Analysis and modeling of ex-vessel underwater cooling processes of debris bed and molten corium pool in interaction with concrete

In the case of a hypothetical nuclear severe accident with partial or extensive core meltdown, the superheated magma made of molten steel and fuel, called corium (T > 2500 K), may threaten the integrity of the reactor pressure vessel and subsequently the reactor containment building, if long-term corium coolability is not assured. The coolability by water injection and successive water penetration into the corium through the upper surface is analyzed for two expected configurations: particle bed, and corium pool overlaying the concrete. The second configuration is linked to the situation of Molten Corium-Concrete Interaction (MCCI), where a crust is formed in the upper corium surface when it comes into contact with water and is later subjected to thermal stresses that lead to its fracturing. The challenge is to characterize the effectiveness of extracting heat by the possible water penetration into the crust. The first configuration can be expected in two different situations: melt fragmentation coming from the rupture of the reactor pressure vessel and expulsion of the corium, or during melt eruption episodes through the corium crust during MCCI via corium entrainment by the concrete decomposition gases. The phenomena linked to the water penetration into the corium for these two configurations are examined through an in-depth analysis of the available experimental results, by the development of an analytical model and finally through the modification and use of the Computational Multi-Fluid Dynamics (CMFD) code MC3D. One dimensional analysis conducts to a better understanding of the minutia of the two-phase countercurrent flow through the porous media and leads the proposal of a simplified heat flux model for the water penetration with corresponding relations applicable for both configurations of interest. Furthermore, the development and the impact of penetrating front instability are studied with the help of 2D MC3D simulations, which show important effects of the initial temperature and the permeability of the corium configuration on the penetration front velocity and appearance of the instabilities. The analytical model is extended to a pseudo-two-dimensional two-zone configuration (with one zone subjected to a two-phase countercurrent flow and another through which monophasic superheated vapor flows) to analyze in greater detail the impact of the penetrating front heterogeneity over the extracted heat flux. The mechanism of water penetration through a fractured crust is revisited. The analysis indicates strong border effects in the SSWICS tests (Argonne National Laboratories) dedicated to the study of this phenomenon. The conclusions of precedent studies on the efficiency of the phenomena could not, therefore, be confirmed due to important uncertainties over the process of fracturing, overly sensitive to the mechanical properties of corium, which in turn are not properly characterized. Finally, the models, and simulations, are applied to real accidental scenarios, including the presence of residual power. For the debris bed, the extracted heat flux, and the cooling capabilities are less than those found using the simplified dry-out heat flux criteria. (author)