The objective of this thesis is to develop an accurate and systematic method for the prediction of criticality and spatial power distributions throughout reactors-especially in those regions where strong material discontinuity occurs. For this purpose, a new method for the transverse leakage treatment such that the neutron``s directional behavior between adjacent nodes to be spatially coupled as the cross terms in evaluating the vertex fluxes of a node is developed. The new transverse leakage model is characterized by the space-dependent neutron flux expanded into the spatially nonseparable polynomial. This new model is then introduced into the diffusion equations based on nodal expansion theory to derive the nodal balance equations.
This new method is first numerically benchmarked against to the numerical reference solutions and proven its accuracy and wide applicability. The results of the practical application to the KNU-1, BOC-1 core show that the new method greatly improves the spatial power distributions of the conventional methods in the regions of the significant material discontinuity such as in the assemblies adjacent to the core baffle and water reflector. Furthermore, with regards to the overall accuracy in predicting the spatial power distributions, the new method improves the results of the conventional methods.