Investigation of Dose Rates Exterior to an Above-Ground Waste Storage Facility Using Radiation Transport Models

Abstract

The dose rate profile at different heights above the ground and as a function of distance from the north, west, and south walls of an above-ground waste storage facility was analyzed using the Monte Carlo n-Particle Transport eXtended (MCNPX) radiation transport code. The waste storage facility houses 9,996 waste barrels of conditioned waste. The facility has concrete shielding added to the building walls on the north, west, and east sides, with no such additional shielding towards the roof or the south side wall; instead, the distance from the first row of barrels to the wall is extended to allow for maneuverability of a crane on the south side. The dose rate is computed as a function of distance using MCNPX and assuming a homogeneous Co distribution in each waste barrel. Different dose regions are identified and analyzed based on graphical features and best-fit functions. The dose rates were expected to be largest at the wall of the facility and subsequently decrease continuously with distance from the repository; however, our analysis indicates a peak in dose rate observed for all heights on the north and west sides of the facility. This peak is likely due to scattering in the shielding material and atmosphere, and possibly could be ascribed to skyshine. The difference between the dose rate at 1 m outside the wall and the peak dose rate is significant, and indicates that the dose rate measured close to the wall may not always be conservative for extended sources, such as an above-ground waste storage facility.