In order to simulate a real cascade performance in terms of the external and internal flow rates and isotopic compositions, it is necessary to solve a system of equations composed of the internal mass balances for the element (U) and for the desired isotopes. Considering the separation of a binary isotopic mixture for a cascade with n stages, we have a system of 4n independent equations with 6n unknowns. This kind of system has infinite solutions unless we introduce practical or theoretical new equations describing the centrifuge separation performance and/or use approximations in terms of restrictions to the stages behavior. Depending on the equations and/or restrictions we use, the simulation results can be quite different. Six different combinations of theoretical equations and stage restrictions are analyzed and compared in this paper using experimental results in order to establish the best mathematical model for the theoretical simulation of a real cascade performance.