A realistic velocity difference scheme has been developed for calculating the drift parameters in both horizontally and vertically oriented sections of the primary heat transport systems of CANDU reactors. This model predicts the unequal velocity effects, spatially and temporally. It can be used to describe the slip in transient and multipurpose thermohydraulic codes. The transient velocity difference equation of this model is an arrangement of the two-fluid model equations. This equation describes the time-dependent relation between the phase velocities. This is a function of the pressure gradient, phase inertias, volume fraction, flow regime, interfacial forces, and additional constitutive relations. In addition, the model includes a package of momentum exchange constitutive laws to calculate the interphase momentum exchange parameters and virtual mass coefficients. The parameters necessary for the integration of this model into CANDU thermohydraulic codes (SOPHT, FIREBIRD) are expressed in terms of the dynamic difference velocity. These parameters are the drift mass flow rate, drift velocity, distribution parameter, flow quality, effective density, and flow enthalpy. Numerical results revealed that the velocity difference model fairly predicted the drift flux parameters when compared with those calculated by existing slipdrift correlations in the SOPHT and FIREBIRD codes, other drift flux models, and with certain experimental data reported in the literature.