A reactive processing method was used to produce dense (499.5% relative density) W/Ta 2 C/Ta 2 WO 8 cermets from Ta 2 O 5 , WC, TaC, and phenolic resin. The powder compacts were reacted under vacuum at 14501C and then densified at 18501C. The microstructure of the cermets was examined using scanning electron microscopy and found to contain fine grains on the order of 1-3 lm in diameter. X-ray diffraction, transmission electron microscopy, and parallel energy electron loss spectroscopy were used to identify the composition and structure of the phases in the composites. The composites were found to contain approximately 47.5 wt% W, 47.3 wt% Ta 2 C, and 5.2 wt% Ta 2 WO 8 . Crack-free and dense W/Ta 2 C-based cermets were prepared by an in situ reactive sintering process without the application of pressure during densification.

Nuclear energy is a mature technology with a small carbon footprint. However, work is needed to make current reactor technology more accident tolerant and to allow reactor fuel to be burned in a reactor for longer periods of time. Optimizing the reactor fuel performance is essentially a materials science problem. The current understanding of fuel microstructure have been limited by the difficulty in studying the structure and chemistry of irradiated fuel samples at the mesoscale. Here, we take advantage of recent advances in experimental capabilities to characterize the microstructure in 3D of irradiated mixed oxide (MOX) fuel taken from two radial positions in the fuel pellet. We also reconstruct these microstructures using Idaho National Laboratory's MARMOT code and calculate the impact of microstructure heterogeneities on the effective thermal conductivity using mesoscale heat conduction simulations. The thermal conductivities of both samples are higher than the bulk MOX thermal conductivity because of the formation of metallic precipitates and because we do not currently consider phonon scattering due to defects smaller than the experimental resolution. We also used the results to investigate the accuracy of simple thermal conductivity approximations and equations to convert 2D thermal conductivities to 3D. It was found that these approximations struggle to predict the complex thermal transport interactions between metal precipitates and voids.

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MARMOT is the mesoscale fuel performance code under development as part of the US DOE Nuclear Energy Advanced Modeling and Simulation Program. In this report, we provide a high level summary of MARMOT, its capabilities, and its current state of validation. The purpose of MARMOT is to predict the coevolution of microstructure and material properties of nuclear fuel and cladding due to stress, temperature, and irradiation damage. It accomplishes this using the phase field method coupled to solid mechanics and heat conduction. MARMOT is based on the Multiphysics Object-Oriented Simulation Environment (MOOSE), and much of its basic capability in the areas of the phase field method, mechanics, and heat conduction come directly from MOOSE modules. While some validation of MARMOT has been completed in the areas of fission gas behavior and grain growth, much more validation needs to be conducted. However, new mesoscale data is required in order to complete this validation.