Radionuclide Transport in an Unsaturated, Fractured Medium

摘要

As part of the effort to evaluate the Yucca Mountain, Nevada, site as a possible location for a repository for high-level radioactive waste, a computer code (TOSPAC) for site-scale calculations of hydrology and radionuclide transport in the unsaturated zone is being developed by the Nevada Nuclear Waste Storage Investigations project at Sandia National Laboratories. For the large-scale calculations needed to assess the safety of the site, it is not feasible to explicitly account for the effects of individual fractures, but yet the fractures cannot be ignored, since radionuclide transport down fractures may be an important transport mechanism. For this reason, we have developed a transport model that considers transport in fractures and in the porous matrix separately, with a coupling term that allows the radionuclides to move from the fracture fluid to the matrix pore fluid and vice versa. By this means, we are able to model transport in a fractured medium with only one spatial dimension, which allows a substantial savings in computer time. In modeling the fracture-matrix transfer term, both advective and dispersive processes are included, as a generalization of the ''matrix diffusion'' effect, which has been studied by several investigators. The results of some sample calculations, with parameter values representative of Yucca Mountain, are presented. These examples, chosen to illustrate various physical regimes, include problems for which there is essentially no fracture flow and problems for which fracture flow is dominant. These calculations show that the fracture-matrix coupling can be a very important effect, slowing the fracture radionuclide transport substantially.