The simulation of melt spreading with THEMA code: Part 1: Model, assessment strategy and assessment against analytical and numerical solutions

In the framework of R&D on severe accidents for PWRs, definition of mitigation means for cooling of the molten core after a vessel melt-through is of utmost importance. In the case of spreading-based concepts, the THEMA code, which considers integrated balance equations over the fluid depth, aims at predicting the spreading extent of non-eutectic mixtures under defined inlet conditions (pouring rate, temperature and composition). In this first part, the models and the assessment strategy of the code are presented. The satisfactory running of the main moduli of the code is then verified by comparing the numerical results against known solutions of basic problems: dam break problem, conduction with or without melting in the basemat, crust formation kinetics on a constant or “free” bottom plate temperature.