Supercritical CO2 (sCO(2)) 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 underexpanded 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 underexpanded 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. (C) 2015 The Institution of Chemical Engineers. Published by Elsevier BM. All rights reserved.