ZrO₂ dissolution by molten zircaloy and cladding oxide shell failure. New experimental results and modelling

Experimental and modelling results of the QUENCH-18 bundle experiment on air ingress, cladding melting and aerosol release

“…Following this, the temperatures continued to increase and stabilized at melting temperature of Zr bearing materials until water injection was initiated at 12330 s. The escalation stopped at the melting point due to the relatively low heat input (≈12 kW consisting of ≈4 kW electric heat and two chemical heat sources each ≈4 kW due to the oxidation of zirconium in steam and air), as well as the high specific heat of fusion of zirconium (h fus 185.7 kJ/kg) in comparison with its specific heat at the stage of temperature escalation (c ≈ 0.27 kJ/kg/K). According to crucible experiments, a noticeable dissolution of zirconium oxide by metallic zirconium begins only from 2300 K (Hofmann et al, 1999); therefore, the cladding melt appearing at ≈2000 K remains localized between the outer oxide layer and the pellet. This is also confirmed by the localization of the melt under the outer oxide layer in the previous QUENCH experiments (Stuckert et al, 2010, Stuckert et al, 2011.…”