Influence of the oxygen potential on the sintering of UO2–45% PuO2

Abstract

The manufacturing of U1−yPuyO2±x mixed oxide fuel is usually based on the powder metallurgy process, and diffusion mechanisms occurring during the sintering of oxide ceramics are affected by the oxygen content of the atmosphere.In the present work, the sintering behaviour of the 55% UO2+45% PuO2 mixture was investigated as a function of the oxygen potential ranging from −524 to −355kJmol−1. Due to the β− decay of 241Pu present in the initial PuO2 powder, 0.72% of 241Am/metal ratio is present within the sample.This study has shown the formation of U0.55Pu0.45O2−x solid solution whatever the oxygen potential applied. The apparent relative density of the ceramics varies with the oxygen potential and goes through a maximal value of 98.7%, for an oxygen potential ranging between −455 and −422kJmol−1. The mean grain size and the intragranular porosity increase with the oxygen potential. A sintering map was established as a function of the oxygen potential. O/M ratio deduced from lattice parameters is close to 2 for all the samples, except for the more reductive conditions (−524kJmol−1) indicating an uncontrolled oxidation of the samples during cooling down, storage and samples preparation for XRD and XAS measurements.Pu distribution within U0.55Pu0.45O2±x ceramics obtained from powders mixing sintered at 1700°C under an oxygen potential of around −425kJmol−1 was studied as a function of dwell time by means of XRD, EPMA and XAS analyses. The optimal homogeneity of the Pu distribution was achieved after 24h sintering. XANES results confirmed the sample oxidation and could demonstrate the only presence of tetravalent Pu whereas Am is partially reduced.