Development of a transient hydrogen generation model for metal-water interactions

A transient model for hydrogen generation in molten metal-water interaction was developed with separate models for two stages of coarse mixing and stratification. The model solves the mechanistic equations for each stage such as the heat and mass transfer correlations, heat conduction equation and the concentration diffusion equation with non-zero boundary conditions. The numerical model was applied to simulate the single droplet experiments performed at the Argonne National Laboratory and the FITS tests on dynamic fragmentation and stratification. The calculation results showed better agreement with the experimental data than those of previous works. It was found from the analyses that the steam concentration at the reaction front might be the main constraint to the extent of the metal droplet oxidation. Also, the hydrogen generation rate in the coarse mixing phase was higher than that in the stratification phase. The particle size was the most important factor in the coarse mixing phase to predict the amount of hydrogen generation. The numerical model was incorporated in the MAAP 4.0 code and applied to a severe accident sequence in PWR. Compared with the original MAAP 4.0 calculation, the modified code slightly underestimated the amount of hydrogen generation. It was also shown that variation of the amount of unreacted metal affected the accident scenario. (author)