The dissolution of UO2 by molten Zircaloy-4 cladding

The dissolution of UO2 by molten Zircaloy-4 cladding, and structural changes to the UO2 matrix, have been investigated using internally heated fuel rod simulators. The simulators were heated in an inert atmosphere at a fixed temperature ramp rate, and held at cladding surface temperatures (1800 degrees C to 2200 degrees C) above the melting point of the cladding (1760 degrees C) for one second to ten minutes. The experiments show that a complex microstructure is formed between the UO2 pellets and the molten Zircaloy-4 cladding. The molten cladding contains varying proportions of uranium and oxygen that come from the dissolution of the UO2 pellets by the molten Zircaloy-4. There is no penetration of Zircaloy-4 into the solid UO2, even along grain boundaries. Measurements of dissolved uranium in the cladding, by scanning Auger microscopy and chemical analysis, show that only small amounts of UO2 (less than 3.5 vol. percent) are dissolved by the molten cladding