ones, respectively.

5.5 5.5 NUREG/CR-647 I
Results of the Analysis

Table 5.10 Volumetric indications in the weldment

Location Location

Through-wall extent Length Width in depth from weld

size size size SNR

ID Mode (mm) Basis (mmn) (mm) Z (mm) X (mm) value Figure

I Det: 9 5 ring around pat. 23 17 116-121 12 51110 A.72a-b

Conf: 8

2 Det: 6 4 ring around pat. 9 NA 213 6 43/13 A.73

Table 5.11 Planar indications in the heat-affected zone

Location Location

Through-wall extent Length Width in depth from weld

size size size SNR

ID Mode (mm) Basis (mm) (mm) Z (mm) X (mm) value Figure

1 Det: 6 34 tip signal pat. 15 NA 135 20 31/4 A.74

2 Det: 8 18 tip signal pat. 75 NA 48 17 38/3 A.75

3 Det: 6 10 tip signal pat. 18 NA 73 -17 22/4 A.76

4 Det: 6 9 cloud-like pat. 21 NA 213 26 43/15 A.77

5 Det: 6 9 tip signal pat. 27 NA 224 24 29/8 A.78

6 Det: 6 8 WP (mode 6) 22 NA 216 26 120/25 A.79

7 Det: 6 7 tip signal pat. 14 NA 76 16 17/5 A.80

8 Det: 6 7 WP (mode 6) 10 NA 214 30 92/25 A.81

9 Det: 6 5 WP (mode 6) 22 NA 216-218 24 88/35 A.82a-b

Conf: 8 4 WP (mode 8)

2 different depths

10 Det: 6 5 WP (mode 6) 19 NA 134 23 57/18 A.83

I1I Det: 6 5 WP (mode 6) 26 NA 208 23 38/10 A.84

12 Det: 6 4 WP (mode 6) 20 NA 215 30 71/20 A.85

13 Det: 6 4 WP (mode 6) 33 NA 232 -28 26/10 A.86

14 Det: 8 4 Cloud-Rie pat. 18 NA 189-192 -14 32/12 A.87a-b

Conf: 6 3 WP (mode 3)

NUREG/CR-647 15. 5.6
Results of the Analysis

Table 5.12 Planar Indications in the base metal

Location Location

Through-wall extent Length Width in depth from weld

size size size SNR

ID Mode (mm) Basis (mm) (mm) Z (mm) X (mm) value Figure

1 Det: 6 26 tip signal pat.WP (mode 8) different depths 23 NA 123-136 28 27/5 A.88a-b

Conf: 8 1

13

2 Det: 6 15 Tip signal pat. WPT (mode 8) different

Conf: 8 3 depths

41 NA 121-122

3 Det: 7 13 tip signal pat. NA 15 145

4 Det: 6 11 WP (mode 6) 30 NA 214

5 Det: 6 8 WP (mode 6) 27 NA 211

6 Det: 8 7 WP (mode 8) 16 NA 113

7 Det: 6 7 WP (mode 6) 20 NA 220

8 Det: 8 7 WP (mode 8) 28 NA 238

9 Det: 8 7 tip signal pat. WP (mode 6) different depths 16 NA 110

Conf: 6 4

24 30/8 A.89a-b

58 51/14 A.90

43 45/12 A.91

31 59/12 A.92

46 65/15 A.93

45 40/14 A.94

31 84/20 A.95

37 62/20 A.96a-b

-159 53/20 A.97

-70 68/25 A.98a-b

25 10/3 A.99a-b

10 Det: 8 6 tip signal pat.

I1I Det: 8 6 tip signal pat;

12 Det: 6 6 tip signal pat. different depths

Conf: 8 1

13 Det: 6 4 cloud-like pat.

14 Det: 8 4 cloud-like pat. different depths

Conf: 6 3

15 Det: 9 4 cloud-like pat.

16 Det: 6 4 cloud-like pat.

20 NA 201

23 NA 203

17 NA 118

30 NA 123 36 58/20 A.100

25 NA 192-195 -49 38/10 Al10lab

NA 18 220

15 NA 122

90 25/7 A. 102

36 52/14 A. 103

Table 5.13 Volumetric indications in the base metal

Location Location

Through-wall extent Length Width in depth from weld

size size size SNR

ID Mode (m m) Basis (mm) (mm) Z (mm) X (mm) value Figure

1 Det: 6, 8 7 Cloud-like pat. WP (mode 6) 19 6 108-115 28 105/2 A.104

Conf: 10 1 -5 a-c

2 Det: 6, 7, 5 WP (mode 6) 19 11 113-118 58 77/17 A. 10

8 a-c

3 Det: 8 5 WP (mode 8) 12 48 86-96 56 35/10 A. 106

Conf: 6, 3-4 WP (modes 6, 7, 9, 10) different depths a-e

7,9, 10 10

4 Det: 7 5 Ring around pat. NA 9 106 108 35/8 A. 107

5 Det: 6 5 WP (mode 6) 28 14 81-85 29 63/13 A. 108

Conf: 7, 1 WP (modes 7, 8) ring around pat. a-b

8 4

6 Det: 6 5 WP (mode 6) different depths 20 9 128-130 31 86/24 A.109

Conf: 10 2 a-b

7 Det: 6 5 Ring around pat. 46 NA 103-104 48 13/6 A.1l10

Conf: 8 2.5 WP (mode 8) different depths a-b

8 Det: 6 4 Ring around pat. 41 NA 119 26 79/24 A.11I1

5.7 5.7 NUREGICR-647 1
z

0~

00

U)

U)

0

(P

U)

U)

TRANSDUCER: 4 MI~z, Dual Element. Normal Beam

mode : pulse-echo f requency (mhz): 4.00 f number :4.8 s tand 0ff(hgt=foc len) (in): 0.390 eel (in/sac) 105000 i .ncident angle (deg): 0.0 skew angle (deg): 0 full b eam angle in metal (ding): 12.0 bean entry dia.(in): 0.125 MATERIAL: Pressure Vessel User Research Facility

material velocity (in/sec): 235000 refracted angle (deg): 0.0 thickness... (in): 9.o000 SAMPLING: Near Surface only

sample period (ns):120.00 number of points : 71 depth (along sound path) in materiel to start sampling (in): 0.000 window (us): 7.42 to 15.82 stop sampling (in): 1.000 SCAN: half wavelength in X and Y scan direction (ding):0

init pos primary axsi (in): -4.5 final: 4.5

init pus secondary axis (in): 352.0 final: 343.0 step size: scan.lnc axes (in): 0.030. 0.030

scan inc length (in): 9.000. 9.000 Nta-scans: 90601 FILE STATISTICS . w... in: 0 max: 131 avg: 1.3 ............. type: ENV

<APLOT> 0

Figure 5.1 Mode 1 detection of planar indication #1 in the near surface zone
6 Distribution of Indications of Flaws in SAFT-UT Data

The characteristics and distribution of the fabrication

flaws in reactor pressure vessels are necessary inputs to

the fracture mechanics calculations for assessments of

reactor pressure vessel integrity. Fracture mechanics

calculations are based on flaws with specific characteristics. In order to provide the high quality detailed empirical data needed for fracture mechanics calculations, this

program concentrated on estimating six different flaw

rates in the PVRUF vessel. The six flaw rates proposed

are all possible combinations of the two flaw types of

interest (planar and volumetric) and three different locations (near surface, weldmnent with HAZ, and base metal).

See Section 1. 1 for further discussion.

The purpose of this section is to present the frequency

distribution of all the indications found in the SAFT-UT

inspections of the PVRUF vessel according to size, location, and type. The performance of the SAFT-UT system

in terms of probability of detection, sizing error, and false

call rate is discussed in Section 7. Flaw rate estimates are

reported in Section 8, with a discussion of the estimation

methodology.

6.1 Joint Frequency Distribution of

Indications in the Near Surface

Zone

Of particular interest are flaws near the inner surface of

the vessel. For pressurized thermal shock (PTS) events,

the calculations indicate that flaws situated outside the

inner 30% of the vessel wall contribute little to RPV failure. Also, flaw depths greater than about 25% of the

vessel thickness are not important, because the thermal

stresses do not extend that far into the vessel. On the

other hand, small flaws of depth less than 10% of the

vessel wall make a substantial contribution to vessel

failure under PTS conditions. It should be noted that

these results concern the embrittled region of the vessel

beltline.

Flaws as small as 6 mm (0.24 in.) can make a sizable contribution to the overall probability of vessel failure during

PTS events. These flaws have been the focus of SAFTUT measurements, since they have a large impact on

vessel integrity. The flaw distributions used in fracture

mechanics calculations tend to focus on welding flaws. In

these calculations, the flaws are typically treated in a

worst-case fashion, by assuming that the flaws are at the

inner surface of the vessel and have a radial orientation.

Welding flaws can occur either within the weld metal

itself, or within the heat-affected zone of the base metal.

These flaws can occur anywhere within the thickness of

the welded cross-section.

Calculations indicate that cracks parallel to the vessel surface have no impact on vessel integrity. In fact, these

results show negative stress intensity factors, meaning

that such cracks will tend to close rather than open. Typical flaws of this type would be inclusions and

laminations.

There are significant driving forces to propagate small

cracks that are entirely within the cladding, or cracks that

extend only slightly into the base metal. Vessel integrity

is quite sensitive to such cracks, and estimates of both the

number and sizes of cracks in the cladding are important.

Therefore, detailed analysis of the cladding and of the

base metal region adjacent to the cladding is reported.

Underclad cracks can be produced by the heat input and

stresses associated with the welding process used to apply

the cladding to the inner surface of the vessel. These

cracks will be in the base metal near the clad-to-base

metal interface. Typical underclad cracks are shallow.

Cracks in the cladding itself can be produced by mechanisms such as stress corrosion cracking. These cracks

will also be shallow, but if conditions are right, they could

occur in large numbers for a given vessel.

Table 6.1 shows the distribution for the near surface indications, characterized by the material zone in which the

indication is located. Four zones were defined to classify

the portion of the vessel where the indications are located:

weldment, heat affected zone, base metal, and cladding.

The latter category, cladding, includes the clad-to-basemetal interface. The characterized indications found in

the PVRUF vessel have been assigned a type of either

volumetric or planar. Planar flaws include those flaws

that are most important to calculations of vessel integrity,

such as vertically oriented cracks, lack of fusion in the

weld, etc. Volumetric flaws include flaws such as slag

inclusions, porosity, and laminations. The method used to

determine the indication type is described in Section 4.

6.1 6.1 NUREG/CR-647 I
Distribution of Indications

Table 6.1 PVRUF: Number of near surface indications by category

PVRUF: Number of indications in the near surface zone

through-wall extent of the indications (DZ)

Total

9/15/95 <2 mm 2-3 mm 3-4 mm 4-5 mmn 5-6 mmn 6-7 mm 7-8 mm >2 mm

Location ya pD V P V P V P V P V P V P

Clad 978 2 0 2 0 0 0 0 0 0 0 0 0 4 0

Weld 87 0 5 0 2 0 0 0 0 2 1 0 1 2 9

Fusion (Haz) 47 0 0 1 1 0 0 0 0 0 0 1 0 2 1

Base 392 4 14 1 4 0 2 0 0 1 0 0 1 6 21

Total 1504 6 19 4 7.0 2 0 ,0 3 1 1 2 14 31

Total number characterized > 2 mmn =45

Total number <2 mm = 1504

Total number = 1549

'Volumetric.

b'Plana

6.2 Joint Frequency Distribution of

Indications Outside the Near

Surface Zone

Flaws can occur anywhere in the zone outside the near

surface. Welding flaws can occur at any location within

the thickness of the welded cross-section. These flaws

can occur either within the weld metal itself, or within the

heat-affected zone of the base metal. Base metal flaws

can be introduced into the original plate or forging prior

to vessel assembly and welding. Typical flaws of this

type would be inclusions and laminations, which are both

most often encountered in the mid-thickness of the vessel

cross-section.

As noted in Section 1.4, bounding calculations for low

temperature overpressurization (LTOP) events show that

there should be concern for flaws that are situated

throughout the thickness of the vessel wall, in addition to

flaws near the inner surface. Calculations also indicate

that about half the calculated vessel failures are due not to

surface flaws, but to flaws within the interior of the vessel

wall.

For LTOP events, flaw depths less than 10% of the vessel

wall thickness are relatively unimportant. Rather, it is the

somewhat larger flaws in the range of 10% to 30% of wall

thickness that contribute most toward vessel failure under

LTOP conditions.

Table 6.2 shows the distribution for the deeper flaw indications, characterized by the material zone in which the

indication is located. In contrast to the near surface, three

zones were defined here to classify' the portion of the

vessel where the indications are located: weldment, heataffected zone, and base metal.

NURiEG/CR-647 162 6.2
Distribution of Indications

Table 6.2 PVRUF: Number of indications outside the near-surface zones by category

PVRUF: number of indications outside the near surface zone

through-wall extent of indications (DZ)

Total

9/15/95 < 4 mm 4-6 mm 6-8 mm 8-10 mm 10-12 mm 12-14 mm > 14 mm >4 mm

Location va P0 V P V P V P V P V P V P

Weld 352 2 11 0 7 0 4 0 2 0 1 0 1 2 26

Fusion -Iaz) 174 0 6 0 2 0 3 0 1 0 0 0 2 0 14

Base 386 6 19 1 7 0 1 0 1 0 1 0 2 7 31

Total 912 8 36 1 16 0 8 0 4 0 2 0 5 9 71

Total number characterized >4 mm =80

total number < 4mm =912

total number = 992

'Volumetric.

6.3 6.3 NUREG/CR-647 1

7 Estimates of SAFT-UT Performance

In this section, the performance of the SAFT-UT system

is described in terms of probability of detection, sizing

error, and false call rate. To properly utilize the SAFTUT results shown in Section 6 in an estimation procedure

for flaw rates, it is necessary to understand the detection

and sizing capabilities of SAFT-UT. This section also

presents some valuable detection and sizing information

from other tests involving the SAFT-UT system and from

the Programme for the Inspection of Steel Components,

Phase II (PISC 11).

7.1 Probability of Detecting a Flaw

Results from the PISC 11 program have shown that three

distinct flaw types should be considered in quantifying the

probability of detection (POD) of flaws in heavy section

steel (Nichols, 1988). These three types are smooth

planar flaws, rough planar flaws, and volumetric flaws.

Smooth planar flaws (e.g., thermal fatigue cracks) were

found to be the most difficult to detect; rough planar flaws

are easier to detect; and volumetric flaws such as slag

inclusions and porosity had significantly higher POD than

the other two types.

In the PISC II exercises, advanced UT techniques were

used to inspect extensive amounts of pressure vessel

material. The POD estimates shown in Table 7.1 were

obtained from a logistic curve fit to this inspection data

(Heasler 1993). As one can see from the table, the

advanced procedures do best with volumetric flaws.

Table 7.2 shows the estimated POD in the SAFT-UT

inspections of the Midland blocks. The POD for volumetric flaws with through-wall extent in the range of I to

2 mm. (0.04 to 0.08 in.) was estimated at 0.7 for both nearsurface and sub-surface flaws. This estimate is based on

the SAFT-UT detection results reported in Appendix B of

this document: viz., three detections of four flaws in the

near-surface zone and two indications of three flaws in

the weld root.

7.2 Sizing Error

SAFT-UT was used in the PISC III program for a full

scale vessel (FSV) test at the Materialprufungsanstalt

(MPA) Laboratory in Stuttgart, Germany. This test

involved the characterization of 12 indications in a fullscale reactor pressure vessel (PISC III report, 1993). The

flaws in that FSV test had a range of 6 to 110 mm (0.24 to

4.33 in.) in through-wall extent (Doctor et al., 1994).

SAFT-UT tended to undersize the flaws by an average of

3.7 mm (0. 14 in.), and the standard deviation of the

SAFT-UT results from the true state was 4.7 mm

(0. 18 in.).

In a destructive analysis of Midland Block 1-8, 13 indications reported by the EPRI Center UT were selected for

examination. The true state for the through-wall extent of

a flaw is taken to be the value reported by the destructive

analysis, as shown in Table 7.3. The through-wall extent

was not determined by the destructive analysis for flaws

numbered 1, 4, 9, 12, and 13; and no comparison can be

made to an SAFT-UT depth estimate. Two flaws (numbered 2 and 10) were not detected in the SAFT-UT data.

One flaw (number 3) was not inspected by SAFT-UT

because it was at the end of the block.

The two small flaws in the weld root (numbered 5 and 6)

were undersized by SAFT-UT. The three small flaws at

the clad-to-base-metal interface (numbered 7, 8, and 11)

were oversized by SAFT-UT.

7.3 False Call Probability

The results of the advanced methods inspections of the

PISC II program gave an upper limit for false call probability (FCP) of 0. 12 for volumetric flaws in thick section

material (0.8 for clad flaws). This value was reported in

Heasler (1993) and was calculated from the number of

detections in material without known flaws. But this

number is only an upper limit because no destructive test

was performed to confirm that the material was blank.

The Midland destructive test results do not directly apply

to a calculation of false call probability for SAFT-UT.

The Midland destructive tests were performed to examine

selected indications in the EPRI NDE Center UT data

(Foulds, 1993). The selection of the locations for destructive analysis was reportedly biased toward the more significant indications (the larger ones) in the UT data.

7.1 7.1 NUREG/CR-647 1
Estimates of Performance

Table 7.1 POD for advanced procedares from fits to PISC-11 data

Flaw type Material POD (2 mm') POD (6 mm') POD (12 mma)

Smooth planar Base b 0.19 0.23

Clad b 0.19 0.25

Rough planar Base b 0.35 0.60

Clad b 0.16 0.40

Volumetric Base 0.30 0.84 0.99

Clad 0.18 0.60 b

'Through-wall extent of flaw.

'No estimation.

Table 7.2 Estimated POD for SAFT-UT inspections of the Midland blocks

Through-wall extent 1-2 mm 6 mm 12 mm

POD for near-surface flaws 0.7 - -

POD for weld and base metal flaws 0.7 0.84 0.99

Table 7.3 True-state table for through-wall extent

EPRI NDE center indication number

1

2

3t

3b

4

5

6t

6b

7

8

9

10Oa

l~b

I1I

True state from destructive analysis

Not determined

1 mm

0.75 mm

3 mm

Not determined

2 mm

2 mm'

3.5 mm

1 mm

2 mm

Not determined

1 mm

1 mm

0.5 mm

Not determined

Not determined

SAFT-UT estimate Material

Not applicable Root

Not detected Root

Not inspected Root

Not applicable

1.3 mm

1.3 mm

*<2.2 mm

< 2.2 mm

Not applicable

Not detected

< 2.2 mm

Not applicable

Not applicable

Root

Root

Root

CBlb

CBI

CBI

CBI

CBI

CBI

CBI

12

13

'Two flaws found and sized but no data provided on spatial relationsi bClad-to-BaseMetal interface zone. hip.

NUREG/CR-647 17. 7.2
8 Flaw Rate Estimates

This section reports the flaw rate estimates derived from

the distributions of indications described in Section 6.

The methodology for fitting a parametric model to the

SAFT-UT data is fully described here. The statistical

confidence intervals on the estimated flaw rates, as determined by the number of indications measured during the

inspections of vessel material by SAFT-UT, are an important part of this analysis.

Tables 6.1 and 6.2 show the number of indications that

were detected and characterized in the SAFT-UT measurements of the PVRUF vessel. These data are presented

by categories of location, type, and size. We use the

amount of material inspected, as given in Table 3.3, to

calculate the indication densities, i.e., the number of indications per unit volume in each category. The estimates

of flaw rates described in Section 7 also depend on-the

measured performance of the SAFT-UT inspection system. The methodology described in this section generates

estimates of the cumulative flaw rate fuinction, denoted A

(s), and defined in Equation 8.1

represents the small flaws. If k0(s) represents the true

flaw rate fuinction, then an inspection procedure with

detection capabilities of POD(s) will produce a data set of

flaws distributed according to the flaw rate function:

?,,(s) = POD(s) ?,0(s) (8.2)

where XI(s) represents the expected number of detected

flaws of size s per unit volume of material.

The fact that the inspection procedure does not furnish the

true size of the flaw fuirther modifies the observed flaw

rate fuinction. We make the assumption that the inspection procedure provides the user with a size estimate that

is contaminated with Gaussian error having a mean of

zero and standard deviation of ay, so that the observed size

is

Sobs =Strue +e (8.3)

These sizing errors tend to "smear out" the flaw rate function. More specifically, the resulting flaw rate function

calculated from inaccurately sized flaws is given by: A(s) = JX(s) ds (8.1)

where X(s) is the flaw rate function, defined as the

expected number of flaws of size s per unit volume, and

where the size s is the through-wall extent of the flaw.

8.1 Use of SAFT-UT Performance

Data

The capabilities of an inspection procedure are quantified

by the probability of detection curve, (POD(s)), and the

root mean square sizing error, cr. The first statistic

describes the detection capabilities of the procedure and

the other summarizes its sizing accuracy.

The POD curve describes the capability of the inspection

procedure to detect a flaw of a certain size. A typical

POD curve is S-shaped; for a small flaw, POD is typically

close to zero and rises to one for large flaws. An inspection procedure with a typical S-shaped POD, as illustrated

in Figure 8. 1, will produce a flaw rate fuinction that underX2 (S) = fo (s - z; ay) X, (z)dz

= fo 4(s - z; ar) POD(z) X0 (z)dz

(8.4)

where ý(z;a) represents a Gaussian density function with

standard deviation of ar and mean of zero.

In practice, Sob, is always positive, even though the model

allows for the possibility of negative flaw sizes. In other

words, the Gaussian model does not produce a completely

accurate description of sizing errors on small flaws. This

is typically not a problem, because small flaws are not

found (POD = 0). Another assumption of this model that

could potentially cause more significant problems concerns the false call ratio. This estimation procedure

assumes that there are no false calls in the data. If the

false call rate of the inspection procedure is significantly

different from zero, the estimate produced by this procedure will be too large, and the shape' of the cumulative

flaw rate function will be incorrect.

8.1 8.1 NUREG/CR-647 1
Flaw Rate Estimates

Equation 8.4 summarizes the basic estimation problem

that must be solved; we want to estimate X0(s), but the

inspection procedure gives data for X2(s). For this particular problem, we have chosen to use the method of maximum likelihood. Maximum likelihood requires us to

specify' a parametric form for the flaw rate function before

a solution can be obtained. We have chosen a Weibull, a

fairly common distribution for reliability work, and one

that has previously been used for flaw size distributions

(see Kennedy, Foulds, and Basin 199 1). The specific

form of the flaw rate function is therefore:

X10(s) = oCO(s;013i'12) 13O13I12sN-exp(-13sI1⁄2) (8.5)

M

X Ho

The method of maximum likelihood produces a solution

to the vector 13 by finding the minimum of the function

y(13) and an ax-level confidence bound for this estimate, ,

which is given by the expression:

ThisR regio cofdec on13 can be trnlae ( n toa)oni(8.9)

dence region on A(s:1P) (or X(s: P)), by using the formulae:

Aupper (s) = max A(s; P) (8.9)

or

kO(S) = 0300(0;11,02) = 13o exp(-P13~l2) (8.6) IISRa

and

where A0(s) = the number of flaws with through-wall

extent greater than s, per unit of weld

volume;

1 = the total number of flaws per unit of

weld volume;

s = the through-wall extent of a flaw; and

P,~ and P2z = the two parameters of the Weibull function that are to be fit to the empirical

data.

8.2 Method of Estimation: Maximum

Likelihood

If the data obtained from an inspection of T units of material includes M flaw indications, and the associated flaw

depths, si, i =1, 2, ... M, then the probability "density" for

this data is given by:

Alower (s) = min A(s; 13)

I3CR.

(8.10)

8.3 Maximum Likelihood Using the

Weibull Distribution

Since the flaw rate function we intend to utilize has a

"Weibull" form as illustrated in Equation 8.6, the specific

log-likelihood formula to be employed is:

'y(3) =2 [log(M!) +T13 0 f POD(z)uo(z;P13112) dz

M I 'ý (8.12)

-Z log(,T13 Jo *(sj - z; o)POD(z)(o(z;P13112)d

Because only three unknown parameters exist in this likelihood, it is possible to find the minimum using a modified exhaustive search. To accomplish this, 13n is

expressed in terms of the two other unknown parameters.

In fact, given specific values for P3, and 132, the value for

130 that minimizes the log-likelihood is:

f.(, 1,S2,--SM M! H1 TX(si) i=1

(8.7)

Assuming that the flaw rate function X(s) is determined

by a vector of unknown parameters 13 = 0

13 , P2, --- On)

then the -2 log likelihood of the density is defined as:

130= 00 M

[T foPOD(z)co(z;P1312) dz]

(8.13)

NUREG/CR-647 18. 8.2
Flaw Rate Estimates

Therefore, the minimum for the log-likelihood expressed

by Equation 8.13 is found by evaluating the likelihood

over a regular grid of P,1 and P32 values.

8.4 Data Requirements for the

Estimation Method

In order to accurately estimate the flaw rate function, one

must inspect enough. material so that a "reasonable" number of flaws are found. In this section, we determine how

many flaws are required to produce a reasonable estimate

of the flaw rate function.

To determine the number of flaws required, maximum

likelihood fits were performed on differently sized simulated data sets. These fits assumed that the inspected

material contained flaws distributed according to the

Marshall distribution (i.e., I31 =1.60 cm-1 and 02 = 1.0). It

was also assumed that the inspection procedure had no

sizing error and fairly good POD characteristics. The

specific cases listed in Table 8.1 were used to determine

the effects of sample size and sizing error on flaw rate

estimation accuracy. In practice, the sample size is

adjusted according to the number of flaws found.

The acceptability of the fit is summarized by the confidence bandwidths associated with the unknown model

parameters. For example, Figure 8.2 displays the 95%,

90%, 80%, and 50% confidence bounds on the P3 parameters in the Weibull distribution for a sample size of 3.

From this plot, one can see that the confidence bounds do

not close and it is quite obvious that 3 flaws do not provide enough information to adequately determine the flaw

rate fuinction. These confidence bounds can be translated

into bounds on the cumulative rate function, as displayed

in Figure 8.3. Figure 8.3 presents the cumulative rate

function surrounded by 95% confidence bounds. As one

can see from this plot, the shape of the curve is quite

uncertain.

Table 8.2 summarizes the critical details of the maximum

likelihood fits. The widths of the 95% confidence bounds

on the two model parameters (PI, P32) are shown for various sample sizes. The results are fairly clear; acceptably

accurate confidence bounds occur for sample sizes above

15, so one can expect good results from a fit to empirical

data with this number of flaws. The table also shows that

10 flaws may be reasonable, but 5 are not.

Confidence bound plots for a sample size of 15 are presented in Figures 8.4 and 8.5. These plots are directly

comparable to the results displayed for the sample size of

3 (Figure 8.2). As one can see, the size of the confidence

interval has been dramatically reduced.

8.5 Near Surface Zone Flaw Rate

Estimates

In the near surface zone, the largest number of planar

indications occurred in the base metal, which had 419

detectable indications. Table 6.1 shows that 21 of these

were greater than 2 mmn (0.08 in.) in size. The inspections

of the weld metal revealed 9 planar indications greater

than or equal to 2 mm in size and a total of 98 detectable

indications. The other categories in Table 6.1 contained 6

or fewer indications greater than 2 mm in size.

In this section, we report the fit of the Weibull distribution to the inspection data in Table 6. 1. The resulting

estimate neglects any detection or sizing errors. This

approach gives the most "optimistic" result that can be

obtained from the data, because the introduction of detection and sizing error will increase the flaw rate function

and widen the confidence interval.

Weibull fits to the empirical data were satisfactory for the

planar indications in the weldment and for the planar indications in the base metal. These two subsets of the

SAFT-UT data have different distributions, as shown by

the parametric model. The Weibull fits were unsatisfactory when the smallest indications (less than 2 mm [0.08

in.] in size) were included in the data, as evidenced by the

model's overprediction of the flaw rate in the size range

of 3 to 5 mmn (0. 12 to 0.2 in.), and underprediction for

flaws greater than 6 mmn (0.24 in.) in size. This effect is

still present in the model's performance without including

the smallest indications, which tends to confirm some

difficulty in fitting this parametric model to the

distribution of the empirical data.

A formulation for the Weibull distribution equivalent to

that used in Equations 8.5 and 8.6 is:

A(s) = 130e( (8.14)

and this form is more commonly encountered in the literature. For this reason, the fits to the SAFT-UT data are

reported in terms of Equation (8.14).

8.3 8.3 NUREG/CR-647 1
Flaw Rate Estimates

Table 8.1 Maximum likelihood fits run to determine sample size

Inspection characteristics Flaw distril

Perfect POD, RMS=0 Marsha

Volumetric POD, RMS=0 Marsha

Volumnetric POD = logit (-4.97 + 5.73s).

Marshall Distribution: = I . exp 1 ft, where s is in cm.

bution

Sample sizes

(no. of flaws)

3,5,10,15,50,100

3,5,16,15,50,100

Table 8.2 Parameter confidence bound widths for different sample sizes

Sample size f = 1.60 cm' CBW (Ij)' 32 =1.0 CBW (P2)

Perfect POD, sizing RMS = 0

3

5

10

15

50

100.

00 3.31

00 2.54

19.95 1.56

12.82 1.17

2.85 0.39

1.42 0.39

Volumetric POD, sizing RMS = 0

3

5

10

15

50

100

CBW = confidence bound width.

00

00

15.675

9.975

2.850

1.425,

2.92

2.34

1.36

0.97

0.39

0.19

Figure 8.6 shows the 50, 80, 90, and 95% confidence

bounds on the a and P parameters from Equation (8.14), in

the Weibull fit to the distribution of the 9 largest indications in the near surface weidment.

Figure 8.7 presents the 95% confidence bounds on the

cumulative flaw rate Weibull function fit to the 9 largest

indications in the near surface weldment. The quality of

the fit is good; that is, the tendency to underpredict the

smaller indications and overpredict the larger ones is not

great. The upper 95% confidence bound on the fit is more

than 200% of the prediction, reflecting the small sample

size of 9 indications. The significant feature of this figure

is the large flaw density that becomes apparent when

expressed in flaws per cubic meter, especially when

compared to the base metal, as described below.

Figure 8.8 shows the confidence bounds on the a and ~ parameters of the Weibull function fit to the 21 largest

indications in the near surface base metal. The

confidence intervals are moderately reduced compared to

Figure 8.6, because the sample size increased from 9 to

2 1. There is considerable overlap in the confidence

regions presented in Figures 8.6 and 8.8, indicating that

the near surface weldment and- base-metal distributions do

not differ significantly (except in terms of gross flaw

density).

Figure 8.9 presents the 95% confidence bounds on the

cumulative flaw rate Weibull function fit to the 21 largest

indications in the near surface base metal. The quality of

the fit is only fair; that is, the tendency to underpredict the

smaller indications and overpredict the larger ones is

apparent. The upper 95% confidence bound on the fit is

reduced to about 150% of the prediction, reflecting the

larger sample size compared to Figure 8.7. The significant feature of this figure is the significantly lower flaw

density for near surface base metal, compared to near

surface weldment as shown in Figure 8.7.

NUREG/CR-647 184 8.4
Flaw Rate Estimates

8.6 Outside the Near Surface Zone

Flaw Rate Estimates

Approximately equal numbers of planar indications

occurred in the weidment and base metal in the zone

below the near surface. There were a total of 380

detectable indications in the weidment and 424 in the base

metal, a difference of only 10%. Table 6.2 shows that

there were 38 indications in the base metal that were

greater than 4 mm in size and 28 in the weldmenL The

HAZ had 14 indications greater than 4 mm in size. The

other categories in Table 6.2 contained 7 or fewer indications greater than 4 mm in size.

In this section, we report the fit of the Weibull distribution to the inspection data in Table 6.2. The resulting

estimate neglects any detection or sizing errors.

Weibull fits to the empirical data were satisfactory for all

3 categories of planar indications--weldment, HAZ, and

base metal--but only when the indications smaller than

4 mm in size were excluded from the fit. A Weibull fit is

also reported for the 7 volumetric indications in the base

metal. The Weibull fits were unsatisfactory when the

smallest indications (less than 4 mm in size) were

included in the data, as evidenced by the model's biased

overprediction of the flaw rate in the size range of 5 to

12 mm, and underprediction for flaws greater than 15 mm

in size.

Figure 8. 10 shows the 50, 80, 90, and 95% confidence

bounds on the a and 03 parameters in the Weibull fit to the

distribution of the 26 largest planar indications in the

weldment. Figure 8.11 presents the 95% confidence

bounds on the cumulative flaw rate Weibull fuinction fit to

the 26 largest planar indications in the weldment. The

quality of the fit is very good; that is, there is no tendency

to underpredict the smaller indications or over-predict the

larger ones. The upper 95% confidence bound on the fit

is about 150% of the prediction, reflecting the larger sample size of 26 indications compared to 9 indications for

Figure 8.7. The significant feature of this figure is the

quality of the fit and the higher flaw rate values compared

to the base metal, as described below.,

Figure 8.12 shows the confidence bounds on the a and f

parameters of the Weibull function fit to the 14 largest

indications in the heat affected zone. The confidence

intervals have increased by a considerable amount,

compared to.Figure 8. 10. This is caused, in part, by one

large indication, planar indication #1 in the heat affected

zone.

Figure 8.13 presents the 95% confidence bounds on the

cumulative flaw rate Weibull function fit to the 14 largest

indications in the heat affected zone. The quality of the

fit is only fair; that is, the tendency to underpredict the

smaller indications and overpredict the larger ones is

apparent. The upper 95% confidence bound on the fit is

increased to more than 200% of the prediction. The

significant feature of this figure is the significantly higher

flaw density for the heat affected zone, compared to the

weldment and the base metal.

Figure 8.14 shows the confidence bounds on the a and P3

parameters of the Weibull function fit to the 7 largest

volumetric indications in the base metal. The confidence

intervals have increased by a considerable amount compared to Figure 8. 10. This is caused, in part, by the fact

that the empirical distribution here is limited to only two

sizes.

Figure 8.15 presents the 95% confidence bounds on the

cumulative flaw rate Weibull function fit to the 7 largest

volumetric indications in the base metal. The quality of

the fit is poor because of the limited distribution of the

empirical data.

Figure 8.16 shows the confidence bounds on the ac and f3

parameters of the Weibull function fit to the 31 largest~

planar indications in the base metal. The confidence

intervals are reduced as expected for the relatively large

sample size of 3 1.1

Figure 8.17 presents the 95% confidence bounds on the

cumulative flaw rate Weibull function fit to the 31 largest

planar indications in the base metal. The quality of the fit

is only fair; that is, the tendency to underpredict the

smaller indications and overpredict the larger ones is

apparent. The upper 95% confidence bound on the fit is

reduced to about 150% of the prediction, reflecting the

larger sample size of 31 indications. The significant

feature of this figure is the significantly lower flaw

density for base metal, compared to the weldment and

heat affected zone.

8.5 8.5 NULREG/CR-647 1
Flaw Rate Estimates

8.7 Comparison to Other Published

Flaw Rate Estimates

Table 8.3 shows the flaw rate parameters for the Weibull

model fits to the six different indication rates extracted

from the SAFT-UT inspections of PVRUF. Table 8.4

shows the flaw rate parameters for Weibull fits that have

been previously published (Foulds et al. 1993)--including

the data from SAFT-UT inspections of the Midland vessel

(Appendix B of this report).

Table 8.3 PVRUF flaw rate function parameters

Location

Near surface

Data set

Planar, weidment

Planar, base metal

Planar, weldment

Planar, heat affected zone

Volumetric, base metal

Planar, base metal

a (mm)

4.11

3.5

8

10.9

5.6

8

2.19

2.68

2.93

1.46

6.5

1.79

P3o flaws/me

1100

280

137

274

25

34

Outside near surface

Table 8.4 Published flaw rate function parameters

Data set a (mm) P3, 13, flaws/m'

Marshall distribution' 6.25 1 0.4-40

SAFT-UT, 31 flaws b 3.81 1.39 966

EPRI NDE center UT, Sandia report' 4.70 1.30 409

-'M-arshall (1982) without preservice inspection and repair.

b Data from Midland report. SAFT-UT inspections of four Midland blocks without adjustment of

POD or sizing error. 'Data reported in Foulds (1993).

Table 8.5 shows the results for the cumulative flaw rate

function for 3, 6, and 12 mm for the six parametric flaw

rate functions fit to the PVRUF data. Table 8.6 shows the

cumulative flaw rates from previously published reports.

In summary, the SAFT-UT data from the PVRUF vessel

provides information on flaw density and distribution as a

function of flaw type and location. The most significant

difference between the flaw rates reported here and those

reported in Marshall 1982 is that the total density of flaws

found by SAFT-UT is higher than that reported in

(Marshall, 1982).

NUREG/CR-647 18. 8.6
Flaw Rate Estimates

Table 8.5 PVRUF estimates of cumulative indication rates from Weibull fits to the indication frequencies

Location

Near

surface

Data set

Planar, weidment

Planar, base metal

A (3mm)

indications/rn 3

660

140

A (6mm)

indications/in 3

110

4.0

A (9mm)

indications/rn 3

4.2

<.01

A (12mm)

indications/rn 3

0.03

<0.01I

A (15mm)

indications/rn 3

<.01

<.01

Planar, weidment 130 89 33 5.2 0.25

Otie Planar, heat 235 180 128 87 55

Outside affected zone

near Volumetric, base 24 5.2 <.01 <0.01 <.01

surface metal

Planar, base metal 29 19 9.9 4.3 1.6

Table 8.6 Published estimates of cumulative flaw rates for weidment using Weibull fit

A (3mml A (6mm) A (9mm) A (12mm) A (15mm)

Data set flaws/rn flaws/rn 3 flaws/rn 3 flaws/rn 3 flaws/in 3

Marshalla (P, = 40 flaws/m' 25 15 9.4 6 3.6

Midland, SAFT-UTb 470 147 36 7 1.2

Midland, EPRIC 230 104 40 14 4.4

--Marshall (1982) without preservice inspection and repair. b SAFT-UT inspections of four Midland blocks without adjustment for POD or sizing error.

C Data reported in Foulds (1993).

8.7 8.7 NUREG/CR-647 1
Flaw Rate Estimates

eq. C

a

0

a

'~1. a

0.0 0.4 0.8 3.2 1.6 2.0

Flaw Depth (em)

Figure 8.1 Typical probability of detection curve

0 10 20 30 40

0, (Cm-')

so 60

(Volumetric POD, No Sizing Error)

95%. bound on P. = (0.80,35.89)

Figure 8.2 Confidence bounds on model parameters~for-sample'size of 3

NUJREGICR-647 18. 8.8
Flaw Rate Estimates

0.0 OA 0.8 12 LO

Flaw Size (CM)

(Volumnetric POD, No Sizing Error)

Figure 8.3 Cumulative flow rate function with 95% confidence bounds for a sample size of 3

a 10 20 30 40

P, (cm')

so 60

95% Confidence Bound on A, = (14.57,80.27)

Figure 8.4 Confidence bounds on model parameters for sample size of 15

8.9 8.9 NUREGICR-647 I
Flaw Rate Estimates

aaI

a0-

0.0 0. [a ]. A 2.0

Flaw Size (cm)

(Volumetric POD, No Sizing Error)

Figure 8.5 Cumulative flaw rate function with 95% confidence bounds for a sample size of 15

L12

0

CM d

I I I I 1

2 4 a 8 10 12

a (MM)

Figure 8.6 Parameter estimates with confidence bounds for simple Weibull fit to the SAFT-UT

data of 9 planar indications in the near surface weld meat

NUREGICR-6471 818.10
Flaw Rate Estimates

I

C

0

0 5 10 is 20 25 30

Plaw slze (nm)

Figure 8.7 Estimate of cumulative flaw rate function with 95% confidence interval for simple Weibull

fit to the SAFT-UT data of 9 planar indications in the near surface weidment

(D -

0

NY2 4 68 10

a (mm)

Figure 8.8 Parameter estimates with confidence bounds for simple Weibull fit to the SAFI7-UT

data of 21 planar indications in the near surface base metal

8.11 8.11 NUREG/CR-647 1
Flaw Rate Estimates

(U

ir

8-

04

8-

0 5 10 15 20 25 30

flaw size (mm)

Figure 8.9 Estimate of cumulative flaw, rate function with 95% confidence interval for simple Weibull

fit to the SAFT-UT data of 21 planar indications in the near surface base metal,

0

I i-%9%

0o80

N -

5 10 15 20 25

a (MM)

Figure 8.10 Parameter estimates with confidence bounds for simple Weibull fit to the SAFT-UT

data of 26 planar indications in the weldment below the near surface

NUREGICR-6471 818.12
Flaw Rate Estimates

8-

a -

0

I 10 15 20 25 30

flaw Shze (MM)

Figure 8.11 Estimate of cumulative flaw rate function with 95% confidence interval for simple Weibull

fit to the SAFT-UT data of 26 planar indications in the weidment below the near surface

NY -

5 10 i5 20 25 30

a (um.)

Figure 8.12 Parameter estimates with confidence bounds for simple Weibull fit to the SAFT-UT

data of 14 planar indications in the heat affected zone below the near surface

8.13 8.13 NUREG/CR-647 I
Flaw Rate Estimates

0

0 5 10 15 20 26 30

flaw SIZO (mm)

Figure 8.13 Estimate of cumulative flaw rate function with 95% confidence Interval for simple Weibull

fit to the SAFT-UT data of 14 planar indications in the heat affected zone below the near surface

0

COCO -

V -

810 i5

a (Mm)

Figure 8.14 Parameter estimates with confidence bounds for simple Weibull fit to the SAFT-UT

data of 7 volumetric indications in the base metal below the near surface

NUREG/CR-6471 81.14
Flaw Rate Estimates

0

I 0

C.J

0 015 20 25 30

Figure 8.15 Estimate of cumulative flaw rate function with 95% confidence interval 'or simple Weibull

fit to the SAFT-UT data of 7 volumetric indications in the base metal below the near surface

(0 -

Sn -

0

6ý :10.48"95%

5 10 15 20 25

a (UM)

Figure 8.16 Parameter estimates with confidence bounds for simple Weibull fit to the SAFT-UT

data of 31 planar indications in the base metal below the near surface

8.15 8.15 NUREG/CR-647 1
Flaw Rate Estimates

2-

8-

U.

aII I I I I I

0 5 10 15 20 25 30

flaw size (mm)

Figure 8.17 Estimate of cumulative flaw rate function with 95% confidence interval for simple Weibull

fit to the SAFT-UT data of 31 planar indications in the base metal below the near surface

NUREG/CR-6471 818.16
9 Discussion of Flaw Rate Estimates

This section discusses the flaw rate estimates given in

Section 8. Two important questions arise from these new

flaw rates. Why were so many flaw indications found in

the vessel by SAFT-UT given the inspections that were

performed on it by the fabricator? What is the origin of

the difference between the number of indications found

by SAFT-UT and the number predicted by the Marshal

distribution?

9.1 SAFT-UT Flaw Indications and

Inspections During Fabrication

The PVRUF vessel had been inspected by RT and several

UT examinations from both the inside and the outside surfaces while it was being fabricated during the late 1970s

and early l980s. There were a number of repairs that were

performed on the vessel based on the NDE results. One

can assume that some large flaws were detected and that

they were repaired. In order to understand why the

SAFT-UT results detected so many flaws, the sensitivity

of the inspections conducted must be under stood.

The SAFT-UT database that was obtained on PVRUF was

created to develop information on the density and distribution of reactor pressure vessel (RPV) fabrication

flaws. The data was not taken based on ASME Code

requirements. It was expected that most of the flaws that

would be detected would be small and thus, the inspection

was conducted at very high sensitivity. The data can be

related to ASME Code requirements by comparing the

response from side drilled holes and end milled slots in

the calibration block that was provided with the PVRUF

vessel. The inspection sensitivity based on these ASME

standard reflectors would be at a DAC of less than 5%.

For the inspections that were conducted by the fabricator,

it is expected that these examinations were performed to

the required ASME Code at that time. Prior to 1986, the

code requirements included using procedures based on

50% DAC inspection sensitivity. For examinations conducted from the inside of the RPV it was common practice to gate out the first inch of RPV material as a result of

inspection transducer ring d own. Transducer ring down

occurs. in single-element transducers when exitation

adversely affects reception during the first few

microseconds. The use of dual element transducers

essentially eliminates this problem and industry has

adopted them for conducting effective examinations of

the RPV near surface zone. Therefore, it is expected that

none of the underclad near-surface flaws in the PVRUF

database would have been detected from the inside. The

use of 50% DAC would not have detected the flaws in the

PVRUF databasý found in the remainder of the vessel

wall because these flaws generally had a low ultrasonic

response. The results from PISC 11 (Nichols and Crutzen

1988) show that for a flaw 50 mm deep the probability of

detection would only be 10% for procedures based on

50% DAC.

In 1986 the ASME Section XI Code was changed based

on PISC 11 results to require that an effective near surface

examination be performed and that a 20% DAC sensitivity procedure be used. Whenever a utility up dates their

151 program to the 1986 or later editions of the ASME

Code, the procedures would have met these requirements.

The requirement of using an effective near surface technique would have meant that they used a transducer technique similar to that used by PNNL but without the SAFT

processing of the data. Therefore, it would be expected

that the larger (high signal-to-noise) flaws in the PVRUF

database would have been detected in the near surface

zone (first 25 mm). For the remainder of the vessel wall

thickness, the use of 20% DAC sensitivitý' effectiveness

must be related through the PISC 11 study. In the PISC 11

study it was found that for flaws 17 mm in depth (largest

validated flaw in the PVRUF database) a procedure that

used a 20% DAC level of sensitivity would have an average probability of detection of 45%.

Finally, inspections that would be performed today for

those personnel, equipment and procedures that have successfully passed the performance demonstration test

specified in ASME Section XI Code Appendix VIII, it is

expected that they would detect all of the larger flaws in

the PVRUF data base. This position is supported by the

PISC II data, which shows for advanced techniques a

probability of detection of 95% for a flaw 17-mmn in

depth.

9.2 PVRUF Flaw Rates and the

Marshall Distribution

A question arises regarding the difference in numbers of

flaws that were found in the PVRUF inspections versus

9.11 9.1. NUREG/CR-647 1
Discussion of Flaw Rate Estimates

those predicted by the Marshall distribution. The PVRUF

results are based on high sensitivity inspections. Work is

in progress and this work will be fully documented in the

Volume 2 of this report, supporting the fact that these

2500 indications are being confirmed to be flaws. Most

of these flaws are very small, are probably not important

to structural integrity, but they are flaws. The Marshall

distribution was developed in the 1 970s and was based on

UT examination of 44 LWR vessels, augmented by

information pertaining to defects in non-nuclear vessels.

The detection efficiency for the UT inspections at that

time was based on expert opinion and led the Marshall

Study Group to recommend that "carefuil experiments be

carried out to determine the efficiency of detection of

cracks in vessels by ultrasonic methods (UKAEA, 1976)."

Early UT examination procedures were not designed to

detect many of the flaws that were found in the PVRUF

inspections. These small flaws are important to

developing distribution and density functions for the

occurrence of flaws in reactor vessels, particularly in the

process of extrapolating the data to describe the flaw

occurrence in the full population of RPVs in the

U.S. In the 1980s some new failure modes became very

important such as pressurized thermal shock, in some of

these cases the smaller flaws became more important.

Although, they do not have as high a probability of causing a vessel failure the probability of their occurrence is

higher. Based on these differences it is not surprising that

there are differences between the 1 990s PVRUF high sensitivity empirical results and those developed by expert

opinion from the 1970s.

NUREGICR-647 192 9.2
10 Validation Plan

This section describes the validation tests planned for the

indications recorded by SAFT-UT. The performance of

SAFT-UT has been measured previously, but additional

confirmation by destructive testing is required to provide

the necessary high quality flaw data for use in probabilistic fracture mechanics analysis, such as pressurized

thermal shock (PTS) analysis.

The SAFT-UT system was used at PVRUF because of the

need for high probability of detection and accurate sizing,

especially for small flaws. Reasonable estimation of the

density and distribution of fabrication flaws in unused

material requires a system with these properties, and a

large data set. More than 2500 indications were acquired

with the SAFT-UT system.

This section describes the identification of material for

removal from the PVRUF vessel and the location of the

largest indications in the vessel. Indications were selected

for destructive testing to validate the flaw rate estimates

in Section 8.

10.1 Removal of Material from

PVRUF for Destructive Testing

The plan for the removal of material from PVRUF proposes that large blocks be cut from the vessel and sent to

PNNL. The size of the proposed blocks is 60 cm across

the weld and 120 cm along the weld, with the weld centered in the width of the block. Approximately 60% of

the beitline weld and 100% of the intermediate-to-uppershell-course weld are to be available for validation of the

SAFT-UT data.

Prior to the removal of any material, the vessel's inside

surface will be marked to identify the material for validation. The locations for cuts will be designed to avoid the

most significant indications (i.e., so that these indications

are deep within the block rather than near a cut), and this

will lead to blocks of different lengths. The lengths will

vary from 60 to 120 cm. The material will be marked

with fiducial points in such a way as to provide enough

information to identify the PNNL material coordinates for

each of the blocks.

Tables 10. 1 and 10*2 show the locations along the welds

for the 15 largest fully characterized indications in a

number of categories for SAFT-UT indications from the

PVRUF vessel.

10.2 Selection of Indications for

Destructive Testing

The objective of the destructive test is to validate the flaw

rates derived in Section 8 of this report. It was established in Chapter 8 that 15 flaws are a reasonable minimum number from which to determine a flaw-rate function. One approach to the validation task would be to

perform destructive tests on the 15 largest indications for

each of the flaw rates estimated in Section 8.

However, one or two of the analysis rules described in

Section 4 determine the size of the largest flaws used to

estimate each of the flaw rate functions. Therefore,

verifying these sizing rules is a more efficient method of

conducting the validation. The recommended validation

steps include the following:

I1. Select the most important indication type for each

flaw rate. This may be the largest indication or it

may be a typical indication from a large group.

2. Use construction radiographs to correlate with SAFTUT images and as a basis for final selection of indications to be destructively tested. These records are

available and their usefulness should be determined

as part of the validation. These images may help

distinguish planar vs. volumetric flaw types before

the destructive test. Also, they may help in extrapolating the destructive test results over the entire

SAFT-UT data set.

10.1 10.1 NUREGICR-647 1
Validation Plan

Table 10.1 Location of the largest indications along the weld, near surface zone

Near surface

Weld Haz. Base Clad

P V V P .V P V

Rank Weld Y(in.) Weld Y(in.) Weld Y(mn.) Weld Y(in.) Weld Y(in.) Weld Y(in.) Weld Y(in.)

1 5 58 5 350 5 479 4 129 4 27 5 473 5 479

2 4 29 5 479 4.52 4 156 5 317 5 473 5 479

3 5 52 5 75 5 247 4 37 5 311 5 483 5 473

4 5 109 4 196 n/a n/a 4 142 4 200 5 478 5 484

5 4 33 n/a n/a n/a n/a 4 127 5 420 n/a n/a 4 539

6 4 59 n/a n/a n/a n/a 4 71 4 209 n/a n/a 4 20

7 4 197 n/a n/a n/a n/a 4 123 5 353 n/a n/a n/a n/a

8 5 371 n/a n/a n/a n/a 4 125 4 49 n/a n/a n/a n/a

9 5 175 n/a n/a n/a n/a 4 167 n/a n/a n/a n/a n/a n/a

10 4 184 n/a n/a n/a n/a 5 200 n/a n/a n/a n/a n/a n/a

11 n/a n/a n/a n/a n/a n/a 5 238 n/a n/a n/a n/a n/a n/a

12 n/a n/a n/a n/a n/a n/a 5 205 n/a n/a n/a n/a n/a n/a

13 n/a n/a n/a n/a n/a n/a 4 33 n/a n/a n/a n/a n/a n/a

14 n/a n/a n/a n/a n/a n/a 4 109 n/a n/a n/a n/a n/a n/a

15 n/a n/a n/a n/a n/a n/a 4 165 n/a n/a n/a n/a n/a n/a

Table 10.2 Location of the largest indication along the weld outside the near surface zone

Full volume

Weld Haz Base

P V P V P V

Rank Weld Y(in.) Weld Y(in.) Weld Y(in.) Weld Y(in.) Weld Y(in.) Weld Y(in.)

1 5 11 4 539 4 220 5 80 5 60 5 181

2 4 215 5 10 5 23 n/a n/a 4 530 5 238

3 5 102 5 29 5 2 n/a n/a 5 312 5 348

4 4 193 5 8 5 152 n/a n/a 5 9 5 214

5 4 194 4 69 4 162 n/a n/a 5 3 5 11

6 5 10 n/a n/a 5 24 n/a n/a 5 261 5 229

7 5 513 n/a n/a 5 3 n/a n/a 5 311 5 343

8 5 103 n/a n/a 4 138 n/a n/a 5 495 5 344

9 5 467 n/a n/a 5 3 n/a n/a 5 515 5 214

10 5 519 n/a n/a 5 4 n/a n/a 5 315 5 180

11 4 113 n/a n/a 4 87 n/a n/a 5 489 5 460

12 5 3 n/a n/a 5 317 n/a n/a 4 47 4 38

13 5 4 n/a n/a 5 6 n/a n/a 5 173 5 114

14 5 29 n/a n/a 4 36 n/a n/a 5 15 5 197

15 5 4 n/a n/a 5 10 n/a n/a 4 86 5 10

3. Validate the location of each selected indication in

the block. This will be done before the block is cut,

using the same inspection mode that originally

detected the indication. A lengthy scan need not be

performed; rather, a simple manual measurement

may be sufficient.

4. Cut the block for normal beam inspection. The first

cut on a large indication will be made to expose a

surface for a normal beam UT inspection of the

indication. This will determine if the large indication

is composed of a number of smaller ones, and will

locate the indication more precisely in the material.

NULREG/CR-6471 10.2 1.
Validation Plan

5. Prepare the indication for metallography if its size is

validated by a normal beam test. After the normal

beam test, a decision can be made as to whether or

not this indication still contributes to the flaw-rate

function. For example, if a large indication is recharacterized as two small ones in the normal beam

test, then there may be no reason to perform metallography. The metallographic process will probably

require that the material containing the indication be

extracted and formed into a cube for grinding,

polishing, and etching.

6. Perform metallographic analysis.

Another method recommended for validation is the use of

the tandem SAFT-UT mode. This inspection can be performed on the blocks removed from the PVRUF vessel

without any fuirther cutting. Tandem mode inspections

can also be used to extrapolate the destructive test results

over the entire SAFT-UT data set.

10.3 10.3 NUREGICR-647 1

11 Reliability Requirements for ISI Systems

Reactor pressure vessel inservice inspections (ISI) produce data that can be used to estimate the number and

sizes of flaws in a vessel. This chapter discusses what

characteristics 151 procedure/equipment/personnel should

have to produce an acceptable flaw inventory for a vessel.

The flaw estimation problem faced during ISI is a bit

different than the problem dealt with in other portions of

this report: an ISI is focused on determining the number

and sizes of flaws in a specific reactor pressure vessel,

while this report has focused on estimating a generic flaw

frequency function for a population of vessels.

In order to perform a series of calculations on the

utilization of ISI results, it is necessary to determine the

amount of RPV weidment that is inspected during an

outage. For PWRs the normal procedure is to perform 151

from the inside of the vessel and in order to do this, all of

the reactor internals are removed. In addition, there are a

number of fixed costs associated with setting up the RPV

inspection system. Consequently, the normal practice is

to only inspect the PW*R RPV once during a ten-year

interval and to inspect all the ASME-required locations

that are accessible. In the case of BWRs, the ISI program

is somewhat more complicated because of the inspection

requirements that must be met and the six different

models of BWRs. Some plants conduct some nozzle

inspections every three years and perform inspections of

the upper portion of the RPV at those times. They only

inspect below the nozzles once during a ten-year interval.

Thus, during each three-year period, differing amounts of

the vessel are examined. The amount of the BWR RPV

that is inspected at any one time will range from about

one third to about two thirds. For this chapter, we will

assume that if ISI is performed that one third of the RPV

weldments will be inspected. Since most of the time

when ISI is performed more than this amount will be

inspected, it is conservative in the analysis to assume the

lower, bound.

The flaws found by inspection typically represent only a

portion of the total population of flaws present. After

inspection, flaws in the vessel can be divided into three

categories:

Furthermore, the reported flaw sizes will differ from the

actual sizes because of sizing error. To evaluate the consequences of imperfect inspection, it is reasonable to concentrate on those flaws not found (i.e., missed + not

inspected), because these are the ones that may potentially

impact the structural integrity of the vessel; it is assumed

the flaws that are found will be repaired or shown to be of

no safety consequence.

Even though inspection provides direct information on

only the first of these three categories of flaws, it is possible to use the inspection data to say something about the

last two categories. In fact, if the inspection is properly

calibrated' the flaws in the last two categories can be

estimated.

In order to evaluate how adequate inspection is, our strategy is to measure how well inspection can estimate the

number of flaws in the last two categories. An adequate

inspection procedure will be one that gives results that do

not differ substantially from perfect inspection, as measured by the mean square error of the estimate. We will

attempt to bound the mean square error so that the

challenge to structural integrity would be evaluated as

essentially the same whether using imperfect inspection

or perfect inspection results.

11.1 Quantitative Model of Inservice

Inspection

The number of flaws of size s in T meters of weldment is

assumed to have a Poisson distribution with mean X(s)T,

where X(s) is the flaw frequency function.

After inspection, the state of the pressure vessel can be

described by two curves, NF(s) and Nu(s). The curve

NF(s) describes the number and sizes of flaws found by

inspection, while Nu(s) describes the flaws not found.

The curves are defined as follows:

NF(S): The number of flaws of size greater than s found by

inspection of T, meters of weldment.

'A calibrated inspection is one with well known POD curve and sizing

error. This is not the typical NDE meaning for calibration.

" Flaws found by inspection,

" Flaws missed by inspection,

" Flaws not inspected.

11.1 11.1 NUREG/CR-647 I
Reliability Requirements

Nu(s): The number of flaws of size greater than s

unfound. This includes flaws missed by inspection and

flaws in the un-inspected T2 meters of material.

Inspection directly produces NF(S), but to conduct a complete structural integrity assessment, one also requires

Nu(s), the flaws not found by inspection. It so happens

that the unfound flaw curve can be estimated from inspection results, using the same estimation methods that have

been used to estimate-generic flaw frequency functions.

The closeness of the estimate Ný U (s) to the desired curve

Nu(s) depends on the inspection's detection and sizing

capabilities, so a reasonable way to evaluate the inspection, capabilities is to evaluate the error between the estimate and the desired value, using an appropriate statistic.

Naturally, one wants to specify the inspection probability

of detection (POD) and sizing capability so that this error

is small. But it.is difficult to specify absolute bounds on

this error, without performing some sort-of analysis that

relates the error to structural integrity. Since a detailed

analysis is beyond the scope of this, evaluation, we will

employ an approximate curve in the analysis. The curve,

R(s) is plotted in Figure 11.1 and is meant to roughly

represent the probability of failure for a single crack

under a PTS event.

From this curve, one can see that the impact on structural

integrity associated with flaws smaller than 3 mm is very

small. Between 3 mm and 8 mm, the impact increases

from 0 to 10-3, a value that is not insignificant. Between

8 mm and 12 mmn, the impact increases dramatically to

one of certain failure (Simonen 1 986a). The benchmark

points on the curve (3, 8, and 12 mm) are connected with

straight lines to produce a simple curve. Given this curve,

it is possible to calculate the expected number of flaws

that would fail during a PTS event, which is:

RMSE(RT)= ,E((kT -RT)) (11.3)

where E(X) denotes the expected value of X. The root

mean square error associated with an (imperfect) inspection will be considered acceptable if it is less than a

certain proportion of the RMSE achieved for perfect

inspection. In this study, we will use the proportion 150%

to define acc eptability.

11.2 Scenarios for Evaluating

Different Aspects of Inspection

Capability

Inspection capability can be divided into four components, which can be separately evaluated. These four

components are the inspection procedure's 1) POD,

2) uncertainty associated with the POD, 3) sizing error,

and 4) uncertainty associated with the sizing error. To

evaluate inspection capability, the following five

scenarios will be utilized, which will allow each of the

four components of inspection to be, compared to a base

case. The scenarios are:

Perfect Inspection: (Base Case) POD(s) = 1 and sizing

error is zero.

Imperfect Detection: POD(s) < 1, but the sizing error is

zero, and POD(s) is assumed to be known.

Imperfectly Calibrated POD: POD(s) < 1, sizing error is

zero, and POD(s) is imperfectly known.

Imperfect Sizing: The sizing error is not zero, but is

assumed to be known; it is Gaussian with mean 0 and a

standard deviation of a,. POD is assumed to be 1.

Imperfectly Calibrated Sizing: POD(s) =1, but the standard deviation (a,) associated with sizing error is imperfectly known.

Each component of inspection capability will be evaluated by comparing the base case RMSE against the

RMSE produced when that component is varied. Acceptable thresholds for that component will be chosen so that

the RMSE is inflated no more than 150%. In the next

sections, the mean square error for RZT will be calculated

for each scenario.

RT = IJR(s) d N,(s)I (11. 1)

.and

k T =IJR(s) d fU(s) 1 (11.2)

This study will evaluate the acceptability of Ný U (s) (and

the corresponding inspection capabilities) by calculating

the root mean square error of RiT , which is defined as:

NUREG/CR-6471 1. 11.2
.Reliability Requirements

The general estimator for &Nu(s) that will be used in this

evaluation is:

Nu(s)= rjPnd(z)Ti+T 2 dN

JPOD(z)T1 z

similar to the curve R(s). Specifically, the POD curve

will have the shape illustrated in Figure 11.2: from 3 mmn

to 8 mm, it will be assumed to be constant with value 013;

from 8 mm to 12 mm, it will rise linearly to P32; and for

sizes larger than 12 mm, the curve will remain constant.

In other words, the POD curve will have a form described

by:

(11.4)

In this equation, POD(z) represents the probability of

detection for a flaw of size z mm, while Pnd(z) 1 -

POD(z). It should be noted that this formula is not.the

only way N U (s) could be estimated, but it is the simplest

estimation formula. This formula accounts for missed

flaws and is unbiased when POD is knownm A formula

very similar to this is used in the steam generator IPC

(Improved Plugging Criteria) inspections to. deal with

unfound flaws.

11.2.1 Perfect Inspection Scenario (Base

Case)

If inspection is perfect, then POD(s) I and the estimator

reduces to: !

POD(sl3) = P IV (S) +.2 1(I - V (s)) (11.8)

Nýu(s)a NF(S) (11.5)

where V(s) = I for any s < 8 mm, V(s) = 0 for any s >

12 mm, and V(s) is linear between 8 mm and 12 mm.

11.2.3 Imperfectly Calibrated POD Scenario

In this scenario, the POD curve originates from a calibration exercise which has fit a curve to detection data. It

has some sort of parametric form, which can be represented by POD(s113). For example, Equation 11.8 represents such a form.

The 13 = (P3~ 13 2) represents the parameter vector estimated

by the regression. The regression determines P3 imperfectly, and this is expressed in terms of an uncertainty

(i.e., Bayesian posterior) distribution f (013J). The uncertainty of the whole curve can be described by the

covariance:

H (z, s) =Coy (POD (zl P), POD (sl13)) (11.9)

The estimate for this, scenario is actually produced by

using the best estimate 13 for 13 inEquation 11. 8:

MSE(R )T+T 2 J2(s sPnd(SI Pý 1+T2 d MSE(T) T, fR s)?(s) POD(s1I1P) ds+(11.10)

(TI + T 2 YJJR(s)X(s)R(z) H(x, Z) " 'dsdz

POD(s 13) Pndz I1P)

For this scenario, the above formula is to be used to determine a bound on H(s,z). This again is an ill-determined

and the MSE for kT is:

MSE (RT - 2T( 1 + 2 JR (s)T(s)ds (11.6)

11.2.2 Imperfect Detection Scenario

In this case, the estimator cannot be simplified. The MSE

associated with the estimator is:

MSER T,=T + T '2 R2 (S) X(s) Pnd(s) T + T2 ds(17 MSE = (RT) TI fRPOD(s) s(17

The objective is to put a bound on MSE and solve for

POD(s). This is an ill-determined problem, unless some

constraint is placed on POD(s); To make the problem

tractable, the POD curve will be assumed to have a shape

11.3 11.3 NUREG/CR-647 1
Reliability Requirements

problem, and to make it tractable, we utilize the functional form for POD introduced previously. If POD has

the form introduced there, then H(s,z) can be expressed

as:

11.2.5 Imperfectly Calibrated Sizing

Scenario

Under this scenario, we assume that the sizing error standard deviation a, is not known, but has to be estimated

by, say as, with v degrees of freedom (i.e., v + 2 data

points in the calibration experiment). This means that the

true and the estimated a, can be related through a conditional distribution of the form:

H(s, z) -= 1 (I - P, )V(s)Vz)

+ ý2 (I - 2 ý1 - VWs) (I1- V(z))

(11.11)

where N represents the sample size used in the regression

experiment. This sample size effectively determines

H(s,z), so the determination of appropriate bounds for

uncertainty of the POD curve can be considered equivalent to the determination of N.

11.2.4 Imperfect Sizing Scenario

Let XF(s) represent the expected value of NF(s), assuming

that flaw sizes are contaminated with Gaussian error. In

other words, the relationship between XF(s) and the true

X(s) is:

%F(S)-= fX (z) JZ~s )dz (11.12)

f (a, as, v) (11.15)

and the MSE is now:

MSE(RT -&s,v)=

JMSE(AT -cr~)nocr I &~~

(11.16)

where 4(x) is the Gaussian density function:

x2) (11.13)

This equation is used to determine an acceptable value for

V.

11.3 Flaw Distribution Function

As one can see from the formulas presented in the previous sections, RMSE (RT) depends on the flaw distribution function X(s). In order to evaluate the desired RMSE,

we must supply approximate values for X(s), and the natural approximations to use are the estimates for PVRUF,

which have been produced in Chapter 8 of this report.

These flaw distribution functions are derived from the

Weibull distribution so that X(s) has the form:

X(s) =00 (0/a)(s/a)O' exp(-(s/ax)P) (11.17)

where the a, P3, and Po are the parameters that determine

the function. The PVRUF inspection has furnished sets of

flaw distributions as presented in Table 11. 1.

These flaw distribution functions are plotted in

Figure 11.3. The objective is to use these eight estimates

to represent flaw distributions that are typical of reactor

The MSE for this scenario is:

2

MSR (kT) = T2 JR2 (s)X(s)ds +T2

i

fR 2 (S)kFds + (T2 fR(s)X(s) - XF (s)ds )

2

(11.14)

For this scenario, the objective is to use the above equation to place bounds on a,,, the sizing error standard

deviation. Equations 11. 12, 11.13, and 11. 14 provide

enough information to accomplish this.

NUREG/CR-6471 1.1.4
Reliability Requirements

Table 11.1 Flaw distributions from PVRUF

X(s) Location/data set a (MM) 031 Po (flaws/in)

?X1(s) Near surface/planar, weldment 4.11 2.19 1100

1%2(s) Near surface/planar, base metal 3.5 2.68 280

XA(5) Outside near surface/planar, weldment 8 2.93 137

X40s) Outside near surface/planar, HAZ 10.9 1.46 274

XAS() Outside near surface/volumetric, base metal 5.6 6.5 25

X605) Outside near surface/planar, base metal 8 1.79 34

vessels generally. We will calculate the RMSE associated

with each different X(s), and choose requirements so that

all or almost all of the associated RMSE are less than the

150% criteria. Ideally, one would like to see that the

RMSE is not strongly influenced by X(s), or if it is, is

influenced in only a simple way.(') We therefore hope

that all the different cases presented above will produce

roughly the same inspection requirements.

It should be noted that a typical reactor contains about a

cubic meter of weldment, so the unit, flaws per reactor

could also be placed on the parameter P0o in Table 11. 1.

11.4 Results from Scenario

Evaluations

In this section, the RMSE formulas are applied for each

scenario to produce tables relating RMSE to each of the

four components of inspection (i.e., POD, POD

uncertainty, sizing error, sizing error uncertainty). The

numbers were calculated by numerical integration with a

computer code.

For these evaluations, it was assumed that only one third

of the weldment in the reactor was inspected (Tj = 1/3

and T2 = 2/3).

11.4.1 Base Case Scenario Results

Evaluating Equation 11.6 for the six flaw distribution

functions yields the results presented in Table 11.2. As

one can see from this table, the RMSE varies considerably

among the six distributions. One can also see that the

RMSE is roughly proportional to average E(RT), so this

Table 11.2 RMSE for base case

X(s) RMSE (P-T) E(RT)

X2(5) 0.026 0.022

?13(S) 5.508 15.173

X405) 14.460 76.215

X5(s) 0.004 0.007

X60s) 3.618 5.280

table shows that there is considerable variability in this

population of six flaw distributions: the population

ranges from a distribution having a small impact on structural integrity (%3(s)) to one that has a large impact on

structural integrity (%4(s)).

It is useful to consider flaw distributions that display this

sort of variability for this evaluation. If the evaluation

produces consistent requirements for inspection using

these disparate flaw distribution functions, we have good

evidence that the requirements are generally valid.

It should also be noted that the units for RT and its RMSE

are "number of flaws important to structural integrity." If

the curve were correct, flaws important to structural

integrity would be important in a PTS accident scenario.

11.4.2 Imperfect Detection Scenario Results

In this scenario, we examine a set of 16 POD curves,

whose lower POD parameter (01,) has been set at 5%,

10%, 30%, and 50%, while the upper POD parameter (032)

has been set at 50%, 70%, 90%, and 95%. The inflation

in RMSE from the base case (i.e., RMSE (Imperfect

Detection)/RMSE (Base Case)) is shown in Table 11.3.

'For example, RMSE might be proportional to X5s().

11.5 11.5 NUREG/CR-647 1
Reliability Requirements

Table 11.3 RMSE (imperfect detection)IRMSE

(base case)

POD(s) Flaw distribution function X(s)

013 P2 )X,(S) )12S) X3(S) X.40) ?15(S) X6(S)

5% 50% 248 344 190 165 522 173

5% 70% 213 309 158 * 134 517 * 142

5% 90% 189 284 * 137 * 114 513 * 121

5% 95% 184 279 * 132 *110 512 *117

10% 50% 228 289 184 164 372 170

10% 70% 198 264 * 154 * 134 369 *140

10% 90% 177 246 * 133 * 113 366 *119

10% 95% 173 242 * 129 * 109 366 *115

30% 50% 183 198 168 160 211 163

30% 70% 163 187 * 142 * 131 210 *134

30% 90% * 148 178 * 124 *111 209 *115

30% 95% * 145 176 * 120 * 107 209 *111

50% 50% 158 158 158 158 158 158

50% 70% * 143 * 151 * 134 * 129 158 *131

50% 90% * 131 * 145 * 117 *110 *157 *112

50% 95% * 128 * 144 * 113 * 106 * 157 *108

Overall, proportions vary from a low of 106% to a high of

522%. The RMSE ratios in the above table that are close

to or less than the 150% threshold are marked with an

asterisk. It is useful to point out that even for a bad case

like f31 = 30% and P32 = 50%, the RMSE inflation of

211% for k5 indicates that it is still a useful inspection;

one could very well argue an inspection with POD as low

as 50% for large flaws would still yield useful results.

It can be seen that distributions X5(s) and X2(s) present the

most difficult estimation tasks for inspection: for these

distributions, the POD curve has to be at 50% for small

flaws and at 90% for large flaws. If we ignore these two

difficult distributions for a moment, one can see that a

POD curve with 13=(30%, 90%) meets the RMSE

criteria.

We will select POD thresholds of 3=(30%, 90%),

although this does not meet the 150% target for the two

most difficult distributions. In other words, POD is

acceptable if it is above 30% for flaws in the 3-8 mm

range, and above 90% for flaws larger than 12 mm.

11.4.3 Imperfectly Calibrated POD Scenario

Results

In this scenario, we examine how much RMvSE is inflated

because the POD curve is imperfectly known. The case

chosen for comparison is not the perfect inspection

scenario, but the case chosen in the last section (P3 =

(30%, 90%)). The uncertainty in the POD curve is represented with a sample number, N, which describes the

number of calibration measurements that need to be taken

to obtain the stated RMSE. The results are shown in

Table 11.4.

All the values in Table 11.4 are below the target value of

150%, indicating a calibration experiment with as little as

10 measurements would be acceptable. These results

indicate that the typical performance demonstration test,

with from 10-30 flaws, is acceptable.

Table 11.4 RMSE (imperfectly calibrated

POD)IRMSE (imperfect detection)

cr(POD) Flaw distribution function X(s)

N XI(S) X2(S) XA3s) X405) X~50) MS6()

10 111 111 116 144 138 104

20 106 106 108 124 121 102

30 104 104 106 116 114 101

11.4.4 Imperfect Sizing Scenario Results

In this scenario, sizing errors of 0.5, 1, 2, and 3 mm were

examined. Table 11.5 presents the relative RMSEs.

Three distributions listed in the table seem to be relatively

sensitive to sizing error (2.1(s), X2(s), and X5(s)); while the.

other three are more insensitive to sizing error. For the

three insensitive distributions, one could tolerate a 3-mm

sizing error; for the sensitive distributions, a sizing error

less than half a mm is required.

Figure 11.4 shows how sizing error smears the flaw distributions. The plot is for a sizing error of 1 mm. In this

case, there are large differences between the different distributions. The sizing error can distort a distribution With

a sharp peak, but will not cause problems in a more diffuse distribution.

Table 11.5 RMSE (imperfect sizing)IRMSE

(base case)

Flaw distribution function X(s)

a, 2.1(S) ?12(S) X3(S) X4(0) XA5S) 2160)

0.5mm 111 160 101 100 453 100

1mm 184 507 106 100 3,165 101

2mm 766 4,475 134 101 20,128 104

3mm 2,071 17,777 179 103 46,517 110

NUREG/CR-647 1 111.6
Reliability Requirements

11.4.5 Imperfectly Calibrated Sizing

Scenario Results

In this scenario, we assess how much RMSE is inflated

due to the uncertainty in the sizing error standard deviation cy,, by comparing the RMSE with that of the imperfect sizing (with cy, perfectly known) case. The sizing

error standard deviation a, has to be estimated in a calibration experiment by &s with v degrees of freedom. We

examine how large a value of v is required for the inflation of RMSE to be within the target value of 150%. The

RMSE is calculated for six situations, using a &, of

0.5 mm or 1 mm, and using a v of 5, 10, and 15.

From Table 11.6, it can be seen that for the three flaw

distributions insensitive to sizing errors (iLe., X3 s),X4 (s),

andk46 s)) the desired uncertainty is achieved with as

little as 7 (i.e., 5 + 2) data points in the calibration experiment, with &S 0.5 or 1 mm. However, in order for all

the flaw distribution functions considered to achieve the

desired accuracy, at least 17 data points are required in

the calibration experiment, with &, = 0. 5 mm.

Table 11.6 RMSE (imperfect calibrated sizing)IRMSE

(imperfect sizing)

Flaw distribution function X(s)

V v X,(S) XA2S) )13(S) X40s) XA5S) X6(S)

0.5mm 5 138 426 101 100 557 100

0.5mm 10 106 130 100 100 201 100

0.5mm 15 103 113 100 100 151 100

1mm 5 242 622 106 100 326 101

1mm 10 144 240 102 100 188 100

1mm 15 122 161 101 100 150 100

11.5 Recommended Requirements for

Is'

From the scenarios presented in previous sections, it is

clear that the detection and sizing properties must be

known, if some estimate for the flaws not-found is to be

made. Since an estimate for the unfound flaws is important in a pressure vessel safety analysis, one can argue

that a fundamental requirement is: for the inspection

procedure to be considered acceptable, the probability of

detection curve (POD(s)) and sizing error ((Y,) should be

known.

This means that estimates (with a known uncertainty) can

be supplied for these two quantities. Ideally, the

estimates should be empirically determined through a

performance demonstration, but it may also be acceptable

to use generic estimates, if the inspection procedure is

particularly well behaved.

It is important to note that one needs to know detection

and sizing information for all sizes of flaws; a test that

estimates detection and sizing for one particular size is

not necessarily adequate. On the other hand, it is always

possible to identify a smallest flaw size of interest; flaws

smaller than this size are of no importance and we need

not worry about the ability of 151 to find these flaws. The

scenarios evaluated in the previous sections allow requirements about the four components of inspection to be generated. These are:

POD Curve: POD should be above 30% for flaws in the

3-8 mm range and above 90% for flaws larger than

12 mm.

POD Uncertainty: A POD curve determined from

approximately 20 well-placed measurements will exhibit

acceptable uncertainty.

Sizing Error: A sizing error standard deviation of 0.5 mm

is required.

Sizing Error Uncertainty: A sizing error standard deviation determined from approximately 20 data points in a

calibration experiment will produce an acceptable

uncertainty.

11.7 11.7 NUREGICR-647 1
Reliability Requirements

q

0

I

C

I

0

0

0

0 5 10 15 20

flaw size (mm)

Figure 11.1 Impact of pressure vessel flaws on structural integrity

8.

CYJ

0 510 15

flaw size (mm)

Figure 11.2 Imperfectly calibrated POD scenario

20

NUREG/CR-6471I1. 11.8
Reliability Requirements

I

S.

S.

3-

0-

S.

S

I

I

0 5 10 Is 20

Flow Size (mm)

0 5 10 15 20

Flaw Size (mm)

a. I 2-

3

I

0.

0.

I

0 -

0.

0 S 10 15 20

Flaw Size (mm)

0 5 10 1s 20

FRaw size (MM)

X6

1

a

0 5 10 Is 20 0 5 10 15

Flew Size (mm) Flaw Size (mm)

Figure 11.3 Flaw distribution functions X1, X2,713, X4, X5, and ?L6 from PVRUF

20

11.9 11.9 NUREG/CR-647 1
Reliability Requirements

I ~

I

2'

S.

I

I

z

a -

0 5 10 Is 20

Flaw Size(a)

0 5 10 Is 20

Flaw Size (mm)

X4

ma -

I

a

~

I

0 -

a

I

I

N,

0.

I

I

0 5 10 1s 20

Flaw Size (man)

0

Flo Siz

05 10

FRow Siz (mm)

IS 20

I

I

0

to Is 20

Flaw Size (mm)

Figure 11.4 Flaw distribution functions: solid curves represent true functions, dashed curves represent

found functions for X1, X2, X3, 14 k, and X6. as = 1 mm is used

NUREG/CR-647 11.0 11.10
12 Conclusions

The SAFT-UT inspection of the PVRUF vessel recorded

a large number of flaw indications and PNNL staff carefully analyzed the entire data set for flaws of interest in

fracture mechanics calculations. The principal conclusions obtained in this project are as follows:

1) The data set provided enough information for the

estimation of six different flaw rate fuinctions that

contain significant information on the size, location,

type, and density of fabrication flaws in reactor

pressure vessels.

2) The density of flaws in the PVRUF vessel is significantly greater than predicted by a Marshall

distribution.

3) The performance of SAFT-UT was consistent with

our expectations and the system produced a high

quality data set.

There were 2500 detectable indications in the SAFT-UT

inspections of the PVRUF vessel, using consistently

applied detection rules. At the start of the inspections,

PNNL expected that 80% of the indications would be

small, that is, on the order of 2 mm in through-wall size.'

This was based on experience gained from the inspections

of the Midland vessel. The data presented in this report

show that 97% of the indications were less than 2 mmn in

through-wall size in the near surface zone, that is, within

25 mm of the inner surface of the PVRUF vessel. For the

portion of the vessel wall beyond 25 mm in depth, 80%

were smaller than 4 mm in through-wall size. The rate of

false detections in this analysis is expected to be low

because the detection rules were developed from the high

correlation of SAFT-UT indications with flaws, as

validated by destructive tests of material removed from

the Midland vessel. The SAFT-UT sizing rules were

created to conservatively size indication zones to ensure

that all potentially large flaws would be included in the

validation plan.

The distribution of the empirical data, reported in Section 6, provided enough information to apply a parametric

model of the cumulative flaw rate to six different subsets

of the data with reasonable confidence bounds on the

results. It was possible to extract distinct distributions

for the weldment and the base metal in the near surface

zone. The comparison of these two distributions shows

significantly higher indication rates in the weldment compared with the base metal in the near surface zone. It was

also possible to extract different distributions for the

weldment, heat-affected zone, and base metal in the portion of the vessel wall beyond 25 mmn from the inner surface. These distributions also show a significantly higher

indication rate in the heat affected zone and weldment as

compared to the base metal.

For the near surface zone, the weldmient has a greater

measured density of flaws than the base metal. The flaw

density ratio of the near surface weldment and the near

surface base metal can be extracted from the analysis

presented in Section 8. For all planar flaws, the j30(s)

values from Table 8.3 yield an indications ratio of 4 to 1

for the weldment compared to the base metal in the near

surface zone. Table 8.5 shows that this ratio increases

when only the larger flaws are considered: 5 to 1 for

A(3mm) and 28 to 1 for A(6mm). This is a reasonable

result, because planar flaws include cracks and lack of

fusion in the weld, and the base metal is thought to contain mostly volumetric flaws with little through-wall

extent. The validation of the SAFT-UT indications by

metallographic analysis is expected to confirmn the relative

densities of the two distinct populations.

For the portion of the vessel outside the near surface zone,

the flaw distribution functions for the weldment and the

heat-affected zone show a greater estimated density of

flaws compared to the base metal. Table 8.3 shows a

ratio of 4 to 1 for the weldment compared to the base

metal. Table 8.5 shows how this ratio changes when only

the larger flaws are considered: 5 to 1 for A(3mm), 5 to I

for A(6mm) and 1 to 1 for A (12mm). For all planar

flaws, Table 8.3 shows a ratio of 8 to 1 for the heat

affected zone compared to the base metal. Table 8.5

shows how this ratio changes when only the larger flaws

are considered: 8 to 1 for A(3mm), 10 to 1 for A(6mm)

and 20 to 1 for A(l2mm). It can be concluded that the

interface between the weldment and the base metal (i.e.,

the heat-affected zone) contains the greatest density of

large planar flaws. Since it was expected that the midwall portion of the base metal contains a population of

volumetric flaws with little through-wall extent, it is clear

that a determination of the nature of the larger SAFT-UT

indications in the outside the near surface portion of the

12.1 12.1 NUREG/CR-6471
Conclusions

vessel can add significantly to our understanding of

fabrication flaws in reactor pressure vessels.

This report contains in Tables 8.3 and 8.5 estimated flaw

rate functions for six different subsets of the SAFT-UT

indications, with confidence bounds. The densities of

flaws predicted by the SAFT-UT inspections are significantly greater in all six cases than those predicted by a

Marshall distribution of flaws.

Finally, the SAFT-UT system and the measurement plan

for PVRUF worked very well. The 10 inspection modes

produced complementary information on the size, type,

location, and density of flaws that is important in fracture

mechanics calculations. The added value of each modality was demonstrated in detection and characterization of

the indications, as described in Sections 4 and 5.

The calibration and vessel scanning procedures were

routinely and successfully applied during the 18 months

of data collection at PVRUF. The image quality of the

SAFT-UT inspections was very high because of the fine

sample spacing specified in the measurement plan, and

the careful execution of the calibration and scanning

procedures.

NUREG/CR-6471 1. 12.2
13 Recommendations

Three recommendations are appropriate at the completion

of this program. First, and most important, validate the

flaw rate estimates derived from the SAFT-UT inspections. Second, develop a methodology to produce generalized flaw densities and distributions for the entire population of vessels or classes of vessels. Third, improve the

SAFT-UT system to make it more efficient for use in the

inspection of reactor pressure vessels.

The indication rates reported in Section 8 should be validated to verify the necessary high quality of flaw statistics

for use in fracture mechanics calculations, such as for

pressurized thermal shock (PTS) analysis. The inspection

data provided enough indications to estimate the six

different indication rates in the RPV, as reported in

Section 8. Relatively few of the larger indications are

determining factors in these distributions. An area of

uncertainty in the empirical data, however, is the accuracy

with which the larger indications have been sized. The

destructive analysis of material removed from the

Midland vessel provided detection and sizing confirmation for small flaws in the depth size range of 0.5 to

2.0 mmn (0.02 to 0.08 in.). This program provides a data

set of fuilly documented indications of real preservice

fabrication flaws that cover a much wider size range. The

flaw rates estimated from this data set should be the

principal focus of the destructive validation discussed in

Section 9 of this report.

To produce generalized flaw density and distribution

fuinctions for the entire population of vessels or classes of

vessels, existing statistical models, such as the U.K.

model (Chapman 1993) investigated under NRC JCN

L2606, should be evaluated for their ability to predict

vessel-specific flaw densities and distributions for use

with and comparison to inservice inspection results from

operating reactors. Future work, in NRC JCN W6275,

will use the data from the SAFT-UT measurements of the

PVRUF vessel to benchmark and calibrate Chapman's

predictive model. A calibrated predictive model, such as

the one developed by Chapman, should provide, when

used with ISI data, a means of extrapolating the flaw rates

from this project .to the entire population of vessels in the

U.S.

In future applications of SAFT-UT to the inspection of

nuclear reactor components, effort should be directed

toward reducing the time and cost of data acquisition, data

processing, flaw detection, and flaw characterization. The

speed of data acquisition could be significantly improved

by designing and building an eight-channel data system

for use on RPVs. The speed of data processing should be

increased to keep up with the data acquisition rate. The

speed of flaw detection should be improved using pattern

matching and on-line analysis rules to provide a near realtime flaw detection and reporting capability. Some of the

significant indications should be characterized using

tandem SAFT-UT and other sizing methods.

13.1 13.1 NUREG/CR-647 1

14 References

ABB-CE Report CEN-622-A. 1996. "Generic Upper

Shelf Values for Linde 1092, 124 and 0091 Weld.

Booth, D. L. 1989. "Material Documentation Report for

the Weld Material Removed from the Consumers Power

(Midland) Reactor Vessel (620-0012-5 1)." The Babcock

and Wilcox Company, Nuclear Power Division,

Lynchburg, Virginia.

Chapman, V. 1993. "Simulation of Defects in Weld

Construction," Pressure Vessel and Piping Conference,

Reliability and Risk in Pressure Vessels and Piping,

PVP-VoI. 25 1, PP 8 1-89. American Society of

Mechanical Engineers, New York.

Doctor, S. R., et al. 199 1. "Progress in Evaluation and

Improvement in Nondestructive Examination Reliability

for Inservice Inspection of Light Water Reactors (LWRs)

and Characterizing Fabrication Flaws in Reactor Pressure

Vessels," Proc. US NRC 19th Water Reactor Safety

Information Meeting, Vol. 1, NUREG/CP-0 119. U.S.

Nuclear Regulatory Commission, Washington, D.C.

Doctor, S. R., G. J. Schuster, L. D. Reid, T. E. Hall.

1995. Development and Validation of a Real-Time

SAFT-UT System for the Inspection of Light Water

Reactor Components, NUREG/CR-45 83, PNL-5 822.

Doctor, S. R., L. J. Angel, A. A. Diaz, R. V. Harris, Jr.,

F. A. Simonen, and G. J. Schuster. 1994. "Reliability of

NDE - Cast Stainless Steel, SAFT-UT Performance, PISC

III Program Status, and Evaluation of Computer-Based

UT/ISI Systems," Proc. of2lst Water Reactor Safety

Information Meeting, Vol. 1, NUREG/CP-0 13 3. U.S.

Nuclear Regulatory Commission, Washington, D.C.

Electric Power Research Institute. 1995. "BWR Vessel

and Internals Project, BWR Reactor Pressure Vessel Shell

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