The effect of fission products Xe and Cs on the thermal conductivity of the U3Si2lattice: a first-principles study.

In the past decades, uranium silicide (U3Si2) as a promising accident tolerant fuel (ATF) has drawn considerable attention in the field of nuclear physics. In comparison with traditional nuclear fuel (UO2), the U3Si2has higher thermal conductivity and uranium density, thereby resulting in lower centerline temperatures and better fuel economy. However, during the nuclear fission reaction, some unexpected fission products, such as Xe and Cs, are released and form the defective states. In this study, we explore the influence of Xe and Cs on the thermal conductivity of the U3Si2lattice from 200 to 1500 K using density functional theory calculations combined with Boltzmann transport equation. Our results reveal that the lattice and electronic thermal conductivities of defective U3Si2are reduced at a constant temperature, as compared with that of ideal system, thus resulting in a decrease of the total thermal conductivity. In the case of Cs occupation at U1 site, the total thermal conductivity (4.42 W mK-1) is decreased by ∼56% at 300 K, as compared with the value of 9.99 W mK-1for ideal system. With U1 and Si sites being occupied by Xe, the total thermal conductivities (4.45 and 6.52 W mK-1) are decreased by ∼55% and 35% at 300 K, respectively. The presented results suggest that the U3Si2has potential as a promising ATF at high temperatures.

Risk and aversion coding in human habenula high gamma activity.

Neurons in the primate lateral habenula fire in response to punishments and are inhibited by rewards. Through its modulation of midbrain monoaminergic activity, the habenula is believed to play an important role in adaptive behavioural responses to punishment and underlie depressive symptoms and their alleviation with ketamine. However, its role in value-based decision-making in humans is poorly understood due to limitations with non-invasive imaging methods which measure metabolic, not neural, activity with poor temporal resolution. Here, we overcome these limitations to more closely bridge the gap between species by recording local field potentials directly from the habenula in 12 human patients receiving deep brain stimulation treatment for bipolar disorder (n = 4), chronic pain (n = 3), depression (n = 3) and schizophrenia (n = 2). This allowed us to record neural activity during value-based decision-making tasks involving monetary rewards and losses. High-frequency gamma (60-240 Hz) activity, a proxy for population-level spiking involved in cognitive computations, increased during the receipt of loss and decreased during receipt of reward. Furthermore, habenula high gamma also encoded risk during decision-making, being larger in amplitude for high compared to low risk. For both risk and aversion, differences between conditions peaked approximately between 400 and 750 ms after stimulus onset. The findings not only demonstrate homologies with the primate habenula but also extend its role to human decision-making, showing its temporal dynamics and suggesting revisions to current models. The findings suggest that habenula high gamma could be used to optimize real-time closed-loop deep brain stimulation treatment for mood disturbances and impulsivity in psychiatric disorders.

First-principles study of the stability and migration of Xe and Cs in U3Si.

In the past several years, the U3Si has been suggested as an alternative nuclear fuel for light water reactors due to its high uranium density and outstanding thermal conductivity. In order to gain fundamental insights into the behavior of fission products in U3Si, the trapping and migration behaviors of the fission products Xe and Cs in U3Si are investigated using density functional theory calculations in this work. UnderU-rich and Si-rich conditions, both the Xe and Cs atoms prefer to substitute for Si andUatoms, respectively. Besides, both Xe and Cs tend to migrate through the vacancy-mechanism. It is noticeable that Xe diffuses faster and forms Xe bubbles more easily than Cs, which is mainly caused by the weaker interaction between Xe and its surrounding atoms.

First-principles study of point defects in U3Si2: effects on the mechanical and electronic properties.

In recent years, U3Si2 has been proposed as an alternative nuclear fuel material to uranium dioxide (UO2) because of its intrinsically high uranium density and thermal conductivity. However, the operation environment in the nuclear reactor is complex and extreme, such as in-pile neutron irradiation, and thus it is necessary to explore the radiation response behavior of U3Si2 and the physical properties of its damaged states. In the present study, first-principles calculations based on density functional theory were carried out to investigate the mechanical and electronic properties of defective U3Si2. Our results showed that the defect stability in U3Si2, except its interstitial defects, is dependent on its chemical environment. When vacancy, antisite or interstitial defects are introduced into U3Si2, its elastic modulus are decreased and its ductility is enhanced. Although the presence of defects in U3Si2 does not change its metallic nature and the electron distribution in its Fermi level, their effect on the partial chemical bonding interaction is significant. This study suggests that under a radiation environment, the created defects in U3Si2 remarkably affect its mechanical and electronic properties.

First-principles study of fission products Xe and Cs behaviors in U3Si2.

In the past several decades, the U3Si2has received much attention for the development of accident tolerant fuel in light water reactors because of its superior thermal conductivity and higher uranium density. In this study, density functional theory calculations have been carried out to study the occupation and diffusion behaviors of fission products Xe and Cs in U3Si2. It is revealed that the occupation sites of Xe and Cs depend on the chemical environment, and both of Xe and Cs are favorable to substitute for U or Si sites. The diffusions of Xe and Cs in U3Si2are predicted to be via the vacancy mechanism and both of Xe and Cs form cluster easily. As compared with Cs, the Xe exhibits a smaller solubility, faster diffusion as well as stronger clustering tendency, which may cause larger bubble size for Xe than Cs under the same conditions in U3Si2. The differences in the diffusion behaviors between Xe and Cs mainly result from their different valence electronic configurations and different atomic radii.

Effect of irradiation-induced cascade mixing on spinodal decomposition in U-Nb and U-Zr alloys: a phase field study.

The spinodal decomposition of the γ-phase in U-Nb and U-Zr alloys under irradiation was investigated using the phase-field method coupled with micro-elasticity theory and rate dependent cascade mixing model. Microstructure evolutions of spinodal decomposition in U-Nb and U-Zr alloys were simulated by considering different initial compositions and dose rates. The volume fraction and composition distribution under different cascade mixing were presented. The simulation results show that the volume fractions and equilibrium composition of the (Nb,Zr)-rich γ2-phase and the rate of spinodal decomposition are influenced by the dose rate and initial alloy composition. The cascade mixing can drive Nb or Zr atoms back into solution until a new equilibrium state between local cascade mixing and spinodal decomposition is reached. The evolution analysis indicated that irradiation-induced cascade mixing acts in opposition to thermodynamically driven spontaneous spinodal decomposition, which can not only slow down the spinodal decomposition but also reduces the composition range of the miscibility gap.

A Density Functional Theory Study of the Hydrogen Absorption in High Entropy Alloy TiZrHfMoNb.

The high entropy alloy is promising for hydrogen storage, especially in regard to its adjustable hydrogen storage properties. Despite several experimental investigations, there still lacks a detailed atomic-level understanding of the hydrogenation process. In this study, based on first-principles calculations, the hydrogen behaviors and microstructural evolution in high entropy alloy TiZrHfMoNb during the hydrogen absorption are investigated systematically. At low hydrogen content, hydrogen atoms prefer to occupy the octahedral interstitial sites of the BCC phase, which is different from that in BCC pure metals; when the hydrogen content reaches 1.08 wt %, the BCC TiZrHfMoNb hydrides transform into FCC phase, and hydrogen atoms are more favorable to occupy the tetrahedral interstitial sites. Further radial distribution function (RDF) analysis indicates that the enhanced disorder of bonds and decreased lattice distortion of the metal structure destabilize the BCC TiZrHfMoNb hydride and eventually induce the BCC → FCC phase transformation, which is quite different from that in conventional alloys; the difference originates from the severe lattice distortion in high entropy alloy. The phonon spectra of different TiZrHfMoNb hydrides show that the hydride with a H/M ratio of 2 dynamically has a stable lattice, corresponding to a hydrogen storage capacity of 1.94 wt %. The present study demonstrates that the high entropy alloys have unique hydrogen absorption ability, which may advance the related experimental and theoretical studies.

Optimizing the thermoelectric transport properties of Bi2O2Se monolayer via biaxial strain.

Recent studies have shown that the two-dimensional semiconductor Bi2O2Se is a promising thermoelectric (TE) material, whereas its TE performance needs to be further improved. By using first-principles methods combined with semi-classical Boltzmann transport theory, we systemically investigate the effects of biaxial strain on the TE transport properties of Bi2O2Se monolayer. Under -2-2% strain, the maximum power factors of 3.63-3.79 and 1.43-1.79 mW m-1 K-2 are found for p-type and n-type doped Bi2O2Se monolayer, respectively. The figure of merit ZT of p-type doped Bi2O2Se monolayer is enhanced to 1.14 by 2% tensile strain at 300 K and it reaches as high as 4.22 at 800 K (as compared with the highest value of 1.42 for bulk Bi2O2Se at 800 K). This study demonstrates that the TE performance of Bi2O2Se can be significantly improved by application of tensile strain and the Bi2O2Se monolayer has great potential as a TE material.

A DFT Study of Hydrogen Storage in High-Entropy Alloy TiZrHfScMo.

In recent years, high-entropy alloys have been proposed as potential hydrogen storage materials. Despite a number of experimental efforts, there is a lack of theoretical understanding regarding the hydrogen absorption behavior of high-entropy alloys. In this work, the hydrogen storage properties of a new TiZrHfScMo high-entropy alloy are investigated. This material is synthesized successfully, and its structure is characterized as body-centered cubic. Based on density functional theory, the lattice constant, formation enthalpy, binding energy, and electronic properties of hydrogenated TiZrHfScMo are all calculated. The calculations reveal that the process of hydrogenation is an exothermic process, and the bonding between the hydrogen and metal elements are of covalent character. In the hydrogenated TiZrHfScMo, the Ti and Sc atoms lose electrons and Mo atoms gain electrons. As the H content increases, the bonding is weakened, and the and bonding are strengthened. Our calculations demonstrate that the TiZrHfScMo high-entropy alloy is a promising hydrogen storage material, and different alloy elements play different roles in the hydrogen absorption process.

First-Principles Study of Thermo-Physical Properties of Pu-Containing Gd2Zr2O7.

A density functional theory plus Hubbard U method is used to investigate how the incorporation of Pu waste into Gd2Zr2O7 pyrochlore influences its thermo-physical properties. It is found that immobilization of Pu at Gd-site of Gd2Zr2O7 has minor effects on the mechanical and thermal properties, whereas substitution of Pu for Zr-site results in remarkable influences on the structural parameters, elastic moduli, elastic isotropy, Debye temperature and electronic structure. The discrepancy in thermo-physical properties between Gd2-yPuyZr2O7 and Gd2Zr2-yPuyO7 may be a result of their different structural and electronic structures. This study provides a direct insight into the thermo-physical properties of Pu-containing Gd2Zr2O7, which will be important for further investigation of nuclear waste immobilization by pyrochlores.