Use of tungsten--2% thoria ultrasonic transmission line and sensor to improve the performance of high temperature ultrasonic thermometry

EG and G Idaho Inc. has been involved in the development of the ultrasonic thermometer for use in measuring nuclear fuel rod centerline temperatures. Development efforts have been applied toward increasing the useful range of the device and in preventing the sheath-to-sensor sticking problem which occurs at high temperatures. Classically, high temperatures are determined ultrasonically by measuring the propagation time between echoes generated at discontinuities placed along a refractory metal ultrasonic transmission line. At temperatures above approximately 1,600/sup 0/C the transmission line often bonds to the protective sheath at contact points. Noise echoes are generated at these bonding points that, at times, mask out the signal echoes. A tungsten-2% thoria ultrasonic transmission line and sensor offer low attenuation characteristics at high temperatures, which allow use of standoffs to provide isolation from the sheath. The ultrasonic characteristics of the tungsten-2% thoria sensor and the standoffs are discussed. Also, decalibration due to transmutations in a nuclear environment is addressed.