Simulation of the air ingress experiment QUENCH-16 with ATHLET-CD

The severe accident analysis code ATHLET-CD is developed in order to analyze the behavior of Light-Water-Reactors under severe accident conditions. The objective of the code development is to provide a reliable tool for safety analysis of LWR, for instance, to assess the effectiveness of accident management measures at different stages of core degradation. Reflooding an uncovered core as a measure to prevent core damage may result in an accelerated zirconium oxidation. Besides a significant hydrogen generation, this reaction may cause a temperature escalation leading to the failure of the claddings. Therefore current research activities focus on the investigation of the behavior of fuel rod bundles during reflooding at high temperatures. In case of air ingress in the course of severe accidents, e. g. in dried-out spent fuel ponds, the high temperature interaction of fuel materials with air results in intense oxidation and nitriding of zirconium components. Under these conditions, severe bundle damages may occur even though the bundle can successfully be quenched by water. Two tests addressing the investigation of the bundle behaviour during air ingress followed by a subsequent bottom flooding with water have been performed in the QUENCH-facility at Karlsruhe Institute of Technology (KIT). The purpose of both air ingress tests, QUENCH-10 and QUNENCH-16, was to provide experimental data needed for model development as well as code validation. In contrast to QUENCH-10, the boundary conditions of QUENCH-16 in terms of the injection of air were chosen to extent the oxygen starvation period in order to investigate the impact of nitride formation on the bundle behaviour. The purpose of the post-test calculation of the experiment QUENCH-16 with the system code ATHLET-CD presented in this compact is to assess the current modeling of zirconium (Zr) oxidation in air. The results are presented and discussed in comparison to the measurements. (orig.)