Review ofTOUGH2 Numerical Modeling of the WCS Facility, Andrews County, Texas

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2007

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The Bureau of Economic Geology (BEG) was tasked with reviewing the TOUGH2 numerical modeling submitted by WCS (two 3D models and one 2D model for the low-level application and one 2D model for the byproduct application). The preliminary step to the review consisted of the lengthy installation of the software and ensuring that the results agreed with those supplied by WCS and the software developers.

The TOUGH2 2D transport modeling by the applicant addressed subsurface-parameter uncertainty in a relatively thorough fashion except for one variable: assumed top-boundary fluxes of ~0.01 inch/yr are too low. Simulations performed for and presented in this report with top flux increases to 0.1 and 1 inch/yr lead to much less conservative results, decreasing breakthrough time from ~14,000 years (applicant's base case) to less than 5,000 years (flux x 10) and less than 1,000 years (flux x 100) for Tc-99. Chloride's breakthrough time, used as a marker for the byproduct facility, also decreases from over 1,000 years (applicant's base case) to less than 200 years in other cases. Note that cases in which simulation results do not meet concentrations suggested by regulations do not necessarily invalidate the site. Because the 2D model used in the simulations is so conservative, a more realistic conceptual model consistent with site geology and hydrology would probably yield results that would be less extreme than some of those presented in this analysis. It is, however, the applicant's charge to develop such models, for example, by modeling the bottom liner (as applicable), by evaluating fracture extent and connectivity, and by better understanding the source (leachate chemical composition was obtained with no credit given to containers; in addition, high water flux is also likely to translate into a much lower radionuclide concentration).

TOUGH2 3D models do provide insight into the behavior of the natural system but should be better calibrated and better constrained to provide arguments to the unproven applicant's contention that the system is and has been at steady state for tens of thousands of years.

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