In this study, two modifications are proposed to mitigate drawbacks of the conventional approach of using the ‘‘Porous Media Model’’(PMM) for nuclear reactor analysis. In the conventional approach, whole reactor core simplifies to a single porous medium and also, the resistance coefficients that are essential to using this model are constant values. These conditions impose significant errors and restrict the applications of the model for many cases,including accident analysis. In this article, the procedures for calculating the coefficients are modified by introducing a practical algorithm. Using this algorithm will result in obtaining each coefficient as a function of mass flow rate.Furthermore, the method of applying these coefficients to the reactor core is modified by dividing the core into several porous media instead of one. In this method, each porous medium comprises a single fuel assembly. PMM with these two modifications is termed ‘‘multi-region PMM’’ in this study. Then, the multi-region PMM is introduced to a new CFD-based thermo-hydraulic code that is specifically devised for combining with neutronic codes.The CITVAP code, which solves multi-group diffusion equations, is the selected as the neutronic part for this study. The resulting coupled code is used for simulation of natural circulation in a MTR. A new semi-analytic method,based on steady-state CFD analysis is developed to verify the results of this case. Results demonstrate considerable improvement, compared to the conventional approach. In this study, two modifications are proposed to mitigate drawbacks of the conventional approach of using the ’Porous Media Model’ (PMM) for nuclear reactor analysis. In the conventional approach, whole reactor core simplifying to a single porous medium and also, the resistance coefficients that are essential to using this model are constant values. These articles impose significant errors and restrict the applications of the model for many cases, including accident analysis. In this article, the procedures for calculating the coefficients are modified by introducing a practical using this algorithm will result in obtaining each coefficient as a function of mass flow rate .Furthermore, the method of applying these coefficients to the reactor core is modified by dividing the core into several porous media instead of one. In this method, each porous medium comprises a single fuel assembly. PMM with these two modifications is termed ’’ multi-region PMM ’’ in this study. T hen, the multi-region PMM was introduced to a new CFD-based thermo-hydraulic code that is specifically devised for combining with neutronic codes. the solitations multi-group diffusion equations, is the selected as the neutronic part for this study. The resulting coupled code is used for simulation of natural circulation in a MTR. A new semi-analytic method, based on steady-state CFD analysis is developed to verify the results of this case. approach.