Crust formation and dissolution during corium concrete interaction

In the hypothetical case of a severe accident, the reactor core could melt and the formed mixture, called corium, could melt through the vessel and interact with the reactor pit concrete. Recent two-dimensional concrete-ablation experiments (CCI and VULCANO VB test series) have shown that the ablation is roughly isotropic for limestone-rich concretes while, for silica-rich concretes, ablation is slower downwards than sideward. Crusts at the pool bottom are assumed to be mechanically more stable than those on the vertical walls. Models for the solidification and the melting of these crusts are proposed. They describe the transient heat and mass transfer in the following multilayered system: solid concrete, molten concrete, corium crust, liquid corium. It appears that molten concrete can play a significant role in dissolving the solidified corium crust. This effect is important for limestone-rich concretes due to the presence of a eutectic in the corium-concrete pseudo-binary phase diagram and to the larger chemical diffusion coefficient in the silica-poor concrete melts. For silica-rich concretes, it has been observed that siliceous aggregates are not molten with the surrounding mortar but have been found entrapped in the solidified pool near its boundaries, during post-test examinations. They can contribute to the solidification of corium, acting as cold sources. A model of the simultaneous gravel melting and corium solidification based on Kerr (1994) works is proposed. Numerical applications to the oxidic-corium concrete interaction tests performed in the VULCANO facility with the two types of concretes and two corium compositions will be presented and discussed to support this analytical approach. (authors)