Chemical interactions between UO2 and Zircaloy-4 from 1000 to 20000C

The chemical interaction between solid and liquid Zircaloy-4 and solid UO2 was examined in the temperature region 1000 to 20000C in argon. The solid/ solid reaction experiments were performed with short light water reactor fuel rod sections with an external pressure of 1 to 80 bar. The annealing times varied between 60 and 9000s. The reaction experiments with liquid Zircaloy were performed in UO2 crucibles between 1800 and 20000C. In addition, the wetting behavior between liquid Zircaloy and UO2 was also examined. The extent of the chemical interaction below the melting point of Zircaloy depends decisively on the solid/solid contact between fuel and cladding. If good contact exists, Zircaloy reduces UO2 to form oxygenstabilized α-Zr(O) and metallic uranium. The uranium reacts with zirconium to form a (U,Zr) alloy, which lies between two α-Zr(O) layers. The UO2/Zircaloy-4 reaction obeys a parabolic rate law. The rate-determining step in the reaction is the diffusion of oxygen into Zircaloy. The growth of the different reaction zones can be represented in an Arrhenius diagram. The extent of the reaction between liquid Zircaloy and UO2 depends on the wetting behavior. A Zircaloy melt rich in oxygen wets UO2 better than a melt poor in oxygen. Molten Zircaloy containing little or no oxygen reacts with UO2 to form a homogeneous (U,Zr,O) melt. As the oxygen content of the melt increases, solid (U,Zr)O /SUB 2-x/ particles precipitate. The technical significance of these out-of-pile UO2/Zircaloy reaction experiments is that Zircaloy cladding can be oxidized by UO2 fuel as quickly as by steam, and that UO2, far below its melting point, can be ''liquefied'' by molten Zircaloy. As a consequence, release of fission gas and volatile fission products is enhanced