Experimental Investigation of the Interfacial Thermal Resistance in Uranium Dioxide and Beryllium Oxide Composites by the Flash Method

Abstract

The addition of a continuous, high thermal conductivity beryllium oxide (BeO) network to the uranium dioxide (UO2) nuclear fuel microstructure has been proposed for improved accident tolerance. The interfacial thermal resistance (ITR) is identified as critical to understanding the effective thermal conductivity of UO2 and BeO composites; however, existing measurements fit the ITR to underdetermined systems with complex percolated microstructures. This work presents a dedicated, combined experimental and analytical approach to quantify the ITR in UO2-BeO composites by the flash method and examines the role of the ITR in the design of microstructures with improved thermal conductivity for nuclear applications. Dense UO2-BeO composites with a dispersed BeO granules were fabricated using methods aligned with industry practice to provide uniform microstructures with distinct thermal properties and limit assumptions to improve confidence in measurement accuracy. A smoothed inversion procedure transformed the observed granule cross-sections to the true diameters. The ITR and component thermal conductivities were fit by an analytical model to the experimental data measured by light flash analysis. The measured ITR, on the order of 10^-6 m^2-K/W, was remarkably near reported values fit to continuous BeO microstructures of varied fabrication technique and thermal conductivity improvement. The method applied here reduced uncertainty compared to the existing literature by fitting the thermal conductivity of UO2 and BeO in application, which were more predictive of the effective thermal conductivity, rather than relying on literature correlations. With the resulting ITR, an effective thermal resistance model for idealized continuous microstructures was applied to quantify the critical particle diameter to maintain thermal conductivity improvement over UO2 across operating and accident conditions, given a constant ITR at these temperatures. A lower limit of 100 µm is identified for a 5 vol.% BeO composite and 40 µm for a 10 vol.% BeO composite with the upper diameter set by practical fabrication limits demonstrated as low as 300 µm. The insight reported with high confidence here improves predictions of the composite thermal conductivity and evaluation of the impact of features such as shape and orientation for the BeO network to design the microstructure for improved fuel performance and accident tolerance.