MODELING OF EARLY BREAKTHROUGH BEHAVIOR OF NEPTUNIUM AND URANIUM IN COLUMN TRANSPORT EXPERIMENTS IN YUCCA MOUNTAIN ALLUVIUM

Column transport experiments were conducted to investigate the transport behavior of Neptunium and Uranium in alluvium obtained from Nye County wells located south of Yucca Mountain, NV. These experiments have indicated that both Np and U often break through much earlier than would be predicted from batch sorption experimental results. However, the first arrival time of the radionuclides is flow rate dependent, and only a small fraction of the total radionuclide mass arrives at early times. A transport model that treats sorption and desorption as processes described by a distribution of residence times on alluvium surfaces was developed and shown to be consistent with the column test results. This model was inspired by and is consistent with uranium desorption measurements conducted using the same alluvium that was used in the columns. The model suggests that the early breakthrough behavior observed in the columns should decrease and radionuclide transport velocities should more closely approach those predicted from batch sorption experiments as time and length scales increase to field scales.