Publications

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Numerical simulation of a repository for heatgenerating nuclear waste in fractured crystalline rock requires a method for simulating coupled heat and fluid flow and reactive radionuclide transport in both porous media (bentonite buffer, surface sediments) and fractured rock (the repository host rock). Discrete fracture networks (DFNs), networks of two-dimensional planes distributed in a three-dimensional domain, are commonly used to simulate isothermal fluid flow and particle transport in fractures, but unless coupled to a continuum, are incapable of simulating heat conduction through the rock matrix, and therefore incapable of capturing the effects of thermally driven fluid fluxes or of coupling chemical processes to thermal processes. We present a method for mapping a stochastically generated DFN to a porous medium domain that allows representation of porous and fractured media in the same domain, captures the behavior of radionuclide transport in fractured rock, and allows simulation of coupled heat and fluid flow including heat conduction through the matrix of the fractured rock. We apply the method within Sandia's Geologic Disposal Safety Assessment (GDSA) framework to conduct a post-closure performance assessment (PA) of a generic repository for commercial spent nuclear fuel in crystalline rock. The three-dimensional, kilometer-scale model domain contains approximately 4.5 million grid cells; grid refinement captures the detail of 3, 360 individual waste packages in 42 disposal drifts. Coupled heat and fluid flow and reactive transport are solved numerically with PFLOTRAN, a massively parallel multiphase flow and reactive transport code. Simulations of multiple fracture realizations were run to 1 million years, and indicate that, because of the channeled nature of fracture flow, thermally-driven fluid fluxes associated with peak repository temperatures may be a primary means of radionuclide transport out of the saturated repository. The channeled nature of fracture flow gives rise to unique challenges in uncertainty and sensitivity quantification, as radionuclide concentrations at any given location outside the repository depend heavily on the distribution of fractures in the domain.

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An important feature required in all geological disposal system modeling is proper representation of waste package degradation and waste form dissolution. These processes are often treated as batch operations, meaning they are zero-dimensional. However, waste package canister degradation or waste form dissolution are affected by near-field conditions, and thus they must be coupled to the computational domain through the exchange of information on local conditions. Accurate waste package and waste form degradation behavior is essential because processes occurring at the batch level also affect far field conditions through heat and mass transport by advection or diffusion. Presented here is the development and performance of the Waste Form Process Model, an integrated module for waste package canister degradation and waste form dissolution developed by Sandia National Laboratories within PFLOTRAN. PFLOTRAN is an open source, massively parallel subsurface simulator for multiphase, multicomponent, and multiscale reactive flow and transport processes in porous media. PFLOTRAN is used to model geologic disposal systems for the Spent Fuel and Waste Science and Technology (SFWST) Campaign under the Spent Fuel and Waste Disposition Program of the U.S. Department of Energy (DOE) Office of Nuclear Energy.

This paper discusses software design choices made during the ongoing development of PFLOTRAN with the intent of improving the longevity and flexibility of the code while maintaining robustness. PFLOTRAN is a massively parallel reactive multiphase flow and transport code employed to simulate subsurface processes in support of radioactive waste management licensing and performance assessment studies. PFLOTRAN's open source licensing, software configuration management, object-oriented design through modern Fortran 2003/2008, approach to coupling scientific process models, and automated testing are discussed in detail.

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The Spent Fuel and Waste Science and Technology (SFWST) Campaign of the U.S. Department of Energy (DOE) Office of Nuclear Energy (NE) is conducting research and development (R&D) on generic deep geologic disposal systems (i.e., repositories). This report describes specific activities in FY 2016 associated with the development of a Defense Waste Repository (DWR)a for the permanent disposal of a portion of the HLW and SNF derived from national defense and research and development (R&D) activities of the DOE.

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The Used Fuel Disposition Campaign (UFDC) of the U.S. Department of Energy (DOE) Office of Nuclear Energy (NE), Office of Fuel Cycle Technology (OFCT) is conducting research and development (R&D) on geologic disposal of used nuclear fuel (UNF) and high-level nuclear waste (HLW). Two of the high priorities for UFDC disposal R&D are design concept development and disposal system modeling (DOE 2011). These priorities are directly addressed in the UFDC Generic Disposal Systems Analysis (GDSA) work package, which is charged with developing a disposal system modeling and analysis capability for evaluating disposal system performance for nuclear waste in geologic media (e.g., salt, granite, clay, and deep borehole disposal). This report describes specific GDSA activities in fiscal year 2016 (FY 2016) 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 and the Dakota uncertainty sampling and propagation code. 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 engineered barriers and natural geologic barriers to the biosphere. Dakota is used to generate sets of representative realizations and to analyze parameter sensitivity.

Permafrost-dominated coastlines in the Arctic are rapidly disappearing. Arctic coastal erosion rates in the United States have doubled since the middle of the twentieth century and appear to be accelerating. Positive erosion trends have been observed for highly-variable geomorphic conditions across the entire Arctic, suggesting a major (human-timescale) shift in coastal landscape evolution. Unfortunately, irreversible coastal land loss in this region poses a threat to native, industrial, scientific, and military communities. The Arctic coastline is vast, spanning more than 100,000 km across eight nations, ten percent of which is overseen by the United States. Much of area is inaccessible by all-season roads. People and infrastructure, therefore, are commonly located near the coast. The impact of the Arctic coastal erosion problem is widespread. Homes are being lost. Residents are being dispersed and their villages relocated. Shoreline fuel storage and delivery systems are at greater risk. The U.S. Department of Energy (DOE) and Sandia National Laboratories (SNL) operate research facilities along some of the most rapidly eroding sections of coast in the world. The U.S. Department of Defense (DOD) is struggling to fortify coastal radar sites, operated to ensure national sovereignty in the air, against the erosion problem. Rapid alterations to the Arctic coastline are facilitated by oceanographic and geomorphic perturbations associated with climate change. Sea ice extent is declining, sea level is rising, sea water temperature is increasing, and permafrost state is changing. The polar orientation of the Arctic exacerbates the magnitude and rate of the environmental forcings that facilitate coastal land area loss. The fundamental mechanics of these processes are understood; their non-linear combination poses an extreme hazard. Tools to accurately predict Arctic coastal erosion do not exist. To obtain an accurate predictive model, a coupling of the influences of evolving wave dynamics, thermodynamics, and sediment dynamics must be developed. The objective of this document is to present the state-of-the-science and outline the key steps for creation of a framework that will allow for improved prediction of Arctic coastal erosion rates. This is the first step towards the quantification of coastal hazards that will allow for sustainable planning and development of Arctic infrastructure.

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