Prototypical experiments on air oxidation of zircaloy cladding at high temperatures

The report presents the results of extensive experimental work on the oxidation of Zircaloy-4 in air at high temperatures. The experimental program was aimed at mechanistic phenomenology of the reaction between Zircaloy and air and investigation of air attack under prototypical conditions for air ingress under the conditions of an hypothetical severe nuclear reactor accident, i.e. at temperatures 800-1500 °C and consideration of mixed air(nitrogen)-steam atmospheres and pre-oxidation. The oxidation in air as well as in air and nitrogen-containing atmospheres leads to strong degradation of the cladding material. The main mechanism for this process is the formation of zirconium nitride and its re-oxidation. From safety point of view, the barrier effect of the fuel cladding is lost much earlier than during accident transients with only a steam atmosphere. Pre-oxidation in steam prevents air attack as long as the oxide scale is intact, i.e. at temperatures above 1050 °C (beyond breakaway regime) and as long as oxidising gases are available (no steam starvation conditions). Under steam/oxygen starvation conditions the oxide scale is reduced and significant external nitride formation takes place. Stronger degradation of cladding tubes was also observed in air-steam and nitrogen-steam mixtures over a wide range of compositions. Regarding modelling of air ingress in severe accident computer codes one conclusion is that parabolic correlations for oxidation in air may be applied only for high temperatures (>1400 °C) and for pre-oxidized cladding (> 1100 °C). For all other conditions faster, more linear reaction kinetics should be applied. The results presented in this report are mainly of phenomenological nature. Therefore, the program will be extended by selected experiments oriented on the determination of kinetic correlations.