Abstract: CORE is a few group 1D/2D/3D steady state nodal code, which can be used for PWR core design and analysis. Core and system integrated engine for design and analysis (COSINE) is the newly approved NPP software R&D key project in China. Core analysis code CORE is an essential subsystem of COSINE code package. In the current version of CORE, the diffusion solver is composed of nodal expansion method (NEM) for 2D/3D problems and the finite difference method for 1D problem. Macro cross sections are interpolated from the pre-calculated XS table. For depletion calculation, the predictor-corrector macro depletion method is used. For pin power reconstruction, the modulate method is adopted. At present, the key modules of CORE have been completed, and the preliminary verification has been done. The result shows that the basic functions and accuracy of the diffusion solution module can meet the requirements.