DEVELOPMENT OF AN INDUCTANCE BASED SENSOR FOR DETECTING EMITTER PORTION OF SELF POWERED NEUTRON DETECTOR IN SHIELDING FLASK

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1 More Info at Open Access Database DEVELOPMENT OF AN INDUCTANCE BASED SENSOR FOR DETECTING EMITTER PORTION OF SELF POWERED NEUTRON DETECTOR IN SHIELDING FLASK Neeraj Gupta, SO/E 1,a, M.B.Jadhav, SO/G 1,b, Jimmy Mathew, SO/H 1,c, D.N. Sanyal, SO/H 1,d 1 Nuclear Power Corporation of India Limited, Anushaktinagar, Mankhurd, Mumbai a neerajg@npcil.co.in, b mbjadhav@npcil.co.in, c jmathew@npcil.co.in, d dnsanyal@npcil.co.in Keywords: SPND (Self Powered Neutron Detector), Eddy Current Testing, Phase discrimination technique, Remote Visual. ABSTRACT The work related to removal of Self Powered Neutron Detectors (SPNDs) from the reactor core at Tarapur Atomic Power Plant-Unit 3 & 4 (TAPS-3&4) requiring special care & remote operation, primarily due to use of Cobalt as the emitter material in the SPND Sensor. Due to long residence time, of the SPND, in the reactor core resulting in high neutron fluence, the Cobalt-60, in the SPND is highly radio-active. Special tools & supporting equipment have been developed for safe & easy removal of SPNDs from reactor core. One of the essential requirements for the tooling is to detect the arrival of SPND Emitter portion (containing Co-60 hence acting as a source of high radiation) inside the Sheilding Flask as the cable is reeled in, for further actions. Simple Remote visual equipment had its limitation due to high radiation fields present. The paper describes the features and details of a simple arrangement using Eddy Current phase discrimination based sensor successfully developed and deployed for the above purpose thus simplifying a complex activity involving high radiation fields by utilizing readily available ECT systems. 1.0 INTRODUCTION SPND consists of a Co Emitter element, which is encapsulated within a metallic sheath with an insulating material in between. A thin conducting wire is taken from the emitter element and taken out for obtaining the emitter signal. This signal lead wire is also encapsulated inside a thin metallic tube with insulating powder in between the two. Upon being placed in a neutron field/ flux, the emitter portion interacts with the neutrons and generates free electrons which are captured by the metallic casing. The electron current (though very small) can be calibrated to provide measurement of neutron flux. Due to long residence time in the reactor core resulting in high neutron fluence, SPND emitter element of Cobalt-59 gets converted to Cobalt-60, which is highly radio-active. Hence, special care & tools are required for removal & handling of this item. Figure 1 shows a photograph of a SPND which was used for sensor development. The development work related to removal of SPNDs from the reactor core of 540 MWe Tarapur Atomic Power Station-3&4 (TAPS-3&4) resulted in several new tools being

2 conceived & developed on diverse principles. One of the major goals was automated, safe & remote operation of the SPND Removal Tool System without any direct human intervention and reduction in man-rem consumption or radiation exposure. For the same, it was necessary to detect the arrival and crossing of the SPND emitter portion at the entry point of the Shielding Flask. Precise position of SPND emitter was required for further processing, which included separation of emitter portion from its extra cable. This made the detection of emitter portion very important EMITTER PORTION Fig-1 : Cobalt SPND used for sensor development Several ideas based on diverse principles were explored such as use of remote visual imaging, contact type proximity sensing, capacitance and optical based sensors etc., but each one had its own limitations. The sensor capability to distinguish between 3.7 mm diameter SPND emitter from 2 mm dia. SPND cable was a challenge. Also, reliability of this sensor performance in presence high gamma radiation from Cobalt added further to the challenge. After studying the various options, it was decided that an inductance based sensor would be suitable due to its simple construction, availability of required components, low probability of failure & easy to use characteristics. Hence, a new idea to detect the emitter portion using a non-contact technique utilizing the principle of magnetic permeability variation was finalized. THEORY Inductance of a coil is the behavior to resist any change of electric current through the coil. As per Faraday's law, the inductance L may be defined in terms of the emf generated in the coil to oppose a given change in current. emf = -L (delta I)/(delta t) (1)

3 The inductance L of a coil of wire, is given by L = mn 2 A/l (2) where, L = Coil inductance, m = Permeability of the core N = Number of turns of the wire A = Cross Sectional area of the coil l = length of the coil Here, it is to be noted that the inductance is a function of core permeability. Also, it is known that two coils placed near each other such that their magnetic fields cut each other have a mutual induction effect. A change in current in any one coil will result in change in magnetic field of both the coils, which results in a reactionary response of developing an emf in the second coil. This mutual inductance M can be defined in terms of the emf generated in the second coil to oppose a given change in current in the first coil as under. emf 2 = -M (delta I 1 )/(delta t) (3) Like the coil self inductance L, mutual inductance M is proportional to number of turns in coil 1 & coil 2 and also the permeability of the space between coil 1 & coil 2. The voltage drop across coil 1, V 1, due to its resistance R 1, self inductance L 1 & mutual induction M is given by V 1 = R 1 I 1 + L 1 di 1 di 2 (4) + M dt dt Similarly, voltage drop across coil 2, V 2, due to its resistance R 2, self inductance L 2 & mutual induction M is given by di 2 di V 1 (5) 2 = R 2 I 2 + L 2 + M dt dt Where I 1 & I 2 are the currents in coil 1 & coil 2 respectively. Here it must be noted that both the self inductance and the mutual inductance are dependent on the permeability of the coil medium and any change in permeability will affect the self inductance and the mutual inductance of the coils. Now consider two coils forming a set of differential coils of an Eddy Current Testing (ECT) machine. The ECT machine display shall show no differential signal as long as both the coils are identical and having identical conditions within the coil core. However, if inductance of one coil is changed due to any reason, a differential signal can be

4 generated in the circuit. In case, the inductance of both coils is changed by the same amount, then again, no net signal is generated. EXPERIMENTAL SETUP After conceptualization of the sensor, initial experiments were performed in the lab with a single coil to check the sensitivity of the coil to the SPND, its emitter and cable portions. While conducting these experiments, an Eddy Current Testing (ECT) machine was utilised for providing excitation voltage as well as display of the return signal. Phase discrimination technique was utilized to ensure unambiguous graphical signal (Lissagous figure of 8 type of display) for the region of interest of the SPND arriving in the sensor portion. The initial experiment consisted of a coil fabricated using a simple cylinder made out of paper with several turns of enameled copper wire wound over it. This experiment provided good results and further development work was initiated. Thereafter, a double coil arrangement was conceived & two coils were operated in differential mode of operation, using the ECT machine. Refer Fig. 2 for the double test coils arrangement attached to ECT Machine and Fig 3 for Cobalt SPND with test coils. This time the coils were constructed using bobbins made from polymer material. The use of double coils gives redundancy during operation & also better signal characteristics and discrimination. The two coil arrangement was also successfully tested. The coil characterstics were optimized to provide an optimal signal for the effective detection of the SPND emitter. ECT MACHINE DOUBLE COILS Fig-2 : Double test coils attached to ECT Machine

5 Fig-3 : Cobalt SPND with test coils attached to ECT Machine Fig-4 : Checking the response with double coils

6 DEVELOPMENT & APPLICATION For the final sensor development, a nylon bobbin was fabricated & the two coils were wound in their respective grooves. This resulted in an arrangement wherein the two coils were placed one on top of the other. The two coils with the optimized parameters suited to the actual available space in the SPND Removal tool in the entire system were installed in a mock-up for trials. The two coils were connected to the ECT machine in a differential operation mode. The frequency, gain and phase were adjusted to obtain clear peaks on the ECT strip chart display. Fig-4 shows the checking of the response with double coils. The system generated a steady straight line signal as long as only SPND cable was moving through it. It was observed that centering of the SPND cable within the coil caused no significant change in the signal. This observation was critical for actual application, as the SPND cable location within the coil would have to be left free without any centering provision, due to radioactive contamination considerations. A clear peak was noted on arrival of the emitter portion of SPND in the coils. The rise in the peak is attributed to the emitter portion entering the first coil & the fall in the peak due to the sensor portion entering the second coil. Similar behavior was noted when the emitter portion leaves the coils, but with a phase difference of 180 degree. Trials were conducted with one coil connected to ECT machine & other coil having open terminals. In this case, an absolute shift in the signal from the coil was noted instead of a peak. Thus, the coil assembly generated clear signal for entry of emitter portion into the coil assembly as well as exit of SPND. The two signals had a distinctive phase difference, from each other. Refer Fig.-5 for the display signal. SPND EMITTER ENTERING THE COIL SPND EMITTER LEAVING THE COIL Fig-5 : ECT response with single coil

7 The distinct peaks in case of double coil arrangement gave a better visual indication as compared to the absolute shift onto one side in case of single coil arrangement. Refer Fig.-6 for the signal with double coil. Double coils also provide inherent redundancy in case of failure of one of the coils during field operation. But even with only single coil working, the arrangement could give clear signal, though different from double coil arrangement. Thus, the double coil arrangement would allow the operation to be completed successfully, even in case of failure of one of the two coils. Another benefit of double coil arrangement noted during the experiment was the signal indication differentiating the direction of SPND movement through the coil. EMITTER ENTERING THE DOUBLE COILS EMITTER EXITING THE DOUBLE COILS Fig-6 : ECT response with Double coil The sensor was fine tuned with several small changes & put into operation during the SPND removal campaign at TAPS-3&4 in 2011 and During the SPND removal procedure, the signal received from the sensor helped the power plant personnel to remotely ascertain the arrival of highly radioactive SPND sensor into the Shielding Flask as well as clearly indicating complete entry of SPND well within the shielding flask. This helped the power plant maintenance crew to carry out a safe & reliable operation with reduced radioactive exposure.

8 RESULTS & DISCUSSIONS The inductance based sensor developed has been used consistently for several SPND removal operations & campaigns. Further, the sensor has been fine tuned to detect other SPND emitter materials such as Vanadium. Field trials for this were also carried out successfully. Further application of the technique is now being explored for other uses, such as Reactivity Devices removal from the reactor core. ACKNOWLEDGEMENTS The authors express their gratitude to the NPCIL management for providing tacit support in exploring the new technique with multiple trials in NPCIL QA lab as well as in converting the technique into an integrated field application with active support from TAPS 3&4 MMU & QA sections and R&D centre. The motivation and support given by Shri S.F. Vhora, Executive Director (Technology Development) and Shri K.P. Dwivedi, Executive Director (QA) worth special mention. REFERENCES [1] J. B. Gupta, A course in Electrical Technolgy-Vol 1 11 th Edn Feb 2002, S.K. Kataria & Sons, Delhi. [2] William M. Flanagen, Handbook of Transformer Applications, McGraw Hill Book Co. ISBN [3] ndt-ed.org. [4] ASM Handbook, NDT Methods. [5] Warren J. Mcgonnagle, Non-Destructive Testing 2 nd Edition 1969, Gordon & Breach Science Publishers. *****