Melting transition measurements in uranium dioxide
The broad objective of research described in this thesis was the experimental determination of the solid / liquid transition in both stoichiometric and hyperstoichiometric uranium dioxide. Experimental information in this field constitutes a necessary reference for the analysis of hypothetical nuclear reactor accidents and, in particular, for the formulation of an equation of state (EOS) for uranium dioxide. A review of previous research on the same topic revealed that only few and uncertain experimental data were available prior to this work, obtained with conventional thermal analysis techniques in which it was not possible, at temperatures close to melting, to prevent sample contamination from the containing material and non- congruent evaporation. The experimental approach in the current research consisted mainly of the development of a laser heating method that enabled fast (tens of ms) melting / freezing experiments under containerless conditions, during which the sample could be kept under a buffer gas at high pressure (tens of MPa) in order to suppress any evaporation. In this way, the main difficulties encountered by previous researchers were overcome. U02~x samples with 0 ≤ x ≤ 0.21 were prepared starting from original nuclear-grade pellets and analysed. Thermograms were recorded with fast pyrometers and interpreted with the help of a one-dimensional codesimulation of the measurements. In addition, a novel method for measurement of phase transition points was developed, based on the precise detection of surface reflectivity variations. The point of congruent melting was determined for the stoichiometric oxide in the pressure range between 0.1 and 250 MPa. Liquidus and solidus lines were established, for pressures between 50 and 250 MPa, in the hyperstoichiometric range investigated. The results obtained, which in a number of cases significantly differed from the existing data, were finally used for a better assessment of the phase diagram of the system U02~x in the vicinity of the melting transition.