Thermal conductivity is a key property that must be known for proper design, test, and application of new fuels and structural materials in nuclear reactors. Thermal conductivity is highly dependent on the physical structure, chemical composition, and the state of the material. Typically, thermal conductivity changes that occur during irradiation are measured out-of-pile using a "cook and look" approach. Repeatedly removing samples from a test reactor to measurements is expensive, has the potential to disturb phenomena of interest, and only provides understanding of the sample's end state when each measurement is made. There are also limited thermophysical property data for advanced fuels. Such data are needed for simulation design codes, the development of next generation reactors, and advanced fuels for existing nuclear plants. Being able to quickly characterize fuel thermal conductivity during irradiation can improve the fidelity of data, reduce costs of post-irradiation examinations, increase understanding of how fuels behave under irradiation, and confirm or improve existing thermal conductivity measurement techniques. This paper discusses efforts to develop and evaluate an in-pile thermal conductivity sensor based on a hot wire needle probe. Testing has been performed on samples with thermal conductivities ranging from 0.2 to 22 W/m.K at temperatures ranging from 20 degrees C to 600 degrees C. Thermal conductivity values measured using the needle probe match data found in the literature to within 5% for samples tested at room temperature, 6% for low thermal conductivity samples tested at high temperatures, and 10% for high thermal conductivity samples tested at high temperatures.
