Sensitivity Analysis of Radionuclide Downward Migration for a Low-Level Radioactive Waste Repository: An Analytical Approach for Performance Assessment

Changbing Yang¹, Kan Tu², Abel Porras², Brad Broussard², Alexander Sun³, Jean-Philippe Nicot&sup4;, and Bridget Scanlon³
¹ University of Texas at Austin, Austin, Texas 78759
² Texas Commission on Environmental Quality, Austin, Texas 78753
³ University of Texas at Austin, Austin, Texas 78758
&sup4; University of Texas at Austin, Austin, Texas 78713–8924

A repository for disposal of low-level radioactive waste in Andrews County, west Texas is being constructed within the low-permeable Triassic Dockum Group mudstones (Cooper Canyon Formation) with sparse siltstone/sandstone interbeds. Radionuclides in the repository can be leached out from the repository and eventually migrate downward to a sandstone layer of the Dockum Group, at depth of ~42 meters beneath the repository. Since groundwater in the sandstone layer may be explored for potential domestic use in future, it is of great interest to assess breakthrough of radionuclides which migrate from the repository. A 1D model was constructed for simulating downward migration of four radionuclides (I-123, Tc-99, C-14, and Cl-36) for a time period of 100,000 years. Radionuclide transport in the 1D vertical domain is assumed to be dominated by processes of diffusion, advection, decay and adsorption. The 1D model was solved numerically with Modflow-Surfact and HYDRUS and also with an analytical approach. The results of analytical approach compare favorably with the numerical results. Since the analytical approach was much more robust than the numerical method for this application, it was then further used to perform comprehensive sensitivity analysis of breakthrough of the four radionuclides at the sandstone layer in terms of different model parameters.

AAPG Search and Discovery Article #90158©2012 GCAGS and GC-SEPM 6nd Annual Convention, Austin, Texas, 21-24 October 2012