First International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurised Components, 20 - 22 October 1998, Amsterdam, Netherlands

TABLE OF CONTENTS

Abstract Introduction Flaws Characteristics Data Collection and Evaluation Stratergy Evaluation of Results at the level of procedures Evaluation at the level of the Flaws Evaluation at the level of Techniques Sizing Performance Sentencing Performance Considering the rejection Criteria conclusions Limitations Overall Inspection Capability of the Steam Generator Tubes Demonstration of Capability Remark and Acknowledgment References

Abstract

This paper summarizes the PISC III report No 41, the full description of the PISC III Action 5 on steam generator tube inspection. The objective of this Action is the experimental evaluation of the performance of examination procedures and techniques available for steam generator tubes inservice inspection in nuclear power plants.
There were several procedures, which demonstrated good detection capability of major flaws in typical locations of the steam generator. For procedures based on eddy current testing (ET), the detection of axial flaws deeper than 40% of the wall thickness T was often good. This capability of detection fell markedly for external flaws with a depth less than 40%T of the wall thickness. In general detection was not effective at the notification level of 20%T. For procedures based on ultrasonic testing (UT) only, detection was excellent for axial flaws deeper than 20%T.
Overall, the flaw sizing results show large dispersion. For some types and sizes of flaws, however, this dispersion was reduced, e.g., for the length sizing of axial flaws being located at the inside surface or deeper than 40%T. There was a wide dispersion of detection and sizing performance from apparently similar procedures and even from similar techniques. It is concluded that capability demonstration is necessary to qualify inservice inspection procedures for steam generator tubes.

Introduction

The PISC programme (Programme for the Inspection of Steel Components) carried out since 1974 under the auspices of the CEC/JRC and the OECD/NEA has the general objective of assessing the capability and reliability of inspection techniques and procedures for Non-Destructive Evaluation of structural components (1). These projects are centered on the Joint Research Center of the European Commission, which, in its roles of Operating Agent and Reference Laboratory, manages the programme. OECD/Nuclear Energy Agency (NEA), provides the Secretariat of the PISC Managing Board, which consists of representatives of 14 countries (10 European Union (EU) and 4 non-EU countries).
The objective of Action 5 of PISC III (2) is the experimental evaluation of the performance of test procedures and techniques available for steam generator tubes in nuclear power plants during in-service inspection. The definition of the programme was based on the interest of the participating countries in a large variety of failure mechanisms. The exercise was characterized by the tube material (Inconel 600), the large number of flaws, and the inspection techniques (predominantly eddy current with a few teams using ultrasonics). The programme (sample matrix and test schedule) of the capability tests on loose tubes was formulated (3) and started in early 1990 with the circulation of training boxes. The Blind Test Tubes of the Round Robin Test (RRT) were circulated to the 29 participating organizations of 9 different countries. Data collection, adaptation, evaluation and discussion took about 3 years. The final report is actually in the approbation phase by the PISC Management Board.

Flaws characteristics

A total of 95 flaws (some of which were combinations of flaws) were introduced into the tubes representing a wide range of types and locations (table 1). The major flaws introduced fell into two principal categories:

• machined flaws simulating typical flaws such as cracking, wastage and pitting.
• chemically introduced flaws validated as realistic simulations of Secondary Water Stress Corrosion Cracks (SWSCC), volumetric Inter-Granular Attack (V.IGA) and pitting.
The tube assemblies contained a range of geometric and structural support features typical of those found in steam generators, including large and short U-bends, expansion transition zones, tubes sheets with short and long crevices, tube support plates and anti-vibration bars (AVB). All the flaws were introduced close to these characteristic features.Destructive examination, guided by special NDE techniques, was performed by the Reference Laboratory on all intended flawed areas of every tube of the Round Robin Test in order to compare the NDE results with the real dimensions and characteristics of the flaws.

	PWSCC=Primary Water Stress Corrosion crack VIGA=Volumetric Intergranular Attack TS=Tube Sheet SWSCC=Secondary Water Stress Corrosion Crack Circum=Circumferential TSP=Tube support plate ASCC=Axial Stress Corrosion Crack Ext=External A VB=Anti Vibration Bars ANS=Axial Narrow Slot int.=internal SrUb= Small radius U Band CNS=Circumferential Narrow Slot Dep=Deposit LrUb=Large Radius U Band
Table 1: Table of the 95 implanted flaws in the RRT Steam Generator tubes.

Data collection and Evaluation strategy

Four kinds of samples circulated: calibration and blank tubes, 3 boxes of training test tubes and 9 boxes of blind test tubes.
The blind test boxes were grouped and circulated in two separate batches to give to the 29 participating teams the concerted inspection time: one week to test each batch by each applied single procedure Ultrasonic Testing (UT) or Eddy Current Testing (ET).
The Reference Laboratory (RL) was responsible for the collection of the inspection data, for the validation of the flaws and for conducting or directing all destructive examinations and also for the analysis and evaluation of results under the guidance of a Data Analysis Group (DAG) following methodologies approved by the Management Board (4).
For the evaluation of the Round Robin Test (RRT) results, the Management Board of the PISC programme approved a strategy based on two independent rejection criteria which are illustrated in Table 2. Criterion 1 considers the defect classification (volumetric flaw or crack) and the depth measurement of the volumetric flaws. Criterion 2 considers in addition the crack length. In the present exercise, owing to the size distribution of flaws, this reference length was fixed at 13 mm.

Table 2a: Acceptance-Rejection Criterion 1(A)

Table 2b: Acceptance-Rejection Criterion 2(B)

Evaluation of results at the level of procedures

Inspection procedures were based on eddy current only or on ultrasonics only or on both groups of techniques. Figure 1 shows, for all teams, the Flaw Detection Frequency for the rejectable flaws, i.e. the capability of the procedures to detect the rejectable flaws. Rejectable are all planar flaws (stress corrosion cracks and narrow slots) and all volumetric flaws deeper than 40% of the wall thickness (rejection criterion 1). This diagram provides one measure of capability for the various teams' procedures.
However, it is important to note that a number of teams did not inspect all the boxes. Whilst the UT only teams did well on the tubes they did inspect, it must be borne in mind that they did not inspect the boxes containing some of the most difficult flaws. Consideration of only the results from teams that inspected all the tubes in all the boxes provides a more homogeneous data sample for the ranking of procedure capability. In this case the average flaw detection frequency for all flaws is about 70% (Figure 2).

Fig 1: Flaw Detection Frequency for all rejectable flaws considering all teams that submitted a full procedure report

Fig 2: Flaw Detection Frequency for all flaws considering all teams inspecting all boxes

Evaluation at the level of the flaws

Fig 3: Flaw Detection Probability for all flaws considering all teams.

Fig 4: Flaw Detection Probability for all axial flaws considering all teams

Flaw Detection Probability (FDP) as a function of flaw size in depth (Figure 3) shows a usual trend. The depth of 40% of the wall thickness appears to represent a turning point in FDP: defects of greater depth are generally reasonably well detected.
Figure 4 shows the FDP for the axial flaws located in the straight portions of the tubes. There is good correspondence between the FDP of axial narrow slots (ANS) and axial stress corrosion cracks (ASCC). Thus, single ANSs are good enough for a first evaluation of detection performance of ET techniques on axial cracks. The evaluation of the detection results of the volumetric flaws indicates that the "Volumetric IGA" are "difficult" flaws compared to wastage, wear and pitting, which were more easily detected.
UT appears to do better than ET procedures for axial flaws. The difference between the two procedures is more noticeable for ANSs smaller than 40% T in depth. UT has good detection capability for pitting and wastage. However most wear flaws were not inspected with UT because they were located in the U bends. IGA as existing in this exercise was very difficult to be detected by UT.
The flaw classification was used in the evaluation only as a switch between crack and non crack declaration. Many teams did not classify the flaws at all. In the cases, where axial flaws were classified, 80 % of them were classified correctly.

Evaluation at the level of techniques

About fifty individual techniques results were considered in the exercise. Such techniques were grouped into categories: ET/Bobbin coils (BP), ET rotating coil (RP) / array coil techniques (AP), and UT Probes.
ET rotating pancake coils, on average, perform better than bobbin coils for ASCC and ANS (Figures 5), but they scan the tubes more slowly. In general, the best combination of techniques appears to include BP plus RP/Multifrequency with or without UT, as far as the defect matrix of this exercise is concerned.

Fig 5a: the Eddy Current BP technique	Fig 5b: the Eddy Current RP or the AP technique.
Fig 5: Flaw Detection Frequency for all axial narrow slots (ANS) considering teams using A and B

Sizing performance

In this PISC exercise, multiple axial stress corrosion cracks or combinations of axial and circumferential crack simulations are considered as one single defective area by the BTB code (4). Some teams were able to identify and report several of the individual cracks (but not all) of a complex cracked area. Depth sizing, when considering the volumetric flaws, was characterised by large dispersion except for wear. No Wastage was depth sized. For length sizing of axial flaws (both ASCC and ANS), this exercise demonstrates possible capability of techniques and procedures, if the flaws are on the inside surface (ID) or external (OD) but deeper than 40%T. Eddy Current RP techniques showed capability to size the artificial ANS flaws but undersized the Axial Stress Corrosion Cracks present in this exercise.
Ultrasonic techniques performed reliable length sizing of the ANS flaws in all cases. Axial Stress Corrosion Cracks are also, as in the case of eddy current RP techniques, undersized by UT techniques. Volumetric flaws or flawed areas were poorly sized in the axial extent.

Sentencing performance considering the rejection criteria

Figure 6 compares the sentencing performance of each team when using criterion 1 and criterion 2 as described in table 2. The Correct Acceptance Frequency (CAF) is about 80% for criteria 1 and 2, while the Correct Rejection Frequency (CRF) is less than 70% for criterion 1 and less than 60% for criterion 2 with large difference of performances between similar procedures.
The Correct Rejection Frequency (CRF) for all flaws and all participating teams using criterion 1 shows that the ISI has less capability for correct sentencing than for detection (Figure 4). The same remark is valid for all ASCC and non-combined ANS.
Eddy Current rotating pancake coils perform better in terms of CRP than bobbin coils for axial flaws (ASCC and non-combined ANS).

Fig 6a: Criterion 1	Fig 6b: Criterion 2 with a reference length for rejection of 13mm.
Fig 6: Correct sentencing of all flaws considering all teams using Note: Teams IC and LC made limited sizing (i.e. depth only or length only)

Conclusions

Limitations
During the PISC III Action 5 the teams and their analysts were not guided by prior knowledge of actual inspection results. The teams received three training tube boxes prior undertaking the blind test box trials. Similarly, the evaluation of results was not guided by the expertise gained on examining pulled tubes. However, the training boxes contained a detailed report describing each flaw in turn. The procedures used for the PISC III trials are not necessarily related to the ones used in-service. The combinations of techniques may be different and new inspection techniques were introduced into the trials. There was a wide variation in experience in teams that participated in PISC III. The working environment during the trials was ideal compared with the more difficult task inspection teams are facing during actual ISI. The effect of radiation fields, time constraints and difficult access were not simulated.

Overall Inspection Capability of the Steam Generator Tubes
There were several procedures which demonstrated good detection capability of major flaws in typical locations of the steam generator. For procedures based on ET, the detection of axial flaws either internal or external and deeper than 40% of the wall thickness was often good. This capability of detection fell markedly for external flaws with a depth less than 40%T of the wall thickness. In general detection was not effective at the notification level of 20% of the wall thickness. For procedures based on UT only, detection was very good for axial flaws deeper than 20% of the wall thickness. Classification of flaws was often not fully reported by the teams. In several cases teams demonstrated their ability to identify the axial flaws. Overall, the flaw sizing results show large dispersion. However for some types and sizes of flaws this dispersion was reduced; e.g. the length sizing of axial cracking which is internal (ID) or deeper than 40% of the wall thickness. There was a wide dispersion of detection and sizing performance from apparently similar procedures and even from similar techniques.

Demonstration of Capability
Conclusions of the exercise indicate that capability demonstration is necessary to qualify in service inspection procedures for steam generator tubes. A simple and probably first level of demonstration of capability could be based on simple flaw simulations demonstrated to be difficult or selective enough to allow a selection or ranking of the inspection procedures or techniques. The logic proposed as a result of the PISC experience could thus be a thorough capability demonstration on narrow slots (designed and parameterized to represent specific difficulties), and other well studied artificial flaws, followed by a very specific demonstration of performance, after adaptation on the precise flaws or damages of the specific steam generator to be inspected.

Remark and Acknowledgment

This document summarizes the PISC III report No 41, full description of the PISC III Action 5 on Steam Generator Tubes Inspection, containing all details and final conclusions, which has been approved by the PISC III Management Board.
This report has been prepared by the Reference Laboratory of PISC under the guidance and with continuous contribution of the members of the Data Analysis Group (DAG) of this PISC III Action 5 whose members C. Birac (Leader Action 5) and R. Comby
(chairman DAG 5), EDF, France; K Bowker, Magnox Electric, UK; A. Garcia Bueno, Tecnatom, Spain; K. Ketelaar, Hoogovens, Netherlands; A. Lipponen, VTT, Finland; G. Maciga and G.L. Zanella, ENEL, Italy; R. Meier, Siemens, Germany; K. Sawaragi and S. Tanioka, Mitsubishi, Japan; M. Bieth, S. Crutzen, J.L. Monjaret and J. Perez Prat, JRC Petten, European Commission. The authors would also like to acknowledge the many contributions and the helpful comments provided to them by the PISC Management Board.

References

• Crutzen S., Jehenson P., Nichols R.W., McDonald N., From Capability Evaluation to Reliability Assessment: A Review of the PISC Projects, Proceedings of the 4th European Conference on Non-Destructive Testing, London, September 1987.
• Birac C., Final Report in the Inquiry for the Preparation of the Programme of the PISC III Action: Steam Generator Tubes Testing (SGT), PISCDOC (87) 2.
• Birac C., Herkenrath H., Miyake Y., Maciga G., The Steam Generator Programme of PISC III, Proceedings of the 10th International Conference on NDE in the Nuclear and Pressure Vessel Industries, Glasgow, June 11-14, 1990.
• Rules of PISC Results Evaluation, PISC III report Nr. 21, EUR Report 15559EN, 1993.

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