Chemical interactions between the UO₂ fuel and the Zircaloy-4 cladding material under power cooling mismatch (PCM) conditions are described. The out-of-pile uranium oxide (U0₂)/Zircaloy-4 reaction experiments were performed with short Zircaloy-4 tubular specimens filled with U0₂ pellets at temperatures between 900 and 1500°C under variable U0₂/Zircaloy-4 contact pressures and under inert gas conditions. The annealing periods varied between 3 and 60 min.

Due to the thermodynamic instability between U0₂ and Zircaloy, U0₂ is partially reduced by Zircaloy at the high temperatures investigated. The extent of the UO₂/Zircaloy reaction as well as the composition of phases depends decisively on whether a good solid contact between U0₂ and Zircaloy has been established. If a solid contact exists, a (U-Zr)-alloy rich in uranium and an oxygen stabilized α-Zr(O) phase are formed.

The degree of chemical interaction between U0₂ and Zircaloy is determined by oxygen diffusion in the Zircaloy and the α-Zr(O)-phase formed. The UO₂-Zircaloy reactions obey a parabolic rate law whose temperature dependence is given in the corresponding Arrhenius plot. The formation of oxygen-stabilized °-Zr(O) implies heavy embrittlement of the Zircaloy cladding above 1200°C. Thin oxide layers on the cladding tube inner surface prevent UO₂/Zircaloy reactions from taking place below 1100°C. Simulated fission products (cesium, iodine, tellurium) in UO₂ exert practically no influence on the extent of the reaction.

A comparison of the out-of-pile test results with those of in-pile experiments exhibits a very good agreement with respect to the phases formed.