Reducing 3D MOC Storage Requirements with Axial On-the-fly Ray Tracing

Abstract

The Method of Characteristics (MOC) is a popular method to solve the multi-group neutron transport equation. While this method is most widely used in two dimensions, extension to three dimensions allows for more accurate calculation of axial leakage and reaction rates. However, the 3D form of MOC can be computationally prohibitive. One concern is the massive memory requirements imposed by storing all segments of 3D tracks. In this study, an alternative approach is presented for axially extruded geometries that only saves segments in two dimensions. This is accomplished by first creating a 2D xy-plane that incorporates all radial detail at every axial level. Then, standard 2D ray tracing is applied to this plane. Axial extruded regions are constructed during segmentation, each containing an axial mesh. During transport sweeps the 3D segments are reconstructed on-the-fly using 2D segment lengths and 1D axial meshes. This strategy implicitly transforms geometries into an axially extruded representation. The resulting algorithm consumes far less memory with minimal computational overhead for common reactor physics problems.