The method of characteristics (MOC) solves the neutron transport equation along each characteristics line. It is independent of the geometry shape of boundary and region. So theoretically, this method can be used to solve neutron transport equation in highly complex geometry. However, we should describe the geometry strictly and calculate intersect points of each characteristics line with every boundary and region firstly. In complex geometry, due to the great difficulty of treating of the arbitrary domain, the selection of geometry shape and efficiency of ray tracing are limited. So in most of the existing MOC codes, the type of outer boundary is limited as rectangular, hexagonal, triangular or the combination of them. And in order to treat reflective boundary condition, the selection of the azimuthal angles number is not freewill for different outer boundary shapes. To wipe off these limitations for using MOC in more complex fuel assembly calculation, the more powerful geometry treatment method is needed. The geometry treatment through the customization of AutoCAD, famous computer aided design software, is given in this paper. Thanks to the powerful capability of AutoCAD in graphics, the description of complex geometry becomes quite convenient. Moreover, with the language VBA (Visual Basic for Applications), AutoCAD can be customized to calculate intersect points of each characteristics line with every boundary and region. Thereby, no matter how complex or what kind of the geometry shape is, it can be treated uniformly by this method as long as it can be drawn in the AutoCAD. In addition, a directional interpolation method for the treatment of reflective boundary condition is introduced in this paper, so the number of azimuthal angles can be selected freely.

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