Nuclear Science User Facilities 66 US DOE-NE programs are investigating new fuels and materials for advanced and existing reactors.A primary objective of these programs is to characterize the irradiation performance of fuels and materials. Examples of the key temperatures needed to evaluate fuel performance, as well as the desired accuracies and resolutions, are shown inTable I [1] . Similar measurement requirements exist for other parameters (i.e. fission-gas pressure). Ultrasonic technologies can be used to measure most of the key parameters of interest, but temperature was selected for initial development, as this is the most common measurement requested of irradiation programs. Development of Ultrasonic Thermometer at INL Joshua Daw – Idaho National Laboratory – Table I. Summary of desired fuel measurement parameters for irradiation testing. Estimated Peak Value Desired Accuracy and Spacial Resolution Fuel Temperature Ceramic LightWater Reactor (LWR): 1400º C 2% 1-2 cm (axially) 0.5 cm (radially) Ceramic Sodium Fast Reactor (SFR): 2600º C Metallic SFR: 1100º C Tristructural-isotropic (TRISO) HighTemperature Gas Reactor (HTGR): 1250° C Cladding Temperature Ceramic LWR: