Uncollided Flux Implementation for Discrete Ordinates Radiation Transport Solutions in Rattlesnake

Simulation of neutron streaming over long distances in optically thin narrow pipes sur- rounded by optically thicker materials that can absorb and/or scatter neutrons presents a simulation challenge to most neutron transport methods. Without sophisticated variance reduction techniques, this problem can be intractable for Monte Carlo techniques. Deterministic methods are often limited due to large angular discretization errors, this phenomenon is commonly known as ray effects. The uncollided component of the angular flux solution is the most anisotropic part and it can be difficult for discrete ordinate methods to accurately represent. The number of discrete angles required to overcome these ray effects is prohibitively large for the hodoscope configuration. Hence, other approaches must be taken to better simulate neutron streaming within the hodoscope collimator slots. In this report, we present an algorithm for semi-analytical calculation of the uncollided flux.