Bartley B. Ebbinghaus

Thermodynamics of gas phase chromium species: The chromium oxides, the chromium oxyhydroxides, and volatility calculations in waste incineration processes☆

The thermodynamics of three gaseous chromium oxides and twelve gaseous chromium oxyhydroxides are assessed for application to chromium volatility calculations in waste incineration processes. For the oxyhydroxides, the thermodynamic properties, S0(298), H0(298) − H0(0), −(G0(T) − H0(298))T, and Cp0(T), are calculated by the molecular constant method using estimated molecular parameters. The enthalpies of formation, ΔHf0(298), of the chromium oxides and the chromium oxyhydroxides are determined either from vapor pressure data when available or from bond energy correlations which use available data on the gaseous chromium oxyhalides. The thermodynamic constants, S0(298), ΔHf0(298), and H0(298) − H0(0), are tabulated for all fifteen gaseous species. Likewise, the thermodynamic functions, −(G0(T) − H0(298))T and Cp0(T), are given in polynomial form as a function of temperature for ranges from 298 to 1000 K and 1000 to 3000 K. Equilibrium vapor compositions over Cr2O3(s) at fixed partial pressures of oxygen (0.10 atm) and water (0.10 atm) and temperatures in the range of 800 to 1600 K show that CrO2(OH)2(g) is the dominant vapor species with small contributions from CrO(OH)3(g), CrO3(g), CrO2OH(g), and CrO(OH)2(g).

Enthalpies of formation of Ce-pyrochlore, Ca0.93Ce1.00Ti2.035O7.00, U-pyrochlore, Ca1.46U4+0.23U6+0.46Ti1.85O7.00 and Gd-pyrochlore, Gd2Ti2O7: three materials relevant to the proposed waste form for excess weapons plutonium

High temperature oxide melt solution calorimetry was used to derive standard enthalpies of formation, ΔH0f (kJ/mol), for three pyrochlore phases: Ca0.93Ce1.00Ti2.035O7.00 (−3656.0±5.6), Ca1.46U4+0.23U6+0.46Ti1.85O7.00 (−3610.6±4.1) and Gd2Ti2O7 (−3822.5±4.9). Enthalpy of drop solution data, ΔHds, were used to calculate enthalpies of formation with respect to an oxide phase assemblage, ΔH0f−ox: CaO+MO2+2TiO2=CaMTi2O7 or Gd2O3+2TiO2=Gd2Ti2O7, and an oxide/perovskite phase assemblage, ΔH0f−pv+ox: CaTiO3+MO2+TiO2=CaMTi2O7, where M=Ce or U. All three pyrochlore samples were stable in enthalpy relative to an oxide assemblage with ΔH0f−ox (kJ/mol) (Gd2Ti2O7)=−113.4±2.8; ΔH0f−ox(Ca1.46U4+0.23U6+0.46Ti1.85O7.00)=−123.1±3.4; ΔH0f−ox(Ca0.93Ce1.00Ti2.035O7.00)=−54.1±5.2. U-pyrochlore was stable in enthalpy relative to an oxide/perovskite assemblage (ΔH0f−pv+ox=−5.1±4.0 kJ/mol). Ce-pyrochlore was metastable in enthalpy relative to the oxide/perovskite phase assemblage (ΔH0f−pv+ox=+21.0±5.5 kJ/mol). A significant metastability field was defined with respect to an oxide/perovskite phase assemblage. However, the proposed waste form baseline composition lies in the stable regions of the phase diagrams.

Thermodynamics of gas phase chromium species: the chromium chlorides, oxychlorides, fluorides, oxyfluorides, hydroxides, oxyhydroxides, mixed oxyfluorochlorohydroxides, and volatility calculations in waste incineration processes

Abstract The thermodynamics of 12 gaseous chromium chlorides and oxychlorides and 12 gaseous chromium fluorides and oxyfluorides are assessed for application to chromium volatility calculations in thermal oxidation processes such as waste incineration. A method is also proposed for estimating the thermodynamics for mixed chromium oxyfluorochlorohydroxide species which contain varying proportions of oxygen, fluorine, chlorine, and hydroxide. The thermodynamic functions, C p 0 ( T ) and −( G 0 ( T ) − H 0 (298))/ T , and the thermodynamic constants, S 0 (298) and H 0 (298) − H 0 (0), for the gaseous chromium chlorides, oxychlorides, fluorides, and oxyfluorides are evaluated by the molecular constant method using molecular parameters taken from the literature or estimated from known constants for related vapor species. The enthalpies of formation of the gaseous chromium chlorides, oxychlorides, fluorides, and oxyfluorides are assessed based on available data in the literature. In some cases, only minimum values of the standard formation enthalpies can be established. The thermodynamic constants, S 0 (298), ΔH f 0 (298), and H 0 (298) − H 0 (0), are tabulated for all 12 chloride and oxychloride and all 12 fluoride and oxyfluoride gaseous species. Likewise, the thermodynamic functions, −( G 0 ( T ) − H 0 (298))/ T and C p 0 ( T ), are tabulated in polynomial form as a function of temperature. In addition, the ΔH f 0 (298) values for many of the chromium hydroxides and oxyhydroxides have been recalculated based on the revised ΔH f 0 (298) values for the chromium chlorides, oxychlorides, fluorides, and oxyfluorides given herein. A general assessment of the volatility of chromium in waste incineration processes at temperatures ranging from 800 to 1600 K with p (HC1) = 0.010 atm and p (HF) = 0.0032 atm yields CrO 2 Cl 2 (g) as the dominant oxychloride and CrO 2 F 2 (g) as the dominant oxyfluoride species except at higher temperatures where CrO 2 Cl(g) and CrOCl 2 (g) are important and where CrO 2 F(g) and CrOF(g) dominate. In addition to these oxychloride and oxyfluoride vapor species, other species such as CrO 2 (OH) 2 (g), CrO 2 ClOH(g), and CrO 2 FOH(g) are expected to be important. At higher temperatures, CrO 3 (g), CrOClOH(g), CrO 2 OH(g), CrO(OH) 2 (g), and CrOOH(g) also contribute to the overall chromium volatility.

The Attractiveness of Materials in Advanced Nuclear Fuel Cycles for Various Proliferation and Theft Scenarios

We must anticipate that the day is approaching when details of nuclear weapons design and fabrication will become common knowledge. On that day we must be particularly certain that all special nuclear materials (SNM) are adequately accounted for and protected and that we have a clear understanding of the utility of nuclear materials to potential adversaries. To this end, this paper examines the attractiveness of materials mixtures containing SNM and alternate nuclear materials associated with the plutonium-uranium reduction extraction (Purex), uranium extraction (UREX), coextraction (COEX), thorium extraction (THOREX), and PYROX (an electrochemical refining method) reprocessing schemes. This paper provides a set of figures of merit for evaluating material attractiveness that covers a broad range of proliferant state and subnational group capabilities. The primary conclusion of this paper is that all fissile material must be rigorously safeguarded to detect diversion by a state and must be provided the highest levels of physical protection to prevent theft by subnational groups; no “silver bullet” fuel cycle has been found that will permit the relaxation of current international safeguards or national physical security protection levels. The work reported herein has been performed at the request of the U.S. Department of Energy (DOE) and is based on the calculation of “attractiveness levels” that are expressed in terms consistent with, but normally reserved for, the nuclear materials in DOE nuclear facilities. The methodology and findings are presented. Additionally, how these attractiveness levels relate to proliferation resistance and physical security is discussed.

Enthalpies of Formation of Gd2(Ti2-xZrx)O7 Pyrochlores

A calorimetric investigation of the enthalpies of formation of Gd 2 (Ti 2-x Zr x )O 7 , where 0≤ × ≤ 2 is underway. All samples exhibit pyrochlore (Fd3m) peaks in their XRD patterns. However, where x=2 significant local disorder is observed in the Raman spectra. Preliminary data for the enthalpies of formation from the oxides in kJ/mol are: x=0, ΔH f = -113.4±2.7; x=0.5, ΔH f = -94.0±3.0; x=1.0, ΔH f = -74.2±4.9; x=1.5, ΔH f = -64.5±2.0; x=2, ΔH f = -52.2±4.8. Two additional samples, Gd 1.80 Zr 2.15 O 7.00 (pyrochlore) and Gd 2.15 Zr 1.87 O 7.00 (fluorite), were also studied. Their enthalpies of formation from the oxides in kJ/mole are -50.9±3.3 and -46.4±3.4 respectively. Replacing Ti with Zr, i.e. when x=2, destabilizes the pyrochlore in enthalpy by approximately 60 kJ/mol. The ΔH mix for the Gd 2 (Ti 2-x Zr x )O 7 solid-solution series is positive and can be described by a regular solution formalism with an estimated interaction parameter, ŝ = +20 kJ/mol. The results of this study suggest that the pyrochlore to fluorite transition enthalpy in Gd 2 Zr 2 O 7 is small, of the order of the configurational entropy contribution due to cation disorder at the transition temperature, TΔS conf. ≍ 10 kJ/mol.

Characterization of a Plutonium-Bearing Zirconolite-Rich Synroc

A titanate-based ceramic waste form, rich in phases structurally related to zirconolite (CaZrTi 2 O 7 ), is being developed as a possible method for immobilizing excess plutonium from dismantled nuclear weapons. As part of this program, Lawrence Livermore National Laboratory (LLNL) produced several ceramics that were then characterized at Argonne National Laboratory (ANL). The plutonium-loaded ceramic was found to contain a Pu-Gd zirconolite phase but also contained plutonium titanates, Gd-polymignyte, and a series of other phases. In addition, much of the Pu was remained as PuO 2-x . The Pu oxidation state in the zirconolite was determined to be mainly Pu 4+ , although some Pu 3+ was believed to be present.

Transpiration studies on the volatilities of PuO3(g) and PuO2(OH)2(g) from PuO2(s) in the presence of steam and oxygen and application to plutonium volatility in mixed-waste thermal oxidation processors

Abstract Transpiration experiments have been carried out to study the volatility of PuO 2 (s) in the presence of oxygen and steam. Vapor pressures have been measured for PuO 3 (g) and PuO 2 (OH) 2 (g) and thermodynamic data established for them. Thus, Δ f H 298 0 is determined to be −(562.8 ± 5.0) kJ/mol for PuO 3 (g) and −(1,018.2 ± 3.3) kJ/mol for PuO 2 (OH) 2 (g). In applying the Pu volatility data to a pilot scale mixed waste (containing both radioactive and hazardous constituents) incinerator with primary and secondary combustion chambers, it is found that Pu volatilization makes a negligible contribution to air emissions if the temperature in the secondary combustor is maintained at less than 1600 K. Chlorine- and fluorine-induced volatilities of plutonium are not considered in this analysis.

Physical Property Changes in Plutonium Observed from Accelerated Aging using Pu-238 Enrichment

We present changes in volume, immersion density, and tensile property observed from accelerated aged plutonium alloys. Accelerated alloys (or spiked alloys) are plutonium alloys enriched with approximately 7.5 weight percent of the faster-decaying {sup 238}Pu to accelerate the aging process by approximately 17 times the rate of un-aged weapons-grade plutonium. After sixty equivalent years of aging on spiked alloys, the dilatometry shows the samples at 35 C have swelled in volume by 0.15 to 0.17 % and now exhibit a near linear volume increase due to helium in-growth. The immersion density of spiked alloys shows decrease in density, similar normalized volumetric changes (expansion) for spiked alloys. Tensile tests show increasing yield and engineering ultimate strength as spiked alloys are aged. (authors)

L1 Report for the Enhanced Surveillance Campaign Experimental Benchmarking of Pu Electronic Structure

The objective of this work is to develop and/or apply advanced diagnostics to the understanding of aging of Pu. Advanced characterization techniques such as photoelectron and x-ray absorption spectroscopy will provide fundamental data on the electronic structure of Pu phases. These data are crucial for the validation of the electronic structure methods. The fundamental goal of this project is to narrow the parameter space for the theoretical modeling of Pu aging. The short-term goal is to perform experiments to validate electronic structure calculations of Pu. The long-term goal is to determine the effects of aging upon the electronic structure of Pu. Many of the input parameters for aging models are not directly measurable. These parameters will need to be calculated or estimated. Thus a First Principles-Approach Theory is needed, but it is unclear what terms are important in the Hamiltonian (H{Psi} = E{Psi}). Therefore, experimental data concerning the 5f electronic structure are needed, to determine which terms in the Hamiltonian are important. The data obtained in this task are crucial for reducing the uncertainty of Task LL-01-developed models and predictions. The data impact the validation of electronic structure methods, the calculation of defect properties, the evaluation of helium diffusion, and the validation of void nucleation models. The importance of these activities increases if difficulties develop with the accelerating aging alloy approach.

Experimental Benchmarking of Pu Electronic Structure

The standard method to determine the band structure of a condensed phase material is to (1) obtain a single crystal with a well defined surface and (2) map the bands with angle resolved photoelectron spectroscopy (occupied or valence bands) and inverse photoelectron spectroscopy (unoccupied or conduction bands). Unfortunately, in the case of Pu, the single crystals of Pu are either nonexistent, very small and/or having poorly defined surfaces. Furthermore, effects such as electron correlation and a large spin-orbit splitting in the 5f states have further complicated the situation. Thus, we have embarked upon the utilization of unorthodox electron spectroscopies, to circumvent the problems caused by the absence of large single crystals of Pu with well-defined surfaces. Our approach includes the techniques of resonant photoelectron spectroscopy [1], x-ray absorption spectroscopy [1,2,3,4], electron energy loss spectroscopy [2,3,4], Fano Effect measurements [5], and Bremstrahlung Isochromat Spectroscopy [6], including the utilization of micro-focused beams to probe single-crystallite regions of polycrystalline Pu samples. [2,3,6]