Advanced Modeling and Experimental Validation of Complex Nuclear Material Forms of Potential Transportation Concern

We present here computer modeling efforts to describe the time-dependent pressurization and gas-phase mole fractions inside sealed canisters containing actinide materials packaged with small (0.12 - 0.5 wt. %) amounts of water. The model is run using Chemkin software, and the chemical reaction mechanism includes gas generation due to radiolysis of adsorbed water, interfacial chemical reactions, and adsorption/desorption kinetics of water on PuO2 materials. The ultimate goal is to provide a verifiable computer model that can be used to predict problematic gas generation in storage forms and assure design criteria for short-term storage and transportation of less than well-characterized (with respect to gas generation) material classes. Our initial efforts are intended to assess pressurization and gas-phase mole fractions using well-defined 3013 container test cases. We have modeled gas generation on PuO2 with water loading up to 0.5 wt. %, at 300 and 525 K, for time frames of 3 years. Estimates of the initial H2 generation rates were determined using RadCalc and employed in the Chemkin model to assess time- and coverage-dependent system behavior. Results indicate that canister pressurization due to radiolysis is a relatively slow process, with pressure increases at 300 K of approximately 1.5 atm. for 5000 g of PuO2 packaged with 0.5 wt. % water. At higher temperatures (> 400 K), desorption of water into the gas phase largely dictates pressurization and the gas-phase mole fractions. These modeling efforts provide a predictive capability for potential gas generation behavior that when augmented and validated by surveillance information will provide a technical basis for safe storage and transportation

Enumeration of microbial populations in radioactive environments by epifluorescence microscopy

Epifluorescence microscopy was utilized to enumerate halophilic bacterial populations in two studies involving inoculated, actual waste/brine mixtures and pure brine solutions. The studies include an initial set of experiments designed to elucidate potential transformations of actinide-containing wastes under salt-repository conditions, including microbially mediated changes. The first study included periodic enumeration of bacterial populations of a mixed inoculum initially added to a collection of test containers. The contents of the test containers are the different types of actual radioactive waste that could potentially be stored in nuclear waste repositories in a salt environment. The transuranic waste was generated from materials used in actinide laboratory research. The results show that cell numbers decreased with time. Sorption of the bacteria to solid surfaces in the test system is discussed as a possible mechanism for the decrease in cell numbers. The second study was designed to determine radiological and/or chemical effects of {sup 239}Pu, {sup 243}Am, {sup 237}Np, {sup 232}Th and {sup 238}U on the growth of pure and mixed anaerobic, denitrifying bacterial cultures in brine media. Pu, Am, and Np isotopes at concentrations of {le}1x10{sup -6} M , {le}5x10{sup -6} M and {le}5x10{sup -4}M respectively, and Th and U isotopes {le}4x10{sup -3}M were tested in these media. The results indicate that high concentrations of certain actinides affected both the bacterial growth rate and morphology. However, relatively minor effects from Am were observed at all tested concentrations with the pure culture

Fermi surfaces of single layer dielectrics on transition metals

Single sheets of hexagonal boron nitride on transition metals provide a model system for single layer dielectrics. The progress in the understanding of h-BN layers on transition metals of the last 10 years are shortly reviewed. Particular emphasis lies on the boron nitride nanomesh on Rh(111), which is a corrugated single sheet of h-BN, where the corrugation imposes strong lateral electric fields. Fermi surface maps of h-BN/Rh(111) and Rh(111) are compared. A h-BN layer on Rh(111) introduces no new bands at the Fermi energy, which is expected for an insulator. The lateral electric fields of h-BN nanomesh violate the conservation law for parallel momentum in photoemission and smear out the momentum distribution curves on the Fermi surface.Comment: 14 pages, 6 figures, 1 table, 1 equation, Accepted for publication in the Special Surface Science issue in honor of Gerhard Ertl's Nobel Priz

Characterizing the dealumination of environmentally relevant zeolites using IR, NMR and neutron diffraction techniques

Results of characterization studies monitoring the sequential chemical bond breaking events, local site symmetry, and long range structural modifications of specific zeolites (H-ZSM-5, TS-1) during hydrothermal treatment of these catalyst materials are described. These characterization techniques include infrared spectroscopy of selected probe molecules, magic angle spinning NMR spectroscopy, and powder neutron diffraction. Information regarding selected examples from each of these techniques is presented and the inherent strengths of each is discussed. The experimental insight into the chemical and structural modifications of high surface area microporous catalyst materials as a function of deactivation conditions (hydrothermal conditioning) is highlighted

An investigation of the chemical and physical properties of pristine, electrochromically damaged, and photochromically damaged KTiOPO{sub 4} (KTP) using surface analytical and optical spectroscopic techniques

A variety of experimental techniques were employed to study the properties of electrochromically (EC) damaged, photochromically (PC) damaged, and pristine KTiOP0{sub 4} (KTP). Additionally, nonlinear optical calculations were performed to complement the experimental work in an effort to elucidate the respective mechanisms operative in producing EC and PC damage to KTP. Several independent experiments indicate that there is Ti deficiency in the EC damaged material, which is due to migration of these ions to the electrode surface. The laser experiments indicate that UV radiation can produce reversible PC damage. UV-producing SFG processes accidentally occurring in SHG cut KTP may lead to macroscopic damage. It must be emphasized that a fundamentally different mechanism is responsible for EC damaged versus PC damaged KTP

Data Reduction and Error Analysis for the Physical Sciences

ABSTRACT Polycrystalline thin films (PTF) of p-WSe2, p-WS2, and p-MoSe2 have been prepared and characterized with respect to their photoelectrochemical properties, p-WS2 showed the highest open-circuit photovoltages and the highest conversion efficiencies in various redox couples. In addition, the band structure of all the films has been determined experimentally and compared to those reported for single crystals. Over the last two decades a great deal of interest has developed in the area of photoelectrochemistry, particularly in the application of photoelectrochemical systems to the problem of solar energy conversion and storage. The interest is to develop new energy sources to supplement and eventually replace fossil fuels. The first photoelectrochemical experiment was performed in 1839 by Becquerel (1), who demonstrated that a voltage and current are generated when a silver chloride electrode, immersed in an electrolytic solution and connected to a counterelectrode, is illuminated. Although the concept of a semiconductor did not exist at that time, it is now clear that the electrode which Becquerel used had semiconducting properties. In 1955, Brattain and Garett (2) used germanium as the first semiconductor electrode in photoelectrochemistry. Since then, the knowledge of semiconductors has grown steadily. Fujishima and Honda (3) were the first to point out the potential application of photoelectrochemical systems for solar energy conversion and storage. They demonstrated that the photo-oxidation of water to 02 was possible by utilizing an n-type semiconducting titanium dioxide photoanode. Since then, there has been a large and rapidly growing international interest in the study of photoelectrochemistry of semiconductors (4). The effective use of solar energy in photovoltaic or photoelectrochemical applications depends in part on the development of materials that can show high conversion efficiencies and long-term stability under operation. In ad-*Electrochemical Society Active Member. **Electrochemical Society Student Member. dition, the desirable materials should have a bandgap that closely matches the solar spectrum and be made of readily available and inexpensive materials. We have focused our attention on the transition metal dichalcogenides (e.g., WSe2, WS2, MoSe2, and others), also known as layered or d-d semiconductors. Tributsch's (5, 6) pioneering work on the use of these materials has stimulated intensive research in this area, and single Crystals of a number of materials have been studied extensively in both aqueous and nonaqueous solvents and in photovoltaic and photoelectrosynthetic cells. The advantages of using these materials are that they have bandgaps (1.1-1.6 eV) that closely match the solar spectrum and exhibit high conversion efficiencies as single crystals. In addition, they can achieve long-term stability due to the fact that the transitions are localized in the nonbonding d orbitals of the metal. These materials consist of metal dichalcogenide sandwiches (e.g., Se-W-Se) held together by van der Waals forces. The fact that there is strong covalent bonding within the layers, but only weak interactions between layers, makes these materials highly anisotropic in their properties. For example, the surface parallel to the C axis (IIC) is more conducting than the surface perpendicular to the C axis (• Therefore, edges and surface imperfections on the surface parallel to the C axis act as efficient recombination centers for photogenerated carriers or products (7

Coadsorption of nitriles and CO on Cu-ZSM-5: An in situ FTIR spectroscopic study

The catalytic opportunity that Cu-ZSM-5 has for potentially reducing NO{sub x} emission through reduction by hydrocarbons has prompted an enormous international examination of this material and the chemical properties it displays. The coadsorption of acetonitrile (ACN) or deuterated acetonitrile (dACN) with CO at Cu-ZSM-5 reveals several interesting features concerning the partial valency of Cu. Specifically, CO binds at Cu{sup +1} and Cu{degree} centers (in the absence of preadsorbed ACN) and exhibits C-O stretching frequencies of 2,157 cm{sup {minus}1} and 2,112 cm{sup {minus}1}, respectively. Carbon monoxide readily adsorbs at an ACN (or dACN) saturated Cu-ZSM-5 and exhibits a C-O stretch of 2,122 cm{sup {minus}1}, a value more consistent with a partially reduced Cu{sup +1} center. Furthermore, the IR cross section for the CN stretch in a number of nitriles (ACN, dACN, and benzonitrile) coadsorbed with CO displays interesting effects attributed to rehybridization and changes in CN dipole moment

ADVANCED MODELING AND EXPERIMENTAL VALIDATION OF COMPLEX NUCLEAR MATERIAL WASTE FORMS OF POTENTIAL TRANSPORTATION CONCERN

We present here computer modeling efforts to describe the time-dependent pressurization and gas-phase mole fractions inside sealed canisters containing actinide materials packaged with small (0.12 - 0.5 wt. %) amounts of water. The model is run using Chemkin software, and the chemical reaction mechanism includes gas generation due to radiolysis of adsorbed water, interfacial chemical reactions, and adsorption/desorption kinetics of water on PuO2 materials. The ultimate goal is to provide a verifiable computer model that can be used to predict problematic gas generation in storage forms and assure design criteria for short-term storage and transportation of less than well-characterized (with respect to gas generation) material classes. Our initial efforts are intended to assess pressurization and gas-phase mole fractions using well-defined 3013 container test cases. We have modeled gas generation on PuO2 with water loading up to 0.5 wt. %, at 300 and 525 K, for time frames of 3 years. Estimates of the initial H2 generation rates were determined using RadCalc and employed in the Chemkin model to assess time- and coverage-dependent system behavior. Results indicate that canister pressurization due to radiolysis is a relatively slow process, with pressure increases at 300 K of approximately 1.5 atm. for 5000 g of PuO2 packaged with 0.5 wt. % water. At higher temperatures (> 400 K), desorption of water into the gas phase largely dictates pressurization and the gas-phase mole fractions. These modeling efforts provide a predictive capability for potential gas generation behavior that when augmented and validated by surveillance information will provide a technical basis for safe storage and transportation

Kinetic studies of competitive adsorption processes related to automobile catalytic converters

This is the final report of a three-year, Laboratory Directed Research and Development (LDRD) project at Los Alamos National Laboratory (LANL). The goal of this project was to study the microscopic details for the adsorption of CO, NO, and O{sub 2} on transition metal surfaces under conditions resembling those present in automobile catalytic converters. Initial sticking coefficients were measured as a function of temperature on transition metal single crystals by using a method originally developed by King and Wells. These measurements were performed under conditions emulating those typical of competitive adsorption, namely, where the substrate is exposed to a mixture of two or more gases simultaneously, or where one molecule is adsorbed on the surface prior to exposure to the second gas. The experimental results were then analyzed by using Monte Carlo computer simulation algorithm in an attempt to better understand the relevant aspects of the adsorption process

Methane Conversion to Fuels and Chemicals: Opportunities and Approaches

This is the final report of a one-year, Laboratory-Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). Methane, the primary component of natural gas, has reserves that are on the order of those of petroleum. Processes that utilize these vast supplies of methane will need to be developed to replace dwindling supplies of petroleum in the future. Processes utilizing natural gas promise to be more environmentally friendly, as natural gas as a feedstock is freer of contaminants and more readily purified than petroleum. Short contact time reactor configurations are likely candidates for this application. The authors objectives are to develop reactor designs and computer models appropriate for short contact time applications. They have succeeded in assembling both an experimental facility for investigating the performance of short contact time reactors, and a computer simulation that includes full mass and heat transport as well as coupled surface and gas phase detailed chemical kinetics