Experience using models imported from CAD software for shielding calculations in MCBEND

*Reactor components (e.g. fuel assemblies) and other equipment (e.g. transport flasks) are designed using Computer Aided Design (CAD) packages. The process of converting CAD models into models that can be used in a Monte Carlo simulation is both time consuming and a source of approximations and errors. It is useful to be able to directly use these geometries in Monte Carlo calculations. MCBEND Version 11A Release Update 0 includes several methods of importing CAD geometries. The first method provides full accurate support for the range of geometry capabilities offered by the Initial Graphics Exchange Specification (IGES) model which can be used in combination with the existing geometry capabilities of MCBEND. All the surfaces in the IGES model are used directly with no approximation. This capability is provided by the OiNC software package developed by Sellafield Ltd as part of a Nuclear Code Development collaboration between Sellafield Ltd and the ANSWERS Software Service. A further two methods provide high performance CAD import options, but with a geometry approximation introduced by first converting the CAD file to a tetrahedral mesh representation or polygon surface representation, known as Standard Tessellation Language (STL), before importing them into MCBEND.