Methodology, applications and performance of the CAD-based geometry type in the serpent 2 Monte Carlo code

The Serpent 2 Monte Carlo code features a CAD-based geometry option for the modeling of complicated and irregular systems. The methodology is based on the STL data format, which is commonly used for computer graphics and 3D printing, and supported by a wide range of software tools. The methodology has been available for several years, but not described in complete detail. This paper presents the geometry routine with its advantages, limitations and known flaws. A brief overview on practical applications and a workflow example involving neutron and photon transport calculations for a spent nuclear fuel storage rack are provided for discussion. It is concluded that the CAD-based geometry type is a convenient option for various neutron and radiation transport problems, and does not suffer from significant deterioration in computational efficiency compared to conventional CSG models. It is also noted, however, that taking full advantage of the methodology requires some level of understanding on the software tools, the STL data format and the Serpent geometry routine to avoid the most common pitfalls.