This work involves the development of physical models for the constitutive relations of a two-fluid, three-dimensional sodium boiling code, THERMIT-6S. Based on a simplified flow regime, namely, concurrent annular flow, constitutive relations for two-phase flow of sodium are derived. The wall heat transfer coefficient is based on momentum-heat transfer analogy and a logarithmic law for liquid film velocity distribution. Mass and energy exchanges are modeled by generalization of the turbulent flux concept. Interfacial drag coefficients are derived for annular flows with entrainment. Code assessment is performed by simulating the W-7b’ test for a low-flow, high-power accident in the liquid-metal fast breeder reactor. While the numerical results for predryout are in good agreement with the data, those for postdryout reveal the need for improvement of the physical models. The benefits of nonhomogeneous, nonequilibrium representation of sodium boiling are studied.