Simulations of the Near-Field Transport of Radionuclides by Liquid Diffusion at Yucca Mountain: Comparisons with and Without Emplacement Backfill.

摘要

The possible set of hydrologic conditions at the Yucca Mountain repository site includes the case where groundwater recharge fluxes are sufficiently low relative to molecular diffusion in the rock so that, in the region around the waste package, the dominant mode of aqueous transport of radionuclides is by diffusion. Although the rock at the repository level is unsaturated, a sufficient amount of pore water could form a contiguous diffusion path from the waste form to the near-field rock if the waste is postulated to be in contact with the rock or emplacement backfill due to failure of the container. Future simulations will have to include conditions where the effects of convective transport in the rock are important in aqueous transport. We consider in this report simplified simulations of one-dimensional transport of radionuclides in the rock due to liquid molecular diffusion in order to determine the effects of an emplacement backfill. Comparison of estimated release rates with and without an emplacement backfill were given in an article by Chambre and Pigford (1984), and these values were referenced in our previous report. Here, we have extended their calculations to include estimates using the physical parameters expected at the Yucca Mountain repository site. Our model is a simple one-dimensional treatment of diffusion in a spherically symmetric geometry that takes into account the sorptive effects of the tuff and backfill through the use of K/sub d/ values. This geometry rather than a cylindrical one was chosen because it is conservative in predicting higher release rates. Radioactive decay is included, and only one species at a time is treated. The dissolution of the radionuclides is assumed to be solubility-limited. 12 refs., 13 figs., 1 tab.