Experimental study of oxidation in oxygen, nitrogen and steam mixtures at 850∘C of pre-oxidized Zircaloy-4

Since the Fukushima Daiichi accident, increased attention is paid to the vulnerability of Spent Fuel Pools (SFPs). In case of an accidental dewatering of the fuel assemblies, the fuel cladding would be exposed to an air-steam atmosphere and its oxidation is a key phenomenon since it drives the fuel assembly heat-up and degradation. In this study, we have investigated the corrosion kinetics of pre-oxidized and as-received Zircaloy-4 (Zy-4) plate samples at 850∘C. The low temperature pre-oxidation aims at simulating the corrosion scale that grows during the in-reactor use of the fuel. High temperature oxidation tests were carried out under mixed oxygen-steam-nitrogen atmospheres. In the different atmospheres investigated, a rather protective effect of the pre-oxide scale regarding subsequent high temperature oxidation has been observed, for limited time periods however. Post-test examinations of the samples demonstrated that the loss of the protection was associated to the spalling of the pre-oxide scale that initiated at sample edges, where the pre-oxide scale was cracked. For the steam partial pressure range investigated in this study (0–8 vol%), there was no noticeable effect of the steam partial pressure on the oxidation rate. Nevertheless, samples hydrogen pick-up were strongly correlated to steam partial pressures. Moreover, 18O isotopic labelling experiments suggested that the contribution of O2 and H2O to the oxidation process corresponds to their respective concentration in the gas phase.