A numerical model for the CO₂-sodium chemical interactions in Sodium Fast nuclear Reactors

Supercritical CO₂(sCO₂) Brayton cycles have gained interest in the frame of Sodium-cooled nuclear Fast Reactors (SFRs), as an alternative to the conventional water Rankine cycles. If CO₂ leaks inside the CO₂-Na heat exchanger, an under expanded CO₂-into-liquid-sodium jet is formed. CO₂ chemically reacts with sodium, following an exothermic reaction which forms mainly solid products. In order to develop a model to numerically reproduce the jet development, a detailed description of the CO₂-Na chemical reaction mechanism must be investigated. In this paper, a chemical reaction model at the dispersed phase scale between CO₂-sodium, inside an under expanded CO₂-into-sodium jet, is presented. The model considers the reaction between a sodium droplet with the surrounding CO₂ environment for the region close to the leakage, where mist flow is supposed to exist, and between a CO₂ bubble with the surrounding liquid sodium environment for the region further downstream the leakage, where bubbly flow takes place. The depletion rate for a single particle (droplet or bubble) is determined, as a function of the main influencing parameters, such as temperature, droplet slip velocity, bulk mass fractions, and the chemical kinetics. (authors)