The environmental dependence of redox energetics of PuO2 and \alpha-Pu2O3: A quantitative solution from DFT+U calculations

We report a comprehensive density functional theory (DFT) + $U$ study of the energetics of charged and neutral oxygen defects in both PuO$_{2}$ and $\alpha$-Pu$_{2}$O$_{3}$, and present a quantitative determination of the equilibrium compositions of reduced PuO$_{2}$ (PuO$_{2-x}$) as functions of environmental temperature and partial pressure of oxygen, which shows fairly agreement with corresponding high-temperature experiments. Under ambient conditions, the endothermic reduction of PuO$_{2}$ to $\alpha$-Pu$_{2}$O$_{3}$ is found to be facilitated by accompanying volume expansion of PuO$_{2-x}$ and the possible migration of O-vacancy, whereas further reduction of $\alpha$-Pu$_{2}$O$_{3}$ is predicted to be much more difficult. In contrast to the endothermic oxidation of PuO$_{2}$,\ the oxidation of $\alpha$-Pu$_{2}$O$_{3}$ is a stable exothermic process.Comment: 5 PLA pages, 4 figure

High Temperature Particle Filtration Technology

High temperature filtration can serve to improve the economic, environmental, and energy performance of chemical processes. This project was designed to evaluate the stability of filtration materials in the environments of the production of dimethyldichlorosilane (DDS). In cooperation with Dow Corning, chemical environments for the fluidized bed reactor where silicon is converted to DDS and the incinerator where vents are cornbusted were characterized. At Oak Ridge National Laboratory (ORNL) an exposure system was developed that could simulate these two environments. Filter samples obtained from third parties were exposed to the environments for periods up to 1000 hours. Mechanical properties before and after exposure were determined by burst-testing rings of filter material. The results indicated that several types of filter materials would likely perform well in the fluid bed environment, and two materials would be good candidates for the incinerator environment

Deep Burn Develpment of Transuranic Fuel for High-Temperature Helium-Cooled Reactors - July 2010

The DB Program Quarterly Progress Report for April - June 2010, ORNL/TM/2010/140, was distributed to program participants on August 4. This report discusses the following: (1) TRU (transuranic elements) HTR (high temperature helium-cooled reactor) Fuel Modeling - (a) Thermochemical Modeling, (b) 5.3 Radiation Damage and Properties; (2) TRU HTR Fuel Qualification - (a) TRU Kernel Development, (b) Coating Development, (c) ZrC Properties and Handbook; and (3) HTR Fuel Recycle - (a) Recycle Processes, (b) Graphite Recycle, (c) Pyrochemical Reprocessing - METROX (metal recovery from oxide fuel) Process Development

Chemical Vapor Infiltration Process Modeling and Optimization

Chemical vapor infiltration is a unique method for preparing continuous fiber ceramic composites that spares the strong but relatively fragile fibers from damaging thermal, mechanical, and chemical degradation. The process is relatively complex and modeling requires detailed phenomenological knowledge of the chemical kinetics and mass and heat transport. An overview of some of the current understanding and modeling of CVI and examples of efforts to optimize the processes is given. Finally, recent efforts to scale-up the process to produce tubular forms are described

Phase equilibria of advanced technology uranium silicide-based nuclear fuel

The phases in uranium-silicide binary system were evaluated in regards to their stabilities, phase boundaries, crystal structures, and phase transitions. The results from this study were used in combination with a well assessed literature to optimize the U-Si phase diagram using the CALPHAD method. A thermodynamic database was developed, which could be used to guide nuclear fuel fabrication, could be incorporated into other nuclear fuel thermodynamic databases, or could be used to generate data required by fuel performance codes to model fuel behavior in normal or off-normal reactor operations. The U3Si2 and U3Si5 phases were modeled using the Compound Energy Formalism model with 3 sublattices to account for the variation in composition. The crystal structure used for the USi phase was the tetragonal with an I4/mmm space. Above 450°C, the U3Si5 phase was modeled. The composition of the USi2 phase was adjusted to USi1.84. The calculated invariant reactions and the enthalpy of formation for the stoichiometric phases were in agreement with experimental data

Stability of High-Level Waste Forms

The objective of the proposed effort is to use a new approach to develop solution models of complex waste glass systems and spent fuel that are predictive with regard to composition, phase separation, and volatility. The effort will also yield thermodynamic values for waste components that are fundamentally required for corrosion models used to predict the leaching/corrosion behavior for waste glass and spent fuel material. This basic information and understanding of chemical behavior can subsequently be used directly in computational models of leaching and transport in geologic media, in designing and engineering waste forms and barrier systems, and in prediction of chemical interactions

Chemical vapor infiltration process modeling and optimization

Chemical vapor infiltration is a unique method for preparing continuous fiber ceramic composites that spares the strong but relatively fragile fibers from damaging thermal, mechanical, and chemical degradation. The process is relatively complex and modeling requires detailed phenomenological knowledge of the chemical kinetics and mass and heat transport. An overview of some of the current understanding and modeling of CVI and examples of efforts to optimize the processes is given. Finally, recent efforts to scale-up the process to produce tubular forms are described

Compatibility/Stability Issues in the Use of Nitride Kernels in LWR TRISO Fuel

The stability of the SiC layer in the presence of free nitrogen will be dependent upon the operating temperatures and resulting nitrogen pressures whether it is at High Temperature Gas-Cooled Reactor (HTGR) temperatures of 1000-1400 C (coolant design dependent) or LWR temperatures that range from 500-700 C. Although nitrogen released in fissioning will form fission product nitrides, there will remain an overpressure of nitrogen of some magnitude. The nitrogen can be speculated to transport through the inner pyrolytic carbon layer and contact the SiC layer. The SiC layer may be envisioned to fail due to resulting nitridation at the elevated temperatures. However, it is believed that these issues are particularly avoided in the LWR application. Lower temperatures will result in significantly lower nitrogen pressures. Lower temperatures will also substantially reduce nitrogen diffusion rates through the layers and nitriding kinetics. Kinetics calculations were performed using an expression for nitriding silicon. In order to further address these concerns, experiments were run with surrogate fuel particles under simulated operating conditions to determine the resulting phase formation at 700 and 1400 C

Forced Chemical Vapor Infiltration of Tubular Geometries: Modeling, Design, and Scale-Up

In advanced indirectly fired coal combustion systems and externally fired combined cycle concepts, ceramic heat exchangers are required to transfer heat from the hot combustion gases to the clean air that drives the gas turbines. For high efficiencies, the temperature of the turbine inlet needs to exceed 1,100 C and preferably be about 1,260 C. The heat exchangers will operate under pressure and experience thermal and mechanical stresses during heating and cooling, and some transients will be severe under upset conditions. Silicon carbide-matrix composites appear promising for such applications because of their high strength at elevated temperature, light weight, thermal and mechanical shock resistance, damage tolerance, and oxidation and corrosion resistance. The development of thick-walled, tubular ceramic composites has involved investigations of different fiber architectures and fixturing to obtain optimal densification and mechanical properties. The current efforts entail modeling of the densification process in order to increase densification uniformity and decrease processing time. In addition, the process is being scaled to produce components with a 10 cm outer diameter

Summary Report Documenting Status of the Rare Earth Oxide Investigation

The goal of this work is to enhance the understanding of ceramic nuclear fuel thermochemistry through a coordinated modeling and experimental approach. This work supports the Advanced Fuels Campaign Feedstock and Fabrication Technology R&D Program and is focused on the following tasks: (1) use existing compound energy formalism-based models to support Los Alamos National Laboratory (LANL) fuel development activities, (2) assess rare earth (RE) oxide systems and begin development of thermochemical representations of U-RE-O systems, and (3) develop a U-Ce-O thermochemical model for the fluorite-structure phase. In support of the experimental efforts at the LANL, an assessment of temperature-oxygen potential conditions for preparing stoichiometric U{sub 1-y}Ce{sub y}O{sub 2} at relatively low values of y (< 0.4) was performed. There is significant agreement in the literature that both the independent urania and ceria phases, and the urania-ceria solution phase are stoichiometric at oxygen-to-metal (O/M) ratios of 2 at 850 C and an oxygen potential of -368 kJ/mol. The oxygen potential value is obtained at a partial pressure of CO/CO{sub 2} ratio of unity at 1 bar total pressure. This information was successfully applied in thermogravimetric analysis experimental efforts at LANL investigating urania, ceria, and blended powders of the two oxides. Data reported in the literature for oxygen potential-temperature-composition for U{sub 1-y}Ce{sub y}O{sub 2-x} was extracted manually and used to generate a data file. Assessment of the data showed both wide error ranges within sets of data as well as inconsistencies between data sets of different investigators. Figure 1, a plot of the extracted data, illustrates the paucity of experimental data with respect to composition, temperature, and O:M space. For example, as shown in Figure 1, the data as a function of temperature are limited to the range 873 K to 1273 K and higher O:M ratios. Furthermore, the compositions studied have focused on higher uranium fractions and very little work has been done at corresponding lower O:M ratios. A compound energy formalism representation has been developed for the (U,Ce)O{sub 2+x} utilizing developed models for the UO{sub 2+x} from Gueneau et al. (2002) and CeO{sub 2-x} of Zinkevich et al. (2006). A three sublattice approach was used to allow for uranium of valences up to +6. Vacancies are considered only on the anion sites. The ionic species are introduced in the sublattice as follows: (U{sup 6+},U{sup 4+},U{sup 3+},Ce{sup 4+},Ce{sup 3+}){sub 1}(O{sup 2-},Va){sub 2}(O{sup 2-},Va){sub 1} Gibbs free energy expressions for each of the derived constituents can be determined from standard state values. Optimizations using all available thermochemical information, e.g., oxygen potentials and phase equilibria, can thus yield the necessary corrections to the Gibbs free energies for the non-standard constituents and derived interaction parameters (L values). While a model is available that includes all the interactions separately among the urania and ceria species, determination of any possible non-ideal interactions between the urania and ceria cations requires optimization from first principles (if possible) and experimental data for the system. Utilizing the best set of data for oxygen potential-temperature-composition for U{sub 1-y}Ce{sub y}O{sub 2-x} the FactSage thermochemical computational software code was used to optimize the system for selected Gibbs free energy functions and interaction parameters. While it was possible to obtain optimized solutions, the resulting parameters did not allow adequate reproduction of the data, as can be seen in Fig. 2. As noted above, the quality of the data among the various investigators is poor and that is a likely cause for the lack of a reasonable representation. The focus for the remainder of the fiscal year will be twofold. There will be collaboration with LANL on the collection of experimental data to resolve inconsistencies in the literature data and to fill some of the gaps in the experimental space. Next the thermochemical data will be used to optimize model parameters in an attempt to achieve a better fit to the data, in particular at higher ceria concentrations and at the lower and upper range of O:M