Thermally Stimulated Liquid Na–CaCO₃ Reaction: A Physicogeometrical Kinetic Approach toward the Safety Assessment of Na-Cooled Fast Reactors

In this study, we characterized the kinetic features of the thermally stimulated reaction of CaCO₃ with liquid Na as a model process of the liquid Na–structural concrete reaction that possibly occurs under a postulated severe accident in a Na-cooled fast reactor. Two sampling conditions characterized by the dispersion of CaCO₃ powders in liquid Na and the limited contact of liquid Na with the CaCO₃ pellet surface were applied to investigate the kinetics and physicogeometrical mechanism of the liquid Na–CaCO₃ reaction using differential scanning calorimetry (DSC) and morphological observations of the products. Under both sampling conditions, the exothermic liquid Na–CaCO₃ reaction started at the initial contact area of the reactants and proceeded while liquid Na penetrated the CaCO₃ powder and pellet, exhibiting the apparent multistep reaction behaviors. Because of the changes in the geometrical constraints of the liquid Na and CaCO₃ contact induced by different thermal and mechanical events in the reacting system, only the initial reaction step at the initial contact area of the liquid Na and CaCO₃ surface exhibited acceptable repeatability to characterize the kinetic behavior. The kinetic information of the first reaction step was extracted from the exothermic DSC peak for the overall reaction using advanced kinetic approaches to the multistep process, providing the kinetic parameters supported by the defined physicogeometrical scheme of the reaction to be used for simulating the liquid Na–CaCO₃ reaction.