Zirconium alloys are worldwide used as cladding materials for the fuel in light water reactors due to their low neutron absorption and excellent mechanical and corrosion properties at operational conditions. However, at higher temperatures relevant for nuclear accident scenarios the oxidation of zirconium becomes severe, thus impairing the barrier effect of the claddings against the release of fission products and being the main source for the release of hydrogen and chemical heat. A significantly accelerating effect of nitrogen on the oxidation kinetics has been observed in the framework of studies on air ingress scenarios during severe accidents in reactors and spent fuel pools. Only very limited information of the influence of nitrogen (used for inertization of BWR containments and as pressurizing gas for emergency core cooling systems) on the oxidation of zirconium in steam-nitrogen mixtures were available.
Isothermal oxidation tests with 1-cm cladding segments of Zircaloy-4 (Zr-1.5%Sn) in steam-nitrogen mixtures have been performed at 600, 800, 1000, and 1200°C. All experiments were conducted in a NETZSCH STA-409 thermal balan ... mehrce with steam furnace coupled with a mass spectrometer (MS) NETZSCH Aeolos for analyses of the off gas composition. The gas compositions were varied between 0 and 100 vol.% nitrogen including 0.1 and 90 vol.% and constant total flow rate of 0.28 mol/h.
The strong effect of nitrogen on the oxidation kinetics of zirconium alloys was confirmed in these tests in mixed steam-nitrogen atmospheres. Already very low concentrations of nitrogen (starting from 0.1 vol%) strongly increase reaction kinetics. Nitrogen reduces transition time from protective to non-protective oxide scale (breakaway). The formation of zirconium nitride, ZrN, and its re-oxidation is the main reason for the highly porous oxide scales after transition.
The results of this study have shown the safety relevant role of nitrogen during severe accidents and, more generally, suggest the need of using well controlled gas atmospheres for experiments on oxidation of zirconium alloys. A complementary paper focusing on the effect of flow rates and partial pressures as well as modelling the effect of nitrogen is submitted in parallel (Grosse, Steinbrueck)