Development of Ultrasonic Sensors for Operation in a Heavy Liquid Metal

This paper is devoted to the development of high temperature,$gamma$, and neutron radiation resistant ultrasonic sensors that must operate continuously in a liquid Pb/Bi alloy up to a temperature of 450$^circhboxC$. The main problems are acoustic coupling of a piezoelectric element to a protector and wetting of the sensor by a heavy liquid metal. The piezoelement was attached to the sensor body by a gold to gold diffusion bonding process, monitored ultrasonically. Long-lasting wetting of the active surface of the sensors was achieved by coating the front face with a protective diamond-like carbon (DLC) layer. Due to the high radiation, only a limited number of materials could be used in the sensor design. The best performance was obtained using bismuth titanate piezoelectric elements, which showed no noticeable changes of pulse responses and transfer coefficients during irradiation and high-temperature tests. The housing of the sensors is made of stainless steel AISI 316 and is laser welded, and a high-temperature$varnothing$1-mm 15-m-long mineral cable is used. The ultrasonic velocity in the liquid Pb/Bi in the temperature range 160$^circhboxC$–460$^circhboxC$was measured using developed sensors, and the signal losses at various distances up to 0.8 m were evaluated.