Dissolution of UO2 in molten Zircaloy-4. Pt. 4. Phase evolution during dissolution and cooling of 2000 to 2500 specimens

For pt.3 see ibid., vol.232, p.1-12, 1996. This report describes metallographic and scanning electron microscopy analyses of specimens from experiments to measure the solubility of unirradiated UO2 fuel in molten Zircaloy-4 at 2300, 2400 and 2500 C. The observed microstructures and individual phase analyses from specimens 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 C experiments. Although slow diffusion of oxygen from residual UO2 to the melt can, in theory, induce subsequent precipita tion of small quantities of a (U, Zr)O2-x phase over long time intervals, no evidence for precipitation was found in the microstructures and phase compositions. Thus, the observations support the contention that UO2 dissolution effectively ceases when the melt reaction path across the U-Zr-O phase diagram intersects the boundary of the two-phase [(U, Zr)O2-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. (orig.)