Application of transient contact method for intended direct determination of thermal properties in irradiated nuclear fuel

The thermal-physical properties of nuclear fuel materials such as thermal conductivity and thermal diffusivity, are key parameters influencing nuclear fuel performance during normal operation and transients, as well as reactor design and safety. The thermal conductivity and thermal diffusivity of uranium dioxide based fuels are known to vary greatly, depending on the local fuel chemistry, porosity and burnup resulting from the irradiation history. Many fuel performance codes use thermal properties data collected from out-of-pile measurements on fresh and irradiated fuel samples but also benchmark their temperature predictions against in-pile fuel centreline measurements from test reactors. However, there is a scarcity of experimental data where both in-pile fuel centreline temperature measurements and out-of-pile thermal properties measurements have been performed on the same fuel rods, either prior to or following the in-pile measurements. The objective is to provide added value to the existing legacy of the centreline temperature database, by providing complementary data for a subset of the fuel rods that have been widely used for fuel performance code benchmarking and validation. The experimental approach development in this work is Transient Plane Source (TPS) technique, where a transient planar heat source/sensor applied to a flat sample surface is used to directly determine the thermal conductivity of the fuel material. The TPS method is utilized for determination of thermal properties of unirradiated UO2, (U,Gd)O2, and ADOPT fuel samples to test its applicability for nuclear fuel materials at normal conditions and potentially for low-scale measurements at elevated temperatures. Fuel performance modelling performed with the FRAPCON code indicates that the thermal conductivity at temperatures below 600 °C is most critical for calculating the fuel temperature in HBWR tests for centreline temperatures up to 1000 °C, justifying the use of TPS method limited to a lower temperature range than widely used Laser Flash technique.