Thermal Properties of Mixed Uranium Nitride Fuels

Sammanfattning

Uranium nitride (UN) is an advanced nuclear fuel candidate offering a uranium density approximately 40% higher than uranium dioxide (UO2) and a thermal conductivity four to eight times greater at the operating temperatures of lead-cooled fast reactors. However, during irradiation, the accumulation of fission products transforms the initially phase-pure ceramic into a complex multi-phase system, degrading thermal transport and narrowing the margin to centreline melting. The thermal conductivity of UN as a function of fission product loading has not previously been characterised systematically, representing a critical gap in the data required for fuel performance modelling and reactor licensing.This work investigates the effect of incorporating representative fission products on the thermal properties of UN using the simulated fuel (SIMFUEL) approach. UN powder was synthesised using the hydriding-nitriding-dehydriding method and mixed with ZrN, NbN, Mo and Ru at 5at.% each, as well as with a multi-element composition containing all four at 1.25at.% each. Ten pellets were densified by spark plasma sintering and characterised using X-ray diffraction with Rietveld refinement, scanning electron microscopy coupled with energy-dispersive spectroscopy, and Archimedes densitometry. Thermal diffusivity was measured by light flash analysis between room temperature and 1200◦C, and thermal conductivity was derived for UN, ZrN, Mo, Ru, UN5ZrN, UN5Mo and UN5Ru. A Maxwell–Eucken model with porosity correction was used to compare the measured data with two-phase model calculations, to isolate the contributions of phase mixing from microstructural degradation. All three composites have a lower thermal conductivity than pure UN across the full temperature range. This indicates that the accumulation of fission products introduces additional scattering mechanisms at phase boundaries, reaction interfaces, and within secondary phases, which offset the enhancement of the higher-conductivity additives. These results constitute the first systematic experimental dataset of thermal conductivity for UN-based SIMFUEL compositions and demonstrate that, while fission product accumulation measurably degrades the thermal performance advantage of UN, it does not fundamentally compromise it.