Recent improvements in the system code package AC² 2019 for the safety analysis of nuclear reactors

Highlights: • New release AC² 2019 contains ATHLET/ATHLET-CD 3.2 and COCOSYS 3.0. • Validation of AC² codes and validation matrices are discussed. • Improvements for ATHLET 3D model, AIDA lower plenum module and SAFT FP transport. • For MCCI simulation in COCOSYS, set concrete decomposition temperatures to 1800 K. • Stable coupling at multiple locations of ATHLET to CFD-codes for single-phase flows. - Abstract: The system code package AC² by GRS for safety analyses of nuclear reactors from normal operation to severe accidents has been updated with a new release. We briefly describe the main modules of AC² 2019: ATHLET 3.2, ATHLET-CD 3.2 and COCOSYS 3.0 and selected improvements in these codes. We illustrate the improved capabilities of AC² with selected examples. The post-test-calculation of the flooding pool of the INKA test facility demonstrated the improved ATHLET 3D model with explicit mixture level model. For ATHLET-CD, we apply the AIDA module with improvements in wall ablation and heat transfer models to a generic AP1000 lower plenum filled with molten corium. And we apply the new SAFT fission product transport module simulating three release paths through steam generator U-tubes. For COCOSYS, we present uncertainty analysis results for two MCCI experiments which justify using an elevated concrete decomposition temperature of 1800 K. Finally, we illustrate the potential of coupling ATHLET to CFD codes for safety analysis with results of a post-test calculation of the ROCOM PKLIIIT1.1 test on 3D mixing phenomena in an RPV.