Nanothermites are interesting energetic systems as their combustion driven by the oxidation of the metallic fuel associated with the reduction of the oxidizer, can produce extremely fast burning rates exceeding hundreds of m.s-1. In addition, by changing the reactant (composition, stoichiometry) geometry and compaction conditions, the control of the burning rate can be achieved, allowing the designer to customize the chemical energy for each application. To date, only rough combustion models exist, most restricting the combustion mechanisms to only condensed phase processes, thus providing an approximative prediction of structure-combustion performance relationships. This work presents a tri-phasic model for the combustion of Al/CuO powder considering 9 gaseous species (Al, Cu, O2, O, Al2O, Al2O2, AlO, AlO2, N2) and 4 condensed species (Al, Cu, CuO, Al2O3) that can be liquid or solid. The reactionnal scheme involves 12 heterogeneous reactions and 2 phase changes based on diffusional kinetics, while gaseous reactions are considered through a chemical equilibrium. A detailed description of the theoretical formulation and numerical method is presented, followed by a discussion of a closed-bomb simulation. This work highlights the great impact of the Al particles initial diameter on the pressure development in the chamber. After the initiation stage, the decomposition of CuO releases gaseous O2, which is spontaneously absorbed on the surface of submicronic Al particles and diffuses through alumina to react with pure Al. At high temperature, gaseous copper, aluminum sub-oxides and aluminum condense on both particle types. By contrast, Al particle micron-size limits the quantity of O2 absorption and gaseous species surface condensation, leading to the formation of a pressure pre-peak 4 times higher than the final chamber pressure.