CHARACTERIZATION OF PLUTONIUM DISTRIBUTION IN THO2–PUO2 MIXED OXIDES BY ELECTRON PROBE MICROANALYSIS

Thoria–plutonia (ThO2–PuO2) pellets with a nominal composition of 9.0 wt% PuO2 were prepared using a fabrication route similar to an industrial process for production of urania–plutonia (UO2–PuO2) mixed oxide fuel. The green fuel pellets were separated into 2 batches and the sintering of each batch was carried out under a reducing atmosphere at 1820 °C or 1750 °C. The distribution of plutonium (Pu) in the sintered pellets was investigated by electron probe microanalysis using X-ray mapping and quantitative point analyses. The results show that the pellet samples consist of Pu-rich agglomerates with Pu content close to that of the mastermix blend and a thorium (Th)-rich matrix. The matrix and the Pu-rich agglomerates are separated by a transition zone with Pu content varying from practically nil to the Pu content of the Pu-rich agglomerates. X-ray maps taken from random regions of the centre of the pellets show different sizes of Pu-rich agglomerates irregularly dispersed in the Th-rich matrix. Image analysis of the Pu X-ray maps indicate that the average diameter of the Pu-rich agglomerates of the material sintered at 1820 °C and 1750 °C were 68 μm and 161 μm, respectively.