Development of a Fast-Running Approach for the Simulation of Debris Beds in the Nuclear Severe Accident Analysis Code AC2

In this work, the development of a fast-running model for the simulation of debris beds in the lower reactor plenum for the code package AC2 is presented. The model uses a simplified parametric approach for the transformation of melt into a porous volume in which the geometric properties of the particles are specified by the user. In addition, the model can be used to simulate the heat transfer between processes in porous media. This includes the simulation of convective heat transfer in single-phase regimes with subcooled or saturated water and superheated steam as well as two-phase heat transfer in consideration of boiling processes. Validation of the model is carried out by simulations of suitable experiments. These include the DEBRIS experiments conducted at the Institute of Nuclear Technology and Energy Systems at the University of Stuttgart. In the experiments, the thermodynamic behavior during quenching as well as the boiling and dryout behavior of flooded debris beds was investigated. The results show that the overall thermal behavior during re-flooding can be simulated in good accordance with the experiment. Further development potential is mainly identified for the representation of further relevant physical phenomena, such as the renewed melting of the particles, to further increase the simulation quality of AC2.