[en] 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