Reactive extraction of nuclear elements from aqueous nitric acid solutions using an organic solvent is an important process operation. A thermodynamic model to predict the equilibrium compositions of the two phases will be useful in optimizing the performance of such processes. A general framework is presented in this work for modeling such systems by dividing the complex reactive metal extraction system into appropriate subsystems, whose models are combined to derive the overall model. This approach reduced the number of parameters to be estimated at each stage, making the framework reliable and accurate. The procedure is illustrated for extraction of plutonium (Pu4+) using tri-n-butyl phosphate (TBP) as the organic solvent. This model for Pu extraction correctly accounts for free TBP, the dissociation of nitric acid in concentrated solutions, and formation of hydrolyzed species of plutonium to predict the distribution coefficient of Pu(IV) accurately over the practical range of nitric acid concentration from 0.1 to 4 N. Equilibrium models for the aqueous solution of the nitric acid subsystem, and the two-phase aqueous nitric acid and TBP sub-system, which are developed as part of this work, will also be useful in developing equilibrium models for extraction of other nuclear elements.