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TECHNICAL JUSTIFICATION IN ACCORDANCE WITH EUROPEAN ENIQ

RECOMMENDED PRACTICE NO. 2 FOR THE ULTRASONIC EXAMINATION OF

DISSIMILAR WELDS AT SURGE LINE NOZZLES

A. Krüger, TÜV NORD EnSys Hannover, Germany B. Rückelt, intelligeNDT, AREVA NDE-Solutions,

Germany

INTRODUCTION

Due to German safety standards defined by the Nuclear Safety Commission (KTA) dissimilar welds in the

primary coolant circuit have to be inspected recurrently. The German safety standard KTA 3201.4 [1] de-

mands qualification of the inspection techniques in accordance with the German ENIQ-Guideline R516

[2]. An important part of an ENIQ-Qualification is the technical justification. With the intention to demon-

strate the advantages of technical justifications in this paper, an example is given for the examination of a

dissimilar weld.

Dissimilar welds are characterized by anisotropic structure of the weld seam leading to a significant

degradation of the ultrasonic signal with strong attenuation, backscattered signal and skewing of the beam.

In this conjunction the technical justification plays a very important role in choosing the suitable inspec-

tion technique and reducing time and effort in search of an optimized technique.

This contribution presents the main parts of the technical justification under consideration of the

German way of qualification of ultrasonic inspection techniques and with point of view of an independent

expert organisation / qualification body. The influential and essential parameters are discussed and the

choice of value for the main parameters such as probe-type, wave-mode, frequencies, angle of incidence,

crystal-size, focal zone, axial and spatial resolution is justified. Experimental results are presented to ex-

plain the choice of the parameters and some instructions for a practical trial on a reference block are given.

MAIN ELEMENTS OF TECHNICAL JUSTIFICATIONS

The ENIQ methodology document defines a technical justification (TJ) as “an assembling convincing evi-

dence on the effectiveness of the test including previous experience of its application, experimental stud-

ies, mathematical modeling, physical reasoning” [3].

The elements of a technical justification are in accordance with German ENIQ-Guideline R516 [2]

and the European ENIQ Recommended Practice No. 2 [3]:

Section 1: Introduction

Section 2: Summary of Relevant Input Information

Component data

Defects

ISI objectives / required inspection performance

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Inspection method

Section 3: Overview of Inspection System

Section 4: Analysis of Influential Parameters

Input parameters

Procedure parameters

Equipment parameters

Section 5: Physical Reasoning (Qualitative Assessment)

Section 6: Prediction by Modelling

(Quantitative Assessment)

Section 7: Experimental Evidence

Results from other qualifications

Results from trials

Experimental studies

Field experiences

Section 8: Parametric Studies

Section 9: Equipment, Data Analysis and Personnel Requirements

Section 10: Review of Evidence Presented

Section 11: Input on Test Pieces for Experimental Trials

Section 12: Conclusions and Recommendations

Section 13: References

The main elements of the TJ are accentuated in bold letters. The development of the main elements

of a new TJ will be shown in the example that follows.

SUMMARY OF RELEVANT INPUT INFORMATION AND INSPECTION TARGET

The target of this TJ is the inspection of the inner surface of the dissimilar weld between surge line and

nozzle at the main coolant pipe of German pressure water reactors from the outside (Figure 1). The inspec-

tion technique chosen is an automated ultrasonic inspection system with phased array transducers.

The input data is all related information such as the information regarding the area or component to

inspect, type and features of expected defects, and qualification objectives. The component and defect in-

fluential variables, which are the subject of our analysis, are summarised in the Tables 1 and 2.

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Figure 1: Surge line nozzle, object to be inspected

The qualification defect is given by Safety Standard KTA 3201.4 (2010-11). The type of the defect

is a sparked notch with size 20 x 3 mm (length x depth) in base material, weld / buttering and cladding,

figure 2. The weld is to be examined from both sides. For detecting of transverse defects the qualification

defect shall be positioned in the weld metal and the buttering. Where the width of the weld metal (includ-

ing the buttering) is less than 20 mm, the notch length shall be limited to the width of the weld metal (in-

cluding the buttering) on the inner surface.

Table 1 – Component data Table 2 – Defect description

Nozzle OD 440 mm Orientation Circumferential and

transverse

Thickness 46 mm Type Planar, postulated

Width of welding +

buttering

about 27 mm Position Inner surface, surface

breaking

cladding 5 mm Tilt ±10 mm

Material of fitting and

cladding

Austenite 1.4550 Qualification defect 20 x 3 mm (l x d) in

base material, weld/

buttering and cladding

Material of nozzle 20 MnMoNi 55

Flatness deviation < 0,5 mm

The demanded recording level is the echo height of the qualification defect (reference reflector) de-

pending on the position plus a sensitivity allowance of 6 dB. The criteria for acceptance of the inspection

technique are stated in Safety Standard KTA 3201.4 section 4.2.3.3.3.

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Figure 2: Scanning directions

ANALYSIS OF INFLUENTIAL PARAMETERS AND IDENTIFICATION OF ESSENTIAL

PARAMETERS (with range of value and tolerances)

The influential parameters are all related parameters affecting the result of the inspection. The influential

parameters are divided into three groups:

Input parameters

Procedure parameters

Equipment parameters

“Those influential parameters whose change in value would actually affect a particular inspection in

such a way that the inspection could no longer meet its defined objectives are defined as the essential pa-

rameters.” [4]

Input data

All input data, already presented in previous section, is essential because input data determines the objec-

tive of the inspection to be qualified. Remarkable changes of the input data would affect the task in such a

way that the proposed UT inspection technique no longer fulfills this task. However, tolerances of the in-

put data may be stated: e.g.

outer diameter: 440mm ± 10%

wall thickness: 46mm ± 10%,

thickness of buttering: 5 mm ± 20%,

temperature of the component: 30°C +20°/-20° etc.

Procedure parameter

The main influential and essential variables of the UT inspection technique, which are subject of our anal-

ysis, are summarised in Table 3. There are some more procedure parameters to be analyzed (e.g. scanning

pattern and scanning speed) but in this paper we reduce the treatment to the main group of UT inspection

parameters. The physical reasoning for the choice of the parameters and their values are explained in the

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following chapter.

Table 3: Influential and essential procedure parameters

Equipment parameters

The equipment parameters are not achieved in this paper. Further information is given in Appendix 2 of

ENIQ Recommended Practice No. 1 [4].

PHYSICAL REASONING (Qualitative Assessment) AND EXPERIMENTAL EVIDENCES

Probe type

The phased array technique and twin transducer probe influences the inspection essentially.

By phased array technique

angle of incidence could be optimized electronically with respect to the welding geometry

(weld-edge preparation),

a number of angles are generated simultaneously best angle is selected at evaluation,

sound flied could be focused electronically to realise focal point at required depth.

A twin-probe reduces

influence of transmitter pulse,

influence of backscattered signals because of separation between scanning direction of transmit-

ter and scanning direction of receiver.

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Frequency

Dissimilar welds are characterized by anisotropic structure of the weld seam leading to a significant deg-

radation of the ultrasonic signal with strong attenuation, deflection and backscattered signal from weld

filler metal and buttering. This influence increases with decreasing wave length or increasing frequency.

Therefore frequency should be as low as possible with respect to sufficient defect detection (detection of

the qualification defect). Figure 3 shows for 1 MHz shear wave in comparison to 2 MHz shear wave a

much more clear C-scan picture of the defects (named “Nut1 – Nut7”). The red colour in the pictures cor-

responds to reflections or backscattering with high amplitude. The colour blue corresponds to low ampli-

tudes. A good compromise is: 1 MHz for shear wave

Figure 3: Selection of frequency

Bandwidth

High resolution (separation of nearby defects) demands short pulse length ( broad bandwidth). A good

signal-noise-ratio (detection of small defects) demands high strength of the pulse ( small bandwidth). A

good compromise is: Bandwidth 30% - 40%.

Angle of incidence:

The sound path through the structure of the weld / buttering should be as short as possible in order to re-

duce disturbance and deflection of sound propagation. Therefore an angle of 50°-56° for shear waves as

preferably flat angle of incidence (see red arrow in figure 4) was chosen. For longitudinal wave mode an

angle of 44°-48° and 60° delivers the best results.

In Figure 4, the C-scan pictures with an angle of incidence of 44°-48° (left side) and 50°- 56° (right

side) are presented. 13 indications from defects at different positions in the weld are clearly recognisable

on the right hand side. At an angle of 44°-48° only 5 indications are clearly recognisable. Red, dark blue

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and black colour in the pictures corresponds to reflections with high amplitude.

Figure 4: Selection of angle of incidence

Essential parameter wave mode

For detection and analysis of defects in the weld seam two wave modes shall be used:

Shear waves show a good signal-noise-ratio and an amplitude dynamic depending on defect depth

in the base material and in the region near-by to the weld metal (adjacent base material), figure 5 upper

blue curve. This effect of amplitude dependence on defect depth is used for defect sizing. Shear waves are

less suitable for defect assessment in the weld metal and buttering due to the bad sound transmission

through anisotropic, coarse-grained materials (austenitic weld metal, buttering and cladding).

It is known from other experiences that longitudinal waves are better suited for sound transmission

through anisotropic, coarse-grained materials, but longitudinal waves possess no amplitude dynamic de-

pending on defect depth, figure 5 yellow curve and figure 6. The reflections from all the defects in the test

block are nearly of the same echo height, figure 6. But longitudinal waves produce diffraction/bending ef-

fects at defect edges (tip echo). In figure 7, the tip echo could clearly be distinguished from the corner

echo. The distance between these two echos is used for defect depth sizing, figure 7. Thus longitudinal

waves are used for defect sizing of defects in the weld metal and buttering.

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Figure 5: Signal amplitude as function of the defect depth

Figure 6: Longitudinal wave for defect detection in the weld metal and buttering

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Figure 7: Defect depth description by tip echo

Essential parameter crystal size

Transducers are selected large enough, so that near-field length is equal or greater than sound path to the

area to be inspected. Thus the sound path could be electronically focused (phased array) to the location to

be inspected. As a consequence, the lateral resolution is increased with simultaneously high sound pres-

sure at location to be inspected (by large transducers).

Essential parameter focal point

The sound field of the phased-array probes is focused electronically to realise focal point at required depth

or sound path length. The sound path to the inner surface at an angle of incidence of 52° amounts to 74

mm. Thus the focal length of the shear wave probe is adjusted to 70 mm.

Influential parameter delay path:

Delay path is affected by the size of the crystal and the angles of incidence to be realised. The dimensions

of the refracting wedge should be as small as possible to reduce the size of the probe housing as much as

possible. The realised values are specified in Tab. 3.

Essential parameter scanning direction

Scanning directions have to be chosen in such a way that postulated defects are struck nearly perpendicu-

lar. This is realised for circumferential defects by axial intromission of sound and for transverse defects by

circumferential intromission of sound.

Influential parameter probe size

The probe housing has to be adapted to the size of the crystal, refracting wedge and transducer backing.

The size of the housing is constructed as small as possible in order to minimise the coupling area. The re-

depth

tip echo

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alised values are specified in Tab. 3.

INPUT ON THE TEST PIECES FOR EXPERIMENTAL TRIALS

A test piece where all the specified parameters could be evaluated is given in Fig. 8. The related defect po-

sitions are stated below the sketch of the test block.

Figure 8: Test block for experimental trials

CONCLUSIONS AND RECOMMENDATIONS

Specialized inspection parameters (frequency, angle of incidence, focal point etc.) with longitudinal and

shear waves are proposed as inspection techniques for dissimilar welds. The inspection techniques deter-

mined in this paper for the respective inspection zones of the dissimilar weld provide a reliable inspection

result concerning the detection of defects. Concerning the depth sizing of possible indications the use of

shear waves in the adjacent base material and longitudinal waves combined with crack tip echo techniques

in the weld area should allow sufficiently exact statements. By using a TJ it was possible to increase the

inspection reliability by compiling all the supporting evidence available. The TJ minimises the reliance on

test pieces and it also allows generalizing specific test piece data very effectively.

REFERENCES

1) Safety Standards of the Nuclear Safety Commission (KTA)

KTA 3201.4 (2010-11); Components of the Reactor Coolant Pressure Boundary of Light Water

Reactors Part 4: In-service Inspections and Operational Monitoring www.kta-gs.de

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2) VGB PowerTech Guideline Methodik für das Vorgehen bei der Qualifizierung von zerstörungs-

freien Prüfungen VGB-R 516, 2010

3) ENIQ Recommended practice 2: Strategy and Recommended Content for Technical Justifications,

Issue 2, ENIQ report nr. 39, EUR 24111 EN, 2010.

4) ENIQ Recommended practice 1: Influential / Essential Parameters, Issue 2, ENIQ report nr. 24,

EUR 21751 EN, 2005.