Characterising the U-Nd-O miscibility gap by an experimental and a theoretical approach

Recent studies performed on (U1−y,Ndy)O2 highlighted the existence of a miscibility gap in this mixed oxide. The objective of this work is to describe this miscibility gap by means of two types of approaches: experimental and theoretical. The first approach involves a set of SEM analyses combined to XRD characterisations. The second approach consists in the thermodynamic assessment of the U-Nd-O ternary system using the CALPHAD method. A good agreement is obtained between the calculated and experimental data for Nd/(U + Nd) ratios from 3% to 28%. Both results show that this miscibility gap appears preferentially when the O/M ratio is less than two and when the Nd atomic fraction of the sample is higher than 0.04. This confirms that the existence of the miscibility gap in neodymium UO2 doped ceramics is consistent with the phase diagram calculated on the basis of a thermodynamic model. Finally, we discuss the conditions leading to the appearance of the miscibility gap with respect to the fabrication process.