Publications

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Appropriate waste-forms for radioactive materials must isolate the radionuclides from the environment for long time periods. To accomplish this typically requires low waste-form solubility, to minimize radionuclide release to the environment. However, radiation eventually damages most waste-forms, leading to expansion, crumbling, increased exposed surface area, and faster dissolution. We have evaluated the use of a novel class of materials-ZrW2O8, Zr2P2WO12 and related compounds-that contract upon amorphization. The proposed ceramic waste-forms would consist of zoned grains, or sintered ceramics with center-loaded radionuclides and barren shells. Radiation-induced amorphization would result in core shrinkage but would not fracture the shells or overgrowths, maintaining isolation of the radionuclide. We have synthesized these phases and have evaluated their leach rates. Tungsten forms stable aqueous species at neutral to basic conditions, making it a reliable indicator of phase dissolution. ZrW2O8 leaches rapidly, releasing tungstate while Zr is retained as a solid oxide or hydroxide. Tungsten release rates remain elevated over time and are highly sensitive to contact times, suggesting that this material will not be an effective waste-form. Conversely, tungsten release rates from Zr2P2WO12 rapidly drop and are tied to P release rates; we speculate that a low-solubility protective Zr-phosphate leach layer forms, slowing further dissolution.

We have investigated cubic zirconium tungstate (ZrW2O8) using density functional perturbation theory (DFPT), along with experimental characterization to assess and validate computational results. Cubic zirconium tungstate is among the few known materials exhibiting isotropic negative thermal expansion (NTE) over a broad temperature range, including room temperature where it occurs metastably. Isotropic NTE materials are important for technological applications requiring thermal-expansion compensators in composites designed to have overall zero or adjustable thermal expansion. While cubic zirconium tungstate has attracted considerable attention experimentally, a very few computational studies have been dedicated to this well-known NTE material. Therefore, spectroscopic, mechanical and thermodynamic properties have been derived from DFPT calculations. A systematic comparison of the calculated infrared, Raman, and phonon density-of-state spectra has been made with Fourier transform far-/mid-infrared and Raman data collected in this study, as well as with available inelastic neutron scattering measurements. The thermal evolution of the lattice parameter computed within the quasi-harmonic approximation exhibits negative values below the Debye temperature, consistent with the observed negative thermal expansion characteristics of cubic zirconium tungstate, α-ZrW2O8. These results show that this DFPT approach can be used for studying the spectroscopic, mechanical and thermodynamic properties of prospective NTE ceramic waste forms for encapsulation of radionuclides produced during the nuclear fuel cycle.

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The DOE R&D program under the Spent Fuel Waste Science Technology (SFWST) campaign has made key progress in modeling and experimental approaches towards the characterization of chemical and physical phenomena that could impact the long-term safety assessment of nuclear waste disposition in deep clay/shale/argillaceous rock. Interactional collaboration activities such as heater tests, particularly postmortem sample recovery and analysis, have elucidated important information regarding changes in engineered barrier system (EBS) material exposed to years of thermal loads. Chemical and structural analyses of bentonite material from such tests has been key to the characterization of thermal effects affecting clay composition, sorption behavior, and swelling. These are crucial to evaluating the nature and extent of bentonite barrier sacrificial zones in the EBS during the thermal period. Thermal, hydrologic, and chemical data collected from heater tests and laboratory experiments has been used in the development and validation of THMC simulators to model near-field coupled processes affecting engineered and natural barrier materials, particularly during the thermal period. This information leads to the development of simulation approaches (e.g., continuum vs. discrete) to tackle issues related to flow and transport depending on the nature of the host-rock and EBS design concept. This report describes R&D efforts on disposal in argillaceous geologic media through developments of coupled THMC process models, hydrothermal experiments and characterization of clay/metal barrier material interactions, and spent fuel and canister material degradation. Currently, the THM modeling focus is on heater test experiments in argillite rock and gas migration in bentonite as part of international collaboration activities at underground research laboratories (URLs). In addition, field testing at an URL involves probing of fault movement and characterization of fault permeability changes. Analyses of barrier samples (bentonite) from heater tests at URLs provide compositional and structural data to evaluate changes in clay swelling and thermal behavior with distance from the heater surface. Development of a spent fuel degradation model coupled with canister corrosion effects has been centered towards its integration with Generic Disposal System Analysis (GDSA) to describe source term behavior. As in previous milestone deliverables, this report is structured according to various national laboratory contributions describing R&D activities applicable to clay/shale/argillite media.

Predicting chemical-mechanical fracture initiation and propagation in materials is a critical problem, with broad relevance to a host of geoscience applications including subsurface storage and waste disposal, geothermal energy development, and oil and gas extraction. In this project, we have developed molecular simulation and coarse- graining techniques to obtain an atomistic-level understanding of the chemical- mechanical mechanisms that control subcritical crack propagation in materials under tension and impact the fracture toughness. We have applied these techniques to the fracture of fused quartz in vacuum, in distilled water, and in two salt solutions - 1M NaC1, 1M NaOH - that form relatively acidic and basic solutions respectively. We have also established the capability to conduct double-compression double-cleavage experiments in an environmental chamber to observe material fracture in aqueous solution. Both simulations and experiments indicate that fractures propagate fastest in NaC1 solutions, slower in distilled water, and even slower in air.

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Cubic zirconium tungstate (α-ZrW2O8), a well-known negative thermal expansion material, has been investigated within the framework of density functional perturbation theory (DFPT), combined with experimental characterization to assess and validate computational results. Using combined Fourier transform infrared measurements and DFPT calculations, new and extensive assignments were made for the far-infrared (<400 cm−1) spectrum of α-ZrW2O8. A systematic comparison of DFPT-simulated infrared, Raman, and phonon density-of-state spectra with Fourier transform far-/mid-infrared and Raman data collected in this study, as well as with available inelastic neutron scattering measurements, shows the superior accuracy of the PBEsol exchange-correlation functional over standard PBE calculations for studying the spectroscopic properties of this material.

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Sandia National Laboratories (SNL) conducted an evaluation of total system performance assessment (TSPA) related computing systems for the previously considered Yucca Mountain Project (YMP). This was done to maintain the operational readiness of the computing infrastructure (computer hardware and software) and knowledge capability for total system performance assessment (TSPA) type analysis, as directed by the National Nuclear Security Administration (NNSA), DOE 2010. This work is a continuation of the ongoing readiness evaluation reported in Lee and Hadgu (2014). The current work examined main components of the computing system identified in the previous work (Lee and Hadgu, 2014) to ensure the operational readiness of the TSPA-LA model capability on the server cluster. The TSPA computing hardware and storage system were replaced in late 2014 to maintain core capability and improve computation efficiency. One floating license of GoldSim Version 9.60.300 was installed on the upgraded cluster head node, and its distributed processing capability was mapped on the cluster processors. Other supporting software was tested and installed to support the TSPA-type analysis on the server cluster. All the TSPA-LA modeling cases were tested and verified for the model reproducibility on the upgraded 2014 server cluster (CL2014). All test runs were executed on multiple processors on the server cluster utilizing the GoldSim distributed processing capability, and all runs completed successfully. The model reproducibility verification was evaluated by two approaches: numerical value comparison and graphical comparison. The analysis demonstrated an excellent reproducibility of the TSPA-LA model runs on the upgraded server cluster. The 2014 server cluster and supporting software systems are fully operational to support TSPA- LA type analysis.

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