Acoustic emission technique for leak detection in an end shield of a pressurised heavy water reactor

Int. J. Pres. Ves. & Piping 36 (1989) 65-74

Acoustic Emission Technique for Leak Detection in an End Shield of a Pressurised Heavy Water Reactor

P. Kalyanasundaram, T. Jayakumar, B. Raj Division for Post-lrradiation Examination and NDT Development, Indira Gandhi Centre for Atomic Research, Kalpakkam, India

C. R. L. Murthy Department of Aerospace Engineering, Indian Institute of Science, Bangalore, India

& A. Krishnan Nuclear Power Corporation, Bombay, India (Received 13 February 1988; accepted 4 March 1988)

ABSTRACT This paper discusses a successful application of the Acoustic Emission Technique (AET).for the detection and location of leak paths present on an inaccessible side of an end shield of a Pressurised Heavy Water Reactor ( P H W R ) . The methodology was based on the fact that air- and water-'leak A E signals have d(fferent characteristic features. Baseline data was generated from a sound end shield of a P H WR for characterising the background noise. A mock-up end shield system with saw-cut leak paths was used to ver(/~v the validity of the methodolog)'. It was found that air-leak signals under pressurisation (as low as 3psi) could be detected by Jkequency domain anaO'sis. Signals due to air leaks from various locations of defective end shield were acquired and anaO'sed. It was possible to detect and locate leak paths. The presence of detected leak paths was further confirmed hy an alternative test. 65 Int. J. Pres. Ves. & Piping 0308-0161/89/$03.50 ~;) 1989 Elsevier Science Publishers Ltd, England. Printed in Great Britain

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INTRODUCTION This paper discusses the successful use of an Acoustic Emission Technique (AET) to locate leaks in one o f the end shields o f a Pressurised Heavy Water Reactor (PHWR) at the Rajasthan Atomic Power Station Unit One (RAPS1). Light water leakage was observed from the south end shield of RAPS-1. A helium leak detection technique was applied to detect the region of leakage. Subsequently AET was applied to locate the leak paths present on the calandria side tube sheet (CSTS) by placing the acoustic emission (AE) sensor on the fuelling machine side tube sheet (FSTS). 1 The end shield system was pressurised with air up to 18 psi and the AE signal due to air leakage through the leak paths was analysed in the time domain. Locations of the leakage were later confirmed by both ultrasonic and visual examination techniques after removing the identified channels. These leak paths were plugged and the reactor was made operational. After a few months of reactor operation, light water leakage was again noticed from the same end shield system. The reactor was shut down and preliminary investigations were carried out to determine the source of leakage. The water level in the end shield was found to be stable at a level above the earlier repaired region. This indicated that there is/are new leak path(s). DP testing was carried out around one of the channels where accessibility for probing on CSTS was available, and a new leak path was located. It was felt necessary to ascertain the presence of any more leak paths in the end shield system. Since all other locations are inaccessible, AET was thought to be the most appropriate technique for finding any further leak paths. In the earlier instance when AET was used, 1 time domain analysis of the AE signal was applied. The end shield had been pressurised with air to 18 psi, which could give a strong AE signal due to an air leak. However, in the present investigations it was felt that time domain analysis would not be feasible because the maximum pressure that could be applied onto the end shield was restricted to below 5 psi due to structural considerations. The weak signal due to leakage at low pressure would be masked in the presence of high background noise due to the running of primary heat transport (PHT) pumps. Hence it was planned to apply frequency analysis for this investigation.

T H E P H E N O M E N O N OF A C O U S T I C EMISSION Acoustic emission is the generation of pressure waves due to dynamic processes such as fluid leakage, crack propagation, plastic deformation and

Leak detection in an end shieM

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fracture in materials. Stress waves of the order of 0-1 microbar generated in a material due to the above-mentioned processes can be detected by this technique. These stress waves are picked up and converted into electrical signals by sensors having a frequency response in the range 100 kHz-2 MHz. These low-level signals are amplified and can be analysed either in the time or the frequency domain. Being highly sensitive, this technique can be used for detection of any microcrack initiation or propagation of any existing microcracks, and also any fluid leakage at inaccessible locations, by picking up signals from accessible locations. Using this technique, on-line monitoring of critical components and structures can give advance warning before an accident or catastrophe takes place.

LEAK D E T E C T I O N A N D A C O U S T I C EMISSION The feasibility of using A E T for leak detection in general depends on three main factors: the energy radiated from the leaks, attenuation between the source and the sensor, and ambient noise. The minimum detectable leakage depends on the type of fluid, the geometry of discontinuity and the sensitivity of the AE system. Various detectability figures such as 100 cma/s of gas leak through a 15-mm orifice, 2 cm3/s of air leakage from a 250-bar high-pressure air valve and 0.3 cm3/s gas leak through a flow control valve at 140 bar have been reported. 1 Signals due to leakage are wideband in general, extending from the low audible range to 1 MHz.

E X P E R I M E N T A L SET-UP F O R A C O U S T I C EMISSION INSPECTION The experimental set-up for the AE inspection is the same for all the different stages of investigation, except the mechanical fixtures used for fixing sensors. For the work on the end shield of the Madras Atomic Power Station Unit 2 (MAPS-2) a hand-held spring-loaded piezoelectric wideband (100kHz2 MHz) sensor was used since the end shield material is non-magnetic and the work was carried out before the reactor had gone critical. A magnetic holder with a spring-loaded sensor was used for RAPS-1 end shield since this is a magnetic material. The signal picked up from the FSTS of the end shield by the sensor was amplified and the amplified signal was immediately recorded on a Biomation i010 transient data recorder (TDR) at a high speed required for recording AE signals. These signals are then transferred onto a magnetic tape using an instrumentation recorder at a slower rate. The

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signals thus recorded are analysed later using a Nickolet Scientific FFT analyser to obtain the frequency distribution of the signal, which acts as a signature for identifying the type of source, such as background noise, air or water leakage, etc. Figure 1 shows the block diagram of the set-up for acoustic emission inspection.

M E T H O D O L O G Y FOR LOCATION OF LEAK PATHS Figure 2 shows a schematic sketch of the calandria and the end shield system of a PHWR. If there were any leak paths above/below water level, air/water would leak out. The signals due to air and water leaks are expected to have different frequency components. However, it was necessary to find out whether air- and water-leak AE signals could be discriminated in the CLOSURE -PLUG

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Leak detection in an end shield

frequency domain. To find out these characteristic frequency components a study was carried out on a mock-up end shield system with artificial cracks. This study would also reveal the levels at which any crack starts or ends. It was also thought that the ligament at which the leak path on the CSTS exists could be determined by finding the location (by moving the probe on the FSTS) at which the intensity of the characteristic frequency of the AE signal is a maximum. As it was necessary to discriminate the leakage signal from the background noise due to fluid turbulence, flow-induced vibrations, etc., baseline data from a sound end shield system of another reactor was analysed before the investigations on the defective end shield system were taken up.

G E N E R A T I O N OF BASELINE DATA FOR MAPS-2 END SHIELD SYSTEM AND PRESSURE TUBES The signals from different locations on the FSTS of the end shield and from the end fittings were recorded while some or all the PHT pumps, moderator pumps and end shield pumps were running, and these signals were analysed later. This campaign served the following purposes. The signals recorded and analysed are the background noise (baseline) due to various sources such as fluid turbulence, flow-induced vibrations, etc., and can act as basic signatures for a particular location under normal reactor working conditions. If future on-line monitoring at these locations indicates deviations from the baseline signature, further analysis has to be done to CH1 1~089m

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identify the source of deviations. Also this work gave us the confidence that the mechanical gadgets and fixtures designed and fabricated for the purpose are adequate, and that the frequency domain analysis would be very useful since the frequency spectrum can act as a signature for determining the condition of the end shield and pressure tubes. Signals from 26 locations on both the north and south FSTS of the end shields and 50 end fittings were recorded and analysed. Figure 3 shows a typical frequency spectrum of the background noise recorded from one of the FSTS locations.

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Leak detection in an end shield

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STUDIES ON M O C K - U P E N D SHIELD SYSTEM Since simultaneous air and water leaks are expected from the end shield under pressurisation, discrimination of the signal was felt essential: Also it was required to know the minimum pressure necessary for successful detection and discrimination of leakage signals. For these purposes a mockup end shield system available at the RAPS site with saw-cut defects depicting existing cracks was made use of. Initially, the system was filled with water above the cracks and pressurised with air to various pressure levels in the range 0--15 psi. Even at a maximum pressure of 15 psi the signal from water leakage could not be detected (Fig. 4(a)). This study indicates that either the signals from water leakage are of very low level or the frequency content is outside the detectable range (100 kHz-2 MHz) of the AE sensor used. Subsequently, water was drained from the mock-up end shield system. One of the cracks was temporarily closed with lead. Then the system was pressurised to a maximum of 6psi. At 4.2 psi a strong signal due to air leakage from the crack, having frequencies in the range 135-145 kHz, was observed. This observation indicates that the air-leak signal from cracks can be detected. Further, the temporarily closed crack was opened partially and again the system was pressurised. This time, at 3 psi, a strong signal having spectral lines in the range 135-145 kHz and 180--190 kHz was observed (Fig. 4(b)). This indicates that (i) the second crack gives a signal at frequencies of 180 and 190kHz, and (ii) the frequency content of the signal due to air leakage depends on the size, shape and morphology of the crack.

I N V E S T I G A T I O N S ON SOUTH END SHIELD SYSTEM OF RAPS-1 Two types of tests (viz. air-leak test and water run-down test) were conducted to detect and locate the leak paths. In the first type, the water level in the end shield system was kept at a certain level so that no water leakage was observed. The end shield was pressurised with air and the pressure was maintained at 5 psi. Twelve different locations on the FSTS were selected in the region of interest, and the AE sensor was fixed at each location in turn and the signals were recorded. On-line time domain analysis of the signals after selecting an appropriate threshold and gain was also attempted during the air-leak test, but it did not give any meaningful information. This confirmed the necessity to carry out frequency domain analysis. The aboverecorded signals were analysed in the frequency domain by computing the averaged auto-power spectrum for each location. A dominant frequency

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component of 250 kHz in the averaged auto-power spectrum was observed for the signal acquired at a specific location. This specific location corresponds to the new leak path on the CSTS detected earlier by DP test. For the surrounding adjacent locations this component is significantly lower in amplitude. The fact that the location giving rise to a dominant frequency component of 250kHz corresponds to the new leak path on the CSTS detected by DP test confirms the validity of the frequency domain analysis approach for the reliable detection and location of leak paths on the CSTS by probing from the FSTS. CHI 1.7089m

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Another signal, having a dominant frequency range of 175-200 kHz, was also observed at the next adjacent ligament towards the west side. The magnitude of this signal was also significantly less when the sensor was kept at surrounding adjacent locations. This indicated the presence of another leak path on the CSTS. The variation in the frequency components of the signals due to various leak paths is attributed to the size, shape and morphology of the leak paths. In order to confirm the additional leak path detected by the above air-leak test, a water run-down test was conducted. The water level in the end shield was raised well above the leak paths. Air at a pressure of 5psi was maintained above the water level and water was allowed to leak through the leak paths. The AE sensor was kept at a selected location on the FSTS at which the AE signals due to both the leak paths could be picked up simultaneously during the above air-leak test, and the signals were recorded continuously until both the leak paths were exposed to an air leak where the water level stabilised. The recorded signals were later analysed chronologically and the following observations were made: (a)

The signal at the beginning of the test was only due to background noise (Fig. 5(a)). (b) The signal having a predominant frequency component of 250 kHz appeared first (Fig. 5(b)) when the leak path detected by DP test became exposed to an air leak. The amplitude of this component slowly increased till the water level reached below the leak path. (c) After the appearance of the 250 kHz component, another component

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P, Kalyanasundaram et al.

in the range 175-200kHz, corresponding to the newly detected additional leak path, appeared (Fig. 5(c)). This indicated that this additional leak path is at a lower elevation compared to the one detected and confirmed by DP test.

CONCLUSION Acoustic emission techniques have been successfully applied for the detection and location of leak paths in the end shield systems of PHWRs. It also emerges that frequency domain analysis has advantages over time domain analysis for the detection and location of leaks occurring relatively at lower pressures and under noisy environments.

ACKNOWLEDGEMENTS We sincerely thank Professor A. K. Rao, Indian Institute of Science, Bangalore, and Dr Placid Rodriguez, Head, Metallurgy Programme and Material Science Laboratory, IGCAR, Kalpakkam, for their encouragement and support. We also thank Mr D. K. Bhattacharya and Mr P. Barat for many useful discussions. We also thank Mr M. R. Bhat, Indian Institute of Science, Bangalore, for his association during this investigation. We sincerely thank Mr C. V. Sundaram, Director, IGCAR, for encouragement and permission to publish this work.

REFERENCE 1. Murthy, C. R. L., Rao, A. K., Seth, V. K. & Krishnan, A., Acoustic emission monitoring of a nuclear reactor end shield for leak detection. In N D E in Relation to Structural Integrity, ed. R. W. Nichols & G. J. Dau. Applied Science Publishers, London, 1983, pp. 357-67.