Detailed investigations on the core meltdown sequence using KESS-2

Results are obtained in the following areas: Comparison of theoretical and experimental results and measurements at TMI-II; limitations due to the use of simple models are demonstrated; consequences of the use of detailed models for the quality of the results are shown. Main results are: 1.) Temperatures and mass behavior can be predicted reasonably. 2.) Reactor Cores remain coolable even if they are partially empty during a longer period of time. 3.) To treat accidents with closed containments (Release Category 6) simple core models are sufficient. 4.) To treat accidents with failure of containment isolation (Release Category 2), detailed models are necessary. Main uncertainties arise from thermohydraulic boundary conditions and the modeling of fission product behavior. 5.) Conditions for steam explosions with catastrophic consequences are very limited. However they cannot be excluded completely up to now. Detailed analyses for the 20 cm2 LOCA and the loss of power accident support these findings. (orig./HP)