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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Activation of methane by U + studied by guided ion beam tandem mass spectrometry and quantum chemistry

Reaction pathways of all products formed in the U + + CH 4 (CD 4 ) reaction were explored as a function of kinetic energy using guided ion beam tandem mass spectrometry and quantum chemical calculations. UH + , UC + , UCH + , UCH 2 + , and UCH 3 + (and their perdeuterated analogues) are formed in endothermic reactions. In both systems, the UCH 2 + (UCD 2 + ) dehydrogenated product was the dominant product in the low-energy region, whereas the UH + (UD + ) hydride product became predominant at high energies. The kinetic energy behavior of the various products is consistent with a common intermediate of H–U + –CH 3 (D–U + –CD 3 ). Here, the kinetic energy dependence of all product cross sections was modeled to obtain experimental bond dissociation energies at 0 K (in eV): D 0 (U + –H) = 2.42 ± 0.10, D 0 (U + –C) = 3.95 ± 0.12, D 0 (U + –CH) = 4.91 ± 0.09, D 0 (U + –CH 2 ) = 4.11 ± 0.04, and D 0 (U + –CH 3 ) = 2.41 ± 0.09. Quantum chemical calculations using the UCCSD(T) and UB3LYP approaches with the cc-pwCVXZ-PP basis set with MDF-60 pseudopotential for U + and the aug-cc-pCVXZ and aug-cc-pVXZ (X = T, Q) basis set for carbon and hydrogen, respectively, validate the experimental bond dissociation energies and outline the potential energy surface for all reactions observed. In addition, spin–orbit corrections of the bond energies for all products were calculated at a CASSCF-CASPT2-RASSI level.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Release of Evaluated 235 U(n,f) Average Prompt Fission Neutron Multiplicities Including the CGMF Model

This report documents an evaluation of the average prompt fission neutron multiplicity, $\overline{v}_p$, of 235 U from 200 keV to 15 MeV that is a potential release candidate for the upcoming U.S. nuclear data library, ENDF/B-VIII.1. This evaluation had to be re-done from "scratch", as the input to the $\overline{v}_p$ evaluation of the previous library, ENDF/B-VIII.0, was lost. That means that all available experimental data were re-analyzed and uncertainties were re-estimated. Another major difference to ENDF/B-VIII.0 is that this evaluation includes model information from the Hauser-Feshbach fission fragment decay code CGMF, while ENDF/B-VIII.0 is based purely on experimental data. CGMF links several fission quantities with each other; $\overline{v}_p$ is predicted by assumptions made on, e.g., pre-neutron emission yields as a function of mass, the total kinetic energy, or spin and parity of fission fragments. This allows to perform two types of validation for the new 235 U $\overline{v}_p$: On the one hand, one can employ evaluated CGMF parameters obtained from fitting to experimental 235 U $\overline{v}_p$ to predict yields as a function of mass, the average total kinetic energy, or the mean energy of the prompt fission neutron spectrum. These model-predicted values can then be compared to experimental and evaluated data. The model-predicted fission-observable values using evaluated parameters obtained here are reasonably close to experimental data indicating the evaluated 235 U(n,f) $\overline{v}_p$ are physical. On the other hand, one can validate 235 U $\overline{v}_p$ with respect to integral responses such as fast ICSBEP critical assemblies or LLNL pulsed spheres. LLNL pulsed-sphere neutron-leakage spectra are minimally impacted by the new 235 U $\overline{v}_p$ as these experimental data are shape data and the $\overline{v}_p$ would mostly lead to a change in normalization of the data as the spheres are relatively thin (0.7 and 1.5 mean-free path) and, thus, mostly depend on 235 U $\overline{v}_p$ from 12-15 MeV. The change in the predicted effective neutron multiplication factor, k eff , of selected ICSBEP critical assemblies, however, is large compared to values using ENDF/B-VIII.0 and experimental k eff : The average bias is 108 pcm across all studied k eff values versus 12 pcm for ENDF/B-VIII.0. A reasonable performance in simulating keff (mean bias of 14 pcm) can be retained by tweaking 235 U $\overline{v}_p$ from 3-5 MeV, and combining it with a recent 235 U PFNS evaluation that is also a ENDF/B-VIII.1 release candidate.

235U↗
DOE OSTI2022

Warped Disk Galaxies. I. Linking U-type Warps in Groups/Clusters to Jellyfish Galaxies

Warped disk galaxies are classified into two morphologies: S and U types. Conventional theories routinely attribute both types to galactic tidal interaction and/or gas accretion, but reproducing U types in simulations is extremely challenging. Here we investigate whether both types are governed by the same mechanisms using the most extensive sample of ~8000 nearby (0.02 < z < 0.06) massive (M*/M ⊙ > 10 9 ) edge-on disks from the Sloan Digital Sky Survey. We find that U types show on average bluer optical colors and a higher specific star formation rate (sSFR) than S types, with more strongly warped U types having a higher sSFR. We also find that while the S-type warp properties correlate with the tidal force by the nearest neighbor regardless of the environment, there is no such correlation for U types in groups/clusters, suggesting a nontidal mechanism could be at play for U types, such as ram pressure stripping (RPS). Indeed, U types are more common in groups/clusters than in fields and they have stellar mass, gas fraction, sSFR enhancement, and phase-space distribution closely analogous to RPS-induced jellyfish galaxies in clusters. We furthermore show that the stellar disks of most RPS galaxies in the IllustrisTNG simulation are warped in a U shape and bent in the opposite direction of stripped gas tails, satisfying theoretical expectations for stellar warps embedded in jellyfishes. We therefore suggest that despite the majority of U types that live in fields being still less explained, RPS can be an alternative origin for those in groups/clusters.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRSDate not supplied

Phosphate (U-Th)/He Thermochronology of Apollo 14 Melt Breccia 14311

Our ability to confidently characterize the impact history of the inner solar system is limited by discrepancies in radiometric dates and age interpretations for lunar rocks, impact melts, and recovered meteorites. It is therefore important to explore different thermally sensitive radiometric systems to unravel the timing and extent of the long term impact flux. Low-temperature thermochronology of lunar samples has the potential to provide more complementary geochronological datasets and further test dynamical models related to the evolution of the inner solar system (e.g., [1]). (U-Th)/He dating is based on the production of 4He atoms by radioactive alpha decay of U and Th (and to a lesser extent, Sm) in a crystal and the thermally activated volumetric diffusion of those 4He nuclides. At high temperatures, the crystal is an open system from which 4He can escape; at lower temperatures, 4He may be retained. This retention temperature depends on factors such as crystal structure and volume fraction of radiation damage in the crystal (e.g., [2, 3]), but is significantly lower for phosphate minerals compared to the diffusion of the radiogenic daughter products in other widely used chronometric systems (e.g., ~75°C in terrestrial apatite (U-Th)/He vs. ~500°C in apatite U-Th-Pb vs. ~900°C in zircon U-Th-Pb chronometry). Phosphate (U-Th)/He dating has been used to decipher peak temperatures and cooling rates related to terrestrial impact events (e.g., [4]). Pairing a low-temperature thermochronometer with higher-temperature approaches (i.e., a combined 207Pb-206Pb and (U-Th)/He approach), can therefore resolve multiple impact ages within a given sample or even grain. However, despite its potential, phosphate (U-Th)/He dating has not been reported on any lunar samples. Here we present the first lunar phosphate (U-Th)/He thermochronology on an Apollo 14 impact melt-breccia.

C A Diaz↗
DOE OSTI2025

Phonon-Spin Scattering from Unpaired f-electrons in U atoms and It’s Influence on Thermal Transport in Uranium-doped Thorium Dioxide Single Crystals

In this work, impact of low level of uranium (U) atom substitution on thermal conductivity of thorium dioxide (ThO2) is investigated. ThO2 is an electronic insulator with a wide optical band-gap and no unpaired electrons whose thermal transport is governed by phonons. U-substitution introduces unpaired f-electrons resulting in paramagnetic behavior of U-ThO2 at room temperature, which significantly suppresses its thermal conductivity. A single crystal of U-ThO2 with graded composition of U is grown using a hydrothermal synthesis method, and thermal conductivity measurements are performed in regions with uniform composition of U at levels of 0%, 6%, 9% and 16%. Measured thermal conductivity profiles over 77–300 K temperature range are analyzed using an analytical expression for phonon-mediated thermal transport based on Klemens-Callaway model. Temperature dependent thermal conductivity is found to deviate significantly from the Rayleigh scattering trend expected for a simple substitutional point defect with a small perturbation to mass and interatomic forces. With the resonant scattering term, observed large suppression of thermal conductivity at low temperatures can be closely reproduced. Additionally, the extracted phonon-spin coupling constants imply a nonlinear relation of phonon-spin interaction intensity with respect to U doping percentage. Our study reveals how phonon-spin scattering contributed by unpaired f-electrons in U atoms influences thermal transport in the U-ThO2 system.

36 - MATERIALS SCIENCE↗
DOE OSTI2024

Evaluation of Sb-Nd and Te-Nd phases within the U-Zr fuel matrix and their interactions with HT9 alloy

Antimony (Sb) and tellurium (Te) were investigated as potential additives for U-10Zr (wt.%) metallic fuel to limit the fuel-cladding chemical interaction (FCCI) with HT-9 alloy. Neodymium (Nd) was utilized to simulate the formation of lanthanide-based solid fission products which are known to play a detrimental role in FCCI. Fuel alloys of U-Zr-Sb-Nd and U-Zr-Te-Nd were evaluated in their annealed condition and compared against their as-cast conditions. Isothermal diffusion couple experiments were performed between U-Zr-Nd, U-Zr-Sb-Nd, and U-Zr-Te-Nd against the cladding alloy HT9 to evaluate the effectiveness of the additives to stabilize Nd within the fuel alloys, as well as investigate the interaction regions that form between the different fuel alloys and HT9. Further, SbNd and Sb 3 Nd 4 , and TeNd are found to be the primary neodymium-based phases formed in the U-Zr-Sb-Nd and U-Zr-Te-Nd alloys, respectively. The zirconium-based phase, Zr 2 Sb, is also found to form within the former alloy. All phases were found to remain stable through the diffusion experiments and exhibited no interaction with HT9 constituent elements. Preferential interaction between Nd with additivities Te and Sb compared to constituting elements in HT9 was further verified based on density functional theory (DFT) calculated enthalpy of mixing.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2020

Materials Data on U(Cr3P2)2 by Materials Project

U(Cr3P2)2 crystallizes in the orthorhombic Pmmn space group. The structure is three-dimensional. U is bonded to six Cr and six P atoms to form distorted UCr6P6 cuboctahedra that share corners with four equivalent CrU2P4 tetrahedra, edges with eight CrU2P4 tetrahedra, and faces with two equivalent UCr6P6 cuboctahedra. There are two shorter (2.97 Å) and four longer (3.01 Å) U–Cr bond lengths. There are four shorter (2.84 Å) and two longer (2.87 Å) U–P bond lengths. There are four inequivalent Cr sites. In the first Cr site, Cr is bonded to two equivalent U and four P atoms to form distorted CrU2P4 tetrahedra that share corners with two equivalent UCr6P6 cuboctahedra, corners with ten CrU2P4 tetrahedra, edges with three equivalent UCr6P6 cuboctahedra, edges with five equivalent CrU2P4 tetrahedra, and faces with two CrU2P4 tetrahedra. There are a spread of Cr–P bond distances ranging from 2.33–2.35 Å. In the second Cr site, Cr is bonded in a 5-coordinate geometry to five P atoms. There are a spread of Cr–P bond distances ranging from 2.39–2.41 Å. In the third Cr site, Cr is bonded to two equivalent U and four P atoms to form distorted CrU2P4 tetrahedra that share corners with four equivalent CrU2P4 tetrahedra, edges with two equivalent UCr6P6 cuboctahedra, edges with two equivalent CrU2P4 tetrahedra, and faces with two equivalent CrU2P4 tetrahedra. There are two shorter (2.31 Å) and two longer (2.35 Å) Cr–P bond lengths. In the fourth Cr site, Cr is bonded in a 5-coordinate geometry to five P atoms. There are four shorter (2.39 Å) and one longer (2.48 Å) Cr–P bond lengths. There are three inequivalent P sites. In the first P site, P is bonded in a 8-coordinate geometry to eight Cr atoms. In the second P site, P is bonded in a 8-coordinate geometry to two equivalent U and six Cr atoms. In the third P site, P is bonded in a 9-coordinate geometry to two equivalent U and seven Cr atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on U(GaFe)6 by Materials Project

UFe6Ga6 crystallizes in the orthorhombic Immm space group. The structure is three-dimensional. U is bonded in a 8-coordinate geometry to twelve Fe and eight Ga atoms. There are four shorter (3.19 Å) and eight longer (3.28 Å) U–Fe bond lengths. There are a spread of U–Ga bond distances ranging from 2.90–3.03 Å. There are two inequivalent Fe sites. In the first Fe site, Fe is bonded to two equivalent U, four Fe, and six Ga atoms to form a mixture of distorted corner, edge, and face-sharing FeU2Ga6Fe4 cuboctahedra. There are two shorter (2.51 Å) and two longer (2.52 Å) Fe–Fe bond lengths. There are a spread of Fe–Ga bond distances ranging from 2.50–2.63 Å. In the second Fe site, Fe is bonded to two equivalent U, four equivalent Fe, and six Ga atoms to form a mixture of distorted corner, edge, and face-sharing FeU2Ga6Fe4 cuboctahedra. There are a spread of Fe–Ga bond distances ranging from 2.60–2.67 Å. There are three inequivalent Ga sites. In the first Ga site, Ga is bonded in a 10-coordinate geometry to one U, six Fe, and three Ga atoms. There are one shorter (2.65 Å) and two longer (2.84 Å) Ga–Ga bond lengths. In the second Ga site, Ga is bonded in a 12-coordinate geometry to two equivalent U, six Fe, and four Ga atoms. Both Ga–Ga bond lengths are 2.98 Å. In the third Ga site, Ga is bonded in a 8-coordinate geometry to one U, six Fe, and three Ga atoms. The Ga–Ga bond length is 2.65 Å.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on U(GePt)2 by Materials Project

U(PtGe)2 crystallizes in the tetragonal P4/nmm space group. The structure is three-dimensional. U is bonded in a 12-coordinate geometry to eight Pt and eight Ge atoms. There are four shorter (3.28 Å) and four longer (3.33 Å) U–Pt bond lengths. There are four shorter (3.26 Å) and four longer (3.33 Å) U–Ge bond lengths. There are two inequivalent Pt sites. In the first Pt site, Pt is bonded to four equivalent U and four equivalent Ge atoms to form distorted PtU4Ge4 tetrahedra that share corners with twelve equivalent GeU4Pt4 tetrahedra, edges with two equivalent GeU4Pt4 tetrahedra, edges with four equivalent PtU4Ge4 tetrahedra, and faces with four equivalent PtU4Ge4 tetrahedra. All Pt–Ge bond lengths are 2.55 Å. In the second Pt site, Pt is bonded in a 9-coordinate geometry to four equivalent U and five Ge atoms. There are one shorter (2.41 Å) and four longer (2.51 Å) Pt–Ge bond lengths. There are two inequivalent Ge sites. In the first Ge site, Ge is bonded to four equivalent U and four equivalent Pt atoms to form distorted GeU4Pt4 tetrahedra that share corners with twelve equivalent PtU4Ge4 tetrahedra, edges with two equivalent PtU4Ge4 tetrahedra, edges with four equivalent GeU4Pt4 tetrahedra, and faces with four equivalent GeU4Pt4 tetrahedra. In the second Ge site, Ge is bonded in a 9-coordinate geometry to four equivalent U and five Pt atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on U(SiNi5)2 by Materials Project

UNi10Si2 crystallizes in the orthorhombic Immm space group. The structure is three-dimensional. U is bonded in a 8-coordinate geometry to sixteen Ni and four equivalent Si atoms. There are a spread of U–Ni bond distances ranging from 2.67–3.09 Å. All U–Si bond lengths are 3.16 Å. There are four inequivalent Ni sites. In the first Ni site, Ni is bonded in a 8-coordinate geometry to one U, eleven Ni, and two equivalent Si atoms. There are a spread of Ni–Ni bond distances ranging from 2.42–2.96 Å. Both Ni–Si bond lengths are 2.54 Å. In the second Ni site, Ni is bonded in a 1-coordinate geometry to one U, eleven Ni, and two equivalent Si atoms. There are a spread of Ni–Ni bond distances ranging from 2.48–2.71 Å. Both Ni–Si bond lengths are 2.40 Å. In the third Ni site, Ni is bonded in a 2-coordinate geometry to two equivalent U, nine Ni, and two equivalent Si atoms. There are four shorter (2.44 Å) and one longer (2.83 Å) Ni–Ni bond lengths. Both Ni–Si bond lengths are 2.50 Å. In the fourth Ni site, Ni is bonded to two equivalent U, eight Ni, and two equivalent Si atoms to form distorted NiU2Si2Ni8 cuboctahedra that share corners with four equivalent SiU2Ni10 cuboctahedra, corners with ten equivalent NiU2Si2Ni8 cuboctahedra, edges with two equivalent SiU2Ni10 cuboctahedra, edges with four equivalent NiU2Si2Ni8 cuboctahedra, faces with four equivalent SiU2Ni10 cuboctahedra, and faces with six equivalent NiU2Si2Ni8 cuboctahedra. Both Ni–Ni bond lengths are 2.40 Å. Both Ni–Si bond lengths are 2.30 Å. Si is bonded to two equivalent U and ten Ni atoms to form distorted SiU2Ni10 cuboctahedra that share corners with six equivalent SiU2Ni10 cuboctahedra, corners with eight equivalent NiU2Si2Ni8 cuboctahedra, edges with three equivalent SiU2Ni10 cuboctahedra, edges with four equivalent NiU2Si2Ni8 cuboctahedra, a faceface with one SiU2Ni10 cuboctahedra, and faces with eight equivalent NiU2Si2Ni8 cuboctahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on U(Fe5Si)2 by Materials Project

UFe10Si2 crystallizes in the orthorhombic Immm space group. The structure is three-dimensional. U is bonded in a 12-coordinate geometry to sixteen Fe and four equivalent Si atoms. There are a spread of U–Fe bond distances ranging from 2.85–3.17 Å. All U–Si bond lengths are 3.09 Å. There are four inequivalent Fe sites. In the first Fe site, Fe is bonded in a 8-coordinate geometry to one U, eleven Fe, and two equivalent Si atoms. There are a spread of Fe–Fe bond distances ranging from 2.38–2.91 Å. Both Fe–Si bond lengths are 2.58 Å. In the second Fe site, Fe is bonded in a 10-coordinate geometry to one U, eleven Fe, and two equivalent Si atoms. There are a spread of Fe–Fe bond distances ranging from 2.44–2.68 Å. Both Fe–Si bond lengths are 2.54 Å. In the third Fe site, Fe is bonded in a 12-coordinate geometry to two equivalent U, eight Fe, and two equivalent Si atoms. All Fe–Fe bond lengths are 2.46 Å. Both Fe–Si bond lengths are 2.56 Å. In the fourth Fe site, Fe is bonded to two equivalent U, eight Fe, and two equivalent Si atoms to form a mixture of distorted corner, edge, and face-sharing FeU2Fe8Si2 cuboctahedra. Both Fe–Fe bond lengths are 2.36 Å. Both Fe–Si bond lengths are 2.36 Å. Si is bonded in a 12-coordinate geometry to two equivalent U and ten Fe atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on U(AlFe)6 by Materials Project

UFe6Al6 crystallizes in the orthorhombic Immm space group. The structure is three-dimensional. U is bonded in a 12-coordinate geometry to twelve Fe and eight Al atoms. There are four shorter (3.16 Å) and eight longer (3.28 Å) U–Fe bond lengths. There are a spread of U–Al bond distances ranging from 2.91–3.02 Å. There are two inequivalent Fe sites. In the first Fe site, Fe is bonded in a 12-coordinate geometry to two equivalent U, four Fe, and six Al atoms. There are two shorter (2.47 Å) and two longer (2.50 Å) Fe–Fe bond lengths. There are two shorter (2.50 Å) and four longer (2.61 Å) Fe–Al bond lengths. In the second Fe site, Fe is bonded to two equivalent U, four equivalent Fe, and six Al atoms to form a mixture of distorted corner, edge, and face-sharing FeU2Al6Fe4 cuboctahedra. There are two shorter (2.62 Å) and four longer (2.64 Å) Fe–Al bond lengths. There are three inequivalent Al sites. In the first Al site, Al is bonded in a 10-coordinate geometry to one U, six Fe, and three Al atoms. There are one shorter (2.62 Å) and two longer (2.79 Å) Al–Al bond lengths. In the second Al site, Al is bonded in a 8-coordinate geometry to one U, six Fe, and three Al atoms. There are one shorter (2.69 Å) and two longer (2.93 Å) Al–Al bond lengths. In the third Al site, Al is bonded in a 12-coordinate geometry to two equivalent U, six Fe, and four Al atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on U(Al3Fe)3 by Materials Project

UFe3Al9 crystallizes in the monoclinic C2/m space group. The structure is three-dimensional. U is bonded in a 12-coordinate geometry to six Fe and fourteen Al atoms. There are four shorter (3.36 Å) and two longer (3.37 Å) U–Fe bond lengths. There are a spread of U–Al bond distances ranging from 3.05–3.35 Å. There are two inequivalent Fe sites. In the first Fe site, Fe is bonded in a 10-coordinate geometry to two equivalent U and ten Al atoms. There are a spread of Fe–Al bond distances ranging from 2.53–2.71 Å. In the second Fe site, Fe is bonded in a 12-coordinate geometry to two equivalent U, two equivalent Fe, and eight Al atoms. Both Fe–Fe bond lengths are 2.53 Å. There are a spread of Fe–Al bond distances ranging from 2.50–2.65 Å. There are three inequivalent Al sites. In the first Al site, Al is bonded in a 12-coordinate geometry to two equivalent U, three Fe, and seven Al atoms. There are a spread of Al–Al bond distances ranging from 2.62–2.91 Å. In the second Al site, Al is bonded in a 10-coordinate geometry to one U, three Fe, and six Al atoms. There are one shorter (2.72 Å) and one longer (2.75 Å) Al–Al bond lengths. In the third Al site, Al is bonded in a 2-coordinate geometry to two equivalent U, two equivalent Fe, and eight Al atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Anionic G•U pairs in bacterial ribosomal rRNAs

Wobble GU pairs (or G•U) occur frequently within double-stranded RNA helices interspersed between standard G=C and A-U Watson–Crick pairs. Another type of G•U pair interacting via their Watson–Crick edges has been observed in the A site of ribosome structures between a modified U34 in the tRNA anticodon triplet and G + 3 in the mRNA. In such pairs, the electronic structure of the U is changed with a negative charge on N3(U), resulting in two H-bonds between N1(G)…O4(U) and N2(G)…N3(U). Here, we report that such pairs occur in other highly conserved positions in ribosomal RNAs of bacteria in the absence of U modification. An anionic cis Watson–Crick G•G pair is also observed and well conserved in the small subunit. These pairs are observed in tightly folded regions.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2022

U–Pb Geochronology and Stable Isotope Geochemistry of Terrestrial Carbonates, Lower Cretaceous Cedar Mountain Formation, Utah: Implications for Synchronicity of Terrestrial and Marine Carbon Isotope Excursions

The terrestrial Lower Cretaceous Cedar Mountain Formation, Utah, is a critical archive of paleoclimate, tectonics, and vertebrate ecology and evolution. Early Cretaceous carbon cycle perturbations associated with ocean anoxia have been interpreted from this succession, as expressed in stable carbon isotopes. However, refining the timing of the observed stable isotope excursions remains a key challenge in understanding how marine anoxia affects the Earth system, and is ultimately recorded in the terrestrial realm. The geochronology and geochemistry of a terrestrial carbonate near the base of this succession, which potentially records the Ap7 global carbon isotope excursion, is studied here. Petrographic and geochemical analyses are used to test plausible mechanisms for U incorporation into the calcite lattice in this sample. Using these methods, the hypothesis that the incorporation of U was at or close to the timing of carbonate precipitation is evaluated. U–Pb geochronology of calcite indicates a plausible Early Cretaceous age. However, comparison of the new U–Pb ages of calcite with detrital zircon maximum depositional ages immediately beneath the studied sample indicates a disparity in the apparent sedimentation rates if both types of geochronologic information are interpreted as reflecting the timing of sediment deposition. The totality of data supports an early, and high-temperature, diagenetic timing of U incorporation, with potential for minor leaching of U in subsequent fluid–rock interaction. The most likely mechanism for U transport and immobilization in these samples is hydrothermal fluid–rock interaction. Therefore, the radiometric ages, and corresponding stable isotope composition of U-bearing carbonate domains in this sample, indicate early subsurface fluid–rock interactions and not a record of atmosphere–soil geochemical reactions.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Understanding neutron capture processes in uranium deposits using combined U-Sm-Nd isotopic compositions

Valuable insights into the history and evolution of a geologic deposit can be found by investigating neutron capture reactions. Thermal neutron capture reactions occur within both the samarium (Sm) and the uranium (U) systems, where 149 Sm and 235 U can capture neutrons to become 150 Sm and 236 U, respectively. Although largely unexplored, paired measurements of 150 Sm and 236 U could be important for understanding neutron capture effects within uranium ore bodies, and such measurements are potentially useful in nuclear forensics for assessing a material's provenance or mineral exploration. In this work, we refined measurement procedures of Sm isotope compositions utilizing MC-ICPMS. While geologic reference materials were found to have indistinguishable Sm isotope compositions, we found significant isotope variations consistent with nuclear field shift among synthetic Sm standards. Here, this observation highlights that future high-precision Sm isotope investigations need to carefully evaluate synthetic standard(s) against geologic reference materials until an unfractionated and agreed-upon standard is identified. Here, we applied this method to a set of nine uranium ores from the South Australian Beverley North uranium deposits. Although 236 U excesses had been previously reported for these U ores, we found no measurable isotopic shifts in 149 Sm- 150 Sm at the current level of precision (±5 parts per million). One possible explanation for this disparity in the observed neutron capture signatures between U and Sm is that the source(s) of the U and Sm in these ores may be decoupled. This is consistent with the finding that these ores have variable 143 Nd/ 144 Nd, thus demonstrating that diverse sources were involved in the formation of the Beverley North deposits. Alternatively, this deposit may be too young (<50Ma) to have accumulated measurable neutron capture effects in Sm to be detected with the methods employed here.

Mineral exploration↗
DOE OSTI2023

The large decline in carbonate $\delta^{238}$U from a PETM section at Tingri (South Tibet) was driven by local sea-level changes, not global oceanic anoxia

Uranium isotope compositions (δ 238 ) and Th/U in carbonates are being explored as paleoredox proxies to study global oceanic anoxia. However, the impact of changes to local depositional environments that might cause a decline in δ 238 U and an increase in Th/U has not been thoroughly investigated. Here we report a decline in δ 238 U (from +0.1 to -0.3‰) and an increase in Th/U from shallow-marine carbonates during the Paleocene-Eocene thermal maximum at Tingri (south Tibet). Changes of δ 238 U and Th/U are not fully coupled. The decline in δ 238 U coincided with a local sea-level fall, whereas the increase in Th/U occurred mainly during a sea-level rise. The decline in δ 238 U likely results from changes in redox conditions of pore waters and in primary carbonate mineralogy. The increase in Th/U is ascribed to reduced authigenic U(IV) accumulation. Here our results suggest that changes in local depositional environments can cause behavior of δ 238 U and Th/U resembling that induced by expanded global oceanic anoxia. This finding calls for caution to consider local factors before applying δ 238 U and Th/U as global paleoredox proxies.

58 GEOSCIENCES↗
DOE OSTI2023

The impact of feedstock size and composition on the hydrothermal growth of (U,Th)O 2

Bulk crystal growth of refractory oxides requires unique approaches, and the U x Th 1-x O 2 solid solution is no exception. Hydrothermal transport reactions of U 0.1 Th 0.9 O 2 onto ThO 2 seeds with feedstocks comprised of U 0.1 Th 0.9 O 2 or mixtures of UO 2 and ThO 2 with a nominal composition of U 0.1 Th 0.9 O 2 are investigated with µ-Raman spectroscopy and X-ray fluorescence. In each case, the trends in stoichiometry as a function of distance from the seed are analyzed and the deviations from the nominal stoichiometry discussed. When the feedstock is composed of mixed oxides, the particle surface area’s influence on the feedstock dissolution rate is the dominant factor and can produce U-rich stoichiometries as high as U 0.75 Th 0.25 O 2 . When composed solely of U 0.1 Th 0.9 O 2 , the total growth amount depends on the particle size, but the obtained stoichiometry is the product of differing solubilities. Although the U0.1Th0.9O 2 feedstock produces the most homogeneous growth, the ending growth of the smallest particle UO 2 /ThO 2 mixture yields the stoichiometry closest to U 0.1 Th 0.9 O 2 . How to obtain desired stoichiometries from mixtures by matching the particle size and solubilities is briefly discussed.

36 MATERIALS SCIENCE↗