Abstract: An algorithm of 3D coarse mesh nodal expansion method (NEM) embedded with 2D fine mesh nodal discrete ordinate method (NDOM) is described. The main idea is to substitute radial part of NEM inner iteration, which is based on coarse mesh diffusion theory, with inner iteration of NDOM, which is based on fine mesh transport theory. Taking advantages of NEM and NDOM, the algorithm models efficiently heterogeneous features in radial planes of an NPP reactor core and takes less computer memory and computation time. Two prototype codes, NSNM and MGNEM, for the algorithm based on 2D multi-group NDOM and 3D multi-group NEM are developed. A computer code named HANWIND is integrated based on former codes. Numerical experiments on benchmark problem OECD/NEA-2D C5GTMOX and a self-estab-lished benchmark problem based on 3D core of 2-loop NPP reactor are shown.

Key words: diffusion theory, transport theory, nodal expansion method(NEM), nodal discrete ordinate method(NDOM), NEM embedded with NDOM