A numerical method for the solution of two-phase flow equations has been developed. The method is based on the commonly used principles for the solution of two-phase flow: a staggered mesh, an upwind treatment of the convection terms, and the implicitness of the transfer terms. The purpose was to develop a robust and fast method for the analysis of nuclear reactors. Consequently, the equations are solved in a one-dimensional form, but the principles of the method are also applicable for multidimensional solutions.