The objective of this paper is to study the 3D reactor physics modeling of a Reduced-Moderation Water Reactor (RMWR) fuel lattice model and investigate the results using two computational codes: WIMS and MONK. Two sets of fuels: Pu-UO2 and Th-UO2 have been studied for this paper. The comparison among the two codes has been carried out with 3D lattice model of a reduced moderation Boiling Water Reactor fuel assembly. The results with a P0 scatter approximation with the transport correction are compared to the results with explicit treatment of P1 scatter. The solution of the multiplication factor, neutron flux and yield in energy are compared by using 172-group cross-sections generated by the use of 2D equivalence theory in WIMS and with cross-sections that have been condensed to 12-group by the use of 2D transport calculations. Therefore, two sets of cross-sections with P0 transport-corrected scattering for WIMS (by adjusting total and the self scatter cross-section) and both P1 and P0 scattering for MONK were used for the comparison of the results for 3D lattice models with both 172-group and 12-group. The 3D calculation route for WIMS (CACTUS3D route) requires the 172-group cross-section generations in a 2D slice of the model. In addition, solving the 172-group problem directly in CACTUS3D is computationally expensive. It has been observed that it is preferable to somehow reduce the number of groups before performing the 3D calculation. Furthermore, there is a reasonably excellent agreement between the codes for the multiplication factor for the model, although the flux is found to be sensitive to the solution method.