Solid Breeder Materials Research Articles

The diffusion and desorption of tritium on Li2TiO3 (-133) surface from first-principles calculations

Li2TiO3 is one of the most promising solid breeder materials for nuclear fusion reactor. Diffusion on the surface is a vital step of the tritium release process. In this work, the diffusion behavior of tritium atoms on the (-133) surface and the desorption of T2 molecules from the (-133) surface are systematically investigated. Possible adsorption sites, local diffusion pathways, and their corresponding activation energies have been identified using first-principles calculations and the climbing image nudged elastic band (CI-NEB) method. The diffusivity and effective activation energy for tritium diffusion on the (-133) surface have been determined using kinetic Monte Carlo (KMC) simulations. The activation energy for T2 desorption from the (-133) surface has also been investigated. The effective activation energy for tritium diffusion on the (-133) surface is found to be 1.06 eV, whereas the activation energy for T2 molecule desorption is 1.46 eV. The (-133) surface exhibits a higher activation energy for tritium diffusion but a lower activation energy for T2 desorption compared to the (001) surface.

Study on hot isostatic pressing conditions of ARAA for fabrication of the breeding blanket first wall

The helium-cooled concept with solid breeding materials is one of the candidates for demonstration fusion power reactor (DEMO) breeding blanket. The first wall (FW) is a core component of the breeding blanket. The FW should keep the structural integrity during the normal plasma operation which is subject to high surface heat flux, neutron wall load, and high coolant pressure simultaneously. Various fabrication methods have been studied for the development of the FW fabrication technologies. This study focuses on FW fabrication technologies with hot isostatic pressing (HIP) bonding which is considered as the most promising method for achieving such complex shape like FW having internal cooling channels. We investigated the HIP bonding conditions using Advance Reduced Activation Alloy, ARAA, which is a reduced activation ferritic-martensitic steel under development in Korea. Temperature, holding time and pressure were considered as the parameters to optimize the HIP bonding conditions. The microstructures around the HIP joints changed with respect to the HIP bonding temperature. The amount of oxide particles around the HIP joints varied depending on the degassing conditions. The tensile strength of the HIP joint was not greatly affected by the presence of the HIP bonding line and degassing conditions. On the other hand, it has been confirmed that the Charpy absorbed energy of the HIP joint was significantly affected by degassing conditions. An increase in oxide particles around the HIP joints occurred when degassing temperature and vacuum level were low, and as the quantity of oxide particles increased, the Charpy absorbed energy of the HIP joint decreased substantially.

Molecular dynamics simulations of the effect of porosity on heat transfer in Li2TiO3

Heat transfer is a key consideration in the development of tritium breeder blankets for future fusion reactors. For solid tritium breeder materials there is a fine balance to be struck between high levels of porosity to encourage tritium release and minimising it to maintain the thermal and mechanical properties. Therefore, in this work we employ molecular dynamics simulations to understand how the introduction of porosity influences the thermal conductivity of lithium metatitanate ceramic breeder material. Our simulations predict that increasing the porosity leads to a decrease in the thermal conductivity which is in good agreement with previous experimental observations. By contrast, we do not observe the increase in the thermal conductivity at high temperatures, that is observed in some experiments. We argue that this increase is a consequence of sintering or some other modification of the experimental sample rather than a fundamental change in the heat conduction mechanism in the crystal matrix.

Solid breeder material crystal structure evolution due to Li burn up–Loss of crystal stability

Solid breeder materials for fusion power plants are subjected to extreme conditions of radiation damage, high temperature and designed component burn-up. The loss of lattice stability of Li2TiO3 due to Li burn-up was investigated using first principles-based approaches to aid evaluation of upper limit of component lifetime. The lattice stability is analyzed using calculated phonon dispersion in the Li2TiO3 supercell, while Li burn-up is modeled by the introduction of Li-vacancies. It has been determined that the studied ceramic can be structurally stable up to ∼40% of Li atoms burn-up. At 50% burn-up, the negative frequency branch in the phonon dispersion spectrum appears indicating the loss of lattice dynamical stability. Moreover, the structure obtained by introduction of unstable frozen phonons with minima energy amplitude also results in a phonon dispersion with negative branches in the structure obtained. Therefore, the system completely loses stability when approximately half of Li atoms are consumed.

Trapping Hydrogen Atoms in Vacancies of Li2TiO3 Crystal: A First-Principles Study.

The hydrogen atom capacity in the vacancies of the Li2TiO3 crystal is systematically studied by the first-principles method to evaluate its tritium release performance as a solid breeder material in nuclear fusion reactors. The adsorption process of adding hydrogen atoms one by one in the vacancy are investigated to find the possible adsorption sites of the hydrogen atoms in the vacancy. The charge transfer and density of states analysis are performed to reveal the form of a hydrogen–hydrogen dimer in the vacancy. Also, the trapping energy and formation energy are defined and calculated to determine the hydrogen atom capacity of the system. According to the simulations, the Ti vacancies have the strongest hydrogen atom capacity followed by Li vacancies, and O vacancies are the weakest. The influence of hydrostatic pressure on the hydrogen atom capacity is also investigated. Our results reveal the hydrogen capacity of vacancies in the Li2TiO3 crystal from the atomic scale, which also provide a theoretical guide to the related tritium release experiments.

Recent Progress in Research of Solid Tritium Breeder Materials Li2TiO3: A Review

During the past decades, fusion reactor fuels such as deuterium and tritium have been extensively investigated due to increasing interest in nuclear fusion energy. Tritium, which is scarce in nature, needs to be fabricated by tritium breeder materials. Among the commonly investigated tritium breeder materials, lithium titanate (Li2TiO3) is recognized as one of the most promising solid tritium breeder materials because of its considerable lithium (Li) atomic density, low activation, excellent chemical stability, and low-temperature tritium release performance. This paper aims to provide a systematic review of the current progress in Li2TiO3 preparation methods as well as the high Li density, tritium release performance, irradiation behavior, and modification technologies of Li2TiO3 pebbles. Li2TiO3 can be synthesized by strategies such as solid-state, sol–gel, hydrothermal, solution combustion synthesis, and co-precipitation methods. Among them, the hydrothermal method is promising due to its simplicity and low cost. Many researchers have begun to focus on composite ceramic pebbles to further improve tritium breeder performance. This will provide a new direction for the future development of Li2TiO3 pebbles. The present review concludes with a summary of the preparation methods currently under development and offers an outlook of future opportunities, which will inspire more in-depth investigation and promote the practical application of Li2TiO3 in this field.

Characterization of cation disorder and oxygen vacancies in Li‐rich Li2TiO3

AbstractLi2TiO3 has a broad stoichiometric range and wide application prospects such as cathode, tritium breeder, and microwave dielectric materials. Particularly, Li2TiO3 shows a faster tritium release performance and exceptional chemical and radiation stabilities, making it one of the most promising solid breeder materials. Li‐rich Li2TiO3 has been developed for years as an advanced breeder material due to its enhanced tritium production and better stability in hydrogen atmosphere. However, the defect chemistry responsible for accommodating excess Li and how the non‐stoichiometry improves the material's performance is still unclear. In this work, the crystal structure and non‐stoichiometry‐induced defects in Li‐rich Li2TiO3 were investigated by means of X‐ray diffraction, electron spin resonance, and X‐ray photoelectron spectroscopy. Results show that oxygen vacancies and cation disorder both play a major role in the incorporation of excess Li. The sintering atmosphere (air/vacuum) has a negligible effect on the formation of the non‐stoichiometry‐induced defects but will influence the concentration of the oxygen vacancies. The better stability in the reduction atmosphere may be ascribed to the oxygen vacancies inside the Li‐rich Li2TiO3 crystals.

Thermo-gravimetric kinetics analysis for the synthesis of lithium aluminate and fabrication of pebbles by solid state reaction process

Lithium aluminate is considered as the solid breeder materials and it can be synthesized by different methods. In the present study, lithium aluminate was synthesized using lithium carbonate and alumina as raw materials, by using solid state reaction process (SSRP). The reaction temperature and time of reaction were optimized using TG-DTA and XRD analysis. The reaction kinetics for the synthesis of lithium aluminate was investigated using TG-DTA data, which is not reported till date. It was found that the reaction is controlled by third order chemical reaction at initial stage and further controlled by diffusion. The activation energy and pre-exponential factor are 292 kJ/mol and 2.29 × 1017 respectively. Lithium aluminate pebbles were fabricated using SSRP and found the process is highly suitable for the synthesis and fabrication of lithium aluminate pebbles. The density of the sintered pebbles was found to be about 90% of theoretical density. The details of the synthesis, pebble fabrication, sintering and kinetic study are discussed here.

First principles study of the stability and thermal conductivity of novel Li-Be hybrid ceramics

Hybrid Li-Be ceramics, combining both tritium (T) breeder (Li) and neutron multiplier (Be) for use as part of the fuel cycle of future nuclear fusion reactors are proposed. The development of such hybrid materials may reduce thermal gradients through better matching of thermal properties, mitigating the detrimental effects that may accompany traditional breeder systems while maintaining acceptable neutron multiplication and T breeding. First-principles methods are used to investigate stability and thermal transport properties of a set of compounds containing both Li and Be. It is demonstrated that BeLi2O4Ge and BeLi2O4Si are mechanically stable and have formation energies comparable with leading candidates for solid state breeder materials, Li2TiO3, Li2ZrO3. It is also demonstrated that similar to the leading candidates, these compounds are insulators with thermal transport defined by phonons. The calculated thermal conductivity of BeLi2O4Ge is slightly higher compared to Li2TiO3 or Li2ZrO3 while in the BeLi2O4Si it is almost three times higher compared to the rest of compounds due to a higher phonon group velocities and increased phonon lifetimes. These results indicate that hybrid Li-Be ceramics offer a potential route towards better matching of thermal properties with minimal functional property degradation, thereby offering better overall fuel cycle performance.

Observation of specific optical phonon modes dominating Li ion diffusion in γ-LiAlO2 ceramic

In γ-LiAlO2 ceramic, Li ion diffusion plays a key role both in tritium release as a solid tritium breeder material in nuclear fusion and in phase stability as a matrix material in molten carbonate fuel cells (MCFC). Yet fundamental understanding of the diffusive process in γ-LiAlO2 is still missing, especially considering the interaction of the lattice dynamics and Li ion motion. Herein, we demonstrated that two specific optical phonon modes are coupled with Li ion diffusion in γ-LiAlO2, by investigating the temperature-dependent lattice dynamics via Raman scattering experiments, as well as first-principles calculations and neutron diffraction experiments (ND). The high-temperature ND experiments showed that γ-LiAlO2 undergoes partial Li ion disordering in Li sublattice at high temperature. Notably, the lattice dynamics studies demonstrated that B1 mode at 226 cm−1 and A1 mode at 263 cm−1 are responsible for Li ion motion through the oscillation of Li–O in <010> and <001> direction, respectively. Our results further suggest that the behavior of tritium release and phase stability in γ-LiAlO2 at elevated temperature may be controlled by tuning the two specific phonon modes through photon/electron-phonon coupling.

Study on the dynamic behavior of solid breeder materials and neutron multipliers under the perturbation of the magnetic field

The functionality of solid blanket pebble bed in a fusion reactor is inevitably subjected to heat flux and electromagnetic forces during the operation. Since the dynamic behavior under the heat flux and forces has strong influence on the packing factor of the pebble bed, and the packing factor is essential to the higher TBR (tritium breeder rate) of the neutron multiplier pebble bed, it’s necessary to study the impact of the dynamic forces on the neutron multiplier pebble bed. Some researchers have investigated the influences of the heat flux and electromagnetic forces on pebble bed. But, till date the influences of the heat flux and electromagnetic forces on the pebble bed of solid breeder materials in presence of neutron multiplier have not been studied yet. Thus, in this paper, the dynamic behavior of the particle system under the magnetic field are investigated based on the neutron multiplier pebble bed in the solid blanket. The mathematical and physical model of the particle pebble bed under the magnetic field was constructed, and the fundamental rule of the neutron multiplier pebble bed under the disturbance of the magnetic field is established. The presented work will provide basic parameters and data for the CFETR (China fusion testing reactor) pebble bed design.

Tritium diffusion in a Li2TiO3 crystal terminated with the (001) surface from first-principles calculations.

The tritium release behavior of the Li2TiO3 crystal has become an important index to evaluate its comprehensive performance as a solid breeder material in nuclear fusion reactors. The tritium diffusion on the surface (surface diffusion) and diffusion from the inside to the surface (hopping diffusion) in Li2TiO3 crystals with a 1/3-Li(001) surface are systematically investigated by the first-principles method. Possible adsorption sites, diffusion pathways and energy barriers of surface diffusion and hopping diffusion have been calculated and analyzed, respectively. Tritium atoms are found to diffuse preferentially along the [100] direction on the surface and two equivalent pathways across the surface were identified. The obtained activation energies are about 0.50 eV for surface diffusion and 1.56 eV for hopping diffusion. The local density of states and Bader charge for typical surface diffusion and hopping diffusion pathways are calculated and analyzed. The results reveal that the tritium (T) atom bonds with neighboring oxygen (O) atoms during the surface diffusion, while the T-O interaction is significantly weakened in the hopping diffusion which results in the higher activation energy than that of surface diffusion. In combination with our previous work, a complete tritium diffusion model for the Li2TiO3 crystal is proposed and the corresponding tritium diffusion coefficients are obtained. Our obtained activation energies are in the same range as previous experimental data and could provide theoretical support for the future related experiments.

The effects of irradiation and high temperature on chemical states in Li2TiO3

In the future D-T fusion reactor, tritium will be bred mainly by the reaction of 6Li (n, α) T, as well as 7Li (n, n’ α) T in tritium breeding materials. Solid breeding materials will experience harsh conditions under both irradiations by energetic particles (neutron, tritium, helium and self-particles) and high temperature. The interactions of irradiations and high temperature on lithium ceramics will influence tritium breeding ratio (TBR). The changes of chemical states and its effects on release behavior of hydrogen isotopes in deuterium-irradiated Li2TiO3 and deuterium-exposed Li2TiO3 at high temperature have been investigated. The peak of O-1s shifted to higher binding energy by both irradiation and deuterium exposure, indicating that O-D bonds formed. The amount of O-D bonds enhanced as increase of irradiation fluence and exposure temperature. The main deuterium atoms were trapped by defects for irradiated samples. Annihilation of E-centers was thought to trigger the release of hydrogen isotopes. O-D bonds were the main deuterium trapping sites in deuterium-exposed Li2TiO3. Deuterium recovered by detrapping O-D bonds would require higher temperature. Both deuterium-irradiation and deuterium-exposure at high temperature could result in the change of chemical states in Li2TiO3. The changes in chemical states had effects on deuterium release. It illustrates that tritium breeding materials in fusion reactor will be modified by both irradiation and high temperature and could result in lower tritium recovery.

Mechanism of vacuum-annealing defects and its effect on release behavior of hydrogen isotopes in Li2TiO3

Li2TiO3 is one of the most promising candidates among solid breeder materials. However, defects introduced into Li2TiO3 will act as the strong trapping sites for tritium. In the present study, mechanism of vacuum-annealing defects and its effect on release behavior of hydrogen isotopes in Li2TiO3 were investigated by means of X-ray diffraction, Raman spectroscopy, electron spin resonance and thermal desorption spectroscopy. The color of samples becomes dark blue and the defects were found to be introduced into Li2TiO3 when annealed in vacuum. This color change suggests the change from Ti4+ to Ti3+ due to decrease in oxygen content. The color recovers to white again after annealing in air. X-ray diffraction and Raman spectroscopy results indicate that there are no modifications on Li2TiO3 crystal phases, but on crystallinity. The main vacuum-annealing defects are E-centers and no other obvious types of defects were observed from electron spin resonance. Based on the experimental results, the production of defects by annealing in vacuum should be satisfied to the following conditions: (1) Li2TiO3 has been exposed in air more than 1 day; (2) Li2TiO3 must be annealed at the temperature higher than 300 °C; (3) Li2TiO3 should be annealed in vacuum lower than 10 Pa. E-centers formed under vacuum-annealing processes have considerable effects on release behavior of hydrogen isotopes investigated by thermal desorption spectroscopy and further should be considered in future fusion reactor. The present work gives some suggestions for future fusion reactors: (1) Li2TiO3 should be preserved in vacuum or kept from water vapor; (2) Li2TiO3 should be annealed at high temperature to remove the adsorbed water before loading into the facility, and must be finished within two days to avoid defects coming from reduction; (3) Li2TiO3 should be improved by adding more oxygen or other elements to refrain from defects introduced by reduction reaction.

First principle study of tritium trapping at oxygen vacancies in Li4SiO4

Tritium trapping at point defects is a major issue concerning tritium extraction efficiency in solid state breeder materials in future fusion reactors. Here, tritium trapping behaviors at oxygen vacancies in Li4SiO4 have been investigated by density functional theory simulations. Formation energies have been calculated to determine the stability of defects in various charge states. Density of states, charge distribution and atomic charge has been calculated to investigate the mechanism of defect formation. The results showed that the neutral and 2 + charged oxygen vacancies are the most stable species and are both diamagnetic. The 1 + charged paramagnetic oxygen vacancy (E′ center) is less stable. In addition, the tritium-trapped O-vacancy is most stable when the defect complex is 1 + charged. The trapped tritium is bonded to Si forming a T-SiO3 unit. Other charge states of the complex may lead to unstable defect structures and high formation energies, forcing tritium to escape the vacancy.

Influence of O-related defects introduced by reduction on release behavior of hydrogen isotopes in Li2TiO3

In fusion reactors, Li2TiO3 is one of the most promising candidates among solid breeder materials. However, Li2TiO3 will be reduced after heating under a reducing atmosphere resulting in a deficiency of oxygen. In the present study, the O-related defects introduced by reduction and their effect on the release behavior of hydrogen isotopes in Li2TiO3 were investigated by means of Raman spectroscopy, electron spin resonance and thermal desorption spectroscopy. O-related defects were confirmed as E-centers by electron spin resonance. The concentration of defects increased as the exposing temperature increased and then decreased when the temperature increased higher above 750 °C. The color of Li2TiO3 samples changed from white to dark blue after heating under deuterium and recovered to white again after annealing in air. This color change suggested a change from Ti4+ to Ti3+ due to a decrease in the oxygen content. Raman spectroscopy results indicated that there are no modifications in Li2TiO3 crystal structure, but on crystallinity. Thermal desorption spectroscopy showed the release behavior of deuterium has been affected considerably by O-related defects.

The Raman effects in γ-LiAlO2 induced by low-energy Ga ion implantation

The tetragonal γ-LiAlO2 crystal, known as a promising solid breeding material in future fusion reactors, has attracted much attention for its irradiation effects. This work focused on the Raman effects in ion-implanted γ-LiAlO2. Ga ions of 30, 80 and 150keV were implanted on the z-cut γ-LiAlO2 sample surfaces at a fluence of 1×1014ions/cm2 or 1×1015ions/cm2. The average ion range varied from 230 to 910Å. The Raman spectra were collected from the implanted surfaces before and after the implantation. Evident changes were reflected in the Raman modes intensities, with abnormal increments for the most detected modes. According to the assignments of Raman modes, the Al-O vibration was enhanced to a greater extent than the Li-Al-O vibration, and the LiO4-AlO4 vibration gained a lesser enhancement. The discussion, including the factors of roughness, crystalline disorder and influence by Ga ions, attempts to explain the increments of Raman intensity.

Experimental investigation of effective thermal conductivity of packed lithium-titanate pebble bed with external heat source and flow of helium

Packed pebble bed of ceramic solid breeder materials viz. lithium titanate or meta-titanate (Li2TiO3), lithium orthosilicate (Li4SiO4) etc. is being considered for fusion blankets. During the breeding of tritium, helium will be produced and also it is proposed to flow helium from outside through the bed to extract tritium, as well as to remove thermal energy from the bed. Experimental determination of thermal property data of the pebble bed under helium gas flow is important for the blanket design. Such study of determination of effective thermal conductivity with flowing helium condition was not performed previously. Though, it is known that with an increase in gas flow rate, the effective thermal conductivity of pebble bed increases, it is necessary to conduct experiments to know the exact values and the degree of variation. Model and experimental setup used in our previous study was used to for this present work. Effects of process parameters viz., helium gas flow rate, bed temperature, pebble size etc. on the effective thermal conductivity of the bed have been studied. Correlations have been developed from these experimental data to estimate effective thermal conductivity at different particle Reynold’s number and bed temperature. It was found that with the use of helium, the effective thermal conductivity of bed increases by many folds. Experimental details and results are discussed in this paper.

DEM-CFD simulation of purge gas flow in a solid breeder pebble bed

Solid tritium breeding blanket applying pebble bed concept is promising for fusion reactors. Tritium bred in the pebble bed is purged out by inert gas. The flow characteristics of the purge gas are important for the tritium transport from the solid breeder materials. In this study, a randomly packed pebble bed was generated by Discrete Element Method (DEM) and verified by radial porosity distribution. The flow parameters of the purge gas in channels were solved by Computational Fluid Dynamics (CFD) method. The results show that the normalized velocity magnitudes have the same damped oscillating patterns with radial porosity distribution. Besides, the bypass flow near the wall cannot be ignored in this model, and it has a slight increase with inlet velocity. Furthermore, higher purging efficiency becomes with higher inlet velocity and especially higher in near wall region.

Density functional study of lithium vacancy in Li4SiO4: Trapping of tritium and helium

Li4SiO4 is a solid breeder material that has important applications in future fusion reactors. The interaction of tritium/helium with lithium vacancy is investigated implementing pseudopotential plane wave method within density functional theory. Models of all types of lithium vacancies and vacancy−tritium/helium defect complexes are created. Possible tritium trap sites in lithium vacancies are examined and the formation energies, vacancy−tritium/helium interaction energies and electronic structures of the defects are calculated. The results indicate that the tritium atom trapped in the lithium vacancy bonds with one of the surrounding oxygen atoms. The formation energies of the vacancy−tritium complexes are in the range of 0.41–1.28 eV under oxygen-rich condition. The interaction energy calculation shows the lithium vacancy has strong tritium trapping capabilities. Moreover, the vacancy−helium complex formation energies are in the range of 1.97–3.43 eV under O-rich condition. The vacancy−helium interaction is relatively weaker, suggesting the helium atom may escape the lithium vacancy more easily.