Laboratory experiments were performed in the temperature range 1800 to 2000°C in inert gas to study the chemical interaction between solid uranium dioxide (UO2) and liquid Zircaloy-4, and the wettability of UO2 by molten Zircaloy as functions of time and Zircaloy oxygen concentration. The experiments were interrupted after various reaction times to determine the extent of the chemical interaction and of the UO2 dissolution. The results show that the dissolution of UO2 by molten Zircaloy is primarily a chemical process, and that the extent of the interaction depends on the wettability of UO2 by molten Zircaloy. The wettability, however, depends strongly on the oxygen content of the Zircaloy and therefore on the time of UO2/Zircaloy contact. The wettability improves with increasing oxygen content. Zircaloy reduces UO2 to form a homogeneous [uranium, zirconium, oxygen (U,Zr,O)] melt at low oxygen concentrations or a heterogeneous (U,Zr,O) melt which contains (U,Zr)O2 particles at high oxygen concentrations. During cooling, the (U,Zr,O) melt decomposes into a (U,Zr) alloy with high uranium content and oxygen-stabilized α-Zr(O). The amount of fuel which can be liquefied by molten cladding depends on the initial oxygen content of the melt. Oxygen-free liquid Zircaloy can dissolve considerably more UO2 than Zircaloy rich in oxygen. The significance of these experiments is that UO2 fuel can be “liquefied” by molten Zircaloy far below the melting point of UO2.

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