Kinetics of precipitation of U4O9 from hyperstoichiometric UO2+x

For safe and reliable operation of fission reactors in space, the phase diagrams and reaction kinetics of systems used as nuclear fuels, such as U–O, U–N, U–C, are required. Diffraction allows identification of phases and their weight fractions as a function of temperature in situ, with a time resolution of the order of minutes. In this paper, we will provide results from a neutron diffraction experiment studying the U–O system. Using the neutron diffractometer HIPPO, the decomposition of UO2+x into UO2 and U4O9 as a function of temperature was investigated in situ. From the diffraction data, the participating phases could be identified as UO2+x, UO2 and U4O8.94 and no stoichiometric U4O9 was found. Results of the experiment were used to improve existing thermodynamic models. The presented techniques (i.e., neutron diffraction and thermodynamic modeling) are also applicable to the other systems mentioned above.