ADVANCED INSPECTION TECHNIQUE FROM INSIDE SURFACE FOR

DISSIMILAR METAL WELD OF STEAM GENERATOR NOZZLE

T. Yamaguchi, T. Shichida, T. Matsuura, Y. Tsuruta, S. Kawanami

Mitsubishi Heavy Industries, Japan

1. Abstract

In 2012, several axial flaws were not detected by Ultrasonic Testing (UT) from outside surface

of nozzle. The flaws were located in Dissimilar Metal Weld (DMW) of an inlet (hot leg) Steam

Generator (SG) nozzle at North Anna Power Station Unit 1. One of the causes of missing flaw

seems to be particular geometry of nozzle outside surface with large taper angle (1).

100% coverage by UT from outside surface was very difficult, because there are some

areas where ultrasonic beam cannot reach. On the other hand, the inspection from inside surface

of nozzle has a merit uninfluenced by the nozzle outside surface geometry. Therefore, we have

developed an advanced inspection technique from nozzle inside.

This technique mainly consists of a new robot to access from inside surface and a new

inspection technique with accurate depth sizing. The robot has high performance movement

characteristic and easy installation structure which make it easy to apply not only UT but also

Eddy Current Testing (ECT) and Visual Testing (VT) to DMW of SG nozzle remotely. This

robot with state-of-art robotic technology has already been applied in the field service in Japan

successfully. Up to date, accuracy in UT depth sizing from defect opening surface had been

insufficient, so we developed both small Transmit-Receive type with L-wave Phased Array

(TRL-PA) UT probe and matrices type with L-wave PA-UT probe. As results, it is verified that

our advanced UT improves defect sizing capability remarkably even for a swallow defect (2) (3).

In this report, we will introduce our new solution from nozzle inside surface and show

experimental results and some field experiences.

2. Background

To prevent the occurrence of PWSCC, Ultrasonic Shot Peening (USP) was applied to SG nozzle

in Japan. Before application of peening, ECT for soundness check was applied. In these

inspections, some axial PWSCCs were detected on the nickel alloy surface (Alloy 600) of SG

nozzle in some plants (shown in Figure 1). After detection of PWSCC, Nuclear and Industrial

Safety Agency (NISA) requested the application of ECT from inside surface for DMW of SG

nozzle once in decade as In-Service Inspection (ISI).

On the other hand, in 2012, 5 Primary Water Stress Corrosion Crackings (PWSCC) were

detected in the DMW in an inlet (hot leg) SG nozzle at North Anna Power Station Unit 1.

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These PWSCCs were not detected by primary UT inspection from outside surface. However

after outside surface machining in preparation for a full structural weld overlay, PWSCCs were

founded by visually. One of the potential causes for the miss of the detection of these PWSCCs

seems to be large taper angle of safe-end configuration (see Figure 2).

100% coverage by UT from outside surface was very difficult, because there are some

areas where ultrasonic beam cannot reach. On the other hand, the inspection from inside surface

of nozzle has a merit uninfluenced by the nozzle outside surface geometry. Therefore, Advanced

inspection technique from inside surface for DMW of SG Nozzle have been developed to fulfill

the ISI request in Japan. This technique is also effective for the case like North Anna Power

Station Unit 1. This technique mainly consists of a new robot to access from inside surface and

a new inspection technique with accurate depth sizing. (see Figure 3).

Figure 1 Location of defect at DMWs of SG

Figure 2 Safe-end configuration of North Anna Power Station Unit 1 SG

Steam Generator

Welding

Inlet Nozzle

Buttering DMW configuration consists of the 11-degree outside-surface-tapered safe-end

Cross-Section view of inlet nozzle (Ref. Pacific Northwest National Laboratory Web site)

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Figure 3 Advanced Robot for DMW of SG nozzle

3. Robot Technique

3.1 Target and feature of Advanced Robot for DMWs of SG nozzle design

The Advanced Robot was designed to achieve 3 functions for domestic inspection requirement

such as quick installation, accurate coverage of inspection area and multiple inspection

techniques. Details of these functions are shown in Table 1.

Table 1 Target and feature of Advanced Robot design

Target Feature

Quick

Installation

• Installation through

primary manhole

• Module structure

• Special tools for installation from

outside of manhole

• Self moving function on tubesheet

Accurate

Coverage

of Inspection

Area

• All circumference

inspection

• High accuracy

inspection

• 100% coverage of inspection area

• Accurate tracking with auto

rectification of sensor position by

laser measurement

Multiple

Inspection

Technique

• Application of ECT,

UT, and VT• Retouchable sensor head

Advanced Robot for DMWs of SG nozzle

Inspection sensors

SG nozzle

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3.2 Quick Installation

To achieve the quick installation, the Advanced Robot consists of four modules. Each module is

lightweight, so easy assembly is possible. Moreover, by using of the special tools for installation

from outside of manhole, working in channel head is unnecessary. Outline of assembly

procedure is shown in Figure 4. After the Advanced Robot is attached on the installation

position, the Robot has to move to the inspection position by itself. Therefore it has moving

capability on tube-sheet remotely by using MHI's SG tube robot technology. The outline of

Advanced Robot movement is shown in Figure 5.

Figure 4. Outline of

assembly procedure of

Advanced Robot (4)

Figure 5 Outline of Advanced Robot movements

Manipulator Base

Manipulator (Root part)

Manipulator (Top part)

Sensor attachment

Install from outside in channel head

Inspection position

Move on tube-sheet

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3.3 Accurate Coverage of Inspection Area

To achieve 100% coverage of inspection area and accurate tracking, seven axes manipulator was

adopted. This manipulator has the wide movable range and high flexibility so that it is possible

to control the sensors smoothly like a human’s arm. Moreover, by laser measurement instrument,

the distance between nozzle inside surface and the sensor attachment is kept constant and the

forcing load to sensors is also controlled even for slight distortion on nozzle with auto

rectification. The apparatus of the seven axes manipulator is shown in Figure 6.

Figure 6 Apparatus of seven axes manipulator

3.4 Multiple Inspection Technique

In inspection, ECT is first applied to detect flaws. When flaws are detected by ECT, VT will be

applied to check the characteristics of them and finally UT will be applied for depth sizing. Thus,

the Advanced Robot can carry the ECT, UT and VT sensor head respectively. Attachments of

each inspection technique are shown in Figure 7. Sensor exchange can be performed without

removing a robot from channel head. The time for sensor exchange is approximately 3 minutes.

Attachment for ECT and UT

Probes

Camera

Attachment for VT

Figure 7 Attachment for each inspection technique

Tip

Camera

Attachment for VT

Probes

Attachment for ECT and UT

Laser measurement instrument

Sensor attachment

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4. ECT Detection Technique

ECT is applied to check the soundness of nozzle weld. Both normal and magnetic biased ECT

probes with cross-coil are used (see Figure 8). The magnetic biased ECT probe can decrease

noises due to permeability change between safe-end weld and stainless overlay.

The results of EDM notch mock-up test are shown in Figure 9 and 10. All EDM notches

including EDM notches with 0.5 mm depth are detected. Moreover, SCC mock-up test was

performed. The result showed 0.5mm depth SCC is detectable (see Figure 11) (5).

Figure 8 Normal and magnetic biased ECT probes with cross-coil

Figure 9 Layout of EDM notch on mock-up

Figure 10 ECT Results of mock-up test

Normal ECT probe Magnetic biased ECT probe

Depth / Length / WidthSUS

EDM notches dimension

TT600 weld material

Buttering

Overlay

Notches on buttering

Notch depth is 0.5mm

Notch depth is 0.5mm

Results of Normal ECT probe Results of Magnetic biased ECT probe

TT600 weld material

Buttering

Overlay

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Figure 11 Cross-section of SC on mock-up

5. Field Application of Robot Technique with ECT

Since 2010, MHI has applied the Advanced Robot successfully in ISI of Alloy 600 DMWs on

SG nozzles of 2 PWR plants as shown in Table 2. ECT was performed in ISI and no indication

was verified. In future, the Advanced Robot will be applied for about 4 plants per year.

Table 3 shows the typical schedule of ECT application in ISI. Installation of Advanced

robot can be carried out in only one day. Then, critical schedule can be finished in about 3 and

half days. The work in the inside of channel head is not required by introduction of the

Advanced Robot. Therefore, as compared with the case using old type equipment for USP,

numbers of total workers have been reduced by about 50%. In addition, amount of radiation

exposure reduced by about 30% compared with the case using old type equipment for USP.

Table 2 Experiences of ISI of SG nozzles

Table 3 Typical schedule of ECT by Advanced Robot (a) (b)

Work 1 2 3 4

Installation of NDT Robot to channel head

Inspection (Data acquisition / Analysis)

Tear down

(days)

(3) (4)

Estimated Condition

(a) Schedule is daytime shift (b) Equipment conveyance is excepted

(c) Inlet nozzle only (d) When there is a problem in data, re-inspection will be carried out

(c) (d)

Unit Type Data

Plant APWR

2 - Loops09/2010

Plant BPWR

2 - Loops10/2011

0.5mm

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6. UT Depth Sizing Technique

In order to improve depth sizing of shallow and deep defects, the PA-UT technique was

developed. A large size matrix PA-UT probe was developed for depth sizing of deep defects.

This probe can utilize 3-Dimensional beam scan. Small size TRL PA-UT probe was developed

for depth sizing of shallow defects. This probe can utilize a large refraction angle. Figure 12

shows these two PA-UT probes and two conventional probes with different refraction angles

used in ISI for the comparison of depth sizing test. The depth sizing test was performed using

artificial SCCs. In this test, a total of 17 test pieces were made. After ultrasonic inspection, all

test pieces were cut and the depth size and shape of cracks was measured. Figure 13 shows a

representative artificial SCC. Results of PA UT and conventional UT probes are shown in

figures 14 (a) and 14(b) respectively. The conventional UT showed a good sizing performance

in depth region less than 15mm, but it became extremely degraded in around 20mm depth

region. On the other hand, the PA-UT technique showed a better accurate sizing result up to

around 40mm depth region. The Root Mean Squire Error (RMSE) of the conventional UT was

9.35mm, the RMSE of the PA-UT with small TRL PA and Matrix PA is 2.54 mm. From these

results, it is verified that the newly developed PA-UT achieved conquest of the weak point

of the conventional UT and has adequate sizing capability up to approximately 40mm depth.

The example of Advanced PA UT sizing result is shown in Figure 15. Just for information,

volumetric inspection by normal UT probe and angle UT probe is also applicable.

Figure 12 UT probes

Figure 13. Representative artificial SCC

Matrix PA probe Specs: 2MHz-96ch Apply to: Middle to deep defects

Small type TRL PA probe Specs: 5MHz-40ch Apply to: Shallow defects

Conventional TRL probe Specs: 3MHz-TR Apply to: All defects

T70, R50

T50, R40

Y+方向

低合金鋼側 ステンレス側

Y=0

最大深さ：23.1mm

位置：X＝0、Y＝0 Max depth: 23.1mm

X=0, Y=0

LAS SUS

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Figure 14 Sizing accuracy of SCC

Figure 15 Example of Advanced PA-UT sizing result

7. Field Application of UT

The PA-UT method is designed to be used with the Advanced Robot. However, we had field

experience of this method for Reactor Vessel (RV) outlet nozzle inspection in 2011 introduced

below.

Two defects were detected and the PA-UT method which is described in section 6 was

carried out for depth sizing. Figure 16 shows the location of the detected defects. Estimated

depths of defects are less than 3mm and 4.7mm respectively. UT results (B-scope) for each

defect is shown in Figure 17. The actual depths of defects estimated in 2.7mm and 4.8mm

respectively as the grinding process of the nozzle for repair. This shows that deviation between

estimated and actual depth of defects is very small. Thus, effectiveness of the PA-UT method in

power plant inspection was proven.

(a) Conventional method (b) Advanced method

Conv. TRLθT70, R50χ

RMSE 9.35

( 1.36 at <20mm)

Advanced method combined Small TRL PA with Matrix PA

RMSE 2.54

0

10

20

30

40

50

0 10 20 30 40 50

実深さ ォmmオ

評価

深さ

ォm

mオ

縦波前後分割ブフ膅フ70キ R50オ

0

10

20

30

40

50

0 10 20 30 40 50

実深さ ォmmオ

評価

深さ

ォm

mオ

小型フRLギPA

マトリックスPA膅浅い欠陥用䐢

マトリックスPA膅深い欠陥用䐢

℧Outline mark means

conventional SCC

面エコーによる評価

Conv. TRL (T70, R50)

Matrix PA (for large defect)

Evaluation using surface echo

Small type TRL PA

Matrix PA (for middle defect)

Actual depth (mm)

Evalu

ation d

epth

(m

m)

Actual depth (mm)

Evalu

ation d

epth

(m

m)

Example of Depth Sizing / Actual depth: 22.9 mm

Conventional Method Advanced Method

Impossible depth sizing (Interim value : 18.8mm)

Evaluated depth : 22.3mm

Probe scanning position

Wall

thic

kness

Probe scanning position

Wall

thic

kness

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Figure 16 Location of SCC indications in Outlet nozzle of RV

Figure 17 UT Sizing Result of Outlet DMWs

8. Conclusion

Advanced Robot for DMWs of SG nozzle and PA-UT (inspection technique from inside

surface) have been developed. Advanced Robot with state-of-art robotic technology has been

applied in the field service in Japan successfully. The PA-UT has been also applied to the RV outlet

nozzle and effectiveness was proven by field experiences. These techniques will be effective ISI

method for particular geometry of nozzle outside surface with large taper angle. MHI will

intend to introduce these advanced techniques to contribute to the maintenance of power plants

hereafter.

膅膅MCPMCP䐢䐢

膅膅R/VR/V䐢䐢

膅膅OutsideOutside䐢䐢 膅膅InsideInside䐢䐢

Stainless overlayStainless overlay

Buttering portionButtering portion

Circ weld portionCirc weld portion

SafeSafe--endend

(Weld portion : 600 series (Weld portion : 600 series

Ni base alloy)Ni base alloy)

Outlet nozzle Outlet nozzle

(low alloy steel)(low alloy steel)

approx.75mmapprox.75mm

View from R/V side View from R/V side

(Top shall be 0 degree)(Top shall be 0 degree)

膅膅MCPMCP䐢䐢

膅膅R/VR/V䐢䐢

膅膅OutsideOutside䐢䐢 膅膅InsideInside䐢䐢

Stainless overlayStainless overlay

Buttering portionButtering portion

Circ weld portionCirc weld portion

SafeSafe--endend

(Weld portion : 600 series (Weld portion : 600 series

Ni base alloy)Ni base alloy)

Outlet nozzle Outlet nozzle

(low alloy steel)(low alloy steel)

approx.75mmapprox.75mm

View from R/V side View from R/V side

(Top shall be 0 degree)(Top shall be 0 degree)

膅膅MCPMCP䐢䐢

膅膅R/VR/V䐢䐢

膅膅OutsideOutside䐢䐢 膅膅InsideInside䐢䐢

Stainless overlayStainless overlay

Buttering portionButtering portion

Circ weld portionCirc weld portion

SafeSafe--endend

(Weld portion : 600 series (Weld portion : 600 series

Ni base alloy)Ni base alloy)

Outlet nozzle Outlet nozzle

(low alloy steel)(low alloy steel)

approx.75mmapprox.75mm

View from R/V side View from R/V side

(Top shall be 0 degree)(Top shall be 0 degree)

No.1 Indication No.2 Indication

Advanced method (Small TRL-PA)

Actual Depth : 2.7mm Actual Depth : 4.8mm

No indication ( < 3mm )

Probe scanning position

Wall

thic

kness

Probe scanning position

Wall

thic

kness

Evaluated depth : 4.7mm

Probe scanning position

Wall

thic

kness

Probe scanning position

Wall

thic

kness

Comparison with Estimated depth and Actual depth

No indication ( < 3mm ) Evaluated depth : 4.7mm

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Reference

1) MT Anderson, AA Diaz, SR Doctor, “Evaluation of Manual Ultrasonic Examinations Applied to

Detect Flaws in Primary System Dissimilar Metal Welds at North Anna Power Station”,

PNNL-21546, Pacific Northwest National Laboratory, 2012

2) T. Yamaguchi, T. Kawashima, M. Takatsugu, K. Unate, “Advanced NDT Robot for

Dissimilar Metal Weld of Steam Generator Nozzles”, Proceeding of the 9th International

Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized

Components, pp.763-769, May 2012.

3) S. Kawanami, T. Kimura, M. Kurokawa, J. Nishida, T. Yamaguchi, T. Matsuura, T. Tsuruta,

“Advanced Phased Array UT Sizing Technique on Stress Corrosion Cracking in Dissimilar

Metal Welds of Nozzle” , Proceeding of the 9th International Conference on NDE in Relation

to Structural Integrity for Nuclear and Pressurized Components, pp.676-683, May 2012.

4) Y. Kohata, J. Fujita, K, Onishi, H. Tsuhari, F. Hosoe, “The Application of Manipulator Robot for

Nuclear Plant Maintenance”, Preprints of 7th Annual Conference of the Japan Society of

Maintenology, Omaezaki, pp.400-405, July 2010. (in Japanese)

5) Y. Asada, K. Tokuhisa, M. Takatsugu, K. Kurokawa, K. Kawata, N. Hirano, T. Sera

“Development of Eddy Current Testing Method for PWR Vessel’s Dissimilar Metal Weld”,

Japan Society of Maintenology, Maintenology, Vol.6-No.4, pp.38-43, 2008. (in Japanese)

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