Nuclear Science User Facilities 68 ), the temperature-dependent acoustic velocity of the sensor mate- rial, c(T), is related to the density, ρ(T), and the elastic (Young’s) modulus, E(T), (both properties are also temperature dependent) of the sensor material through the following equation: A typical multisensor UT system with key components identified is shown in Figure 1. As indicated in this figure, a narrow ultrasonic pulse is generated in a magnetostrictive rod by a short-duration magnetic- field pulse produced by an excitation coil.The ultrasonic pulse propagates to the sensor wire, where a frac- tion of the pulse energy is reflected at each discontinuity (notches or diameter change). Each reflected pulse is received by the excitation coil, transformed into an electrical signal, amplified and evaluated in a start/stop counter system.The time interval between two adjacent echoes is evaluated and compared to a calibration curve to give the average temperature in the corre- sponding sensor segment.When a number of notches are available on the wire sensor, the various delay time measurements give access to a temperature profile along the probe. IrradiationTesting Until recently, INL-developed UTs had been tested at high temperatures in furnace environments (i.e., inert gas or vacuum atmosphere), but not in an irradiation environment. In-core qualification of a new sensor is a necessary step prior to deployment in irradiation test campaigns. ULTRA To generate and receive ultrasonic pulses and signals, two of the most commonly used technologies are piezoelectric and magnetostrictive transducers. Only the magnetostrictive transducers will be discussed here.The current capabilities of magnetostric- tive transducers are typically limited c(T) = (1) E(T) ρ(T) Figure 1. Schematic diagram of magnetostriction based ultrasonic thermometer.