Ultrasonic Inspection of High Heat Flux (HHF) Tested Tungsten Monoblock Type

Divertor Test Mock ups

Kedar Bhope1, M ayur M ehta

1 and S.S.Khirwadkar

1

1Institute for Plasma Research, Bhat, Gandhinagar-382 428, India

kedar@ipr.res.in, mayur@ipr.res.in, sameer@ipr.res.in

Abstract: IPR has recently installed High Heat Flux Test Facility (HHTF) to simulate the thermal

fatigue scenario on divertor target for ITER like environment. In which actively cooled divertor test mock up is subjected to cyclic heat loads by 200 KW Electron Gun. One of the quality control steps during thermal fatigue test of these components is the ultrasonic testing of bonded region between

Tungsten and Copper alloy tube. Two no. of small scale Cu-W monoblock mock ups has been tested in HHTF for more than 200 thermal cycles of incident heat flux of 20M W/m

2 at IPR and High Heat Flux

testing of one sample was stopped after it got subjected to a “Loss Of Coolant Accident” (LOCA) scenario. This paper highlights the successful application of developed ultrasonic immersion C-scan imaging technique for Cu-W joints of these monoblocks before and after their thermal fatigue test.

Ultrasonic C-scan results of these mock ups are compared to check the degradation of W-Cu and Cu-Cu interface joints. Ultrasonic testing results of Cu-W monoblocks are able to detect, locate and size

the degradations in two sample joints. This paper presents the detailed comparison of ultrasonic test results of Cu-W mono-block divertor assembly before and after HHF Tests.

Keywords: Ultrasonic Testing, Cu-W M onoblock, Thermal fatigue, High Heat Flux test, Divertor

Introduction

Divertor targets are the Plasma Facing Components(PFCs), which are responsible for effective removal of heat from the divertor system and also capable to withstand the steady state heat flux up to

10 M W/m2 for ITER while 20M W/m

2 for DEMO reactor [1].During cyclic operation of a fusion reactor

large thermal expansion coefficient difference between tungsten and CuCrZr create the thermal stress at

joint interface, it might leads to component failure and also affect the heat transfer property of the component [2].With taking this in account, lifetime of a divertor PFC is measured in terms of number of steady state cyclic heat loads sustained by PFCs. High Heat Flux (Thermal fatigue) tests are actually

simulating steady state and transient heat loads using electron beam which is faced by PFCs during operation of a fusion reactor and high heat flux (HHF) test with desirable heat loads is a crucial

qualification test to check the heat transfer performance of divertor [3].In order to do so, IPR has

recently installed High Heat Flux Test Facility (HHTF) to simulate the thermal fatigue scenario on divertor target for ITER like environment.

The monoblock type geometry is a most potential design for PFCs of divertor. The monoblock configuration, consist essentially of armor blocks made of Tungsten (W) with a hole in which a cooling

tube, made of a copper alloy (CuCrZr), is joined by different technologies. In these cooling tubes,

pressurized water is used as coolant [4].The W/Cu joint quality for this type of components is particularly important. The ultrasonic testing for metal/metal joints NDT is universally accepted

because of easy result interpretation. In this paper the application of developed ultrasonic C-scan testing

of monoblock Cu-W joints during the mock ups manufacture and also after their thermal fatigue testing is reported. This procedure was applied on two small scale W monoblock mock ups to check the

degradation of Cu-W and Cu-Cu interface joints that were manufactured in the NFTDC labs. They were finally tested to thermal fatigue at Institute for Plasma Research (IPR) in HHFT facility .

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Tungsten (W) Monoblock mock up

In the frame of M OU with NFTDC [5], nine monoblock mock-ups were manufactured by Hot

Radial Pressing (HRP) technology in NFTDC Laboratory, Hyderabad. The supplied monoblock divertor assembly consists of Tungsten tiles of dimension (30×30×10) mm3 thick with a central hole of

diameter 17 mm. Oxygen Free High Conductive (OFHC) Cu casted in 17 mm hole of W tile to create a

hollow Cu tube of diameter 15 mm and 1.0 mm thickness. Five W monoblock tiles with OFHC copper layer were assembled such that to keep gap of 0.5 mm between two subsequent monoblock W tiles

during joining process. Subsequently this OFHC Cu is bonded with a CuCrZr alloy of inner diameter

(ID) of 12 mm and 1.5 mm thickness using Hot Radial pressing. All the dimensions of a mock-up are as shown in Fig. 1.

Fig.1. M onoblock mock-up geometry with 5 tiles Ultrasonic immersion C-scan test procedure

An ultrasonic pulse-echo method using high frequency side looking probe of 20 M Hz and normal incidence of the ultrasonic beam to the joint interface have been developed [6]. In this method,

reflected echo amplitude is used for Cu-Cu alloy de-bond detection and reflected echo phase is used to detect de-bond between Cu and W. Monoblock divertor assembly placed in a water immersion tank on a circular rotating table is shown in Fig. 2 (a). UT probe is placed inside the monoblock through index

axis (Z-axis) to detect the de-bonding as viewed from sideways.

Fig.2. (a) Ultrasonic C-scan setup for Cu-W M onoblock (b) M arking on Mock-up to locate the defect

30

30

Ø17 – OHFC Copper

Ø15 – CuCrZr tube OD

Ø12 – CuCrZr tube ID

(a) (b)

00-360

0 90

0

1800 270

0

θ Scan

Face 1

Face 2

Face 3

Face 4

WT 1

WT 2

WT 3

WT 4

WT 5

Z- Axis Index

Water path distance is maintained such that ultrasonic beam focused on interface. To examine the entire circumference (θ Scan) monoblock assembly is rotated by a rotary table and index in Z-direction. In

order to precisely identify the location of the defect on C-scan, W tile assembly has been divided in

four different faces viz., Face 1, Face 2, Face 3 and Face 4. Each face contained an angle ranges from

0°- 90°, 90°- 180°, 180°- 270°, 270°-360˚ respectively and each W tile is marked with numbers as shown in Fig.2 (b). Calibration of the ultrasonic flaw detection system carried out by detecting four Flat Bottom Holes (FBHs) of 2mm dia. at depths from 0.75 to 2.55 mm with 0.5 mm step. C-Scan is

calibrated by detecting the 0.5mm FBH at Cu-W interface and the same defect is used as reference

defect. Ultrasonic test parameters used for inspection are represented in Table 1.

Table 1. UT Test Parameter

Technique Pulse-echo immersion C-scan technique Probe Side looking V3343, 20M Hz, 0.125’’FPF, F0.5’’

Static gain 55.00 dB

Range 4 mm

Immersion medium Distilled water

Scan length & resolution 360 ̊, 0.1˚

Index length & resolution 65mm, 0.1mm

Water path 4.7mm (±1.6 mm)

Acquisition mode RF (Synchronized)

Instrument OM NISCAN MX

Thermal Fatigue (High Heat Flux) Testing

Two small scale W monoblock targets named as "Test mock-up-I" and "Test mock-up-II" were tested

in the HHTF at Institute for Plasma Research (IPR), India. The main part of the facility is 200kW

electron gun with an acceleration voltage of 45kV with 10 kHz scanning frequency of circular electron beam. PFCs up to 1 meter length can be loaded in the system. The cooling supply of the facility allows

a cooling water flow up to 85 liter per minute at an inlet pressure of 18 bars and inlet temperature of 20° C. One infrared camera, two pyrometers and 4 thermocouples are installed as diagnostics to

monitor surface temperatures, mock up temperature and cooling water temperatures.

Three tiles with total area 900 mm2 of the Test mock-up-I was exposed to incident heat flux 19 MW/m

2

(Corresponding 20kW power on 30 mm X 30 mm area) for 15 sec ON & 5 sec OFF. When test was

reached to the incident heat flux 22 MW/m2, LOCA condition was occurred. Before LOCA condition ~

200 cycles of thermal cyclic test were completed on Test mock-up-I. LOCA was occurred due to

absence of coolant flow in heat sink tube of the test mock-up-I. Three Tiles of Test mock-up-II with

total area 900 mm2 was cyclically loaded with incident heat flux 19 M W/m

2 for 15 sec ON & 5 sec

OFF. Thermal fatigue tests for 15 sec ON time & 5 sec OFF time performed on the test mock-up-I and

test mock-up-II are summarized in Table 2.

Table 2. HHF test details of both test mock-ups

Test mock up-I Test mock up-II

Initial Screening up to 19M W/m2

200 cycles at 20 M W/m2

Initial Screening up to 20M W/m2

165 cycles at 20 M W/m2

LOCA occurred at 22MW/m2 -

Ultrasonic Inspection Results and Discussion

Ultrasonic testing of Test mock-up-I and Test mock-up-II has been carried out before and after HHF

test to check the integrity of the Cu-W bonding area as well as to monitor health of the samples [7]. Fig. 3 indicates comparison of sample face before and after HHF test. It clearly shows that middle tiles of

both samples are affected.

(a) (b) (c) (d)

Fig.3. Photographs of samples: (a, c) Before HHF Test (b, d) After HHF Test

UT Results of Test mock-up-I. Ultrasonic image as shown in Fig. 4 was used to decide which face can be possible to take for HHF test for investigate the performance of mock-up. It clearly resembles

that face 3 contained less defects and it is to be used for HHF test. On the basis UT results, an HHF test performed on the sample. Comparing the ultrasonic C- scan images for before and after HHF test, it is

noted that defects are increased rigorously on region other than face 3. Investigating the HHF test parameter, LOCA (Loss of Coolant Accident) was found responsible to increase the defects as shown in Fig. 5 (a, b). 5

(a) (b) Fig.4. Ultrasonic C-scans of Test mock-up-I Before HHF Test at (a) Cu-Cu joint & (b) Cu-W joint

(a) (b)

Fig.5. Ultrasonic C-scans of Test mock-up-I After HHF Test at (a) Cu-Cu joint & (b) Cu-W joint

00 270

0

WT 1

WT 3

WT 4

WT 5

Face 1 Face 2 Face 3 Face 4

WT 2

900 180

0 360

0 00

2700

WT 1

WT 3

WT 4

WT 5

Face 1 Face 2 Face 3 Face 4

WT 2

900 180

0 360

0

00 2700

WT 1

WT 3

WT 4

WT 5

Face 1 Face 2 Face 3 Face 4

WT 2

900 180

0 360

0 00

2700

WT 1

WT 3

WT 4

WT 5

Face 1 Face 2 Face 3 Face 4

WT 2

900 180

0 360

0

Test mock-up-I Test mock-up-I Test mock-up-II Test mock-up-II

White dashed marked region in the Fig.5 (a) represents zone of interest and the good region on face 3 after HHF test. Bluish-white region shows good bond in Cu-Cu alloy C-scan. However, Bluish color in

Fig 4 (b) and Fig.5 (b) is due to 22% echo height (i.e. acoustic impedance mismatch reflected

amplitude) shows good bond at Cu-W joint. Area other than face 3 for all tiles found red because de-bond present at Cu-Cu alloy interface which hide the information about Cu-W joints. Fig. 6 represents

the B-scan at white marked region which gives clear idea about the defect depth at indicated region. Tile no 1 and 5 are less affected heat regions and contained segregated isolated defects.

Fig.6. B-scan image of WT 2 at 22 mm

Fig. 5(b) shows C-scan at a depth of Cu-W interface, signal amplitude greater than 60% (of FSH) at location of WT 5 on face 2 in C-scan indicates the presence of defects. B-scan as represented by Fig.7

shows de-bonds at Cu-W interface from 90°-180° scan length. Corresponding A-scan also shows

reflected echo phase change. Visual examination of sample were carried out before and after HHF test which reveals that bending occurred in sample and gap between W tiles get slightly increased.

Fig.7. B-scan image of WT 5 at 52 mm

UT Results of Test mock-up-II. On the basis of UT inspection conducted prior to HHF test as shown in Fig.8, Face 2 of the test mock-up was identified to be used for carrying out the HHF tests. A

thermal cycle with incident heat flux as mentioned in table 2 was applied on Test mock-up II. In this sample attention has been made to study the response of defects which are present at WT 2, 3, & 4

before HHF test. Fig.9 represents C-scan image of sample after HHF test with white mark region of

interest. As it can be seen by comparing the C-scan images, no appreciable deterioration could be observed in Cu- Cu joint and Cu-W joint.

(a) (b) Fig.8. Ultrasonic C-scans of Test mock-up-II Before HHF Test at (a) Cu-Cu joint & (b) Cu-W joint

00 270

0

WT 1

WT 3

WT 4

WT 5

Face 1 Face 2 Face 3 Face 4

WT 2

900 180

0 360

0 00

2700

WT 1

WT 3

WT 4

WT 5

Face 1 Face 2 Face 3 Face 4

WT 2

900 180

0 360

0

90° 180° Face 2

WT 5 (52 mm)

180̊ 270˚ Face 3

WT 2

(22mm)

(a) (b)

Fig.9. Ultrasonic C-scans of Test mock-up-II After HHF Test at (a) Cu-Cu joint & (b) Cu-W joint

An irregularity at inner wall of tube observed at WT4 in test mock up-II before HHF test. Fig.10 shows

C-scan, B-scan and surface morphology of tube inner wall and this type of defects does not reflect ultrasonic beam in the direction of the probe. These inner wall defects were kept in observation for

before and after HHF test but at applied thermal cyclic load conditions does not make change in size of

this defect as presented in Fig.8 (a) & Fig. 9 (a).

4

(a) (b) (c) Fig.10. Effect of inner wall damage on results (a) C-scan (b) B-scan & (c) Inside view of mock-up II

However, For both test mock-ups it has been noticed that C-scan images after the HHF test (Fig. 5 & 9) the amplitudes of the received ultrasonic signals are smaller than the amplitudes before the HHF test

(Fig. 4 & 8); this reveals micro structural changes in copper and copper alloy layers that give place to

ultrasonic attenuation increment but does not affect on defect detection. For Test mock-up-I, % defective area is increased ~50% due to LOCA while for Test mock-up-II no appreciable defect

increased in assembly. Fig.11 shows summary of % defective area.

Fig.11. Summary of % Defective area in both mock-ups

Inner wall damage

00 270

0

WT 1

WT 3

WT 4

WT 5

Face 1 Face 2 Face 3 Face 4

WT 2

900 180

0 360

0 00

2700

WT 1

WT 3

WT 4

WT 5

Face 1 Face 2 Face 3 Face 4

WT 2

900 180

0 360

0

Conclusion

Ultrasonic testing of HHF (High Heat Flux) tested two Cu-W Monoblock assembly was carried out

successfully on Test mock-up-I and Test mock-up-II. Ultrasonic testing is able to detect, locate and size the defect present at Cu-Cu alloy and Cu-W joint. Comparison of ultrasonic results has been made

between before and after HHF test. It is concluded that % defective area in Test mock-up-I is increased

~50% due to LOCA while for Test mock-up-II no appreciable defect increased in assembly with incident heat flux of approx 20M W/m2. Test mock-up-II successfully withstands 20M W/m2 heat loads

with acceptable defects. Bending occurred in Test mock-up-I and gap between W tiles get slightly

increased after HHF test. Inner wall damage in Test mock-up-II were kept in observation and found unchanged after HHF test. It has also been noticed that after HHF test micro structural changes in the

copper and copper alloy layers producing an ultrasonic attenuation increment but does not affect on ultrasonic results.

Acknowledgement

The authors would also like to thank all members of High Heat Flux Test Facility of Institute for

Plasma Research for their support during various activities involved in this work. The authors would also like to thank scientists, engineers and technical staff of NFTDC, Hyderabad, India for development

of monoblock type of mock ups.

References

[1] M . M erola et al, “EU activities in preparation of the procurement of the ITER divertor”, Fusion

Engineering and Design 81(2006) 105-112.

[2] J. H. You et al, “Thermo mechanical Behavior of actively cooled,brazed divertor components under

cyclic high heat heat flux loads”, Journal of Nuclear M aterials,. 250 (1997) 184-192 [3] J. Linke et al., “High Heat flux testing of Plasma Facing materials and components-Status and

perspectives fro ITER related activities ”, Journal of Nuclear M aterials, 367–370 (2007) 1422–1431.

[4] Selanna Roccella et al., “Development of an ultrasonic test method for the non-destructive examination of ITER divertor components”, Fusion Engineering and Design, 84 (2009) 1639–1644.

[5] M OU with NFTDC on “DEVELOPMENT OF FABRICATION TECHNOLOGY AND PROTOTYPES OF DIVERTOR TARGET ELEM ENTS”, Ref. No.MOU/3/IPR/NFTDC/2008-09.

[6] K.Bhope et al., “Simulation Study and Development of Ultrasonic Inspection Technique for Cu-W M onoblock Divertor Assembly”., Preceding of Asia Pacific Conf. on Non-Destructive Testing- 2013,21-25 November, M umbai, india.

[7] Rafael M artínez-O˜na et al., “Ultrasonic techniques for quality assessment of ITER Divertor plasma

facing component”,Fusion Engineering and Design 84 (2009) 1263–1267.