Nuclear Science User Facilities 74 by the UT and by several thermo- couples located near the UT.The thermocouple labeled TCSPND4 is closest to the UT, and offers the best comparison.The standard deviation of the UT temperature is ~2°C at the maximum test temperatures. Future Activities The results of testing to date have been very promising, but some work remains in order to consider the UT completely qualified for in-core deployment.The issue observed in the ULTRA2 test is likely due to changes made to a ceramic cement used to fill the transducer housing, but this has not been verified through PIE.This issue may be solved by identifying a better potting compound or by making the coil wire more robust, either by changing materials or wire diameter. A planned NSUF-sponsored irradia- tion test, DISECT, will be performed in the Belgian BR2 reactor in collaboration with Studiecentrum voor Kernenergie/Centre d’Étude de l’énergie nucleaire (SCK•CEN). DISECT is meant to study metallic fuel foils arranged along a ~1 meter test vehicle. Multi-point temperature measurements along the length of the DISECT capsule are needed in order to fully characterize the experi- ment. Multipoint thermocouples are planned as primary instrumentation, but an INL UT will also be included in the test, along with a promising fiber-optic sensor.The expected temperatures are relatively low for a UT, less than 300ºC for the first phase of testing, but the need for a temper- ature-profile measurement makes the test ideal for demonstration of the performance of the UT because the temperatures measured by the single UT can be directly compared to those of the multipoint thermocouples.The UT designed for this application will have 10 measurement zones along the length of the test, making this the most complicated irradiation yet for the sensor. This article documents the develop- ment of a multipoint ultrasonic thermometer and the progress, to date, toward regular deployment in irradiation experiments.The largest hurdle a new in-core sensor must overcome to be considered qualified is demonstration in prototypic irra- diation conditions.The reactor access provided by NSUF has been critical in progressing the UT through several stages of design improve- ment, and has shown that the UT is a viable option for making multi- point temperature measurements in extreme irradiation environments such as those experienced in the ATR. Ultrasonic technologies offer the potential for high accuracy and resolution for in-pile measurement of a range of parameters,including geometry changes,temperature,crack initiation and growth,gas pressure and composition,and microstructural changes.