COCOSYS and ASTEC Analyses of Iodine Multi-Compartment Tests in the ThAI-Facility

Post test calculations with the accident codes COCOSYS-AIM and ASTEC-IODE were performed on the iodine multi-compartment tests of the German ThAI program (Thermal hydraulics, Aerosols, Iodine). In the tests transport and adsorption/desorption behavior of gaseous I₂ were measured in the 60 m³ vessel with a five-compartment configuration under severe accident conditions. The thermal hydraulic modules in COCOSYS and in the containment part of ASTEC (CPA) are nearly identical but not the iodine modules AIM and IODE. The adsorption/desorption model in AIM is based on ThAI data whereas in IODE correlations derived from laboratory-scale tests are used. A 50-zone nodalisation of the ThAI vessel was used with both codes. COCOSYS-AIM and ASTEC-IODE describe qualitatively correctly the I₂ concentration differences of several orders of magnitude in periods with stratified atmosphere and the slow homogenization in a convective mixed atmosphere. However, both codes overestimate the gaseous I₂ concentration at high relative humidity. The most likely reason is a slow reaction of deposited I₂ to the non-volatile FeI₂ on the steel surfaces, which has not been modeled sufficiently yet. Further experimental investigations in the ThAI facility are envisaged. A considering of the ThAI data in the I₂ adsorption/desorption correlations may improve future ASTEC-IODE results. Nevertheless, the analyses of the large-scale ThAI iodine tests have been an important validation step for COCOSYS-AIM and ASTEC-IODE demonstrating the capability of multi-compartment I₂ treatment.