Publications

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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), Office of Fuel Cycle Technology (OFCT) is conducting research and development (R&D) on geologic disposal of spent nuclear fuel (SNF) and high level nuclear waste (HLW). Two high priorities for SFWST disposal R&D are design concept development and disposal system modeling (DOE 2011, Table 6). These priorities are directly addressed in the SFWST 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, shale, and deep borehole disposal).

This report provides an update to Sassani et al. (2016) and includes: (1) an updated set of inputs (Sections 2.3) on various additional waste forms (WF) covering both DOE-managed spent nuclear fuel (SNF) and DOE-managed (as) high-level waste (HLW) for use in the inventory represented in the geologic disposal safety analyses (GDSA); (2) summaries of evaluations initiated to refine specific characteristics of particular WF for future use (Section 2.4); (3) updated development status of the Online Waste Library (OWL) database (Section 3.1.2) and an updated user guide to OWL (Section 3.1.3); and (4) status updates (Section 3.2) for the OWL inventory content, data entry checking process, and external OWL BETA testing initiated in fiscal year 2017.

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This paper considers concepts for disposal of canistered high-level (radioactive) waste (HLW) in large diameter deep boreholes. Vitrified HLW pour canisters are limited in diameter to promote glass cooling, and constitute a large potential application for borehole disposal where diameter is constrained. The objective for disposal would be waste packages with diameter of 22 to 29 inches, which could encompass all existing and projected HLW glass inventory in the United States. Deep, large diameter boreholes of the sizes needed have been successfully drilled, and we identify other potentially effective designs. The depth of disposal boreholes would be site-specific, and need not be as deep as the 5 km being investigated in the Deep Borehole Field Test. For example, a 0.91 m (36 inch) diameter borehole drilled to 3 km could be used for disposal from 2.5 to 3 km (8, 200 to 9, 840 ft). The engineering feasibility of such boreholes is greater today than was concluded by earlier studies done in Sweden and the United States. Moreover, the disposal concept and generic safety case have evolved to a point where borehole construction need not be as elaborate as previously assumed. Each borehole in the example could accommodate approximately 100 waste packages containing canisters of vitrified HLW. Emplacement of the packages would be through a 32-inch (0.81 m) guidance casing, installed in two sections to reduce hoisting loads, and forming a continuous pathway from the surface to total depth. Above the disposal zone would be a nominal 1 km (3, 280-ft) seal interval, similar to previously published concepts. Following those concept studies, the seal system would consist of alternating lifts of swelling clay, backfill and cement. Above the seal zone the borehole would be plugged with cement in the conventional manner for oil-and-gas wells. The function of seals in deep borehole disposal is to maintain the pre-drilling hydrologic regime in the crystalline basement, where groundwater is increasingly saline, stagnant, and ancient. Seals would resist fluid movement and radionuclide transport during an early period of waste heating, but after cooling little fluid movement is expected. Thus, the function of seals could be less important with HLW that has low heat output, and sealing requirements could be limited. The safety case for deep borehole disposal relies on the prevalence of groundwater that is increasingly saline with depth, stagnant, and ancient, in crystalline basement rock that has low bulk permeability and is isolated from surface processes. The minimum depth for disposal depends on sitespecific factors, and may be less than the 2.5 km example. Rough-order-of-magnitude cost estimates show that deep borehole disposal of HLW would be cost-competitive with the lowest cost mine repository options. Thinner overburden, and shallower development of conditions favorable to waste isolation, could make drilling of large-diameter disposal boreholes even more cost effective. The dimensions of the disposal zone and seal zone would be site specific, and would be adjusted to ensure that both are situated in unaltered crystalline basement rock.

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.

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.