Melting of stoichiometric and hyperstoichiometric uranium dioxide

The solid–liquid transition in stoichiometric and hyperstoichiometric UO 2 was investigated by means of advanced techniques. Laser heating enabling fast melting and freezing processes was used under container-less conditions and buffer gas pressures up to 250 MPa, making non-congruent evaporation ineffective. Pulse thermograms of UO 2+ x with 0 ⩽ x ⩽ 0.21 were recorded with fast pyrometers and interpreted with computer simulations. In addition, a novel method for identification of phase transitions was implemented, based on the detection of surface reflectivity variations. The melting line of UO 2.00 was for the first time determined at pressures between 10 and 250 MPa, and the melting temperature of the stoichiometric oxide was measured to be 3147 ± 20 K, in fair agreement to previous measurements reported in the literature. The liquidus and solidus lines of UO 2+ x differ from the currently recommended data, which substantially underrate the effect of oxygen on melting.