DISSOLUTION OF UO2 IN MOLTEN ZIRCALOY-4 .4. PHASE EVOLUTION DURING DISSOLUTION AND COOLING OF 2000 TO 2500-DEGREES-C SPECIMENS

This report describes metallographic and scanning electron microscopy analyses of specimens from experiments to measure the solubility of un irradiated UO2 fuel in molten Zircaloy-4 at 2300, 2400 and 2500 degree s C, The observed microstructures and individual phase analyses from s pecimens heated for up to 30 min at each temperature indicate that all melts became effectively saturated after a few minutes at temperature , in agreement with our findings from previous 2000-2200 degrees C exp eriments. Although slow diffusion of oxygen from residual UO, to the m elt can, in theory, induce subsequent precipitation of small quantitie s of a (U, Zr)O-2-x phase over long time intervals, no evidence for pr ecipitation was found in the microstructures and phase compositions. T hus, the observations support the contention that UO2 dissolution effe ctively ceases when the melt reaction path across the U-Zr-O phase dia gram intersects the boundary of the two-phase [(U, Zr)O-2-x + L] field . The saturated melt compositions have been used to revise the pseudo- binary Zr(O)-UO2 phase diagram, allowing more realistic prediction of the products formed by fuel/molten cladding interaction.