Actinide Materials Research Articles

Potential benefit of amorphization in the retention of gaseous species in irradiated pyrochlores

Understanding the structure-property relationship for materials destined for irradiation extremes is a key step in developing materials with reliable, long-term performance. One crucial relationship is the ability of a material to withstand or accommodate amorphization, as this dictates its potential use as a nuclear waste form. Pyrochlores are one such class of materials for consideration as waste forms and there has been significant work examining how both the crystal structure and chemistry impacts amorphization resistance, leading to the important conclusion that the amorphization resistance of pyrochlores (A2B2O7) is very sensitive to the nature of the B cations. For example, pyrochlores with BTi amorphize much more readily than BZr compounds. However, there are still questions regarding how these types of materials respond to prolonged or high-dose irradiation conditions. In this work, Gd2Ti2O7 (GTO) and Gd2Zr2O7 (GZO) pyrochlores were implanted with 400 keV Kr++ ions at room temperature to calculated peak damages of 119 and 135 displacements per atom (dpa), respectively. As expected, GTO amorphized completely under irradiation. However, discrete bubbles of Kr coalesced within the amorphous matrix without micro-cracking or spallation. In contrast, GZO transforms to a disordered fluorite structure under irradiation with no indications of localized amorphization. But, the accumulation of Kr within the host material leads to sub-grain structures, extended defects, and the development of micro-cracks. Thus, while GTO readily amorphizes even under low dose irradiations, the resistance of the amorphous GTO matrix to micro cracking and gas release, even in the presence of large bubble formation, suggests an enhanced propensity to retain gaseous species. Consideration of long-term dose accumulation effects in nuclear waste forms would suggest reconsideration of amorphization processes in pyrochlores and related materials as a potential beneficial effect for immobilization and long-term storage of actinide materials.

Investigating structure, magneto-electronic, elastic and thermoelectric properties of alkaline earth actinide perovskite oxide (BaBkO3) from first principle calculations

First principle calculations with highly precise spin-polarized density functional theory (DFT) have been performed to study the structural stability, mechanical and magneto-electronic properties of cubic perovskite BaBkO3. The properties were studied under the generalized gradient approximation (GGA) and onset Coulomb interaction by WEIN2k package. The DFT and analytically calculated values of Goldschmidt tolerance (GT) factor in addition to stable-phase optimization show stability of the present material in the ferromagnetic cubic phase with a higher magnetic moment of 7 μB. This is because the nature of magnetism of 5f actinide materials (l=3) is governed by the large spin-orbit interaction and the hybridization of 5f electron orbitals with other states. The value of exchange and correlation potential were treated with different approximations: GGA and GGA + U calculations. Contribution of electronic states was studied through total and partial density of states using GGA and GGA + U approach. GGA + U calculations reveal an indirect band gap of 3.15 eV for BaBkO3 in the spin down channel supporting the half metallic nature and spin-polarized electronic band structure encourages complete spin polarization of the material with metallic character in spin up state. Mechanical properties like stability, stiffness, hardness, brittleness and ductility were discussed on the basis of elastic parameters obtained. The oxide perovskite BaBkO3 exhibit higher value of Seebeck coefficient and power factor at room temperature with a value of 150 μVK−1 and 5.1×1012 μWcm−1K−2s−1respectively. The properties of half-metallicity and higher Seebeck coefficient makes this material a promising candidate for thermoelectric and spintronic device applications.

Numerical evaluation of fast-sensitive microstructured semiconductor neutron detectors for TREAT hodoscope

Presented is the numerical study of fast-sensitive, actinide and hydrogenous microstructured semiconductor neutron detectors (MSNDs) for the hodoscope at the Transient REActor Test facility (TREAT) using Geant4 and MCNP6. Neutron converters considered were 237Np, 235U, natural uranium, and 232Th for actinide MSNDs and paraffin wax for hydrogenous MSNDs (H-MSNDs). Paraffin wax was found to have a larger fission-spectrum-weighted macroscopic cross section (0.32 cm −1) than the actinide materials (the best being 0.067 cm −1 for 237Np). However, actinide reactants were found to allow higher lower-level discriminator (LLD) settings due to the large energy of fission fragments. Actinide MSNDs filled with 235U, natural uranium, and 232Th were evaluated in Geant4 using the fission fragment generator. With the LLD set to 5 MeV, the intrinsic neutron-detection efficiency of the 235U-filled MSNDs was 1.2% for a 2-cm device length and saturated at 2.6% for lengths beyond 14 cm, where 20-μm trench and 10-μm wall widths were assumed. The deposited energy in 235U predicted by Geant4 differed from the MCNP6-predicted value by about −0.7%. With the LLD set to 300 keV, Geant4 and MCNP6 predicted an efficiency of about 10% for a 2-cm long H-MSND with 20-μm trench and 10-μm wall widths and an efficiency of about 26% for a detector length of 20 cm. For a LLD set to achieve a signal-to-noise (S/N) ratio of 100 when including gamma-ray noise, the best-case, Geant4-predicted efficiencies were 2.5% and 9.6% for 2-cm and 20-cm long devices with 60-μm trench and 40-μm wall widths.

Employing an Unsaturated Th4+ Site in a Porous Thorium-Organic Framework for Kr/Xe Uptake and Separation.

Actinide based metal-organic frameworks (MOFs) are unique not only because compared to the transition-metal and lanthanide systems they are substantially less explored, but also owing to the uniqueness of actinide ions in bonding and coordination. Now a 3D thorium-organic framework (SCU-11) contains a series of cages with an effective size of ca. 21×24 Å. Th4+ in SCU-11 is 10-coordinate with a bicapped square prism coordination geometry, which has never been documented for any metal cation complexes. The bicapped position is occupied by two coordinated water molecules that can be removed to afford a very unique open Th4+ site, confirmed by X-ray diffraction, color change, thermogravimetry, and spectroscopy. The degassed phase (SCU-11-A) exhibits a Brunauer-Emmett-Teller surface area of 1272 m2 g-1 , one of the highest values among reported actinide materials, enabling it to sufficiently retain water vapor, Kr, and Xe with uptake capacities of 234 cm3 g-1 , 0.77 mmol g-1 , 3.17 mmol g-1 , respectively, and a Xe/Kr selectivity of 5.7.

Employing an Unsaturated Th4+ Site in a Porous Thorium–Organic Framework for Kr/Xe Uptake and Separation

AbstractActinide based metal–organic frameworks (MOFs) are unique not only because compared to the transition‐metal and lanthanide systems they are substantially less explored, but also owing to the uniqueness of actinide ions in bonding and coordination. Now a 3D thorium–organic framework (SCU‐11) contains a series of cages with an effective size of ca. 21×24 Å. Th4+ in SCU‐11 is 10‐coordinate with a bicapped square prism coordination geometry, which has never been documented for any metal cation complexes. The bicapped position is occupied by two coordinated water molecules that can be removed to afford a very unique open Th4+ site, confirmed by X‐ray diffraction, color change, thermogravimetry, and spectroscopy. The degassed phase (SCU‐11‐A) exhibits a Brunauer–Emmett–Teller surface area of 1272 m2 g−1, one of the highest values among reported actinide materials, enabling it to sufficiently retain water vapor, Kr, and Xe with uptake capacities of 234 cm3 g−1, 0.77 mmol g−1, 3.17 mmol g−1, respectively, and a Xe/Kr selectivity of 5.7.

Dual roles of f electrons in mixing Al 3p character into d -orbital conduction bands for lanthanide and actinide dialuminides

Correlated electron phenomena in lanthanide and actinide materials are driven by a complex interplay between the $f$ and $d$ orbitals. In this study, aluminum $K$-edge x-ray absorption spectroscopy and density functional theory calculations are used to evaluate the electronic structure of the dialuminides, $M{\mathrm{Al}}_{2}$ ($M$ = Ce, Sm, Eu, Yb, Lu, U, and Pu). The results show how the energy and occupancy of the $4f$ or $5f$ orbitals impacts mixing of Al $3p$ character into the $5d$ or $6d$ conduction bands, which has implications for understanding the magnetic and structural properties of correlated electron systems.

Potential Benefit of Amorphization in the Retention of Gaseous Species in Irradiated Pyrochlores

Understanding the structure-property relationship for materials destined for irradiation extremes is a key step in developing materials with reliable, long-term performance. One crucial relationship is the ability of a material to withstand or accommodate amorphization, as this dictates its potential use as a nuclear waste form. Pyrochlores are one such class of materials for consideration as waste forms and there has been significant work examining how both the crystal structure and chemistry impacts amorphization resistance, leading to the important conclusion that the amorphization resistance of pyrochlores (A2B2O7) is very sensitive to the nature of the B cations. For example, pyrochlores with B=Ti amorphize much more readily than B=Zr compounds. However, there are still questions regarding how these types of materials respond to prolonged or high-dose irradiation conditions. In this work, Gd2Ti2O7 (GTO) and Gd2Zr2O7 (GZO) pyrochlores were implanted with 400 keV Kr ions at room temperature to calculated peak damages of 119 and 135 displacements per atom (dpa), respectively. As expected, GTO amorphized completely under irradiation. However, discrete bubbles of Kr coalesced within the amorphous matrix without micro-cracking or spallation. In contrast, GZO transforms to a disordered fluorite structure under irradiation with no indications of localized amorphization. But, the accumulation of Kr within the host material leads to sub-grain structures, extended defects, and the development of micro-cracks. Thus, while GTO readily amorphizes even under low dose irradiations, the resistance of the amorphous GTO matrix to micro cracking and gas release, even in the presence of large bubble formation, suggests an enhanced propensity to retain gaseous species. Consideration of long-term dose accumulation effects in nuclear waste forms would suggest reconsideration of amorphization processes in pyrochlores and related materials as a potential beneficial effect for immobilization and long-term storage of actinide materials.

Unraveling 5f-6d hybridization in uranium compounds via spin-resolved L-edge spectroscopy

The multifaceted character of 5f electrons in actinide materials, from localized to itinerant and in between, together with their complex interactions with 6d and other conduction electron states, has thwarted efforts for fully understanding this class of compounds. While theoretical efforts abound, direct experimental probes of relevant electronic states and their hybridization are limited. Here we exploit the presence of sizable quadrupolar and dipolar contributions in the uranium L3-edge X-ray absorption cross section to provide unique information on the extent of spin-polarized hybridization between 5f and 6d electronic states by means of X-ray magnetic circular dichroism. As a result, we show how this 5f-6d hybridization regulates the magnetism of each sublattice in UCu2Si2 and UMn2Si2 compounds, demonstrating the potentiality of this methodology to investigate a plethora of magnetic actinide compounds.

Analysis of External Surface Irregularities on Fukushima-Derived Fallout Particles

Three large high radioactivity particulate fragments, each several hundred micron in diameter, have been recovered from the region immediately surrounding the Fukushima Daiichi Nuclear Power Plant. Through the application of high-resolution electron and ion-beam methods, this work has sought to investigate the structure and composition of this fibrous surface morphology. By evaluating this, a potential material source can be determined, alongside important information relating to the conditions / events at the time of the reactor explosions and catastrophic release of radioactive materials. The results of this study show that the fibrous features associated with these large radiocesium-containing particles share a common elemental composition. With respect to the surrounding particle, the fibres are enriched in Si, Cl and Fe, whilst depleted in both Zn and Al. Based on composition, these fibres are ascribed to thermal insulating material used within the plant, which was sufficiently heated during the Loss of coolant incident (LOCI) at the plant to be incorporated into the molten ejecta material that rapidly solidified upon quenching in air. Elemental analysis of these fibres does not reveal any evidence of leaching or the presence of actinide materials.

In Situ Raman Spectroscopy for Nuclear Material Monitoring in Molten Salt Systems

Molten salt systems are under consideration for both energy generation and used fuel processing as part of the nuclear fuel cycle. Generation IV nuclear reactor designs use molten salts as both coolants and fuel media. Advanced reprocessing technologies, such as the pyrochemical reprocessing of used nuclear fuel, rely on molten chloride melts as the electrolyte during electrochemical processing of used nuclear fuel. Before any of these technologies can reach the industrial scale, techniques for the monitoring of nuclear material in a nondestructive and in situ manner must be further developed for molten salt systems. Real-time monitoring of nuclear material allows for process optimization, along with addressing nuclear security concerns associated with the diversion of actinide materials. In this study, we explore the use of Raman spectroscopy, employed using a fiber-coupled system, as a method for the in situ­ analysis of molten salt. We present the development of an automated, in situ­ Raman system for analysis of electroactive species in molten LiCl-KCl. The Raman modes of lanthanide chlorides, serving as actinide surrogates, were investigated in LiCl-KCl eutectic at 500°C and compared with the available literature. This work was performed under the auspices of the US Department of Energy (DOE) under contracts DE-NE0008262, DE-NE0008236 and DE-NE0008572, and the US Nuclear Regulatory Commission (NRC) under contract NRC-HQ-13-G-38-0027. Dr. Kenny Osborne serves as the program manager for the DOE awards and Ms. Nancy Hebron-Isreal serves as the grants program officer for the NRC award.

Tricritical point from high-field magnetoelastic and metamagnetic effects in UN

Uranium nitride (UN) is one of the most studied actinide materials as it is a promising fuel for the next generation of nuclear reactors. Despite large experimental and theoretical efforts, some of the fundamental questions such as degree of 5 f–electron localization/delocalization and its relationship to magneto-vibrational properties are not resolved yet. Here we show that the magnetostriction of UN measured in pulsed magnetic fields up to 65 T and below the Néel temperature is large and exhibits complex behavior with two transitions. While the high field anomaly is a field-induced metamagnetic-like transition and affects both magnetisation and magnetostriction, the low field anomaly does not contribute to the magnetic susceptibility. Our data suggest a change in the nature of the metamagnetic transition from first to second order-like at a tricritical point at Ttri ∼ 24 K and Htri ∼ 52 T. The induced magnetic moment at 60 T might suggest that only one subset of magnetic moments has aligned along the field direction. Using the results obtained here we have constructed a magnetic phase diagram of UN. These studies demonstrate that dilatometry in high fields is an effective method to investigate the magneto-structural coupling in actinide materials.

Solid-phase extraction microfluidic devices for matrix removal in trace element assay of actinide materials

Advances in sample nebulization and injection technology have significantly reduced the volume of solution required for trace impurity analysis in plutonium and uranium materials. Correspondingly, we have designed and tested a novel chip-based microfluidic platform, containing a 100-µL or 20-µL solid-phase microextraction column, packed by centrifugation, which supports nuclear material mass and solution volume reductions of 90% or more compared to standard methods. Quantitative recovery of 28 trace elements in uranium was demonstrated using a UTEVA chromatographic resin column, and trace element recovery from thorium (a surrogate for plutonium) was similarly demonstrated using anion exchange resin AG MP-1. Of nine materials tested, compatibility of polyvinyl chloride (PVC), polypropylene (PP), and polytetrafluoroethylene (PTFE) chips with the strong nitric acid media was highest. The microcolumns can be incorporated into a variety of devices and systems, and can be loaded with other solid-phase resins for trace element assay in high-purity metals.

Application of the Uranium-Helium Chronometer to the Analysis of Nuclear Forensic Materials.

Radiochronometers are used to constrain the manufacturing and processing history of actinide materials for nuclear forensic investigations. This paper describes U-He ages and He diffusion kinetics obtained from a metallic, highly enriched uranium sample. The average U-He age is 8% older than the known casting date, which indicates that excess He is present and is likely due to incomplete degassing of pre-existing He during the casting process. Although the U-He age is older than expected, the accuracy is comparable to other chronometers that have been applied to this material. Diffusion kinetics obtained from the uranium metal indicate that He is quantitatively retained under plausible storage conditions.

Thermoelectric power as a probe of density of states in correlated actinide materials: The case ofPuCoGa5superconductor

We present measurements of the thermoelectric power of the plutonium-based unconventional superconductor ${\mathrm{PuCoGa}}_{5}$. The data is interpreted within a phenomenological model for the quasiparticle density of states of intermediate valence systems, and the results are compared with results obtained from photoemission spectroscopy. The results are consistent with the intermediate valence nature of $5f$ electrons; furthermore, we propose that measurements of the Seebeck coefficient can be used as a probe of density of states in this material, thereby providing a link between transport measurements and photoemission in strongly correlated materials. We discuss these results and their implications for the electronic structure determination of other strongly correlated systems, especially actinide materials.

Nuclear Forensics of a Non-Traditional Sample: Neptunium

ABSTRACT Recent nuclear forensics cases have focused primarily on plutonium (Pu) and uranium (U) materials. By definition however, nuclear forensics can apply to any diverted nuclear material. This includes neptunium (Np), an internationally safeguarded material like Pu and U, that could offer a nuclear security concern if significant quantities were found outside of regulatory control. This case study couples scanning electron microscopy (SEM) with quantitative analysis using newly developed specialized software, to evaluate a non-traditional nuclear forensic sample of Np. The results of the morphological analyses were compared with another Np sample of known pedigree, as well as other traditional actinide materials in order to determine potential processing and point-of-origin.

ORNL actinide materials and a new detection system for superheavy nuclei

The actinide resources and production capabilities at Oak Ridge National Laboratory (ORNL) are reviewed, including potential electromagnetic separation of rare radioactive materials. The first experiments at the Dubna Gas Filled Recoil Separator (DGFRS) with a new digital detection system developed at ORNL and University of Tennessee Knoxville (UTK) are presented. These studies used 240 Pu material provided by ORNL and mixed-Cf targets made at ORNL. The proposal to use an enriched 251 Cf target and a large dose of 58 Fe beam to reach the N = 184 shell closure and to observe new elements with Z = 124, 122 and 120 is discussed.

Actinide targets for the synthesis of super-heavy elements

Since 2000, six new super-heavy elements with atomic numbers 113 through 118 have been synthesized in hot fusion reactions of 48Ca beams on actinide targets. These target materials, including 242Pu, 244Pu, 243Am, 245Cm, 248Cm, 249Cf, and 249Bk, are available in very limited quantities and require specialized production and processing facilities resident in only a few research centers worldwide. This report describes the production and chemical processing of heavy actinide materials for super-heavy element research, current availabilities of these materials, and related target fabrication techniques. The impact of actinide materials in super-heavy element discovery is reviewed, and strategies for enhancing the production of rare actinides including 249Bk, 251Cf, and 254Es are described.

Electrodeposition of actinide compounds from an aqueous ammonium acetate matrix: Experimental development and optimization

Electrodeposition is a technique routinely employed in nuclear research for the preparation of thin solid films of actinide materials used in accelerator beam bombardments, irradiation studies, or as radioactive sources. This study investigates the deposition of both lanthanides and actinides from an aqueous ammonium acetate electrolyte matrix. Electrodepositions were performed primarily on stainless steel disks, with yield analysis evaluated using γ-spectroscopy. Experimental parameters (run time, current density, voltage, electrolyte concentration, and initial analyte mass) were studied and modified to optimize the uniformity and adherence of the deposition while maximizing yield. The procedure utilized samarium as the plating material, both with and without a radioactive tracer. Surface characterization studies were performed by scanning electron microscopy, electron microprobe analysis, radiographic imaging, and x-ray diffraction.

Redox response of actinide materials to highly ionizing radiation

Energetic radiation can cause dramatic changes in the physical and chemical properties of actinide materials, degrading their performance in fission-based energy systems. As advanced nuclear fuels and wasteforms are developed, fundamental understanding of the processes controlling radiation damage accumulation is necessary. Here we report oxidation state reduction of actinide and analogue elements caused by high-energy, heavy ion irradiation and demonstrate coupling of this redox behaviour with structural modifications. ThO2, in which thorium is stable only in a tetravalent state, exhibits damage accumulation processes distinct from those of multivalent cation compounds CeO2 (Ce(3+) and Ce(4+)) and UO3 (U(4+), U(5+) and U(6+)). The radiation tolerance of these materials depends on the efficiency of this redox reaction, such that damage can be inhibited by altering grain size and cation valence variability. Thus, the redox behaviour of actinide materials is important for the design of nuclear fuels and the prediction of their performance.

ChemInform Abstract: Exploring Actinide Materials Through Synchrotron Radiation Techniques.

AbstractReview: 387 refs.