Uranium Interaction in a Porous ZrO ₂ -MgO Matrix

During the fabrication of fuel elements for research reactors, uranium silicide alloys are melted at temperatures close to 1800°C using porous ZrO2-MgO crucibles. The controlled porosity required to withstand thermal shock also promotes infiltration of approximately 1% to 2% of the molten alloy into the pore network, retaining uranium within the ceramic matrix. In the present work, the interaction between a molten U-Si alloy and porous stabilized zirconia was investigated through microstructural, crystallographic, and diffusion analyses.The ZrO2-MgO matrix exhibited a total porosity of ~18%, while molten alloy infiltration reached nearly 50% of the crucible wall thickness. X-ray microtomography revealed the preferential filling of larger pores, reducing the maximum equivalent pore diameter from ~400 to ~250 µm after infiltration, with a geometric tortuosity of τ = 1.907. The uranium concentration profiles obtained by energy-dispersive X-ray spectroscopy allowed for the estimation of an effective diffusion coefficient on the order of 10−5 cm2/s. X-ray diffraction and Rietveld refinement revealed the formation of a previously unreported disordered cubic pyrochlore–type structure crystallizing in the Fd3̅m space group with stoichiometry Zr2(Zr0.8125 U0.1875)2Si3O4.The results indicate that uranium infiltration is primarily governed by metallostatic pressure and pore connectivity, while simultaneous diffusion and chemical interaction promote the formation of uranium-containing pyrochlore/defective fluorite–type structures relevant for actinide immobilization applications.