The Use of a Thermochemical Database To Model Stratification In a Molten Pool During a PWR Severe Accident
M. Barrachin, M. Salay, F. Fichot.
11th Symposium Thermodynamics of Nuclear Materials (Karlsruhe, Germany), September 2004.
In case of a hypothetical severe accident in a pressurised water reactor (PWR), the destruction of fuel rods and melting and relocation of materials may lead to the formation of a molten pool in the lower plenum of the vessel. The standard analysis identified that in this configuration, the heat-focusing effect of a molten metal layer located on top of the oxide one could be considered as an important cause for vessel failure. A comprehensive model involving both thermodynamical and thermalhydraulic aspects has been developed to predict the stratification of oxide and metal liquids in a molten pool. The thermodynamical NUCLEA database provides the equilibrium composition of both phases. The predictions are consistent with previous analytical tests. The physical local properties, in particular the densities are calculated from local compositions and temperature. The relative motion of the two phases is determined with a drift flux model. This provides the velocities of materials in the pool and therefore, the global transport of species. A one dimensional transient with successive steel injections is presented to demonstrate the existence of a transient 3-layer configuration. The oxide pool is then surrounded by two metallic layers with different compositions. It corresponds to a global non equilibrium situation which may remain for a certain time, depending on species transport across the oxide pool. This morphology which was not foreseen in the standard analysis could threaten the vessel integrity.