DISSOLUTION OF UO2 IN MOLTEN ZIRCALOY-4 .2. PHASE EVOLUTION DURING DISSOLUTION AND COOLING

Metallography and scanning electron microscopy analyses have been perf ormed on samples from a previous study of CANDU (CANada Deuterium Uran ium) UO2 fuel dissolution in molten Zircaloy-4 [1]. Under conditions o f good convective stirring within the melt, rapid UO2 dissolution occu rs, leading to melt saturation with respect to U after a few minutes i n the temperature range 2000 to 2200-degrees-C. Melt saturation is pos tulated to occur when the melt composition path across the U-Zr-O phas e diagram intersects the boundary of the two-phase [(U,Zr)O2-x+L] fiel d. The point of intersection depends significantly on the melt oxygen content. Contraction of the two-phase field at higher temperatures cau ses a corresponding increase in fuel solubility.