Development of level measuring sensor for high temperature applications

The Bhabha Atomic Research Centre is currently working on development of a compact high temperature reactor (CHTR). CHTR is a 233U-Thorium fuelled, lead-bismuth cooled and beryllium oxide moderated reactor. The reactor, being designed, will demonstrate technologies for high temperature process heat applications such as hydrogen production from water. As a part of CHTR development and to facilitate R and D studies on thermal hydraulics, material compatibility and high temperature instrumentation on Pb-Bi coolant, a high temperature natural circulation based Liquid Metal Loop was setup. The heart of the liquid metal loop is the Instrumentation and Control (I and C) system. Many of the conventional and commercially available instruments and sensors do not work in the range of 400 deg C to 600 deg C for molten Pb-Bi coolant. Direct level, flow and pressure measurements is a great challenge as conventional techniques are not feasible with liquid Pb-Bi coolant. R and D efforts for the development of direct level measuring sensor working with eddy current and mutual induction principles were taken up. A level probe sensor based on mutual induction principle was fabricated in-house for online level measurement of molten lead-bismuth at high temperature. The sensor was inserted from the top flange of the expansion tank of liquid metal loop. Experiments were conducted by exciting primary with AC current at various frequencies with fixed level of liquid metal. The sensor was also calibrated with reference to liquid metal level measured by argon purging system and maintaining the AC signal at 105 mA amplitude, 35 kHz. The data were recorded at different temperatures from 300 to 410 deg C by varying the level of the liquid metal for the calibration purpose. Calibration showed almost linear variation of sensor RMS voltage output with level. For the same level, a small difference in output at different temperatures was observed. The paper deals with the design of sensor and experiments performed. (author)