Behaviour of oxide layer of zirconium-based fuel rod cladding under steam starvation conditions

This report presents the results of tests relating to the long-term annealing of preoxidised Zircaloy cladding tubes in an inert atmosphere at annealing temperatures between 1250 and 1500 C and a corresponding model. The tests were aimed at investigating the reduction kinetics of the oxide layer in the steam starvation phase by the progression of core drying during a severe accident. Homogeneous formation of α-Zr(O) precipitations inside the oxide layer and formation of α-Zr(O) scale on the outer cladding surface were detected in addition to a decrease in the oxide layer thickness. This phenomenon is supposed to have a strong influence on intensive hydrogen release during the following quench phase. (orig.)