Chemical Interactions Between UO₂ and Zircaloy-4 from 1000 to 2000°C

The chemical interaction between solid and liquid Zircaloy-4 and solid UO₂ was examined in the temperature region 1000 to 2000°C 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 9000 s. The reaction experiments with liquid Zircaloy were performed in UO₂ crucibles between 1800 and 2000°C. 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 oxygen-stabilized 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 oxyen 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_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.