Application of Infrared Thermography in NDT of Plasma-Facing Components for Tokamaks

 Pan QI, Qiang LI, Guang-Nan LUO

Institute of Plasma Physics

Chinese Academy of SciencesP.O.Box 1126, Hefei, 230031 China

17th World Congress of Non-Destructive Testing Exhibition Center, Shanghai, China, October 25 – 28, 2008

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Outline

• Background

• Infrared thermography applied in PFCs testing

• R & D at ASIPP for NDT of PFCs

• Summary and future work

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Outline

• Background

• Infrared thermography applied in PFCs testing

• R & D at ASIPP for NDT of PFCs

• Summary and future work

Experimental Advanced Superconducting Tokamak (EAST)

Background

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International Thermonuclear Experimental Reactor (ITER)

The background of my work is magnetic confinement fusion tokamakITER being constructed in France The mini tokamak EAST constructed in our instituteBy the end of september 2006, the engineering missions were completed

The inner vacuum vessel with stainless-steel PFCs (initial phase)

The inner vacuum vessel with graphite tiles PFCs (first phase)

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The cross-section of EAST in-vessel components

Divertors

High field plate

Passive plateLow field plate

The vacuum vessel is the important part of tokamak. It contains divertors, passive plate, high field plate and low field plate. These parts make up of plasma facing components (PFCs). The components consist of plasma facing materials (PFMs) and Cu alloy heat sink. In the initial phase, stainless steel was used as PFM. Doped graphite tile is the main choice in the first phase (now) and tungsten will be chosen as candidate in the future phase. the purpose of PFCs is to protect the vacuum vessel, injection power system and diagnostic components.

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Problems of PFCs

•Engineering constraints •Industrial constraints

bonding/joint

Ensure and control the quality of manufacturing (the keypoint for successful delivery of PFCs)

(1)

(1) M. Missirlian et al. Qualification of high heat flux components: application to target elements of W7-X divertor, in: 11th PFMC, Greifswald, Germany,october11th, 2006.

When tokamak is working, PFCs are the first wall facing plasma, subjected to high heat flux, bombardment of different particles and high magnetic filed. In this working conditions, PFCs will meet the problems, which contain engineering constraints and industrial constraints. But the transition (bonding/joint) between PFMs and heat sink is the most critical issue. How do we ensure and control the quality of this part during manufacturing is the key point for successful delivery of PFCs.

ASIPPExamination on quality of bonding/joint of PFCs

Destructive method Nondestructive method

High heat flux(HHF) testing(very important)

Purpose

qualification of HHF PFCs design & modeling

qualification of advanced nondestructive tests

Method

screening test-heat load limit

fatigue test-fatigue limit

critical heat flux test-safety margin for heat transfer

Metallography

qualify the quality of the braze joint

Systematic fracture shear stress measurements

qualify the quality of the braze joint

X-ray tomography - a sample X-ray examination allowed assessment of the different copper infiltration rates inside CFC tiles, which can change the global thermal conductivity of the HHF element

Ultrasonic inspection– applied to test the defect existing in PFMs/heat sink interface

Transient thermography – applied at the final stage of the fabrication and provide a global information about the thermal performance

Lock-in thermogaphy – applied to test PFCs throughout manufacturing process

Pulsed thermography – this approach is under development

ASIPPComparison of the methods

Methods HHF test Ultrasonic X-ray test IR test

Nondestructive No Yes Yes Yes

Quantitative Yes Yes Yes conditional

Test cost High Low High Low

Portable No Yes No

decision by method

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Outline

• Background

• Infrared thermography applied in PFCs testing

• R & D at ASIPP for NDT of PFCs

• Summary and future work

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Transient thermography - SATIR

Tank: hot water

Control system (loop)

Reference element

Tested element

Pump

(SATIR : French acronym for Infra Red Acquisition and Data Processing Device)

(1)

In 1994,an infrared nondestructive test-bed SATIR was developed for the PFCs quality control by CEA (France fusion institute). The principle is based on the comparison of the surface temperature evolution between reference elements and tested elements during a transient heat load, which is generated by hot and cold water flowing successively in the cooling channel drilled in heat sink. Interface defects are detected by a slower temperature surface response. The surface temperature transition due to the heating and cooling cycle is measured by means of an infrared camera.The test-bed of SATIR

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This is the process of transient monitoring. The temporal evolution of the surface temperature difference at each corresponding pixel of both elements is calculated and the areal maxima (so-called DTref_max) distribution can be obtained. A bonding flaw between PFMs and heat sink can be judged from the distribution on the areas with the values of DTref_max higher than a critical value verified by high heat flux (HHF)testing. The disadvantage of the method is the necessity of a reference component. An innovative data processing based on spatial autocorrelation and on energy distribution at PFMs and heat sink bonding are being developed allowing inspection of a component without the necessity to get a reference component.

Presence of defect

(2)

(2)

(2) A.Durocher et.al. Qualification and Nondestructive examination methods of high heat flux plasma facing components, in: the 24th SOFT, Warsaw, Poland, Sept,12th, 2006

No necessity of a reference component

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Manufacturing of calibrated flaws at the doped graphite tile/copper interface

Trans.IR calibration and validation

HHF test

Transient validation

(1)

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Lock-in thermography

The Lock-in is a method based on propagation in the solids of modulated thermal waves imposed at the surface of element. It is used for evaluating the heat transfer capabilities of tiles, mainly the PFM/heat sink bonding

Magnitude and phase-shift of surface temperature vary according to thermal properties underneath. Phase-shift is less sensitive to flux homogeneity and surface emissivity

Properties: easy to operate, no cooling, less emissivity dependence

Lock-in thermography device (movable) applied in TS Tokamak

•Investigations on possibility to detect damage during Tokamak shutdowns•Features – Portable – In-field – Health check – Annual check – Global check – Suspicious tiles monitoring(1

)

(3)

(3) X. Courtois et al, In-situ monitoring of actively cooled plasma facing components using acoustic and thermal methods, Phys. Scr. T128(2007)189-194

ASIPPPulsed thermography – ARGUS (Advanced infRared thermoGraphy Unit for inSpection)

The ARGUS setup

Principle: For each pixel, a frequency analysis based on the fast Fourier transform algorithm(FFT) is calculated on the cool down behavior after the flash. The defect detection is not based on the power spectrum of the FFT, but on the phase-shift of the basic frequency of the FFT(f1)and its higher harmonics(f2,f3,…). The basic frequency(f1) is adjusted to obtain a penetration depth reaching the foil/heat sink interface, whereas the penetration of the first harmonic frequency(f2) is limited to the PFM/foil interface. The means that the phase image of each frequency addressed by the FFT carries information about depth of the defect detected

Application to AMC tiles inspection

(4)

(4) X. Courtois et al, In-situ monitoring of actively cooled plasma facing components using acoustic and thermal methods, Phys. Scr. T128(2007)189-194

ASIPPComparison of three thermography methods

Methods Trans.IR Lock-in IR Pulsed IR

Aim Quality control and the final acceptance test

Quality control and the final acceptance test

Quality control and the final acceptance test

Technology maturity Mature Immature Immature

Reference element Yes No No

Data processing Simple and less data Complex and large amount of data

Complex and large amount of data

Equipment costs Moderate Cheap Cheap

Test costs Cheap Cheap Cheap

Mass testing Ok Ok Ok

Quality criteria Quantitative Quantitative Quantitative

Determination of criteria

Comparison of HHF calibration and complex

Comparison of HHF calibration and complex

Comparison of HHF calibration and

complex

Portable No Yes Yes

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Outline

• Background

• Infrared thermography applied in PFCs testing

• R & D at ASIPP for NDT of PFCs

• Summary and future work

ASIPPConstruction of test-bed

The experimental set-up at our institute

Hot water tankCold water tank

Pump

Infrared camera

Tested sample

Benefiting from the experience of SATIR, we have constructed the test-bed based on the method of transient thermography.

Technical indicator

Hot water flow rate: 3.9m/sCold water flow rate:3.4m/sHot water temperature:98℃Cold water temperature:25℃Volume of hot water tank:100LVolume of cold water tank:150LInfrared camera: ThermaCAM®PM595PAL, 320×240 pixels, 50 Hz

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Max power: 10kW; accelerate voltage: 10kV; cooling water: 0.4MPa,2 m3/h;Max scanning area:>30×30mm

The test equipment in ASIPP

Technical indicator

The results of NDE performed on the PFCs with calibrated defects must be compared with the high heat flux (HHF) test. So the facility is important and necessary for infrared thermography

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Infrared image with defect in flexible graphite layer

Our works

Simulation by ANSYS

The IR image is used to visualize the heat transfer between the PFMs and heat sink, which enables to identify the calibrated defects in thermal contact. Three dimensional thermal finite element analyses have been performed to simulate the defect. The experimental observations obtained from IR thermography have confirmed the FE simulation.

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Edge detection for defect by canny operator

Outline

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• Background

• Infrared thermography applied in PFCs testing

• R & D at ASIPP for NDT of PFCs

• Summary and future work

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Plasma-facing component is one of the most important in-vessel components in a tokamak. High reliability and stability are necessary for high performance plasma operations.

Good bonding/joint quality between the plasma-facing material and the heat sink is a keypoint to realize the high reliability and stability, which requires reliable examinations during manufacturing and at final batch reception.

Infrared thermography has proved to be a reliable and effective method in the examination and been chosen as a candidate method in the manufacturing of the ITER PFCs.

R & D work is underway to establish reliable NDTs in future manufacturing of the next-step EAST PFCs, mainly including IR thermography and ultrasonic inspections.

Summary

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Future work

Continuous efforts in developing IR transient thermography NDT, i.e., completion and improvement of the system, esp. calibration with comparison to HHF testing

More NDT methods, e.g., ultrasonic inspection, will be examined for their use in the manufacturing of the future EAST PFCs

The acceptance criteria for the NDTs must be determined via dedicated and reliable testings, usually destructive ones

Portable Lock-in or Pulsed IR thermography test-bed will be useful for in-field examination, may be used as an effective tool for routine health check and service life prediction of the PFCs

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