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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Theoretical study of the electronic, thermodynamic, and thermo-conductive properties of y-LiAlO2 with 6Li isotope substitutions for tritium production

Lithium aluminate has attracted researchers’ interests due to its wide applications. The structural, electronic, optical, lattice phonon thermodynamic, and thermo-conductive properties of ?-LiAlO2 with 6Li-isotope substitution are investigated by density functional theory and lattice phonon dynamics. The calculated results show that ?-LiAlO2 possesses a wide band-gap of 4.63 eV. The valence band is mainly formed by p orbitals of Li, O, and Al. The calculated lattice thermal conductivity of ?-LiAlO2 is found to be in good agreement over a wide range of temperatures with the available experimental data. The calculated dielectric matrix shows anisotropic behavior along z and x (or y) axes. Due to the wide band-gap, the optical conductivity of ?-LiAlO2 is mainly contributed from the electron hopping between valence-conduction bands. Substitution of 7Li with 6Li isotope in ?-LiAlO2 leads to observable differences in the lattice phonon frequencies at higher frequencies (>8THz) and to slight changes in corresponding thermal conductivity and the infrared and Raman spectra which are in good agreement with the measured data. The relevance of these properties for tritium production through 6Li absorbing neutron in nuclear reactors is discussed.

ab initio thermodynamics, ?-LiAlO2 and 6Li-isotope↗
DOE OSTI2021

Neutron Irradiation Induced Changes in Isotopic Abundance of 6Li and 3D Nanoscale Distribution of Tritium in LiAlO2 Pellets Analyzed by Atom Probe Tomography

Tritium, a radioactive isotope of hydrogen is produced by neutron irradiation of 6Li enriched LiAlO2 pellets in Tritium producing burnable absorber rods (TPBARs). Three-dimensional nanoscale mapping of Tritium in irradiated pellets is critical to understand its spatial variation and its association with various microstructural features in the pellets. Spatially resolved analysis of 6Li isotopic enrichment in such ceramic pellets before and after irradiation can provide insights to heterogeneities in Li isotopic distribution. However, Li being a light isotope, its spatially resolved compositional or isotopic analysis is a challenging task to most analytical methods. Here we used atom probe tomography, to evaluate the Li isotopic enrichment in both the LiAlO¬2 matrix and the Li deficient secondary phase LiAl5O8 before irradiation. These results were then compared with 6Li isotopic ratio measured by APT analysis of neutron irradiated pellets to clearly demonstrate the unique capability of APT to quantitatively analyze the isotopic enrichment of 6Li as well as for other light elements present in these materials. Evidence for heterogenous nanoscale distribution of 3H and O3H within irradiated pellet microstructure is also provided, which is attributed to 3H trapping in irradiation induced vacancy clusters or voids within the LiAlO2 matrix. The characterization results given here, now prove the feasibility for using APT for analyzing nanoscale spatially resolved isotopic enrichment of Li as well as other light elements such as Tritium in TPBARs. This work also paves way to adopt APT for analyzing light elements such as Li and tritium in materials used for nuclear fusion reactor applications, geochemistry, astrophysics as well as for energy storage applications.

Devaraj, Arun↗
DOE OSTI2021

Light Propagation Considerations For Internally Clad Sapphire Optical Fiber Using the 6Li(n,)3H Reaction

Optical frequency domain reflectometry measurements in internally clad single crystal sapphire fiber have received attention in recent years due to their high temperature distributed sensing potential. As work with these fibers has proceeded, there have been some inconsistencies in the results. Deeper investigation and testing has identified two critical considerations for the proper functioning of these fibers. First, users must address the propagation of multimode light along the outer surface of the fiber. Here, by observing the far-field image of an internally clad sapphire fiber when adding index matching fluid to the outer fiber surface, we demonstrate the effects of removing the higher order modes from these fibers. The addition of index matching fluid resulted in nearly single mode performance where multimode performance was previously observed. Second, users must address the effect that coupling the fiber to the interrogator via silica based fiber has on the internally clad sapphire fibers performance. Direct fusion splicing of silica to sapphire, as has been used in the recent work with these fibers, has a mode filtering effect which can be beneficial towards the modal behavior of the fibers. However, in this paper we demonstrate that the splicing can cause a sensing failure due to little or no low order mode light, that is useful for sensing, returning to the detector. The positive results from recent years have demonstrated that optical frequency domain reflectometry sensing performance will be successful in clad sapphire fiber; but only when the considerations described herein are addressed properly.

47 OTHER INSTRUMENTATION↗
DOE OSTI2024

6Li as a three-body system in the (p, 3He) reaction at astrophysical energies

Several astrophysical processes are governed by the occurrence of nuclear reactions involving light nuclei at energies below the Coulomb barrier. Among other effects, their understanding is challenged by the appearance of clustered structures in the ground-state configuration of some of these nuclei, which may have a significant impact on the reaction cross section. In this contribution, we focus on the Li 6 (p, He 3 ) He 4 reaction, to probe the role of clustered configurations of Li 6 . In particular, we consider a three-body ab-initio calculation, based on the hyperspherical harmonics (HH) method, of the Li 6 wave function (WF), together with a more phenomenological three-body model. We observe that the HH WF entails a degree of clustering much larger than obtained from the phenomenological WFs. However, the corresponding reaction cross section, evaluated as a direct two-nucleon transfer in distorted-wave Born approximation, still follows the scaling with the clustering strength already pointed out in a previous work [1] and exhibits an energy trend very similar to that obtained with realistic phenomenological WFs. This opens up interesting perspectives towards constraining

Perrotta, S.↗
DOE OSTI2019

Be 7 and Li 7 nuclei within the no-core shell model with continuum

The production of $^7Be$ and $^7Li$ nuclei plays an important role in primordial nucleosynthesis, nuclear astrophysics, and fusion energy generation. The $^3He(α,γ) ^7Be$ and $^3H(α,γ) 7Li$ radiative-capture processes are important to determine the $^7Li$ abundance in the early universe and to predict the correct fraction of $pp$ -chain branches resulting in $^7Be$ versus $^8B$ neutrinos. The $^6Li(p,γ) ^7Be$ has been investigated recently hinting at a possible cross section enhancement near the threshold. The $^6Li (n,^3H) ^4He$ process can be utilized for tritium breeding in machines dedicated to fusion energy generation through the deuteron-tritium reaction, and is a neutron cross section standard used in the measurement and evaluation of fission cross sections. In this work we study the properties of $^7Be$ and $^7Li$ within the no-core shell model with continuum (NCSMC) method, using chiral nucleon-nucleon interactions as the only input, and analyze all the binary mass partitions involved in the formation of these systems. The NCSMC is an ab initio method applicable to light nuclei that provides a unified description of bound and scattering states and thus is well suited to investigate systems with many resonances and pronounced clustering like $^7Be$ and $^7Li$. Our calculations reproduce all the experimentally known states of the two systems and provide predictions for several new resonances of both parities. Some of these new possible resonances are built on the ground states of $^6Li$ and $^6He$, and thus represent a robust prediction. We do not find any resonance in the $p + ^6Li$ mass partition near the threshold. On the other hand, in the $p + ^6He$ mass partition of $^7Li$ we observe an $S$ -wave resonance near the threshold producing a very pronounced peak in the calculated $S$ factor of the $^6He (p,γ)$ $^7Li$ radiative-capture reaction. While we do not find a resonance near the threshold in the $p + ^6Li$ channel, in the case of $^6He + p$ reaction a resonant S -wave state is predicted at a very low energy above the reaction threshold, which could be relevant for astrophysics and its implications should be investigated. We note though that this state lies above the three-body breakup threshold not included in our method and may be influenced by three-body continuum correlations.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2023

Phase Boundaries, Isotope Effect and Superconductivity of Lithium Under Hydrostatic Conditions

We present theoretical and experimental studies of superconductivity and low temperature structural phase boundaries in lithium. We mapped the structural phase diagram of 6Li and 7Li under hydrostatic conditions between 5 – 55 GPa and within the temperature range of 15 – 75 K, observing the FCC → hR1 → cI16 phase transitions. 6Li and 7Li show some differences at the structural boundaries, with a potential shift of the phase boundaries of 6Li to lower pressures. Density functional theory calculations and topological analysis of the electron density elucidates the superconducting properties and interatomic interactions within these phases of lithium.

Deemyad, Shanti↗
DOE OSTI2022

Scintillation Hydro-Gel for Isotopic Neutron (SHINE): Eco-Friendly Quantum Dot Neutron Detectors

The development of new neutron detectors to replace helium-3 (3He) detectors is imperative due to a worldwide shortage of 3He following the draw down in nuclear weapons production since the end of the Cold War. The United States Department of Homeland Security would like to deploy monitors for the detection of neutron emissions from shipping containers housing illicit nuclear material; however, this effort has been put on hold until new replacements for 3He detectors can be developed. Scintillation Hydro-Gel for isotopic Neutron Emitters (SHINE) is a unique, first of its kind, 6Li-loaded quantum dot gel scintillator developed at INL. By incorporating 6Li with quantum dots in a gel matrix, SHINE displays the best properties of liquid and solid scintillators without their disadvantages such as continuous filtering to keep liquids free of contaminates, slow throughput of containers, higher base component costs, ‘dead’ voids in solid scintillators, and a high loading of 6Li without compromising on light transparency. Additionally, SHINE is completely eco-friendly, a breakthrough in high-efficiency detection systems. SHINE is a unique combination of 6LiCl, a highly water-soluble compound, and InP/ZnS core/shell quantum dots, which are poured into a gel-form using cross-linking polymers. In this presentation, SHINE has been successfully tested for neutron detection and shows promise as both a replacement for current 3He neutron detectors as well as potential use in handheld, compact neutron detection units and antineutrino detection.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Calibration-free analysis of Li isotope ratios using laser ablation and laser absorption spectroscopy

We introduce a rapid, calibration-free, all-optical method for high-precision lithium isotope ratio measurements in solid materials using laser ablation combined with tunable laser absorption spectroscopy. A new asynchronous acquisition method is used to acquire time-resolved, high-resolution spectra of the 6Li and 7Li D1 and D2 transitions near 671 nm. Isotope ratios and atomic column densities are extracted from measured spectra via a physics-based fitting model including hyperfine structure. Under 1 Torr air, spectra recorded = 0.75 ms after plasma onset exhibit narrow linewidths corresponding to Doppler temperatures = 400 K, enabling resolution of the isotope peaks with high signal-to-noise ratios. Analysis of LiAlO2 samples with varying 6Li:7Li ratios demonstrates isotopic precisions of 0.6–1.7% for spectra acquired in 30 s. Isotope ratios determined from the spectral fits show accuracy within –0.3% to –1.7% of reference ICP-MS measurements without requiring calibration to external standards. By eliminating sample preparation and enabling spatially resolved isotopic mapping, this method offers a rapid analysis approach to lithium isotope determination in solid materials relevant to nuclear energy, safeguards, and geochemistry.

Phillips, Mark C.↗
DOE OSTI2019

Unraveling Gibbsite Transformation Pathways into LiAl-LDH in Concentrated Lithium Hydroxide

The intercalation of lithium (Li+) ions into metallic hydroxides (e.g. gibbsite, Al(OH)3) facilitates the synthesis of layered double hydroxides. Caustic environments, such as in lithium hydroxide (LiOH) solutions, present additional pathways beyond the intercalation of ions, specifically dissolution/precipitation processes. Herein, the phase transformation Al(OH)3 to lithium aluminum hydroxide dihydrate, Li[Al(OH)3)]2OH•2H2O in partially deuterated LiOH solutions is tracked through multinuclear, magic angle spinning, high-field nuclear magnetic resonance spectroscopy (27Al and 6Li MAS-NMR) , X-ray diffraction, and scanning electron microscopy of harvested solids. At high magnetic fields (19.975 T), 27Al MAS-NMR enabled detection and quantification of the reactive interface between Al(OH)3 and Li[Al(OH)3)]2OH•2H2O. 6Li MAS-NMR spectra collected on the harvested solids exhibited exchange mediated narrowing as the reaction time progressed, consistent with transport of Li+ ions between the interface and the bulk Li[Al(OH)3)]2OH•2H2O. Complimentary in-situ 27Al MAS-NMR at lower magnetic fields (7.046 T) utilized unique rotors capable of simultaneous resolution of solution and solid state Al-species in caustic environments. The reaction between Al(OH)3 (0.5 M) and LiOH (3 M) initially exhibited greater dissolution and formation of Al(OH)4- than the precipitation. Over several hours, the supersaturated Al(OH)4- solution precipitated Li[Al(OH)3)]2OH•2H2O. Simultaneous, in-situ observations of the solid component tracked the formation of Li[Al(OH)3)]2OH•2H2O evident by an increase in signal and narrowing of the Oh resonance. These results provides insight into Al precipitation in alkaline media with applications ranging from the reduction of Al content in Hanford site, high-level radioactive waste (HLRW) to the precipitation of LDH materials for application as catalyst materials, drug delivery devices, and ion-ion exchangers.

Graham, Trenton R.↗
DOE OSTI2021

First-principles Studies of Tritium Species Formation on and Desorption from Surfaces of $γ- LiAlO_2$ Pellets with Related Secondary Phase

The $γ- LiAlO_2$ ceramics enriched with the $^6Li$ isotope are used in the form of annular pellets in tritium-producing burnable absorber rods (TPBARs). When irradiated in a pressurized water reactor (PWR), the $^6Li$ absorbs neutrons, simulating the nuclear characteristics of a burnable absorber rod, and produces tritium $(_{1}^{3}H, T)$. Once the $_{1}^{3}H$ is produced in the bulk of $γ- LiAlO_2$, it diffuses to its surface, forming different $_{1}^{3}H$ species (possibly $T, OT, T_2, or T_2O$). The $T_2O$ is expected to be generated from experiment, to avoid $_{1}^{3}H$ leakage. However, up to now, it is still not clear which $_{1}^{3}H$ species are formed on the $γ- LiAlO_2$ surface and travel to the getter. Besides, with the dramatic Li loss under irradiation, the secondary phase (possibly $LiAl_5O_8$) precipitates and participates into the $_{1}^{3}H$ diffusivity and $_{1}^{3}H$ species formation. Therefore, exploring $_{1}^{3}H$ species formation on and desorption from surfaces of $γ- LiAlO_2$ with related secondary phases becomes a very important task to provide a better understanding of $_{1}^{3}H$H species escaping from pellets.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2022

A backing detector for order-keV neutrons

In this work, we have designed and tested a large-area (0.15 m 2 ) neutron detector based on neutron capture on 6Li. The neutron detector design has been optimized for the purpose of tagging the scattering angle of keV-scale neutrons. These neutron detectors would be employed to calibrate the low-energy (<100 eV) nuclear recoil in detectors for dark matter and coherent elastic neutrino nucleus scattering (CE$\textit{v}$NS). We describe the design, construction, and characterization of a prototype. The prototype is designed to have a tagging efficiency of ~25% at the relevant $\mathcal{O}$(keV) neutron energies, and with a mean capture time of ~ 17 μs. The prototype was characterized using a 252 Cf neutron source and agreement with the simulation was observed within a few percent level.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2023

6 Li-loaded liquid scintillators produced by direct dissolution of compounds in diisopropylnaphthalene (DIPN)

Here, this paper describes preparation of 6 Li-loaded liquid scintillators by methods involving direct dissolution of 6Li salts in the commercial diisopropylnaphthalene (DIPN) solvent, without the formation of water-in-oil emulsions. Methods include incorporation of 6 Li that, unlike previously reported formulations, does not require additions of water or a strong acid such as hydrochloric acid (HCl). Results of the conducted experiments show that dissolution of aromatic and aliphatic 6 Li salts in DIPN can be easily achieved at 0.1%–0.3% by weight of atomic 6 Li, using small additions of waterless surfactants, or mild carboxylic acids. An alternative way suggests incorporation of 6 Li as a part of a surfactant molecule that can be dissolved in DIPN without any solubilizing additions. Proposed methods enable preparation of efficient 6 Li-loaded liquid scintillators that, at a large scale of 50 cm, exhibit good pulse shape discrimination (PSD) properties combined with up to 107% of light output and up to 115% of the attenuation length measured relative to standard undoped EJ-309 liquid scintillator.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Performance stability of plastics for neutron-gamma pulse shape discrimination

The introduction of the first commercially available plastic scintillators with pulse shape discrimination (PSD) offered by Eljen Technology marked progress towards potentially replacing liquid scintillators in neutron detection. However, use of these plastics over several recent years has revealed an important flaw in these materials: the eventual degradation of scintillation light output and PSD performance. Here, studies described in this paper considered possible reasons for this degradation. Experiments conducted with numerous lab-prepared and commercial EJ-276 samples showed that the main factor affecting the instability is oxidation that involves highly reactive radicals generated during the polymerization process or from the further breakdown of polymer chains under oxygen/air exposure. Based on the obtained results, the stability of scintillation performance for PPO (2,5-Diphenyloxazole)-based PSD plastics has been improved through modifications of the composition via the utilization of scintillation dyes and compounds with antioxidant properties that diminish the effects of oxidation. Elements of these studies were used in the commercial production of the most recent EJ-276D PSD plastic version for fast neutron detection and 6Li-loaded plastics for thermal neutron and antineutrino detection applications. Performance projections of the new PSD plastics indicate a likely degradation of less than 10 % over 5–10 years in comparison to previous EJ-276 that might lose up to 30–40 % of the scintillation light during 1–2 years of storage or deployment under ambient conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2019

First-principles study of the surface properties of y-LiAlO2: Stability and tritium adsorption

y-LiAlO2 enriched with 6Li isotope exhibits excellent performance under neutron radiation and is used in tritium-producing burnable absorber rods (TPBARs) to produce tritium (T, 3H) in pressurized water reactor (PWR). To better understand the 3H formation and diffusion on the surfaces, the first-principles calculations are performed to study the stability of y-LiAlO2 surface and the 3H adsorption on the surface. By considering all possible symmetrical stoichiometric and nonstoichiometric low-index surfaces, two most stable surfaces, one stoichiometric (100) and one nonstoichiometric (101), were identified. The 3H adsorption and diffusion on the most stable (100) surface were further explored, and the obtained results show that there are four stable sites (three O and one Al atop) for 3H adsorption. The calculated Bader charges of atoms reflect that 3H adsorption on the top of O site is a reduction process, while on the top of Al site is an oxidation one. Under low adsorption coverage of 1/4 per surface unit cell, 3H atoms are adsorbed on the most stable OA atop site. With increasing 3H diffused from bulk to surface, the AlG site is the second choice for 3H adsorption, and then the OB site. The coverage of 2/4 is the most favorable 3H adsorption state due to the counteracted charge transfer. When the coverage is up to 4/4, the interaction between 3H atoms leads to partial formation of T2 molecule that desorbs from surface. Therefore, T2 molecule should be the main product from the perfect surface of y-LiAlO2 at high 3H coverage.

Stability of y-LiAlO2 surface, Tritium-producing b↗