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MODELING OF STRESS CORROSION CRACKS FOR THE UT EXAMINATION OF

REACTOR VESSEL CLEVIS OF FRENCH 900 MWE AND 1300 MWE PWR

Pierre Peureux, Bertrand Chassignole, EDF, France

ABSTRACT According to the RSE-M code dedicated to In-Service Inspection (ISI), the reactor vessel clevis of the

French 900 MWe and 1300 MWe Pressurized Water Reactor (PWR) are inspected at each ten years

outage by Visual Testing (VT). Following to a new French Safety Authority request, additional UT

examination techniques is required in order to detect and characterize potential Stress Corrosion Cracks

(SCC) located in the clevis alloy 182 welds between the guide block (Alloy 600) and the buttering pad

(Alloy 182) during the 3rd ten years outage. The UT technique has been evaluated on notches.

In order to improve the knowledge of the behavior of stress corrosion crack regarding the UT

examination, we define a design for such a crack which is implemented in modeling software. This design

is based on metallurgical analyses of the alloy 182 weld in a full scale mock up.

Modeling software allows us to compare our SCC design with EDM. Modeling results are also

compared with experimental ones obtained on the full scale mock up containing emerging notches. These

results confirm the ability of our process to detect stress corrosion cracks.

INTRODUCTION

Context

According to the RSE-M code dedicated to In-Service Inspection (ISI), the internal structure radial guides

of Reactor Vessel of the French 900 MWe and 1300 MWe Pressurized Water Reactor (PWR) are

inspected at each ten years outage by Visual Testing (VT). The objective of this automated and under

water VT examination is to check for any mechanical disorder.

Some components of French Pressurized Water Reactor (PWR) are realized in Alloy 600 and

generally welded with nickel-based Alloy 82 or 182 (depending of the welding process), such as the

reactor vessel clevis. International feedback shows that such components are potentially sensitive to Stress

Corrosion Cracking (SCC). Consequently those components are under consideration in EDF maintenance

strategy. The main goal is the prevention of unavailability with the early detection of SCC defects.

Following to a new French Safety Authority request, additional UT examination techniques have to

be developed and implemented during the 3rd

ten years outage in order to detect and characterize potential

Stress Corrosion Cracks (SCC) located in the radial guide welds in alloy 182 between the guide block

(Alloy 600) and the buttering pad (Alloy 182).

A noxiousness study led to define three types of defects (Figure 1), each one has been approximated

by an emerging notch in the technical justifications of the UT process. UT examination technique is

employed to detect and characterize defects of type 1 and type 2. However, due to the restricted

accessibility of the examination area, VT examination technique is used to detect defect of type 3.

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Figure 1 : From left to right: defect type 1, 2 and 3 (red)

This paper is focused on the UT examination which is dedicated to the detection of defects type 1

and type 2. Those defects are surface breaking defects due to the PWSCC (Primary Water Stress

Corrosion Cracking) phenomenon.

The first one, type 1 defect, is emerging in the spacing, precisely in the weld root. Its orientation is

circumferential and the crack orientation path is vertically oriented. The second one, type 2 defect, is

emerging in the middle of the weld radius. Its orientation is circumferential too and the crack orientation

path has a tilt angle from 0° to 90° in comparison to horizontal plane.

Objectives

In order to accurately predict the beam to defect interaction in modeling codes, the purpose of this study is

to define a realistic design of a SCC crack. This design is dedicated to the ultrasonic examination of

reactor vessel clevis. During the year 2011, the UT examination has been qualified according to the

French requirement. The corresponding technical justifications are based upon notches.

The main purpose is to identify the crack orientation and morphology with respect to the general

texture of the macrostructure of the weld. In our case, the weld is in Alloy 182 and exhibits an anisotropic

and heterogeneous structure [1]. After that, the cracks are implemented in modeling codes (Civa software,

Finite Elements Athena 2D code) in order to evaluate the performances of UT examination.

CONSTRUCTION OF THE CRACKS

The step by step procedure

The purpose of the first step is to analyse the Alloy 182 weld in order to describe it with a set of

anisotropic and homogeneous media characterized by a specific grain orientation. For each medium,

Voronoï tessellations are used to model the weld at the grain scale. This is the second step. The average

grain size is still the same in each medium. The last step is the estimation of the crack orientation path

using a probabilistic method.

Analysis of the weld in Alloy 182

The weld in Alloy 182 has been characterized from metallurgical analysis which leads to obtain a mapping

of the grain orientations (Figure 2). Then, the weld is divided in a set of anisotropic and homogeneous

Alloy 600

Weld in alloy

182

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media, each one characterized by a specific grain orientation. The average size of the columnar grains is as

well estimated from metallurgical analysis:

- Average length: 3 mm

- Average width: 0.3 mm

Figure 2 : From left to right: metallurgical analysis of the weld; mapping of the grains orientation;

description of the weld in a set of anisotropic and homogeneous media

Decomposition of the weld

Two weld areas are under consideration to define type 1 and type 2 defects (see Figure 1).. Knowing the

average size and the orientation of the columnar grains in the weld, we have the input parameters to map

each medium at the grain scale with a Voronoï diagram.

A Voronoï diagram is a special kind of decomposition of a given space, e.g., a metric space

represented by each anisotropic medium in our study, determined by distances to a specified family of

objects (subsets) in the space (Figure 3). These objects are usually called the sites or the generators and to

each site is associated to a corresponding Voronoï cell, namely the set of all points in the given space

whose distance to the given object is not greater than their distance to the other objects.

Figure 3 : examples of Voronoï diagrams, using Euclidian distance (left) and Manhattan

distance (right)

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Then, a small area near the spacing potentially sensitive to SCC is divided in a finite number of

media defined in the previous paragraph, In the case of type 1defect, the result is illustrated in Figure 4.

The image on the left shows the region of interest, i.e. the area in which the defect type 1 is defined.. Each

medium is approximated by a rectangular domain and remeshed thanks to Voronoï tessellations (right-

hand image)

Figure 4 : left : region of interest for type 1 defect and zoom ;right : mapping with Voronoï

diagrams

Estimation of the crack orientation path

Assuming that the crack orientation path is intergranular, the crack propagates along the grain boundaries

defined by the grain-scale model. We make several assumptions for the propagation as follows:

- The crack cannot go back, it goes only forward,

- At each nod, each direction of propagation is equally probable,

- The first point is randomly chosen, in the spacing for type 1 defect and approximately in the

middle of the radius for type 2 defect.

Figure 5 : An example of crack propagation through a Voronoï Diagram (from left to right)

Figure 5 shows an example of propagation through a Voronoï diagram such as described above.

First of all the initial point is chosen randomly. On the left-hand and on the middle pictures, two directions

of propagation are equally probable at each node represented by a red triangle. On the right-hand image of

Figure 5, there is only one possibility as the crack cannot go back.

In the case of type 1defect, the results are illustrated on Figure 6. After repeating several times the

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process of propagation, the results can be filed in three classes. In each group the cracks have

approximately the same shape. First of all, the three families have the same orientation, that is to say the

cracks are globally vertically oriented, at least for the first 15 mm. Moreover, each family presents

irregular facets. At last, the difference depends on the initial point which leads to a sharp change in the

orientation when leaving the weld root area.

Figure 6 : three kinds of shape for defect type 1

In the case of type 2defect, the results are presented on Figure 7. As for type 1defect, the process of

propagation is repeated several times and after analyzing the results we obtain two crack designs with the

same global orientation. The number of facets differentiates the two families. On the right- hand image of

Figure 7, the crack exhibits a higher roughness.

Figure 7 : two kinds of shape for type 2 defect

The estimation of the crack orientation path for the two kinds of defects is in accordance with the

noxiousness study which led to define them.

EVALUATION OF PERFORMANCES OF UT EXAMINATION

The UT examination for type 1 & type 2 defects

The principle of the UT examination is based on four types of focused probes used in immersion

technique, with various incidences (0°, 15°, 30° and 45°) in order to detect and characterize defects with

various orientations. The UT probes are implemented on the VIROLE tool of the “MIS” (Machine

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d’Inspection en Service, in-service inspection machine).

The inspection is conducted with 1.5MHz longitudinal waves. The UT examination is performed

from the vertical face of the radial guide (see Figure 8 on the right hand side) by Areva/Intercontrôle.

Figure 8 : The UT examination: the manipulator (left), the UT probes implemented on the

Virole tool (center) and the Virole tool in front of a reactor vessel clevis (right)

Modelling results

In technical justifications, type 1 and type 2 defects are approximated by notches with the same

characteristics in terms of orientation, localization and so on. In order to improve the knowledge of the

impact of SCC morphology regarding the UT examination, numerical studies are performed. In particular,

a comparison between the results obtained for a notch and for a rough crack is proposed. We mainly focus

in this paper on type 1 defect.

Two softwares are used in this study: CIVA 10.0 developed by CEA and ATHENA 2D developed

by EDF R&D. The first one is a 3D modeling software relying on semi-analytical methods [2]. In our

study, CIVA allows computing a 3D reactor vessel clevis but the alloy 182 weld is not taken into account.

ATHENA is a Finite Elements code which fully solves the propagation equations inside a meshed

configuration, especially in complex mediums such as anisotropic and heterogeneous austenitic welds

[3;4]. Nevertheless, the current version of ATHENA is limited to 2D configurations. Both codes include

beam propagation and flaw scattering for various probes and various flaw types.

Results with Civa software

The reference is the amplitude of a 2 mm side drilled hole at 190 mm depth.

Figure 9 presents modelling results obtained with the CIVA software for the type 1 defect. In this

case, the UT examination is conducted from the front vertical face using the probe with 1.5 MHz

longitudinal waves at normal incidence. The amplitude of the specular echo is analyzed, both for the notch

and for the rough crack. CIVA predicts that the amplitude is 2 dB lower for the complex crack

morphology.

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Figure 9 : Examples of results for type 1 defect using Civa

Figure 10 : results using 1.5 MHz L15 transducer

Results for longitudinal waves with a 15° incidence are presented on Figure 10. For the notch

configuration, corner and diffraction echoes are clearly visible on each flaw tip. In case of the rough defect

(right-hand image), an additional echo appears and the amplitude of the diffraction echo significantly

decreases.

Figure 11 : Impact of the disorientation between beam and defect for a notch (red) and for a

rough crack (blue and green)

Figure 11 shows the impact of the misorientation between the beam and the defect for two types of

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type 1 defect. In both examples, the two curves are almost similar even if the amplitude decreases slowly

for the positive angles in case of the type 1defect with irregular facets presented on the left hand side of

Figure 11 and it is the contrary on the right hand side. Moreover, on the right hand side, the amplitude

increases slowly for angles greater than 10° due to the specific configuration of the irregular facets.

Results with Athena 2D code

The previous configurations are then simulated with the ATHENA 2D code. This time, the weld structure

is taken into account and the weld modelling corresponds to the left-hand image of Figure 4. A

comparison between CIVA and ATHENA results is proposed in Table 1. The amplitude of the specular

echo is around 2 dB lower with ATHENA calculations. This weak attenuation is mainly imputed to the

influence of the alloy 182 weld.

Orientation Amplitude with

Civa

Amplitude with

Athena

Attenuation due to the

weld

-10° 9.5 dB 8.5 dB 0.9 dB

0° 12.2 dB 10.5 dB 1.8 dB

10° 3.7 dB 1.4 dB 2.3 dB

Table 1 : Values of the amplitude of the specular echo for type 1 defect: Comparison between

Civa and Athena results

Figure 12 : Example of beam propagation computed with ATHENA 2D

CONCLUSIONS AND PERSPECTIVES This paper presents a first simplified approach to define a design for a stress corrosion crack propagating

in the weld of a reactor vessel clevis. The method is based upon metallurgical analyses of the weld which

gives the average grain size and their orientation.

With those input data and mathematical modelling of the weld at the grain scale (Voronoï

tessellations), the crack orientation path is estimated using a probabilistic propagation along the grain

boundaries. Then, using modelling codes dedicated to ultrasonic propagation, the influence of the SCC

morphology on the UT examination performances can be estimated. The results in terms of defect

detection are in agreement with the technical justifications based upon notches.

A way to go further with this study is to evaluate the impact on the defect characterization,

especially for the type 2 defect. In this case a specific attention will be devoted to the prediction of the

weld influence in modelling codes because of the defect location involving a significant propagation in the

anisotropic and heterogeneous structure.

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REFERENCES

1) B. Chassignole, O. Dupond and L. Doudet, “Ultrasonic and Metallurgical examination of

2) an alloy 182 welding mold”, 7th ICNDE, 2009, pp. 228-235.

3) S. Mahaut, M. Darmon, S. Chatillon, F. Jenson, P. Calmon, “Recent advances and current trends of

ultrasonic modelling in CIVA”, Insight 51 (2), pp 78-81, 2009.

4) E. Becache, P. Joly, C. Tsogka, SIAM J Numer Anal 37, 1053-1084 (2000).

5) B. Chassignole, V. Duwig, M.A Ploix, P. Guy and R. El Guerjouma, Ultrasonics 49, 653-

6) 658 (2009).

7) A Clemençon, P Peureux, B Chassignole, O Dupond, Y. Bouveret, “New ultrasonic inspection

technique for the examination of reactor vessel clevis of French 900 MWe and 1300 MWe PWR”,

International Pressurized Water Reactor Conference and Exhibition EPRI 2010.