Materials issues in high temperature ultrasonic transducers for under-sodium viewing.

Liquid sodium is used as the coolant in some fast spectrum nuclear reactors. This material is optically opaque. To facilitate operations and maintenance activities, an ultrasonic under-sodium viewing system has been developed. In the USA, the technology was successfully demonstrated in the 1970s and, over the intervening 30+ years, the capability was lost. This paper reports materials challenges encountered in developing both single-element and linear phased-array 2-MHz transducers that must operate at temperatures up to 260°C. The critical issues are fundamentally material selection: the ability of a transducer to be immersed into liquid sodium and function at 260°C, to achieve wetting and transmission of ultrasound into the sodium, and to be able to be removed and re-used. [ABSTRACT FROM AUTHOR]