The out-of-pile bundle experiment QUENCH-16 on air ingress was conducted in the electrically heated 21-rod QUENCH facility at KIT in July 2011. The oxidation of the Zircaloy-4 claddings in air following a limited pre-oxidation in steam was examined. Three contributors for intensive hydrogen production during the final reflood were identified: 1) re-oxidation of nitrides, 2) secondary oxidation of cladding metal by steam penetrated through the porous re-oxidized scales and 3) melt oxidation
