A lumped parameter model of continuous/dispersed phases mass transfer for simulating in-vessel corium stratification transients

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In the frame of in-vessel corium modelling, this paper proposes an enhanced integral model in order to simulate the liquid phases stratification transients associated with the miscibility gap in the O,U,Zr,steel system. An important feature of this model is that it explicitly takes into account mono-dispersed metal phases in the continuous oxide phase. These droplets can either come from a direct molten steel relocation or be detached by Rayleigh–Taylor instabilities at the interface between the heavy or light metal layers and the oxide layer of the stratified pool. At all interfaces between metal and oxide phases, a same consistent local equilibrium assumption (based on a same simplified thermodynamic representation of the liquid miscibility gap) is made.A limited parametric analysis is reported. It highlights the possible impact of direct molten steel relocation into the oxide phase in comparison with a relocation on the top of the pool. Indeed, the U,Zr enrichment of the metal droplets during their residence time in the oxide phase significantly affects the remainder of the stratification transients in all cases. The parameters that affect this mass transfer are shown to have a significant impact on the overall stratification kinetics. These results also highlight an important qualitative difference between the two kinds of stratification transients: heavy metal phase formation or stratification inversion. In the latter, the heavy metal phase average composition always stays close to its interface composition equilibrium with the oxide and, accordingly, when the density criterion associated with the Rayleigh–Taylor instability is met, the heavy metal layer is, in most cases, completely relocated at the top of the pool. During the heavy metal phase formation, the situation is drastically different as the light metal layer is not initially close to equilibrium with the oxide in such a way that several intermittent droplets detachment are predicted.These different trends are reported taking into account the model underlying hypotheses and the path to move forward and increase knowledge about stratification kinetics is discussed.