Revision 0 iii EXECUTIVE SUMMARY This study addresses the issue of waste and cover subsidence caused by corrosion of the non-crushable waste containers defined as containers with significant void space that will not be stabilized by dynamic compaction of the E-Area Slit Trenches. Concentrations at the hypothetical 100-m well were evaluated for 1,000 years and compared with the base case value for compliance. To generalize the results, a hypothetical, no-decay radionuclide characterized by a Kd

more »

... 5 ml/g) that would be most problematic was selected. The vadose zone analyses employed two distinct modeling grid layouts to better account for dissimilar flow fields between the edge trench and the middle or crest trench. These grid layouts provide higher-fidelity simulations of the actual trenches, and therefore would deliver more accurate flow fields and activity fluxes. As in the 2004 Special Analysis performed by Collard and Hiergesell (Collard and Hiergesell 2004), the waste zone thickness was modeled as a variable that changes from 16 ft (before subsidence) to 2.5 ft (after subsidence). Although the non-crushable containers will not be stabilized by dynamic compaction, these containers will gradually corrode, eventually collapse after placement of the final closure cap and cause the cap to subside resulting in an increase of the infiltration rates. Using the HELP model, infiltration rates for each trench (i.e., edge trench, middle trench and crest trench) were estimated for both intact and subsided conditions over a 10,000-year period. Applying these infiltration rates in all time intervals up to 1,000 years, steady-state flow fields were generated for all scenarios.