Theoretical analyses of interactions between core melt and coolant

The code IVA3 will be utilized for analysing the physical processes related to steam explosions, i.e. the premixing phase preceeding the explosion and the explosion itself. The programme models the thermo- and fluiddynamics of multi-phase, three-component systems, esp. the movements of molten material, water and vapour with one velocity field for each of the three components. The following activities are reported on: 1. Modifications of the model and code corrections: An improved model for the heat transfer between the three components has been inserted and a number of code errors were corrected. 2. The programme was made more user-friendly concerning its input, its graphical output and the programme management. 3. A FARO-experiment (investigating the behaviour of molten material falling into water) was recalculated with IVA3 in order to test the new model. The initial pressure increase is very well reproduced by the code, whereas the results at later times indicate a too effective heat transfer and thus the need for some more model refinement. 4. The COMET-experiments planned at KfK are designed to simulate the behaviour of molten core material cooled from below with water. They were precalculated with an intermediate version of IVA3 (June 1993). Recently one of the calculations was repeated with the newest code version. The interaction of melt and water turned out to be much more benign in the new calculation. However, a numerical problem still prevents the calculation to proceed to large times. This problem is being analysed; a recalculation of all experiments will have to be performed after its solution. (orig.)