Publications

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This present work describes a quantum-mechanically based model of the electron- and proton-induced ionization of isolated pyrimidine molecules. The impact energies range from the target ionization threshold up to ~1 keV for electrons and from 10 keV up to 10 MeV for protons. The cross-section calculations are performed within the 1st Born approximation in which the ejected electron is described by a Coulomb wave whereas the incident and the scattered projectiles are both described by plane waves. The pyrimidine target is described using the Gaussian 09 software package. Furthermore, our theoretical predictions obtained are in good agreement with experimental absolute total cross sections, while large discrepancies are observed between existing semi-empirical models and the present calculations.

The reaction of Tc(+7) with H2O2 has been studied in H2SO4 and the speciation of technetium performed by UV–visible and 99-Tc NMR spectroscopy. UV–visible measurements show that for H2SO4 ≥ 9 M and H2O2 = 0.17 M, TcO3(OH)(H2O)2 reacts immediately and blue solutions are obtained, while no reaction occurs for H2SO4 < 9 M. The spectra of the blue solutions exhibit bands centered around 520 and 650 nm which are attributed to Tc(+7) peroxo species. Studies in 6 M H2SO4 show that TcO4− begins to react for H2O2 = 2.12 M and red solutions are obtained. The UV–visible spectra of the red species are identical to the one obtained from the reaction of TcO4− with H2O2 in HNO3 and consistent with the presence of TcO(O2)2(H2O)(OH). The 99-Tc NMR spectrum of the red solution exhibits a broad signal centered at +5.5 ppm vs TcO4− and is consistent with the presence of a low symmetry Tc(+7) molecule.

Several numerical codes for proton and electron transport in water - a commonly used surrogate of the living matter - have been reported in the literature. In the current work, we report on a home-made step-by-step Monte Carlo code, called TILDA-V, based on a complete set of multiple-differential and total cross sections for describing all the inelastic processes occurring throughout the slowing-down of protons in water and DNA.

The interaction of Cs adatoms with mono- or bi-layered graphene (MLG and BLG), either free-standing or on a SiO2 substrate, was investigated using density functional theory. The most stable adsorption sites for Cs are found to be hollow sites on both graphene sheets and graphene-veiled SiO2(0001). Larger dipole moments are created when a MLG-veiled SiO2(0001) substrate is used for adsorption of Cs atoms compared to the adsorption on free-standing MLG, due to charge transfer occurring between the MLG and the SiO2 substrate. For the adsorption of Cs on BLG-veiled SiO2(0001) substrate, these differences are smoothed out and the binding energies corresponding to different sites are nearly degenerate; smaller dipole moments created by the Cs adatoms on BLG compared to MLG are also predicted.

Uranyl nitrate is a key species in the nuclear fuel cycle, but is known to exist in different states of hydration, including the hexahydrate [UO2(NO3)2(H2O)6] (UNH) and the trihydrate [UO2(NO3)2(H2O)3] (UNT) forms. Their stabilities depend on both relative humidity and temperature. Both phases have previously been studied by infrared transmission spectroscopy, but the data were limited by both instrumental resolution and the ability to prepare the samples as pellets without desiccating it. We report time-resolved infrared (IR) measurements using an integrating sphere that allow us to observe the transformation from the hexahydrate to the trihydrate simply by flowing dry nitrogen gas over the sample. Hexahydrate samples were prepared and confirmed via known XRD patterns, then measured in reflectance mode. The hexahydrate has a distinct uranyl asymmetric stretch band at 949.0 cm-1 that shifts to shorter wavelengths and broadens as the sample dehydrates and recrystallizes to the trihydrate, first as a blue edge shoulder but ultimately resulting in a doublet band with reflectance peaks at 966 and 957 cm-1. The data are consistent with transformation from UNH to UNT since UNT has two non-equivalent UO22+ sites. The dehydration of UO2(NO3)2(H2O)6 to UO2(NO3)2(H2O)3 is both a morphological and structural change that has the lustrous lime green crystals changing to the dull greenish yellow of the trihydrate. Crystal structures and phase transformation were confirmed theoretically using DFT calculations and experimentally via microscopy methods. Both methods showed a transformation with two distinct sites for the uranyl cation in the trihydrate, as opposed to a single crystallographic site in the hexahydrate.

The elastic properties and mechanical stability of zirconium alloys and zirconium hydrides have been investigated within the framework of density functional perturbation theory. Results show that the lowest-energy cubic Pn3m polymorph of δ-ZrH1.5 does not satisfy all the Born requirements for mechanical stability, unlike its nearly degenerate tetragonal P42/mcm polymorph. Elastic moduli predicted with the Voigt-Reuss-Hill approximations suggest that mechanical stability of α-Zr, Zr-alloy and Zr-hydride polycrystalline aggregates is limited by the shear modulus. According to both Pugh's and Poisson's ratios, α-Zr, Zr-alloy and Zr-hydride polycrystalline aggregates can be considered ductile. The Debye temperatures predicted for γ-ZrH, δ-ZrH1.5 and ε-ZrH2 are D = 299.7, 415.6 and 356.9 K, respectively, while D = 273.6, 284.2, 264.1 and 257.1 K for the α-Zr, Zry-4, ZIRLO and M5 matrices, i.e. suggesting that Zry-4 possesses the highest micro-hardness among Zr matrices.

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Electron-induced ionization of the tetrahydrofuran molecule, the commonly used surrogate of the DNA sugar-phosphate backbone, is theoretically described in this study within the 1st Born approximation. Comparisons between theory and recent experiments are reported in terms of doubly and singly differential cross sections.

This report documents the development, demonstration and validation of a mesoscale, microstructural evolution model for simulation of zirconium hydride {delta}-ZrH{sub 1.5} precipitation in the cladding of used nuclear fuels that may occur during long-term dry storage. While the Zr-based claddings are manufactured free of any hydrogen, they absorb hydrogen during service, in the reactor by a process commonly termed ‘hydrogen pick-up’. The precipitation and growth of zirconium hydrides during dry storage is one of the most likely fuel rod integrity failure mechanisms either by embrittlement or delayed hydride cracking of the cladding. While the phenomenon is well documented and identified as a potential key failure mechanism during long-term dry storage (NUREG/CR-7116), the ability to actually predict the formation of hydrides is poor. The model being documented in this work is a computational capability for the prediction of hydride formation in different claddings of used nuclear fuels. This work supports the Used Fuel Disposition Research and Development Campaign in assessing the structural engineering performance of the cladding during and after long-term dry storage. This document demonstrates a basic hydride precipitation model that is built on a recently developed hybrid Potts-phase field model that combines elements of Potts-Monte Carlo and the phase-field models. The model capabilities are demonstrated along with the incorporation of the starting microstructure, thermodynamics of the Zr-H system and the hydride formation mechanism.

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Report RWEV-REP-001, Analysis of Postclosure Groundwater Impacts for a Geologic Repository for the Disposal of Spent Nuclear Fuel and High Level Radioactive Waste at Yucca Mountain, Nye County, Nevada was issued by the DOE in 2009 and is currently being updated. Sandia National Laboratories (SNL) provided support for the original document, performing calculations and extracting data from the Yucca Mountain Performance Assessment Model that were used as inputs to the contaminant transport and dose calculations by Jason Associates Corporation, the primary developers of the DOE report. The inputs from SNL were documented in LSA-AR-037, Inputs to Jason Associates Corporation in Support of the Postclosure Repository Supplemental Environmental Impact Statement. To support the updating of the original Groundwater Impacts document, SNL has reviewed the inputs provided in LSA-AR-037 to verify that they are current and appropriate for use. The results of that assessment are documented here.

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