Unreviewed Disposal Question Evaluation: Subsidence Study for Non-Crushable Containers in Slit Trenches (U) Page: 4 of 49
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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 (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.
For a better representation of the waste footprint, an aquifer model with a refined mesh of 20 ft x 20 ft in
plan view was designed. The 2004 Special Analysis used a coarser mesh of 200 ft x 200 ft. The fine
mesh allows more precise allocation of contaminant source cells into each individual slit trench. This
allocation scheme makes analyses of any partial or total subsidence of any trench or a combination of
trenches among the slit trenches possible.
To study the potential effect of trench subsidence on the well concentrations within the 1,000-yr time
window, two high-impact cases were considered. In the first case, trenches subsided right after dynamic
compaction (i.e., at 125 years). In the second case, trenches subsided at 419 years to make the peak
concentrations from both the subsided and unsubsided area align in space and time.
The study shows that the first case presented no compliance problem for the subsidence of up to two
trenches. In the second case, even a single trench subsidence caused the well concentration to be out of
compliance. The peak concentration exceeded the base case value by 9% (subsidence of 14-5 trench) and
15% (subsidence of 14-5 and 14-4 trenches)
Wilhite (2003) states that "DOE guidance for PA maintenance establishes a criterion for the significance
of changes in PA results. The criterion is that changes of about 10 percent in the dose or impact are
considered to be insignificant." Using this criterion, the following recommendations are made:
1. In general, the amount of the waste area that contains non-crushable containers should not
exceed 10 percent for two adjacent Slit Trench disposal units. The waste area is defined as
the trench area where waste is disposed, not the overall area of the disposal unit. This
requirement can be satisfied in either of two ways:
a. Each Slit Trench disposal unit does not exceed 10 percent, or
b. If one Slit Trench disposal unit exceeds 10 percent, then when combined with each
neighboring disposal unit, the combination does not exceed 10%. For example if the
waste area that contains non-crushable containers is 15% for one disposal unit, then
neither adjacent disposal unit can exceed 5%.
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THONG, HANG. Unreviewed Disposal Question Evaluation: Subsidence Study for Non-Crushable Containers in Slit Trenches (U), report, March 15, 2005; South Carolina. (digital.library.unt.edu/ark:/67531/metadc782012/m1/4/: accessed February 18, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.