Publications

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This report describes specific GDSA activities in fiscal year 2015 (FY2015) toward the development of the enhanced disposal system modeling and analysis capability for geologic disposal of nuclear waste. The GDSA framework employs the PFLOTRAN thermal-hydrologic-chemical multi-physics code (Hammond et al., 2011) and the Dakota uncertainty sampling and propagation code (Adams et al., 2013). Each code is designed for massively-parallel processing in a high-performance computing (HPC) environment. Multi-physics representations in PFLOTRAN are used to simulate various coupled processes including heat flow, fluid flow, waste dissolution, radionuclide release, radionuclide decay and ingrowth, precipitation and dissolution of secondary phases, and radionuclide transport through the engineered barriers and natural geologic barriers to a well location in an overlying or underlying aquifer. Dakota is used to generate sets of representative realizations and to analyze parameter sensitivity.

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The calculation of radionuclide concentrations in a receptor well of a generic reference aquifer is highly dependent on the mixing that occurs in the aquifer and well. This study tests two simple analytical approaches for estimating well concentrations: a Generic Aquifer Transport and Capture (GATC) approach and an adapted IAEA ERB1B model. These approaches are tested against a 3-D generic repository model simulated numerically using PFLOTRAN. Because the PFLOTRAN model in this study simulates a diffusive release to the aquifer and the ERB1B model is designed for advective releases, the ERB1B model had to be adapted to accept a diffusive release. The results show that for this PFLOTRAN simulation the GATC equations accurately predict the initial breakthrough and peak concentration at the well but somewhat overestimate concentrations between these points. The adapted ERB1B model only predicts the peak accurately. Similar studies with broader aquifer domains and different types of advective and diffusive releases to the aquifer are needed to fully test the ranges of these analytical approaches.

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directly, rather than through simplified abstractions. It also a llows for complex representations of the source term, e.g., the explicit representation of many individual waste packages (i.e., meter - scale detail of an entire waste emplacement drift). This report fulfills the Generic Disposal System Analysis Work Packa ge Level 3 Milestone - Performance Assessment Modeling and Sensitivity Analyses of Generic Disposal System Concepts (M 3 FT - 1 4 SN08080 3 2 ).

An analysis of the Waste Isolation Pilot Plant (WIPP) colloid model constraints and parameter values was performed. The focus of this work was primarily on intrinsic colloids, mineral fragment colloids, and humic substance colloids, with a lesser focus on microbial colloids. Comments by the US Environmental Protection Agency (EPA) concerning intrinsic Th(IV) colloids and Mg-Cl-OH mineral fragment colloids were addressed in detail, assumptions and data used to constrain colloid model calculations were evaluated, and inconsistencies between data and model parameter values were identified. This work resulted in a list of specific conclusions regarding model integrity, model conservatism, and opportunities for improvement related to each of the four colloid types included in the WIPP performance assessment.