Life cycle impact and cost analysis of quarry materials for land-based enhanced weathering in Northern California
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32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
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Nature and coordination geometry of geologically relevant aqueous Uranium(VI) complexes up to 400 ºC: A review and new data
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Kalintsev, Alexander↗
Identifying recharge processes into a vast “fossil” aquifer based on dynamic groundwater 81 Kr age evolution
Water in deep aquifers in arid regions is often considered to be "fossil" when modern recharge rates are negligible relative to the reservoir capacity. Over the past five decades, the Nubian Sandstone Aquifer (NSA) in the arid region of the Sinai Peninsula (Egypt) and the Negev Desert (Israel) has been considered to contain fossil water based on 14 C dating, which revealed 14 C ages of about 30 kyr over most of the aquifer. However, this relatively homogeneous age distribution contradicts the expected increase in groundwater age in the direction of decreasing piezometric head along the flow trajectories. Here, dating results with the longer-lived 81 Kr radioisotope (t 1/2 = 229 ± 11 kyr) are presented, highlighting a wide age range of 40 kyr to 630 kyr in the confined sections of the aquifer, all with very low 14 C activity (<1 pmC). Elevated 81Kr and 14 C activities were only observed within or close to the system's recharge areas. These findings support a new perception of groundwater replenishment during different epochs from the early mid- Pleistocene to the Holocene. By tracking the downstream age evolution, rejuvenation was identified in places where the confinement had been breached. At other locations, the existence of an older groundwater body contributing to the aquifer was detected by means of strongly depleted 81 Kr activity. High spatial heterogeneity in groundwater ages close to the discharge zone of the system is attributed to pronounced age stratification with depth. Calculated ages in the more isolated sections of the system were used to assess regional flow velocity, hydraulic conductivity, and their agreement with present recharge rates. We conclude that regional aquifers in which low 14 C activity suggests homogeneous ancient groundwater age should be reevaluated with 81Kr. With an effective age range beyond one million years, this may enable the restoration of recharge history and provide crucial information for the management of groundwater resources.
54 ENVIRONMENTAL SCIENCES↗
Large-scale paleo water-table rise in a deep desert aquifer recorded by dissolved noble gases
Illuminating past hydrological and climatological conditions in arid regions may provide insights into future trends in groundwater availability. The main objective of this study is to explore paleorecharge processes in the deep regional Nubian Sandstone Aquifer (NSA), which stretches below the hyperarid deserts of the Sinai Peninsula (Egypt) and the Negev (Israel), using dissolved noble gas data. Extremely high amounts of dissolved excess air (Ne concentrations up to 4 × solubility equilibrium) were observed in the ancient ( 81 Kr-depleted) groundwater that was recharged during past pluvial epochs in the basin. Here, the observed unique excess air signal and the clear spatiotemporal structure of the noble gas compositions in the aquifer are hypothesized to reflect two major characteristics of the groundwater system: (a) large-scale, long-term rises in the water table that facilitate the entrapment and dissolution of air bubbles under increased hydrostatic pressure, and (b) the geological settings in the southern recharge area in Sinai, including an almost horizontal position of the layers and intercalation of low-permeability rock formations within the aquifer, which inefficiently reject air bubbles from groundwater, thereby allowing for substantial entrapment of bubbles that dissolve as the water table rises. Enhanced dissolution of entrapped air was also observed in other paleo-groundwater-containing regional sandstone aquifers across North Africa, which, together with the recent findings from the NSA, suggest that largescale water table fluctuations have likely extensively occurred during past pluvial periods over these (and possibly other) arid regions. A preliminary assessment of noble gas recharge temperatures (NGTs) indicates an apparent decoupling of surface and water table temperatures in the case of deep aquifers with a thick (hundreds of m) unsaturated zone. This observed decoupling calls for a re-evaluation of previously obtained NGT records and a need for future work to consider the modification of NGTs due to soil air fractionation and geothermal heating of the underlying deep unsaturated zones.
54 ENVIRONMENTAL SCIENCES↗
Magnetic structure and magnetodielectric behavior of the chiral magnet CoTeMoO 6
We investigate the magnetic structure and magnetodielectric behavior of the chiral magnet CoTeMoO 6 (CTMO) through neutron diffraction, magnetization, and magnetodielectric measurements, complemented by density functional theory (DFT) calculations. Our findings reveal a canted magnetic structure with moments confined to the ab plane, giving rise to weak ferromagnetism under an external magnetic field. Additionally, we observe magnetodielectric coupling that strongly correlates with the magnetic ordering temperature and magnetic structure. Furthermore, these results are discussed in the context of potential mechanisms involving spin-dependent p-d hybridization and spin-phonon coupling.
Canted magnetism↗
Magnetic properties of (Mo 2/3 Dy 1/3 ) 2 AlC arc melted polycrystalline samples
Here, this study investigates the structural and magnetic properties of arc-melted (Mo 2/3 Dy 1/3 ) 2 AlC polycrystalline samples, a member of the i-MAX phase family. Temperature-dependent magnetization and specific heat measurements confirm the low-temperature antiferromagnetic transitions around 14 K and 17 K. Neutron diffraction data collected at 4 K reveal the emergence of magnetic Bragg peaks that are not allowed in the paramagnetic space group C2/c, further confirming the presence of antiferromagnetic ordering. The detection of a secondary phase, DyAl 2 , is complicated by overlapping Bragg peaks with the monoclinic phase of (Mo 2/3 Dy 1/3 ) 2 AlC in powder XRD patterns. However, magnetization and neutron diffraction data suggest the presence of DyAl 2 , evidenced by a ferromagnetic phase transition around 62 K.
36 MATERIALS SCIENCE↗
Effect of nitriding on mechanical and microstructural properties of Direct Metal Laser Sintered 17-4PH stainless steel
In this work, the effect of the nitriding process on microstructure and mechanical properties of additively manufactured (AM) 17-4PH stainless steel is investigated. The nitriding was performed at 530 °C, 560 °C, and 580 °C for 2 h. The nitriding process improves the hardness and surface roughness of the AM 17-4PH steel. Detailed microstructural characterizations of both as-built and nitride samples are performed using an optical microscope, scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction technique. It reveals that the nitride layer thickness increases with nitriding temperature. A distinct transition layer between the substrate and nitride layer is observed in the 560 °C and 580 °C nitride samples. The nitriding process develops almost equiaxed grain microstructure with new secondary phase precipitates, whereas in the as-built material, the grains are primarily columnar along the AM process build direction. Specifically, the nitriding process introduces γ-Fe4N, ε-Fe3N, CrN, and Ni3N precipitates. The increase in Ni- and Cu-rich precipitates with the nitriding temperature explains the observed improvement in the hardness and surface roughness. Furthermore, the nitriding process does not alter the substrate's initial weak crystallographic texture.
17-4PH steel↗
Formation of multicomponent alloy particles in doped ceria under I 2+ ion irradiation and thermal annealing
Multicomponent alloys resembling the e-phase particles in nuclear fuel have the potential to immobilize 99 Tc in a stable waste form. Formation of such multicomponent alloys under extreme conditions is studied in a non-activated model system: CeO 2 doped with 2 wt% Mo, 1.5 wt% Ru, 0.75 wt% Pd, 0.5 wt% Re and 0.25 wt% Rh. The doped CeO 2 films were irradiated with I 2+ ions (610°C, 10 16 and 5×10 16 I 2+ /cm 2 ). For selected samples post-irradiation heat treatment was conducted (900°C, 1100°C). Analytical TEM revealed the formation of 5-20 nm precipitates containing Pd, Mo, and Re sometimes associated with cavities. Iodine was found to segregate to grain boundaries and cracks. After heat treatment at 1100°C, the CeO 2 matrix had recrystallized. Precipitates containing Pd, Mo, Re, and Ru were observed near the interface with the polycrystalline YSZ substrate. It follows that Pd is the most mobile element, followed by Mo and Re. Finally, while irradiation promotes precipitation, high-temperature effects are more significant.
36 MATERIALS SCIENCE↗
Molecular dynamics simulations of radiation response of $\mathrm{LiAlO_2}$ and $\mathrm{LiAl_5O_8}$
Here, we report the findings of a computational investigation of defect production and migration in two lithium aluminate ceramics: LiAlO 2 and LiAl 5 O 8 . Although the first ceramic, LiAlO 2 has been widely investigated computationally, especially via molecular dynamics (MD), there have been no MD studies done on LiAl 5 O 8 up to date, due to the lack of interatomic potentials for this system. This work implemented an existing set of Li-Al-O coulombic and Buckingham potential parameters for the LiAl 5 O 8 system from the literature, followed by validating it based on experimental values of density, crystallinity, bond lengths and melting point. The Li + diffusion in the two ceramics was examined and was found to be one order of magnitude slower in LiAl 5 O 8 (D 0 = 3.17 x 10 -11 m 2 /s) as compared to that in LiAlO 2 (D 0 = 4.02 x 10 -10 m 2 /s) at 600 K. The lithium vacancy migration barrier of 4.15 eV in LiAl 5 O 8 was more than three times that in LiAlO 2 (1.31 eV) possibly due to stronger ordering in the Li-O 6 octahedra of LiAl 5 O 8 than in Li-O 4 tetrahedra of LiAlO 2 . The Li displacement threshold energy (E d ) in LiAl 5 O 8 was found to have a median value of 68 eV which is much higher than that in LiAlO 2 (40 eV). The greater E d for Li signifies a lower likelihood for defect formation and clustering and thus lower tendency for amorphization in LiAl 5 O 8 . The presented results show the difference in defect dynamics in the two ceramics that may help us understand the susceptibility of the two ceramics to amorphization caused by irradiation.
36 MATERIALS SCIENCE↗
Molecular dynamics study of primary damage in the near-surface region in nickel
In this work, we carried out a large-scale molecular dynamics (MD) simulation to elucidate the effect of the free surface on the defect production of displacement cascades in pure nickel. These MD simulations were performed both in the bulk and near-surface regions with primary knock-on atom (PKA) energies of E PKA = 1, 5, or 10 keV and at temperatures T = 300, 425, or 525 K. Additionally, for every (T, E PKA ), near-surface cascade simulations were performed as a function of depth. In the near-surface simulations, assuming isotropic neutron fluence, the PKAs were initiated in random directions, including toward and parallel to the free surface. For both the near-surface and bulk cascades, the effect of E PKA and T on the defect production is similar. In both cases, the defect production increases with E PKA , but the temperature has minimal effect. However, the production and clustering of vacancies are higher for near-surface cascades, and they decrease with increasing depth. In contrast, the production and clustering of self-interstitial atoms are lower and increase with depth. Eventually, the production and clustering of both defect types approach bulk cascade-like behavior, and the depth at which this occurs increases with E PKA , but is independent of temperature.
36 MATERIALS SCIENCE↗
Insights into radiation resistance of titanium alloys from displacement cascade simulations
Radiation damage in beam window materials limits the use of high power proton beams in high energy physics research. The alloy Ti-6Al-4V is presently used as a beam window material but a prospective alternative, Ti-15V-3Cr-3Sn-3Al has been proposed. Since both these alloys contain dual phases at room temperature, we compare the radiation damage in the α and ß-phases of these two materials via primary knock-on atom (PKA) cascade simulations in the 10-40keV energy range. At PKA energies 30 and 40keV, the number of Frenkel pairs in the ballistic stage is higher in the ß-phase of Ti-6Al-4V than that in the ß-phase of Ti-15V-3Cr-3Sn-3Al almost by a factor of 2. The α-phase, of both these alloys, by far outperforms the ß-phases of the two alloys, both in terms of damage during the ballistic stage and in terms of the surviving defects. The average displacement threshold energy (E d ) in the α-phase of both alloys was found to be 66eV while that in the ß-phase of Ti-15-3 was 55 eV and in the ß-phase of Ti-6-4 was 46. Further, while the number of surviving defects is almost equal in both alloys, the vacancy and interstitial clustering mechanisms differ notably, which can impact the degree of radiation hardening and loss of ductility. Our simulations show larger vacancy and interstitial clusters form in Ti-6Al-4V as compared to that in Ti-15-3-3-3 alloy. These results indicate that Ti-15-3-3-3 alloys may be a promising candidate for next generation beam window material with a higher radiation tolerance than the existing Ti-6-4 alloy.
36 MATERIALS SCIENCE↗
Cluster dynamics simulations of tritium and helium diffusion in lithium ceramics
Tritium (T) and He diffusion in LiAlO 2 and LiAl 5 O 8 phases influences the performance of tritium producing burnable absorber rods (TPBARs) by affecting the gas release, swelling and thermal conductivity of Li-bearing ceramic pellets. Frenkel pair defects and clusters created by irradiation can attract T and He interstitials and form clusters of the type He i x Li, He i x Al, He i x O, T i x Li, T i x Al, and T i x O, 1 ≤ x ≤ 4 in a Li, Al or O vacancy site (notation denotes x He or T atoms in a 1 Li, 1 Al or 1 O vacant site). The concentration and mobility of each of these clusters collectively contribute to the diffusion of the He and T gases in LiAlO 2 and LiAl 5 O 8 . Here, in this work, free energy cluster dynamics simulations implemented in the Centipede code, are used to obtain the concentration and diffusivities of these clusters which are then used to calculate the total diffusivity of T and He gases in LiAlO 2 and LiAl 5 O 8 . The results show that diffusivity of T is at least one order of magnitude higher in LiAlO 2 as compared to that in LiAl 5 O 8 whereas He diffusion is 2–13 orders of magnitude higher in LiAlO 2 as compared to that in LiAl 5 O 8 . There is a higher concentration of highly diffusive species (T interstitials and T i 03 Li for the case of tritium and He i 01 Li, He i 02 Li, and He i 03 Li for the case of He) in LiAlO 2 than in LiAl 5 O 8 which increase the total diffusion of T and He in LiAlO 2 .
36 MATERIALS SCIENCE↗
Dynamic data-driven multiscale modeling for predicting the degradation of a 316L stainless steel nuclear cladding material
Here, we have developed a long short-term memory stacked ensemble (LSTM-SE) surrogate modeling approach that can provide rapid predictions of microstructural evolution and the resultant mechanical properties of American Iron and Steel Institute (AISI) 316L series stainless steel (316LSS) fuel cladding under conditions of varying temperature and radiation dose rate. To acquire training data, we developed and implemented a kinetic Monte Carlo (KMC) model to simulate precipitation kinetics of M 23 C 6 , γ', and G phases within SS316L cladding. Experimentally reported precipitation kinetics of SS316L in literature were linked to the kinetic parameters of the simulated precipitation in our KMC model. The model was then used to simulate microstructure evolution under synthetically generated treatments of varying temperature and radiation dose rate, for periods of up to 3000 hours. Changes in volume fraction, number density, and particle size of precipitates were recorded, and particle area fractions were correlated using statistical methods to develop the surrogate model. Simultaneously, the mechanical properties of the simulated microstructures were evaluated using microstructure-based finite element method (FEM) analysis to determine the elastic modulus, yield stress, ultimate tensile strength, and elongation to failure of the aged microstructures. Using this approach, our surrogate model can predict precipitation behavior within 0.25% volume fraction and mechanical properties within 6% relative error from the values predicted by the KMC and FEM models using 50 training simulations as input. The trained recurrent neural network-based model can return estimations of precipitation kinetics and mechanical properties ~1000 times faster than the physics-based codes. This work demonstrates, as a proof of concept, that reactor material service lifetimes under variable service conditions can be predicted for a statistics-based model from a practicably obtainable dataset.
36 MATERIALS SCIENCE↗
Effect of Mg and Ni impurities on tritium diffusion in lithium ceramics through cluster dynamics simulations
This study investigates the impact of Mg and Ni doping on tritium diffusion in LiAlO 2 and LiAl 5 O 8 ceramics, that are used in tritium-producing burnable absorber rods (TPBARs). Utilizing Centipede simulations across a broad temperature range (500 K to 1250 K), we explore the interplay between defect dynamics, cluster formation, and tritium mobility. In LiAlO 2 , Mg doping significantly enhances tritium diffusivity by increasing tritium interstitial concentrations and diffusion coefficients of key species, thereby doubling the overall tritium diffusivity. Ni doping, while shifting the dominant defect to Li vacancies, maintains high tritium mobility due to the low binding energy of Li vacancy-tritium complexes, which ensures effective tritium migration. In LiAl 5 O 8 , Mg and Ni doping results in a slight reduction in the diffusion coefficients of key species, yet the dramatic increase in tritium interstitial concentrations compensates, leading to a net small increase in tritium diffusivity. In conclusion, the findings highlight the critical role of defects in tritium transport and the effect of Mg and Ni defects on the performance of these ceramics in demanding nuclear environments.
Cluster dynamics↗