HGSYSTEM/UF{sub 6} model enhancements for plume rise and dispersion around buildings, lift-off of buoyant plumes, and robustness of numerical solver

The HGSYSTEM/UF{sub 6} model was developed for use in preparing Safety Analysis Reports (SARs) by estimating the consequences of possible accidental releases of UF{sub 6} to the atmosphere at the gaseous diffusion plants (GDPs) located in Portsmouth, Ohio, and Paducah, Kentucky. Although the latter report carries a 1996 date, the work that is described was completed in late 1994. When that report was written, the primary release scenarios of interest were thought to be gas pipeline and liquid tank ruptures over open terrain away from the influence of buildings. However, upon further analysis of possible release scenarios, the developers of the SARs decided it was necessary to also consider accidental releases within buildings. Consequently, during the fall and winter of 1995-96, modules were added to HGSYSTEM/UF{sub 6} to account for flow and dispersion around buildings. The original HGSYSTEM/UF{sub 6} model also contained a preliminary method for accounting for the possible lift-off of ground-based buoyant plumes. An improved model and a new set of wind tunnel data for buoyant plumes trapped in building recirculation cavities have become available that appear to be useful for revising the lift-off algorithm and modifying it for use in recirculation cavities. This improved lift-off model has been incorporated in the updated modules for dispersion around buildings