Porflow Implementation of Vadose Zone Conceptual Model for Slit and Engineered Trenches in the E-Area Low Level Waste Facility Performance Assessment

Abstract

ScopeA summary of the conceptual models to be used in representing slit (ST) and engineered trenches (ET) in PORFLOW (ACRi 2018) simulations as part of the performance assessment (PA) of the E-Area Low Level Waste Facility (ELLWF) is presented. Key details that are discussed include: model geometries, spatially dependent hydro-stratigraphic representations, model dimensionality, and boundary conditions (i.e., infiltration rates, cover overhangs, subsidence). Accounting for differences in the percent of noncrushable materials, eighteen unique models, defined by seven hydro-stratigraphic groupings, will be used to represent slit and engineered trenches. DiscussionThe relevance of several hydro-stratigraphic and design features specific to the ELLWF has been evaluated using PORFLOW to assist in the development and implementation of the vadose zone conceptual models to be used in the PA. In the following subsections, a summary of these evaluations is presented to highlight the actions and decisions in the development and implementation of the proposed conceptual models. Disposal Unit LengthThe longest footprint in the ELLWF is ST1 (675.3 feet). While the length of the majority of STs and ETs fall within 3% of the length of ST1, the lengths of ST10, ST11, ST21, ST22, and ET3 are substantially different (i.e., greater than 10%, and up to 33% different). Therefore, a series of PORFLOW simulations were performed to confirm that a generic model of any length could be used to represent any trench without impacting the flux to the water table. A generic 2D model with waste uniformly distributed across 600-, 500-, 400-, 300-, 200-, and 100-foot lengths was used to calculate steady-state flow and transient transport solutions to obtain the flux to the water table for the list of nine radionuclide species investigated in Hamm et al. 2018. Because the specified quantity of the radionuclide species is the same regardless of length, and no solubility limitations are imposed, all differences in the peak flux to the water table were negligible (<~3%) and attributed to numerical dispersion. Therefore, one generic waste zone length can be used for all disposal units (DUs) in the ELLWF.

PORFLOW Implementation of Special Waste from Models for Slit and Engineered Trenches in the E-Area Low Level Waste Facility Performance Assessment

“…The ELLWF hydrostratigraphy, updated in 2017 by Bagwell et al, was used as a basis for the development of the ST/ET conceptual models (Danielson, 2019). The STs and ETs containing these SWFs will be modeled as having the depth to the water table and a clayey thickness beneath the waste zone as specified by Danielson (2019).…”

“…, and ST05 will be assigned the waste zone chemical and hydraulic properties obtained from Kaplan (2016) and Nichols (2020), and the depth to the water table and the clayey thickness beneath the waste zone will be modeled as specified for each trench unit by Danielson (2019). Table 4 Special waste forms to be modeled using the "Solubility-Controlled Release" implementation.…”

“…The ELLWF hydrostratigraphy was updated in 2017 by Bagwell et al, which was used as a basis for the development of the ST/ET conceptual models (Danielson, 2019). ST09, where the HXs are located, is to T. L. Danielson SRNL-STI-2020-00162, Rev.…”

“…Tall box burial is limited to the southern-third (i.e., 220 feet) of ST08 and ST09 and the southern 180 feet of ST10. ST08-10 are part of the hydrostratigraphic grouping that is considered to have a depth to the water table of 75 feet and 11 feet of clay thickness beneath the waste zone (Danielson, 2019). Tall box models will be implemented in three dimensions and will incorporate a portion of a ST segment that is 35 feet deep (i.e., the maximum depth of the tall boxes), where the waste zone is 31 feet and four feet of backfill material is placed on top.…”

Component-In-Grout Model Implementation for the E-Area Low-Level Waste Facility's Performance Assessment

“…The cross-sectional material layout for the CIG unit model is shown in Figure 2-2. The hydrostratigraphic surfaces present around and beneath the CIG units are taken directly from Danielson (2019) and are based on the work of Bagwell and Bennett (2017). The thickness of grout surrounding the waste zone is 1-foot thick on all sides and the waste zone is 18 feet wide by 14 feet tall (excluding the grout encapsulating layer).…”