Development of an optical surface method for transport calculations

The Optical Surface Method (OSM)is a new technique for performing neutron and photon transport calculations for three-dimensional room-like structures An iterative process is used to determine flux values and/or detector responses at points defined by the user. The uncollided fluxes are determined analytically, and one-dimensional discrete ordinates calculations are used to determine transport of radiation through, and reflection from, the walls of the structure The results of these discrete ordinates calculations are stored in response matrix files and are pre-calculated by the MAKERT utility during the problem setup For transport of particles through open space, a simple 1/47πr² calculation is used, under the assumption that the air in the room can be treated as a void in most situations

The geometry for a given problem is defined as a series of walls. Each wall is defined in terms of its dimensions and material composition. Additionally, the walls are divided into a grid of “patches,” the size and shape of which are determined by the user of the GSM code. The use of these patch grids allow the inclusion of the spatial distribution of the incoming and outgoing radiation fields for each wall of the structure.

Results for the initial test cases show close agreement with detector fluxes and responses predicted by MCNP, while using only a fraction of the computer time required for an MCNP calculation. Detector points placed exceedingly close to a wall do not yield accurate results, due to the violation of several assumptions made in the coding of the GSM. This issue will be addressed in future versions of the GSM code.