Oxidation and hydrogen uptake during high-temperature reaction of zirconium alloys in steam-nitrogen mixtures

Zirconium alloys are worldwide used as cladding materials for fuel in light water reactors with excellent performance during operation due to their low neutron absorption and very good 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 recent studies on air ingress scenarios during severe accidents in reactors and spent fuel pools over a wide range of boundary conditions [1-5].
The presented poster summarizes results on the oxidation of Zircaloy-4 in steam-nitrogen atmospheres at temperatures between 600°C and 1200°C. Experiments were conducted using thermo-gravimetry (TG) under gas compositions between 0 and 100 vol% nitrogen including 0.1 and 90 vol% and total gas flow rates of 0.278 mol/h. The strongest effect of nitrogen on the oxidation kinetics was seen at 800-1000°C with higher kinetics in all mixtures compared to the annealing in pure steam. ... mehrFigure 1 shows the TG curves for the test series at 800°C as an example. Even 0.002 % nitrogen in steam, which corresponds to the solubility of nitrogen in water, resulted in an increased oxidation rate compared to pure steam. 1 % nitrogen increased the oxidation by a factor of 3-4, and higher nitrogen contents caused increased kinetics by more than one order of magnitude under these conditions. The reaction in pure nitrogen was by far the slowest at all temperatures.

Fig.1: Mass gain during oxidation of Zircaloy-4 in steam-nitrogen mixtures at 800°C.
The effect of nitrogen was less pronounced at lower and higher temperatures. Generally, the formation of zirconium nitride, ZrN, and its re-oxidation is the main reason for the highly porous oxide scales after transition because of the strongly different molar volumes of the involved phases. Nitrogen also influenced the time of the kinetic transition from (sub )parabolic to linear or even accelerating kinetics.
The increased oxidation kinetics results also in an increased hydrogen production. Hydrogen is released to the ambient and it may be absorbed by the metal phase. The uptake of hydrogen by the metal was determined using neutron radiography. A strong correlation was found between the transition of the oxidation kinetics and enhanced hydrogen absorption. Much higher hydrogen concentrations in the metal were observed because of the enrichment of hydrogen in pores and cracks near the metal-oxide interface [6].
From the viewpoint of nuclear safety, the effect of nitrogen must be taken into account because it strongly degrades mechanical properties of the zirconium alloy claddings and significantly increases the oxidation kinetics and hence the hydrogen source term during nuclear accidents. A more general conclusion from this work is that experiments on the oxidation kinetics of zirconium alloys must be conducted very carefully especially with respect to well controlled gas atmospheres. Nitrogen (air) must be excluded from the reaction gas as long as they are not used by intention.

References:
[1] Steinbrück, M. et al., “Experiments on air ingress during severe accidents in LWRs,” Nuclear Engineering and Design 236, 1709-1719 (2006).
[2] Duriez, C., Steinbrück, M., Ohai, D. Meleg, T., Birchley, J. and Haste, T., “Separate-effect tests on zirconium cladding degradation in air ingress situations,” Nuclear Engineering and Design 239, 244-253 (2009).
[3] Steinbrück, M., “Prototypical experiments relating to air oxidation of Zircaloy-4 at high temperatures,” Journal of Nuclear Materials 392, 531-544 (2009).
[4]. Steinbrück, M. and Böttcher, M., “Air oxidation of Zircaloy-4, M5® and ZIRLO™ cladding alloys at high temperatures,” Journal of Nuclear Materials 414, 276-285 (2011).
[5] Lasserre, M. et al., “Qualitative analysis of Zircaloy-4 cladding air degradation in O2-N2 mixtures at high temperature,” Materials and Corrosion 65, 250-259 (2014).
[6] Grosse, M., Lehmann, E., Steinbrück, M., Kühne, G. and Stuckert, J., “Influence of oxide layer morphology on hydrogen concentration in tin and niobium containing zirconium alloys after high temperature steam oxidation,” Journal of Nuclear Materials 385, 339-345 (2009).