Modeling the Effect of Sedimentation on Cesium Transport in Fourmile Branch

摘要

The major mechanisms of radioactive material transport and fate in surface water are (1) sources, (2) dilution, advection and dispersion of radionuclides by flow and surface waves, (3) radionuclide decay, and (4) interaction between sediment and radionuclides. STREAM II, an aqueous transport module of the Savannah River Site emergency response WIND system, accounts for the source term, and the effects of dilution, advection and dispersion. Although the model has the capability to account for nuclear decay, due to the short time interval of interest for emergency response, the effect of nuclear decay is very small and so it is not employed. The interactions between the sediment and radionuclides are controlled by the flow conditions and physical and chemical characteristics of the radionuclides and the sediment constituents. The STREAM II version used in emergency response does not model the effects of sediment deposition/resuspension to minimize computing time. This study estimates the effects of sediment deposition/resuspension on radionuclide aqueous transport. For radionuclides that adsorb onto suspended sediment, the omission of deposition/resuspension effects overestimates the downstream radionuclide peak concentration and is therefore conservative. For the case of cesium transport in the Fourmile Branch, the calculated reduction in peak concentration as the cesium is transported downstream is greater with sediment deposition modeled than without. For example, including the effects of sediment deposition/resuspension in the STREAM II calculation results in a 72 percent reduction in the downstream (5075 meters downstream from H-Area) peak cesium concentration. It is important to note that the high partition coefficient appropriate for cesium enhances the importance of sediment deposition/resuspension; the reduction in the calculated peak concentration would be less for radioisotopes with lower partition coefficients.