Computed Radiography of Resistance Temperature Sensor for Indian PHWR

Transcription

1 National Seminar & Exhibition on Non-Destructive Evaluation, NDE 2014, Pune, December 4-6, 2014 (NDE-India 2014) Vol.20 No.6 (June 2015) - The e-journal of Nondestructive Testing - ISSN Computed Radiography of Resistance Temperature Sensor for Indian PHWR Sanjoy Das, Benny Sebastian, D.Mukherjee & K.K. Abdulla Atomic Fuels Division Bhabha Atomic Research Centre Mumbai, India Abstract Resistance temperature detector(rtd) sensor is widely used for measurement of temperature. In Indian Pressurized Heavy Water Reactor ( PHWR ), it is used to monitor the inlet and outlet temperature of coolant in channel. It was noticed in some channel, RTD sensors showed considerable drift from actual temperature after prolonged use in reactor. Investigation was carried out on such damaged RTD sensors to find out the cause. Computed radiography was also carried out to find the internal health of such sensors. The digital version of image offers a substantial advantage over conventional film radiography in terms of dynamic range, linearity, reusability of image recording medium etc. Also several image filtering operation can be applied to improve the structural feature of interest and suppress the irrelevant surroundings. Due to complex configuration of the sensors, the image obtained with computed radiography system is quite noisy. Therefore noise removal operation was also performed to remove image noise. This paper describes the computed radiography as a technique to find the cause of the malfunctioning of temp monitoring RTD sensors. Keywords: Computed radiography, Image Processing, Digital radiography 1. Introduction Radiography produces two dimensional images of objects under test. Sometimes jobs are not simple in shape. In such cases, background is more complex in nature than a piece of plate. Radiographic exposure technique is governed by shape of the job, penumbra, back scattering phenomenon, radiation intensity etc. In industrial radiography, images of objects are recorded on a film, which is examined by using a film viewer. The image thus produced is permanent in nature which cannot be modified. The recent trend in inspection technology is to digitize the inspection data, which can be further processed to gain the additional information. Digital radiography is emerging field in inspection method, which produces a digital image on monitor rather than on a film. There are several techniques to produce the digital radiograph. At present the available technologies in digital radiography are fluoroscopy system, linear detector array, flat panel detector and computed

2 radiology system for direct production of digital image. Each system has it s own distinct operation principle. In computed radiography, a special phosphor coated flexible image plate is exposed to radiation beam. Then the image plate is scanned in special scanner to produce electronic image. This technique does not include any wet chemical processing, hence three is no associated chemical hazard. PHWR is horizontal pressure-tube type reactor, where fuels clusters are loaded inside pressure tube. There are 306 nos. pressure tube in a typical 220 MWe Indian PHWR. The rated inlet and outlet temp. of pressure tube are 249 o C and 293 o C respectively. Resistance temperature detector(rtd) sensor, i.e. figure 01, is used to monitor the inlet and outlet temperature of heavy water coolant. It was observed after prolonged use, some of the RTD sensors showed considerable drift in temperature. Computed radiography on such damaged sensors was carried out in assisting investigation of such damaged RTD sensors. Figure 01: Resistance Temperature Sensor 2.0 Resistance Temperature Detector(RTD): Resistance temperature detectors (RTDs), are sensors used to measure temperature by correlating the resistance of the RTD element with temperature. The RTD element is made from a pure material, typically platinum. Platinum is the best metal for RTDs because it follows a very linear resistance-temperature relationship and it follows the R vs T relationship in a highly repeatable manner over a wide temperature range. The unique properties of platinum make it the material of choice for temperature standards over the range of -200 C to 900 C. Coil design has a wire coil which can expand freely over temperature, held in place by some mechanical support which lets the coil keep its shape. This strain free design

3 allows the sensing wire to expand and contract free of influence from other materials. This coil resembles a filament in an incandescent light bulb. The housing or mandrel is a hard fired ceramic oxide tube with equally spaced bores. The coil is inserted in the bores of the mandrel and then packed with a very finely ground ceramic powder. A schematic diagram of RTD sensors is shown in figure- 02. Figure 02: Diagram of Resistance Temperature Detector. 3.0 Computed Radiography Imaging System: Computed radiography refers to conventional projection radiography, in which the image is acquired in digital format using an imaging plate rather than on film. The imaging plate looks very similar to fluoroscopy screen, but it works on different principle. The special phosphor of image plate interacts with radiation beams, absorbs and stored part of it s energy in it s structure, called electron-trap. After exposure, the image plate which contains latent image is scanned in a special scanner with the help of fine laser beam. The amount of photo stimulated luminescence(psl) received is proportional to amount of radiation energy stored in image plate. A photo multiplier tube(pmt) measures the light intensity, which is used to produce a digitized image. As the surface of the image plate is scanned by the fine laser beam, the light quanta emitted from each point from image plate is converted into digital values for each pixel and stored in the computer as a digital image. This process is described in figure 03. After completing the scanning, the image can be presented on monitor and can be evaluated by experts. A number of

4 image processing operation can be performed on the digitize image for effective evaluation and quantification. Figure 03: Principle of operation of Inage Plate base CR System 4.0 Computed Radiography of RTD: The RTD sensors head received in our laboratory is approx 1.5 mm dia. and 15 mm long. They are exposed at 60 kv using mini-focus x-ray machine of focal spot dimension 0.4x0.4 mm. At first, damaged sensors received from the plant was computed radiographed and the digital image so obtained was processed to obtain noise free, edge enhanced sharp digital radiograph. Also some fresh new RTD sensors were computed radiographed and were processed to obtain high contrast digital image. Both the images were evaluated and it is found that the coiling of Pt wire is uniformly spaced in fresh RTD sensors, i.e. figure 04. The dia. of wire is few tens micron and each turn is separated from adjacent turn clearly. Whereas in case of damaged RTD sensors, the coiling is compressed at some locations and dilated at other locations as shown in figure 05. The sensors are subjected to large amount of vibration at high temp. in reactor. This can adversely affect the coiling of wire after prolonged use.

5 (A) (B) (C) Figure 04: Computed radiograph of fresh RTD sensor. (A) Complete view. (B) Connector end view. (C) Extreme end view. Dilated Coil Compressed coil 5.0 Conclusion: Figure 05: Computed radiograph of damaged RTD sensor.

6 In the damaged sensors, the coiling of Pt wire is not regularly spaced. At some location it was expanded which does not cause any change in resistance value. But in the collapsed region, the turn of coils are in contact with other which might cause apparent change in resistance value. This can be source of error in temp. measurement. References 01. ASNT, Nondestructive Testing Handbook, third edition: Vol. 4, Radiographic Testing, Columbus,Ohio, American Society for Nondestructive Testing, Das, Sanjoy, D.M ukherjee and B.K. Shah, Weld Testing by Image Plate Based Computer Radiography, M aterial Evaluation, Americans Society for Non-destructive Testing(ASNT), Columbus, USA, M arch-2012,vol.70/issue Sanjoy Das, D. M ukherjee and B.K.Shah, Advanced Digital Radiography Systems and Image Processing Operations in Radiography Inspection, Inspectioneering Journal, Vol.16/Issue6,2010,USA.