Design and Analysis of a Mutual Inductance Level Sensor for Liquid Metals

This article describes the design and analysis of an electromagnetic level sensor for use in high-temperature liquid metal systems. The mutual inductance level sensor (MILS) described in this work was fabricated using two single-conductor mineral insulated cables wrapped in a bifilar fashion around a stainless steel tube core and was housed in an isolating thimble that preserved the pressure boundary of the test vessel. Two sensor variations were fabricated that differ only in active length, 1016 mm and 1778 mm. Experimental data were collected using the 1016 mm sensor (MILS-MKII-040) that demonstrated a sensitivity of 9.2 μV/mm in a room-temperature testing stand that used solid aluminum as a surrogate for liquid metal. Experimental data were collected using the 1778 mm sensor (MILS-MKII-070) that demonstrated a sensitivity of 6.9 μV/mm in the high-temperature (300°C) liquid sodium environment at the Mechanisms Engineering Test Loop of Argonne National Laboratory. The sensor performance was found to be repeatable over the course of several months, with roughly ±1% deviation from nominal output. Finite element models were developed in COMSOL Multiphysics that fully describe each test setup, and the models were validated using experimental data. The validated COMSOL models were used to perform an array of analyses that examined the performance of the sensor in differing environments. Maximizing the coil diameter inside the isolating thimble was found to maximize the signal and sensitivity of the sensor. An optimal operating frequency was found near 1000 Hz using both experimental data and COMSOL. The influence of a metallic thimble surrounding the sensor and a metallic sensor core was quantified and found to be negligible at the optimal operating frequency. The sensitivity of the sensor was quantified when monitoring the level of additional liquid metals. These include lead, lead bismuth eutectic (LBE), sodium potassium alloy (NaK), and lithium (in addition to sodium). The sensitivities were quantified using liquid metal properties at 350°C and 650°C. The geometry of the test stand model, all material properties used in the model, and the results are presented in a manner that the reader can replicate the model and perform additional analyses.