Dissolution of UO2 in molten Zircaloy-4. Pt. 3. Solubility from 2000 to 2500 C

For pt.2 see ibid., vol. 208, p.43, 1994. The solubilities of unirradiated UO2 in molten Zircaloy-4 and in a molten Zircaloy-4 alloy containing 25 at.% of oxygen have been determined from crucible-based experiments at 100 C intervals over the temperature range 2000-2500 C. In all cases, U-saturated melts are formed within a few minutes at temperature. Fuel solubilities increase 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 that relate fuel solubility and the volume fraction of dissolved fuel to the fuel/cladding mass ratio over the entire temperature range studied. These equations can be applied to different fuel assembly geometries 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. (orig.)