Temperature-Dependent Acoustic Properties of Magnetostrictive Iron-Gallium Alloys

Abstract The precise and continuous measurement of reactor core temperature is crucial for the safe and efficient operation of light water reactors. Current sensor technologies are limited in their capabilities for continuous monitoring, linearity, and multilocation detection. Magnetostrictive materials, which deform in response to magnetic fields or exhibit magnetization variation when stressed, offer a promising solution through ultrasonic waveguide thermometers. This study prototyped a high-temperature and radiation-tolerant UT consisting of a solenoid and a Galfenol waveguide, and quantified its performance as a thermometer up to 300 °C. The impact of waveguide diameter, ambient temperature, and thermal treatment on UT performance was then thoroughly assessed. Galfenol waveguides with diameters of 0.5 mm, 0.8 mm, and 1.0 mm showed uniform temperature-dependent behavior with minimal hysteresis error when cycled between RT and 300 °C. The acoustic attenuation coefficient decreased with increasing wire diameter, likely due to the combined effects of eddy currents and magneto-mechanical energy conversion. Although thermal annealing at 900 °C for an hour in a nitrogen environment caused significant surface damage to the waveguides, it effectively relieved internal stress, thus minimizing the nonlinearity in the acoustic attenuation coefficient.