Experimental results of the QUENCH-16 bundle test on air ingress

• Formation of zirconium nitrides under oxygen starvation conditions.
• Growth of nitrides inside the oxide layer and at the boundary to the α-Zr(O) layer.
• Massive steam penetration through the porous oxide/nitride scales during reflood.
• Melting of claddings after reflood initiation due to sharp temperature escalation.
• Very high hydrogen production during reflood.

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. It was performed in the frame of the EC supported LACOMECO program. The test scenario included the oxidation of the Zircaloy-4 claddings in air following a limited pre-oxidation in steam, and involved a long period of oxygen starvation to promote interaction with the nitrogen. The primary aim was to examine the influence of the formed oxide layer structure on bundle coolability and hydrogen release during the terminal flooding phase. QUENCH-16 was thus a companion test to the earlier air ingress experiment, QUENCH-10, which was performed with strongly pre-oxidized bundle. Unlike QUENCH-10, significant temperature escalation and intensive hydrogen release were observed during the reflood phase. Post-test investigations of bundle cross sections reveal residual nitride traces at various elevations. The nitrides were formed at upper bundle elevations characterized by steam starvation conditions. The external part of the oxide scale is of porous structure due to re-oxidation of nitrides during reflood. Relative thick internal oxide scales underneath this porous layer and residual nitrides were formed during reflood. At lower bundle elevations frozen partially oxidized melt was detected, relocated from upper elevations. Three contributors for the high hydrogen production during the reflood were recognized: re-oxidation of nitrides, secondary oxidation of residual cladding metal due to massive steam penetration through the porous oxide/nitride layer and melt oxidation.