In a computer modeling method for the core of a nuclear reactor the core is divided into a plurality of primary grids in the form of coarse mesh cells. A sub-section of the core relating to at least one grid is assigned to that grid. The sub-section includes that grid and a buffer zone that surrounds the grid horizontally and contains at least the grid that lies immediately adjacent to the first grid. The sub-section is sub-divided into a plurality of secondary grids--fine mesh cells--that are smaller than the primary grids. In a first computing step, a model of the core is calculated using a nodal calculation method, based on a respective dataset that is assigned to each primary grid. In a second computing step, a respective second dataset is assigned to each secondary grid of a sub-section and a model is then calculated for the sub-section, based on the dataset and the flows on the border of the sub-section that have been calculated in the first computing step.