Synthesis of nuclear waste monazites,ideal actinide hosts for geologic disposal

Monazite, an orthophosphate mineral of the lanthanides (Ln) and the actinides (An) U and Th, is a model for an ideal synthetic mineral waste form for geologic disposal of long-lived nuclear waste actinides. Natural monazites are known to have survived many of the conditions that might be inflicted on a nuclear waste repository by geological disruptions. High Th and U monazites with compositions typical of nuclear wastes have been synthesized with a routine calcination-pelletization-crystallization procedure. Charge balance for the Th⁴⁺ → Ln³⁺ substitution can be provided by either an equimolar Ca²⁺ → Ln³⁺ or Si⁴⁺ → P⁵⁺ substitution. For U⁴⁺ → Ln³⁺, only the Ca²⁺ → Ln³⁺ substitution resulted in a phase-pure monazite. Unit cell parameter data were obtained for each nuclear waste monazite phase.     

Radioactive and nonradioactive waste intended for disposal at the Waste Isolation Pilot Plant

Transuranic (TRU) waste generated by the handling of plutonium during research on or production of U.S. nuclear weapons will be disposed of in the Waste Isolation Pilot Plant (WIPP). This paper describes the physical and radiological properties of the TRU waste that will be deposited in the WIPP. This geologic repository will accommodate up to 175,564 m³ of TRU waste, corresponding to 168,485 m³ of contact-handled (CH-) TRU waste and 7079 m³ of remote-handled (RH-) TRU waste. Approximately 35% of the TRU waste is currently packaged and stored (i.e. legacy) waste, with the remainder of the waste to be packaged or generated and packaged in activities before the year 2033, which is the closure time for the repository. These wastes were produced at 27 U.S. Department of Energy (DOE) sites in the course of generating defense nuclear materials. The radionuclide and nonradionuclide inventories for the TRU wastes described in this paper were used in the 1996 WIPP Compliance Certification Application (CCA) performance assessment calculations by the Sandia National Laboratories/New Mexico (SNL/NM).     

[NZP], a new radiophase for ceramic nuclear waste forms

The NZP structure is a candidate for immobilization of certain types of nuclear waste. It will incorporate ¹³⁷Cs, ⁹⁰Sr, and a range of other nuclides. The leach resistance of CsZr₂(PO₄)₃ appears to be comparable with that of other phases under consideration for radio-Cs immobilization. This phase can be formed by sintering at ～850°C; it is reasonably refractory, and it is compatible with monazite, a favored immobilizing agent for waste actinides.     

New mesoporous silica/carbon composites by in situ transformation of silica template in carbon/silica nanocomposite

Hard template-based fabrication of mesoporous carbon unavoidably goes through the removal process of the template to generate template-free carbon replica, including troublesome disposal of template waste often accompanied by toxic etchant, which not only increases the fabrication cost of materials but also raises serious environmental concerns. As a novel strategy to overcome such problem, a direct in situ synthesis approach using silica waste in carbon/silica nanocomposite as a silica source and cetyltrimethylammonium bromide as a porogen under basic condition is reported in this study for the generation of a new composite composed of mesoporous MCM-41 silica and hollow carbon capsule. The resultant MCM-41/carbon capsule composite offers a 3-D interconnected multimodal pore system, which discloses a wide pore range of ordered uniform mesopores (ca 2.3?nm) resulting from MCM-41 silica and disordered uniform mesopores (ca 3.8?nm) and macropores (ca 300?nm) from hollow mesoporous carbon, respectively. The composite has a high specific surface area (ca 909?m²/g) and large pore volume (ca 0.73?cm³/g). The in situ transformation approach of silica waste into valuable mesoporous silica is considered as a promising scalable route for efficient new multi-functional composites useful for a wide range of applications such as adsorption of volatile organic compounds and radioactive wastes produced in a nuclear facility.     

Synthesis of LTA zeolite beads using alum sludge and silica rich wastes

This study successfully recycled alum sludge with either lithium slag or bottle glass to make high purity zeolite LTA beads. The basic synthesis approach was to fuse the waste material with sodium hydroxide then complete hydrothermal crystallization. Separate fusion of individual wastes promoted zeolite LTA formation compared to combined fusion. Preferred synthesis conditions were: 10 Na₂O:Al₂O₃:2.5 SiO₂:300 H₂O; hydrothermal temperature = 80 °C; time = 5 h. Zeolite LTA purity was between 80 and 85 wt% from the two waste combinations. Use of pseudoboehmite and carboxymethyl cellulose favoured granulation/extrusion/spheronization of zeolite LTA powder. However, the beading process decreased calcium exchange capacity (CEC) from 99.5 to 37 mg Ca²⁺/g for alum sludge + lithium slag and from 64.5 to 37 mg Ca²⁺/g for alum sludge + waste glass. Additionally, when using 20 wt% pseudo-boehmite as a binder the ion-exchange equilibrium time increased from 60 to 300 min. The importance of not only making zeolite powder from waste materials but also to extend studies to shaped forms was evident. It is recommended that future studies should also focus on making extrudates or beads as these are the forms used commercially.     

Synthesis,characterization and structural refinement of polycrystalline uranium substituted zirconolite

Ceramic precursors of Zirconolite (CaZrTi₂O₇) family have a remarkable property of substitution on Zr^{4 +} cationic sites. This makes them potential material for nuclear waste management in ‘synroc’ technology. In order to simulate the mechanism of partial substitution of zirconium by tetravalent actinides, a solid phase of composition CaZr_0.95U_0.05Ti₂O₇ has been synthesized through ceramic route by taking calculated quantities of oxides of Ca, Ti and nitrates of uranium and zirconium respectively. Solid state synthesis has been carried out by repeated pelletizing and sintering the finely powdered oxide mixture in a muffle furnace at 1050°C. The polycrystalline solid phase has been characterized by its typical powder diffraction pattern. Step analysis data has been used for ab initio calculation of structural parameters. The SEM and EDAX analysis also confirm that zirconolite acts as a host material for uranium. The powder diffraction data of 3500 points between 2θ = 10–80° has been analysed by GSAS (general structure analysis system) software to obtain the best fit of the observed data points. The uranium substituted zirconolite crystallizes in monoclinic symmetry with space group C2/c (#15). The following unit cell parameters have been calculated: a = 12.4883(15), b = 7.2448(5), c = 11.3973(10) and β = 100.615(9)°. The calculated and observed values of the intensities, lattice parameters and density measurement shows good agreement. The Rietveld analysis and GSAS based calculations for bond distance Ti—O, Ca—O, Zr—O, and O—M—O bond angles have been made. The structure was refined to satisfactory completion.The and Rp and Rwp are found to be 7.48 and 9.74 % respectively.     

Crystalline Host Phases for Actinides,Obtained by Self-Propagating High-Temperature Synthesis

Host matrices for actinides prepared by self-propagating high-temperature synthesis are studied. The matrices consist of a pyrochlore or fluorite phase and metallic molybdenum. The factor determining the structural type of the crystal lattice of the target phase is the ionic radius ratio. When the difference in the ionic radii is insignificant, as in the case of Y^{3 +} (r 0.102 nm) and Zr^{4 +} (r 0.084 nm), the oxide Zr_{1 - x}Y_xO_{2 - 0 . 5 x} with a fluorite structure is formed, in which the cations occupy the eight-coordinate sites. This structure permits incorporation of heavy lanthanides and tetravalent actinides: U^{4 +} (r 0.10 nm), Np^{4 +} (r 0.098 nm), and Pu^{4 +} (r 0.096 nm). When the difference in the ionic radii is more considerable, as in the case of Y^{3 +} and Ti^{4 +} (r 0.061 nm), a pyrochlore-related structure is realized. In this case the cations occupy different (eight- or six-coordinate) sites. The pyrochlore structure is preserved if the radii of ions occupying different structural sites change in parallel. This structure is typical of zirconates of trivalent actinides and light REEs. The decision on the major host phase for actinides is determined by the waste composition. At low content of light REEs and americium an oxide with a fluorite-related structure shows promise. At high content of these elements, zirconates and titanates with the pyrochlore structure are more stable.     

Flower-like NiCo2S4 nanosheets with high electrochemical performance for sodium-ion batteries

A three-dimensional flower-like NiCo₂S₄ formed by two-dimensional nanosheets is synthesized by a facile hydrothermal method and utilized as the anode for sodium-ion batteries. Studies have shown that materials can achieve the best performance under the ether-based electrolyte system with voltage ranging from 0.3 to 3 V, which could effectively avoid the dissolution of polysulfides and over-discharge of the material. Here, sodium storage mechanism and charge compensation behaviors of this ternary metal sulfide are comprehensively investigated by ex situ X-ray diffraction. Moreover, ex situ Raman spectra, ex situ X-ray photoelectron spectroscopy and transmission electron microscopy measurements are used to related tests for the first time. Additionally, quantitative kinetic analysis unravels that sodium storage partially depends on the pseudocapacitance mechanism, resulting in good specific capacity and excellent rate performance. The initial discharge capacity is as high as 748 mAh·g⁻¹ at a current density of 0.1 A·g⁻¹ with the initial coulomb efficiency of 94%, and the capacity can still maintain at 580 mAh·g⁻¹ with the Coulomb efficiency close to 100% after following 50 cycles. Moreover, by the long cycle test at a high current density of 2 A·g⁻¹, the capacity can still reach at 376 mAh·g⁻¹ after over 500 cycles.

     

Thermal conductivity measurements of pacific illite sediment

Results are reported for effective thermal conductivity measurements performed in situ and in core samples of illite marine sediment. The measurements were obtained during a recent oceanographic expedition to a study site in the north central region of the Pacific Ocean. This study was undertaken in support of the U.S. Subseabed Disposal Project, the purpose of which is to investigate the scientific feasibility of using the fine-grained sediments of the sea floor as a repository for high-level nuclear waste. In situ measurements were made and 1.5-m-long hydrostatic piston cores were taken, under remote control, from a platform that was lowered to the sea floor, 5844 m below sea level. The in situ measurement of thermal conductivity was made at a nominal depth of 80 cm below the sediment surface using a specially developed, line-source, needle probe. Thermal conductivity measurements in three piston cores and one box core (obtained several kilometers from the study site) were made on shipboard using a miniature needle probe. The in situ thermal conductivity was approximately 0.91 W · m^–1 · K^–1. Values determined from the cores were within the range 0.81 to 0.89 W · m^–1 · K^–1.Paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

A facile one-step hydrothermal method to produce graphene–MoO3 nanorod bundle composites

We describe a facile in situ hydrothermal fabrication of graphene–MoO₃ nanorod bundle composites utilizing sodium salicylate. The structure, morphology and composition of graphen–MoO₃ composites were investigated by means of field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy and thermogravimetric-differential scanning calorimetry (TG-DSC). FESEM and TEM studies show that the presence of ordered MoO₃ nanorod bundles in composites, the characterization results of XRD, Raman spectra and TG-DSC analysis confirm the reduction of graphite oxide (GO) to graphene accompanying by the formation of MoO₃ nanorod bundles in the hydrothermal process. Due to characteristics of MoO₃ and graphene–MoO₃ composites, our findings may have implications in the synthesis and fabrication of well-defined functional graphene–MoO₃ hybrid materials. It may also provide a general approach for the preparation of graphene–metal oxide hybrid materials.     

Facile fabrication of integrated three-dimensional C-MoSe<Subscript>2</Subscript>/reduced graphene oxide composite with enhanced performance for sodium storage

Scrupulous design and fabrication of advanced electrode materials are vital for developing high-performance sodium ion batteries. Herein, we report a facile one-step hydrothermal strategy for construction of a C-MoSe₂/rGO composite with both high porosity and large surface area. Double modification of MoSe₂ nanosheets is realized in this composite by introducing a reduced graphene oxide (rGO) skeleton and outer carbon protective layer. The MoSe₂ nanosheets are well wrapped by a carbon layer and also strongly anchored on the interconnected rGO network. As an anode in sodium ion batteries, the designed C-MoSe₂/rGO composite delivers noticeably enhanced sodium ion storage, with a high specific capacity of 445 mAh·g^-1 at 200 mA·g^-1 after 350 cycles, and 228 mAh·g^-1 even at 4 A·g^-1; these values are much better than those of C-MoSe₂ nanosheets (258 mAh·g^-1 at 200 mA·g^-1 and 75 mAh·g^-1 at 4 A·g^-1). Additionally, the sodium ion storage mechanism is investigated well using ex situ X-ray diffraction and transmission electron microscopy methods. Our proposed electrode design protocol and sodium storage mechanism may pave the way for the fabrication of other high-performance metal diselenide anodes for electrochemical energy storage.

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Stability of actinide(III,IV) and lanthanide(III,IV) complexes with P<Subscript>2</Subscript>W<Subscript>17</Subscript>O<Stack><Subscript>61</Subscript><Superscript>10−</Superscript></Stack> heteropolyanions

The total stability constants of Th⁴⁺, U⁴⁺, Np⁴⁺, Pu⁴⁺, Am⁴⁺, Cm⁴⁺, Ce⁴⁺, Tb⁴⁺, Pr⁴⁺, Tb³⁺, and Pr³⁺ complexes with P₂W₁₇O ₆₁ ^10? heteropolyanion in 1 M sodium salt solutions at pH ≥ 5.5 (i.e., when the anion is not protonated; so-called “absolute” constants), were determined experimentally or calculated from published data. Plots of constants vs. ionic radius of the f element were considered for solutions with ionic strength 1 (1 M acid or sodium salt solutions at pH ≥ 5.5). The correlations found confirm that the interaction of counterions in the complex is predominantly electrostatic. At the same time, different contributions of the covalent interaction for actinides and lanthanides were suggested.     

Removal of technetium-99 from contaminated groundwater with sorbents and reductive materials

Pertechnetate oxyanion (TcO₄⁻), which is highly soluble in water and readily mobile in the environment, can be immobilized through an ion exchange /adsorption process and chemical reduction followed by adsorption and/or precipitation. Previous studies have focused on the separation and removal of ⁹⁹TcO₄⁻ from high-level waste streams; however, little information is available for ⁹⁹TcO₄⁻ removal from only slightly contaminated groundwater. This paper describes treatment of ⁹⁹TcO₄⁻-contaminated groundwater with both batch and column flowthrough experiments. Synthetic resins and sponges, and zero-valence iron filings were used to evaluate their capacities and the rates of ⁹⁹TcO₄⁻ removal. The toxicity characteristic leaching procedure (TCLP) was applied to evaluate the leachability of ⁹⁹Tc adsorbed or co-precipitated on iron. Results suggest that both iron and synthetic resins remove ⁹⁹TcO₄⁻ from groundwater and that at a high flow rate (with residence time of less than 1 min), ⁹⁹TcO₄⁻ removal capacity is greater for iron filings than for the synthetic resins on a volume basis. Additionally, the rate of ⁹⁹TcO₄⁻ sorption on the sponge is slow (approximately 3 days), and the capacity is relatively low. No appreciable amount of ⁹⁹Tc can be leached out from the spent iron filings by the TCLP test. Overall, zero-valence iron filings provide fast reaction and high removal capacity for ⁹⁹TcO₄⁻ in groundwater. The high removal efficiency, low cost, and the small waste production of zero-valence iron are attractive for remediation of ⁹⁹TcO₄⁻-contaminated groundwater.     

In situ growth of Co3O4 nanoneedles on titanium mesh for electrocatalytic oxygen evolution

Designing high-efficient and low cost of electrodes with seamless integration of substrate and electrocatalyst particles is of significant concern for electrocatalytic water splitting. In this study, we actualized in situ growth of Co₃O₄ nanoneedles on titanium (Ti) mesh (denoted as Co₃O₄@Ti) by a simple combination of hydrothermal approach and subsequently calcination treatment under relatively low temperatures. The as-prepared Co₃O₄@Ti samples were evaluated as anodes for electrocatalytic oxygen evolution reaction (OER) in alkaline electrolyte. It demonstrates that the optimized Co₃O₄@Ti electrode displayed good OER activity with a small overpotential of 416 mV at a current density of 20 mA cm^?2, which is on a par with commercial RuO₂ catalyst (overpotential of 403 mV at 20 mA cm^?2). The satisfactory OER performance of Co₃O₄@Ti electrode is largely attributed to the seamless integration of conductive Ti mesh substrate and the direct growth of Co₃O₄ nanoneedles on Ti mesh with sufficient active sites. This study suggests the potential application of Co₃O₄@Ti electrode as preeminent OER catalyst.

     

Thermodynamic modeling of thermal processes involving actinides (U,Am, Pu) in the course of heating radioactive graphite in steam

The behavior of actinides (U, Am, Pu) in the course of combustion of radioactive graphite in steam was studied by thermodynamic modeling. Thermodynamic modeling was performed using TERRA program in the temperature interval from 373 to 3273 K to determine the possible composition of volatile actinide species formed in the course of graphite utilization by heating in steam. The modeling shows that the actinides at high temperatures are present in the following forms: U, as UO₃ and UO₂ vapors and as UO₃ ^– and UO₂ ⁺ ions; Am, as Am vapor; and Pu, as PuO₂ and PuO vapors and as ionized PuO⁺. The main reactions within separate phases and at the interface were revealed, and their equilibrium constants were determined.     

3He-CMN boundary resistance

The boundary resistance between liquid and solid³He and CMN is calculated taking the coupling to be due to the nuclear spin-electron spin dipolar interaction. The boundary resistance is calculated using a relationship betweenR _K ^–1 and the longitudinal relaxation timeT ₁. We find qualitative agreement with the results of Leggett and Vuorio for the temperature dependence and impurity dependence ofR _K ^–1 . Quantitative agreement with experiment is possible using a number of plausible assumptions. An important insight emerges: the relative rates of motion of the spins in³He and CMN are crucial to their coupling. Solid³He at low pressure is in near synchronism with CNM. As a consequence its boundary resistance should be very low.Supported in part by the National Science Foundation and the A. P. Sloan Foundation.     

Microstructural characterisation and thermal stability of in situ TiB2/60Si–Al composite synthesised by displacement reactions and spray forming

Abstract

A novel in situ reactive technique has been employed for preparing 2·0 wt-%TiB₂/60Si–Al composite. The kinetic equations and the Arrhenius type equation were applied to compute the coarsening rate constant and the activation energy for grain growth for the composite when it was heated at semisolid state for partial remelting. Experimental results have shown that the in situ TiB₂ particles can refine effectively the primary Si phase and restrain the Si phase growth. The cubic coarsening rate constant for the composite was computed to be in the range of 75–148 μm³ min^?1 at temperatures in the range of 600–700°C, which was much less than that for the 60Si–Al alloy (1323–4523 μm³ min^?1). The value of activation energy for grain growth for the composite was about twice of that for the 60Si–Al alloy. The composite exhibited a higher thermal stability than that of the 60Si–Al alloy, suggesting that the in situ TiB₂ particles can effectively pin the grain boundaries and arrest the migration of liquid film in the semisolid state of the composite.     

The electrical resistivity and specific heat of americium metal

The electrical resistivity and specific heat of samples of ²⁴¹Am and ²⁴³Am are reported. The electrical resistivity at room temperature is reduced compared with the value for plutonium, while the power law exponent at low temperatures is increased. The electronic specific heat coefficient is lower than for the lighter actinides. These features lead to the conclusion that americium is the first of the actinides in which the 5 f electrons are essentially localized. Anomalies occur in the electrical resistivity and in the specific heat in the region of 60 K, the cause of which is unknown.     

129I targets for studies of nuclear waste transmutation

Nuclear incineration of long-lived fission products and minor actinides is being investigated as an alternative means of reactor waste disposal. ¹²⁹I is of particular interest because of its long half-life and high mobility in the environment. Lead iodide targets of ¹²⁹I for neutron capture cross-section measurements were prepared from 210 l fuel reprocessing waste solution containing 1.3 g l⁻¹ iodine and other fission products. The iodine was separated by oxidation to I₂ and extraction into chloroform, reduction to iodide by sodium sulphite and re-extraction into an aqueous phase. Iodide was precipitated using lead nitrate and dried. The chemistry was carried out batch-wise using 400 ml starting solution each time and recycling the chloroform. An extraction efficiency of about 90%, determined by γ-ray spectrometry, was achieved.     

Effect of Hf ion implantation on grain growth in Ni nanocrystalline electrodeposits

Abstract

The microstructural stability of Ni nanocrystalline electrodeposits was investigated to verify general principles underlying the suppression of grain growth by microalloying with elements of very low solid solubility. Hf ions at 300 keV energy were implanted in Ni nanocrystalline foils at low (5·8 × 10¹⁵ ions cm^?2) and high (3·0 × 10¹⁶ ions cm^?2) doses. Their effects on grain growth at 550°C were studied in situ by transmission electron microscopy at 1·25 MeV and by selected area electron diffraction. Grains roughly doublled in size during implantation, but grain growth during subsequent heat treatment was dose dependent and significantly less than in specimens without implantation. Observation on implanted Ni single crystals revealed clustering and the formation of fine Ni₅Hf precipitates. A possible mechanism of grain growth suppression is discussed.