HEXNEM nodal neutronics code for two dimensional multi group diffusion calculations in hexagonal geometry

A computer code HEXNEM based on a Nodal Expansion method has been developed for solving of the neutron diffusion equation for two-dimensional hexagonal geometries. The nodal equations are derived using higher order polynomial approximations to the spatial dependence of the flux within the hexagonal node. The final equations which are cast in the form of inhomogeneous response matrix equations for each group, involve spatial moments of the node interior flux distribution and surface averaged partial currents across the faces of the node. These equations are solved using a conventional fission source iteration accelerated by coarse mesh rebalance and asymptotic source extrapolation. Numerical calculations for models of reactor cores like PFBR or VVER designs have shown accuracy of the nodal schemes to be superior to that the coarse mesh finite difference method. The higher order axial approximations in the nodal method permit the use of an axial mesh which is at least four times coarser than a typical finite difference mesh. Two benchmark problems have been solved and the results are presented. This report describes the mathematical development and numerical solution of the nodal equations, as well as the input options used in the computer code. (author)