ANEMONA — a neutron transport code for general geometry reactor assemblies based on the method of characteristics and R-function solid modeler

The method of characteristics (MOC) solves the transport equation along straight lines, called characteristics, of the system. Along these lines the di€erential operator of the Boltz- mann equation reduces to a total derivative. The MOC methodology does not impose any limitation on geometry and allows for an accurate treatment of highly heterogeneous systems. However, the actual treatment of arbitrary domains in terms of their ¯exible description and ecient ray tracing does impose diculties which limited broader application of MOC in reactor analysis. Most of the existing MOC codes describe the geometry by lines and arcs with extensive input data, have diculty in sub-meshing, require a pre-de®ned condition of closed rays at starting points limiting the boundary shape, and need a large number of polar angles which prolong the execution time. A new MOC code, ANEMONA, has been developed to remove all these geometrical limitations. This uni®ed and easy-to-use method is based on an R-function solid modeler approach that can account for all heterogeneities with full ¯exibility in domain description, ray map generation, ray tracing and boundary types. The ANEMO- NAʹs methodology has many advantages: an easy description of arbitrary domains and meshing; allowed mismatch of the re¯ected rays; calculation of the re¯ected angular ¯ux as the boundary mesh average; use of only two adequately chosen polar angles; and the reduc- tion in computational time by energy-dependent azimuthal ray map. ANEMONA has been tested on a number of benchmark problems with excellent results.