Extension of analytic function expansion nodal method to multigroup problems in hexagonal-Z geometry

The analytic function expansion nodal (AFEN) method has been successfully applied to two-group neutron diffusion problems. However, the current AFEN method cannot treat complex eigenmodes, which appear in the general multigroup equations. The AFEN method is extended such that complex eigenmodes are treated within the framework of the original AFEN method for any type of geometry. Also, a suite of new nodal codes based on the extended AFEN theory is developed for hexagonal-z geometry and applied to several benchmark problems. Numerical results obtained attest to their accuracy and applicability to practical problems.