E-Journal of Advanced Maintenance Vol.9-3 (2017) 173-184
Japan Society of Maintenology

Development of Ultrasonic Inspection for Main Coolant Line Welds in EPR
Nuclear Power Plant
Huaidong CHEN1,*, Weiqiang WANG1, Guanbing MA1, Lipeng ZHOU1, and Ming LI1
1

CGN Inspection Technology Company. 191 Yang Pu Road, SIP, Suzhou 215021, P.R.China

ABSTRACTThe main coolant line (MCL) is a part of the primary coolant boundary and it is the key factor for the reactor
safety. In order to guarantee the integrity of the primary circuit, full volumetric ultrasonic examination isrequired for the MCL welds of the European Pressurized Reactor (EPR) according to the RSE-M code 2010
Version. The difficulties for application of the ultrasonic inspection on the MCL welds are mainly caused bylimited access space and coarse austenitic grain structure. In this study, inspection system including automatedscanner and transmitter/receiver longitudinal wave technique has been developed in consideration of themultiple constraints. The qualification process of the ultrasonic examination on the test mock-ups including flaw
sizing results is also presented and commented.
KEYWORDS

ARTICLE INFORMATIONEPR; Main coolant line; Ultrasonic inspection; Austenitic stainless steel

Article history:
Received 7 November 2016
Accepted 29 May 2017

1.

Introduction

The European Pressurized Reactor (EPR) is a third generation pressurized water reactor (PWR),
developed based on the experience from the French N4 reactor and German Konvoi reactor [1,2]. The
main coolant line (MCL) is a part of the reactor coolant pressure boundary, it takes coolant from thereactor pressure vessel to the steam generator, and then provides cold coolant back to the vessel
through a reactor coolant pump. Different from the cast austenitic stainless steel main coolant pipingof the Chinese existing PWR, the pipes of the EPR plant are forged by austenitic stainless steel, andan automatic narrow gap gas tungsten arc welding (GTAW) has been implemented for the butt
welding with ER316L filler metal. The MCL in the EPR plant consists of four loops, each loop hasnine homogenous welds and two dissimilar metal welds.
 homogenous welds between primary pipes homogenous welds between primary pipe and pump dissimilar metal welds between primary pipe and steam generator
 dissimilar metal welds between primary pipe and reactor vessel
Fig. 1 shows the typical locations of MCL welds and a schematic of pipe-to-pipe weld.1*

Corresponding author, E-mail: chenhuaidong@cgnpc.com.cn
ISSN-1883-9894/10 © 2010 – JSM and the authors. All rights reserved.

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Huaidong CHEN, et al.
Development on Ultrasonic Inspection for Main Coolant Line Welds
in EPR Nuclear Power Plant(a)(b)

Fig. 1 Typical locations of MCL welds (a) and a schematic of pipe-to-pipe weld (b)It is very important to evaluate the structural integrity of the main coolant line using highly
reliable non-destructive examinations (NDE). Ultrasonic techniques (UT) are extensively used in theinspection of the main primary system because of their capabilities to detect and size potential
in-depth flaws. According to the RSE-M code 2010 version, automated ultrasonic examination has
been introduced on the MCL welds of the EPR plant as a major NDE method in lieu of radiographic
examination (RT). The volume to be inspected for each weld extends through the full thickness of thematerial and to the fusion faces of the welds. However, there are some constraints which must beregarded for the development of ultrasonic inspection the MCL welds:
 Access limitations. The pipes may be vertical, horizontal or inclined, and some straight pipes
are joined to elbow pieces or nozzles (Fig. 2). Due to the space and location limit, many welds
have some access restrictions, this makes it difficult to design the inspection scanner.
 Various dimensions of MCL pipes. The outer diameter of the MCL pipes range between
932mm to 974mm and the thickness ranges between 76mm to 97mm (see Table 1). To meetthe requirement of the complete weld volume inspection, each weld shall be scanned inaccordance with a dedicated scan plan.
 Accuracy of UT characterization. It is difficult to detect and size defects by ultrasonicinspection due to the coarse grained and anisotropic crystal structure of the weld [3-5]. The
large grains could scatter the sound energy leading to a high degree of attenuation, and often
to beam-distortion.

Fig. 2 Examples of the environmental constraints

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Development on Ultrasonic Inspection for Main Coolant Line Welds
in EPR Nuclear Power Plant
Table 1 Dimensions of the main coolant line

WeldOuter Diameter

Thickness

C2932mm

76mm

C1935mm

77.5mm

U4960mm

90mm

U3938mm

76mm

U2938mm

76mm

U1974mm

97mm

H3974mm

97mm

H2938mm

76mm

H1938mm

76mm

To achieve the inspection requirements, it was needed to develop ultrasonic technology withqualified procedure, scanner and personnel. In this study, a specific manipulator taking intoconsideration the environmental constraints was developed to inspect thick-walled pipes with varing
diameters. A demonstration of the capabilities of the ultrasonic techniques has been done on the test
mock-ups.

2.
2.1.

Inspection System
Description of the scanner design

CGN inspection technology company has developed an inspection system for the MCL welds
according to the structure characteristic and examination requirements. Schematic representation of
the inspection system is shown in Fig. 3. The system consists of scanner, control system, removableguide rail, and couplant recovery system.

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Development on Ultrasonic Inspection for Main Coolant Line Welds
in EPR Nuclear Power Plant

Fig. 3. The inspection system

Fig. 4 shows the structure of the scanner, the scanner is positioned on the pipe to be inspected byusing removable guide rails installed on the pipe. An array of probes mounted in the scanner areplaced on the pipe and the weld area is inspected as the scanner is moved along the weld. The probes
can be moved around and along the external surface of the pipe using either circumferential or axial
raster scan pattern. An encoder will record the probe position in relation to the distance traveled.

Fig. 4. A sketch of the scanner

In order to overcome the axial movement restrictions such as the elbow sections, three different
stroke lengths (220 mm, 380 mm and 460 mm) of the axial movement modules are designed.
Removable guide rail is made up of two semicircle arch supports. It can be easily installed on theouter surface of the different diameter pipes with seven adjustable clamping screws to the pipe (Fig.
5). For some special locations which are difficult to fix, the rails also use several auxiliary fixtures toinstall on the pipe, as shown in Fig. 6.

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Development on Ultrasonic Inspection for Main Coolant Line Welds
in EPR Nuclear Power Plant

Fig. 5. Guide rail(a) Steam generator(b) Reactor coolant pump

Fig. 6. Auxiliary fixtures used for steam generator (a) and reactor coolant pump (b) installation

2.2. Ultrasonic techniques
Due to the weld configuration such as material and thickness, the inspection area shown in Fig. 7
is separated into three areas: the near-surface area A, the middle area B, the bottom area C. To inspect
the whole weld it is necessary to use several focus TRL (Transmitter/Receiver Longitudinal) wave
probes with different beam angles scanned over the surface (Table 2). In addition, a 0° longitudinal
wave probe was used to monitor on the back wall reflection echos.

AB
CFig. 7. Inspection area
Table 2. The parameters of probes for ultrasonic test
Probe

Frequency

Focus sound path

177

Crystal type

Huaidong CHEN, et al.
Development on Ultrasonic Inspection for Main Coolant Line Welds
in EPR Nuclear Power Plant
0°

1.5MHz

10 mm

TRL

60°

1.5MHz20 mm

TRL45°

1.5MHz

40 mm

TRL

45°

1.5MHz

90 mm

TRL

0°

2 MHz/Single crystal

Simulations were utilized for verify the parameters of ultrasonic techniques in relation to thematerial type and thickness. In this study, CIVA software was used to evaluate their ability to detect
defects. The ultrasound simulation in the CIVA software includes two basic modules: Beam field
predictions using the elastic-dynamics pencil method and defect response predictions using SOV,
Kirchhoff and GTD models[6].
Obtained beams of the probes simulated are shown in Fig. 8. Theoretical focal depths asmeasured from these simulations, are 4 mm for the 0° FS10 probe, 10 mm for the 60° FS20 probe, 22mm for the 45° FS45 probe and 58 mm for the 45° FS90 probe. Sensitivity was obtained by setting
the reflection of 2mm side-drilled hole of different focus depths to 0 dB, respectively. Rectangulardefects with 20 mm×5 mm, positioned in different depths, were used to simulate the cracks and thecurves of the ultrasonic responses. The results appear as shown in Fig. 9-12 below:
 The inspection area of 0° FS10 can cover a range from 0 to 35 mm;
 The inspection area of 60° FS20 can cover a range from 0 to 30 mm;
 The inspection area of 45° FS40 can cover a range from 10 to50 mm;
 The inspection area of 45° FS90 can cover a range from 20 to 100 mm.
The results indicated that the probes are able to perform a full coverage examination of theinspection volume, and have sufficient sensitivity to the respective focusing area to meet therequirements of ultrasonic inspection of the austenitic stainless steel weld.

(a) 0° FS10(b) 60° FS20(c) 45° FS40(d) 45° FS90178Huaidong CHEN, et al.
Development on Ultrasonic Inspection for Main Coolant Line Welds
in EPR Nuclear Power Plant
Fig. 8. Sound field simulations

10Amplitude（dB）

88.6

66.8
5.2

4
3.1
2

1.5
0.2

0-1.9-2-41015202530
Defect depths（mm）

3540Fig. 9. Ultrasonic responses (0° FS10)

65.6

5Amplitude （dB）

4.7

43

2.9

2
1

0.9
0.5

010

152025De fe ct de pths （ mm ）17930

Huaidong CHEN, et al.
Development on Ultrasonic Inspection for Main Coolant Line Welds
in EPR Nuclear Power Plant

Fig. 10. Ultrasonic responses for different defect depths (60° FS20)

12Amplitude（dB）

109.7

864.4

4220-1.5-2-410305070

Defect depths（mm）

Fig. 11. Ultrasonic responses for different defect depths (45° FS40)

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Huaidong CHEN, et al.
Development on Ultrasonic Inspection for Main Coolant Line Welds
in EPR Nuclear Power Plant
1210.1

Amplitude （dB）

10886.1

64.5

42

2.4020406080
De fe ct de pths （ mm ）100

Fig. 12. Ultrasonic responses for different defect depths (45° FS90)

3.

Inspection Qualification Progress

3.1

Methodology
The purpose of the qualification is to confirm that an inspection system has the ability to solve its
required examination task. The qualification of the EPR reactor is carried out according to the RSE-M
2010 code. There are 3 types of qualification defined on the base of RSE-M Appendix 4.3:
conventional, general and specific qualification.
 Conventional qualification. No defect is postulated, Evidence justifying the techniqueperformance (detection, localization) to be compiled in a technical file.
 General qualification. Presence of defects is suspected. Evidence justifying the techniqueperformance (detection, localization, characterization) by a combination of technicaljustification and practical tests based on realistic defects.
 Specific qualification. Presence of specific defects has been observed. Justifying evidence(detection, localization, characterization) by a combination of technical justification andpractical tests based on real or similar defects.
Due to the main coolant line weld is one type of safety class 1 area of the main primary
components, the qualification level is “special” type, determined by the China National NuclearSecurity Administration (NNSA). The qualification of the main coolant line also adopted the ENIQ
(European Network for Inspection Qualification) methodology as a complement to the RSE-M
Appendix 4.3. The main steps of qualification are based on: Technical justification which include theoretical and laboratory evidences for theeffectiveness of the inspection, and inspection procedures for the MCL welds examinationmust be qualified.
 Open and blind practical test on full-scale mock-ups, the mock-ups used for the qualification
tests have a representative character in size, geometry and welding technology.
3.2Demonstration of ultrasonic inspection
The qualification was performed with practical trail on an “open test” mock-up and finallyvalidated on a “blind test” mock-up to verify the detection and sizing techniques.
The purpose of the open test is to verify the UT procedure and equipment capabilities. Figure 10
shows the open test mock-up for the MCL welds. The mock-up has the same dimensions and material
with the components to be inspected in the nuclear power plant. Distribution of the flaws wasimplanted throughout the entire weld volume. The flaws were typical of those produced during
welding fabrication, including cracks, inclusion, and porosity.

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Development on Ultrasonic Inspection for Main Coolant Line Welds
in EPR Nuclear Power Plant

Fig. 13. Open test mock-up

Length and depth sizing of defects were performed by further evaluation after the dataacquisition was completed. Length sizing was determined by using the drop-of-signal techniquewhich sets the endpoints where the signal can be discriminated from the background noise. As for theheight sizing, tip diffraction method was used by analysis the signals from the top and bottom edgesof the flaw on the B-scan or D-scan image.
Practical example of defect sizing results on B-scan image was shown in Fig. 14. The results ofthe detected flaws are listed in the Table 3 The ASME BPV Code, Section XI, Appendix VIII [7],requires that the acceptable length sizing root mean square (RMS) is within 19mm and depth sizing iswithin 3mm. The error calculated shown in Table 4 present good enough for the requirement. The
result showed good detection ability and acceptable sizing accuracy for full-scale mock-up. The
inspection system was qualified and has been used on site in the site inspection of plant.

Fig. 14. Example of evaluation layout

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Development on Ultrasonic Inspection for Main Coolant Line Welds
in EPR Nuclear Power Plant
Table 3. Flaw sizing results

No.

Actual value

DefectMeasured value

Length/mm

Height/mm

Length/mm

Height/mm

1

Crack

20

522

5.7

2

Crack

28

729

9.2

3

Crack

21

823

7.4

4

Crack

40

1845

20.2

5

Crack

25

1027

12.1

6

Crack

35

1538

13.8

7

Crack

76

2984

32.1

8

Crack

29

1732

15.2

9Slag inclusion

1010149.2

10Porosity

301033

12.3

Table 4. Root mean square error of defect measurement
Sizing techniqueCalculated RMS error

ASME acceptable RMS error

Flaw length

3.75mm19mm

Flaw depth

1.87mm3mm

Fig. 15. Performance demonstration (a) and scanner installed on pipe at site (b)4. Conclusion
Due to the coarse austenitic grain structure and limited access space, the ultrasonic inspection ofMCL welds of the EPR plant represents a new challenge. In this study, a customized inspection
system was successfully designed to perform automated ultrasonic examination from outside surfaceof the MCL welds. The scanner and the auxiliary fixtures were designed to deal with the difficultaccess conditions. The evaluation and experimental results confirm that used focus TRL techniques

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Development on Ultrasonic Inspection for Main Coolant Line Welds
in EPR Nuclear Power Plant

can cover a full range of inspection volume. Full-scale mock-ups were used to demonstrate detection
and sizing capabilities. Performances of the inspection system had been verified according to theRSE-M code and local regulations.

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