DISSOLUTION OF UO2 IN MOLTEN ZIRCALOY-4 .3. SOLUBILITY FROM 2000 TO 2500-DEGREES-C

The solubilities of unirradiated UO2 in molten Zircaloy-4 and in a mol ten Zircaloy-4 alloy containing 25 at.% of oxygen have been determined from crucible-based experiments at 100 degrees C intervals over the t emperature range 2000-2500 degrees C. In all cases, U-saturated melts are formed within a few minutes at temperature. Fuel solubilities incr ease significantly with rising temperature and, at any one temperature , are highest in initially unoxidized Zircaloy-4 and when the initial UO2/Zircaloy-4 mass ratios are low. Regression equations are derived t hat relate fuel solubility and the volume fraction of dissolved fuel t o the fuel/cladding mass ratio over the entire temperature range studi ed. These equations can be applied to different fuel assembly geometri es with unoxidized or partly oxidized cladding to place an upper limit on the quantity of fuel dissolved at any temperature during a severe fuel damage accident. Sample calculations for 37 element CANDU bundles are used to illustrate their applicability.