The inorganic and physical chemistry of reactants (e.g., impurities) produced during the reduction of spent light water reactor fuel in a hot cell has been analyzed. Two source terms were identified that influence the composition and quantity of these impurities in the salt matrix. One source comes from the reduction process, which occurs between the fuel and the Li/LiCl salt matrix, and the other from chemical reactions that occur between the hot cell atmosphere and the salt matrix. The spent-fuel-oxide chemistry and energy of formation for the reactants were evaluated. Most of the rare-earth-oxide reactions were not thermodynamically feasible with molten lithium, except when nitrogen was present during the reduction process. A model of the reaction at a vapor-liquid interface was developed and applied to the pilot-scale oxide reduction device design. A predominance diagram for the Li-O-N reactions was constructed to determine the possible reactions during operation of the device, and from these results, the mass accumulation was determined from hot cell conditions.