On the use of CALPHAD-based enthalpy-temperature relations in suboxidized corium plane front solidification modelling

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 the state 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 the model. 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 to obtain models with an explicit temperature formulation. The enthalpy-temperature relation is then considered as an "Equation-of-State" (EOS) that can be written as: H : T, (w j) j∈S → h along with its reciprocal relation T : h, (w j) j∈S → T where (w j) j∈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 modelling of suboxidized in-vessel corium plane front solidification. Along with theoretical considerations on the equations formulation, 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.

Modeling of the corium crust of a stratified corium pool during severe accidents in light water reactors

“…Thus, as a first approach, we have chosen constant liquidus temperatures for the oxide and metallic layers. In [20], it is also shown that the radial variation of composition in the solid crust remains small (maximum of ≈ 9.5 % for the mass fraction of ZrO 2 ) in comparison to the variation of composition in the liquid. However, the crust model and its coupling with the pool were developed in such a way that more complex models could be easily plugged in, e.g.…”

“…At this stage, the crust composition treatment and thermodynamic closures remain simple while being qualitatively coherent (and probably quantitatively at the first order) to regain the expected physical behavior of the coupling at the interface between the pool and its crust. Indeed, it is shown in [20] that, for a given pool composition, the variation of the liquidus temperature of the oxide pool remains small (less than 50 K). Plus, we show in the following that for usual reactor case calculations, the metallic pool layers do not lose enough power in the crust in such a way that the interface temperature of its facing oxidic crust stays well above the solidification temperature.…”

“…However, the crust model and its coupling with the pool were developed in such a way that more complex models could be easily plugged in, e.g. models allowing thermodynamic treatments consistent with CALPHAD data as proposed in [20,21].…”