**Abstract** : The knowledge of in-vessel corium behaviour and associated risk of vessel failure are matters of prime interest within the framework of Severe Accident studies in a Light Water Reactor. Core meltdown during a severe accident results into formation of a corium pool at the bottom of the reactor vessel. This corium pool undergoes a density dependent liquid separation, thus exhibiting stratification into different phases. The distribution of phases depends on the time since the phase density depends on both temperature and composition of the phases. The evolution of stratification determines the impact of heat flux distribution at the lateral vessel wall, thereby determining the chances of success of the in-vessel retention strategy.
Within integral codes, the modelling of corium behaviour involves the coupling between lumped parameter thermal models and thermochemical models. Integral thermal models consist of mass and energy conservation equations that require inputs related to thermochemical properties of the materials, which are closely related to thestate variables. In particular, the closure of energy conservation equations requires enthalpy-temperature relations.In the framework of multicomponent systems, the dependence of such relations to chemical composition is of importance and should be treated adequately to obtain a more accurate description of the phases depicted by themodel. An approach to do so is to keep a general formulation of energy conservation equations in terms of specific enthalpies instead of substituting simplified enthalpy-temperature relations on a case-by-case basis in order toobtain models with an explicit temperature formulation. The enthalpy-temperature relation is then considered as an Equation-of-State (EOS) that can be written as $\mathscr {H}$ : $T$, ($w^j$) $_{j\in S}$ $\rightarrow h$ along with its reciprocal relation $\mathscr {T}$ : $h$, ($w^j$) $_{j\in S}$ $\rightarrow T$ where ($w^j$ ) $_{j\in S}$ is the composition of the phase in terms of species mass fraction, $h$ the mass enthalpy and $T$ the temperature.
The present study seeks to explore the use of CALPHAD-based enthalpy temperature relations in the modeling of suboxidized in-vessel corium plane front solidification. Along with theoretical considerations on the equationsformulation, this paper discusses the numerical results based on a model developed in the PROpagation of Corium (PROCOR) platform interfaced with the Open-Calphad thermodynamic software.