2018 | ANNUAL REPORT 67 Project Description Ultrasonic thermometry Ultrasonic thermometry has the potential to improve upon tempera- ture sensors currently used for in-pile fuel temperature measurements. Current methods for in-pile tempera- ture detection primarily rely on either thermocouples or post-irradiation examination methods (such as melt wires). Commercially-available ther- mocouples (e.g.,Type K,Type N,Type C, etc.) are widely used and cover a wide temperature range. However, their use is limited.Type K andType N thermocouples decalibrate at tempera- tures in excess of 1100°C. Material transmutation causes decalibration in tungsten/rhenium (e.g.,Type C) or platinum/rhodium (e.g.,Type R or S) thermocouples in neutron-radiation environments.Although larger- diameter, multipoint thermocouples are available, most thermocouples only measure temperature at a single loca- tion. Melt wires and other post-irradi- ation methods only allow estimation of maximum test temperatures at the point of installation.The labor and time to remove, examine, and return (if necessary) irradiated samples for each measurement also makes this out-of-pile approach very expensive. Prior ultrasonic thermometry applica- tions have demonstrated the viability of this technology, but in-pile applica- tions were primarily limited to high- temperature fuel damage tests, which ceased several decades ago [2]. Theory of Operation Waveguide based ultrasonic ther- mometers (UTs) work on the principle that the speed at which sound travels through a material (acoustic velocity) is dependent on the temperature of the material.The average acoustic velocity of a mate- rial can be measured by sending an ultrasonic pulse through a thin rod of known length and measuring the time between the initial pulse and the reflection of the pulse from the opposite end of the rod. By introducing acoustic discontinuities such as notches or sudden diameter changes into the rod, the probe may be segmented into multiple zones (the average acoustic velocity of each segment derived from timing of the successive reflections). If the ultra- sonic waves are non-dispersive (the rod having a diameter of less than one tenth of the signal wavelength