2018 | ANNUAL REPORT 85 Figure 2. Microscope images of printed 3ω sensors. in-pile thermal conductivity measure- ments.The majority of the fuel thermophysical properties measure- ments to date, including thermal conductivity, have been performed out of pile in a hot cell. In-pile measure- ment of thermal conductivity presents a significant challenge due to the space constraints, the difficulties to realize nonintrusive sensor implemen- tation and the extreme environment of in-pile irradiation and temperature. Hence, very few thermal conductivity methods have been implemented in-pile, and these methods are often intrusive and fail to yield accurate fuel thermal conductivity. Project Description The goal of this project is to develop advanced 3ω sensors that can be tightly integrated with nuclear fuel system using advanced manufac- turing methods to perform in pile thermal conductivity measurement.To determine the thermal conductivity, we will use the 3ω method.A metal sensor directly printed onto substrates representative of nuclear fuel materials serves as both a heater and a tempera- ture sensor.The heater is driven by AC current at frequency ω, which produces a localized alternating temperature change through periodic Joule heating at frequency 2ω, with tunable heat penetration depth controlled by the current frequency.The 2ω tempera- ture change of the heater results in changes of its electrical resistance at frequency 2ω and a corresponding third harmonic component of the heater voltage (3ω voltage) which we can measure using a lock-in amplifier. This frequency-modulated thermal conductivity measurement offers real advantages over other methods for obtaining temperature-dependent thermal conductivity since we can confine the AC temperature field to the region of interest and minimize the influence of radiation heat loss. The project outcome will advance