First-principles calculations with the advanced SCAN functional were carried out to investigate the defect formation energies and migration barriers of specific neutral point defects in 6H-SiC including monovacancy, antisite, and self-interstitial defects, as well as the lattice swelling effect caused by them. Our calculation results show that the CSi antisite is the easiest formed point defect in the C-rich environment and the SiC antisite has the lowest formation energy in the Si-rich environment, while the ISi interstitial is the most difficult to be generated in both cases compared with other point defects. In addition, the VC monovacancy and the CSi antisite will cause the lattice shrinkage, while the VSi monovacancy, the SiC antisite, the IC and the ISi interstitials will induce the lattice swelling. Furthermore, the minimum migration barrier among these defects is about 0.97 eV for the IC interstitial, while the migration barrier of the ISi interstitials is about 1.7 eV and the migration barriers of other defects are greater than 3 eV, which indicates that the diffusion of C interstitials plays a major role in the recovery process of defective 6H-SiC crystals. Overall, our work provides systematic insights into the formation and recovery mechanisms of various point defects in 6H-SiC.

In this work, both the first principles calculations and molecular dynamics simulations are adopted to investigate the formation ability of various intrinsic point defects in 4H-SiC as well as the electronic band structure changes and the swelling effects caused by them. Our results demonstrate that the classical MEAM potential has an unsatisfactory performance on the defect formation energies as compared with the ab initio calculations based on the advanced SCAN functional, but provides a reasonable description on the volumetric swelling caused by single point defect. Both the SCAN and MEAM calculations indicate that the VC monovacancy and the CSi antisite defect will cause the lattice shrinkage, while the VSi monovacancy, the SiC antisite, the IC and the ISi interstitials will induce the volumetric swelling. Besides that, first principles calculations reveal that monovacancies and interstitials would induce defective electronic states near the Fermi level, while antisites have negligible influence on the electronic band structures as compared with the pristine case. Furthermore, the molecular statics relaxations are employed to explore the influence of point defect densities on the swelling behaviors of 4H-SiC, which reveals that the volume swelling ratios present good linear relationships with the increased density of the VC monovacancy, CSi antisite, SiC antisite and IC interstitial defects, but express complex non-linear tendencies with the VSi monovacancy and ISi interstitial defects. Therefore, our research deepens the understanding of the point defects’ effects on the electronic properties and swelling behaviors of 4H-SiC.

Effects of neutron irradiation on the microstructure and mechanical properties of CMRR (China Mianyang Research Reactor) structural material pure aluminum and 6061 alloy were studied by scanning/transmission electron microscopy (S/TEM) and tensile tests. The fast neutron (E > 0.1 MeV) fluences of 1.09–2.89 × 1022 n•cm−2 produced damage doses of 1.6–4.2 displacements per atom (dpa). Voids and faulted 1/3<111> dislocation loops were observed in both pure aluminum and 6061 alloy. In particular, voids tended to preferentially aggregate on coarse Cu-rich and fine Mg2Si precipitates which contributes to the higher irradiation tolerance in terms of void formation for 6061 alloy, suggesting that the uniformly distributed precipitates can act as trapping sites for defect clusters during irradiation and thus promote recombination. The size and density of Mg2Si precipitates showed a slight increase and Cu-rich precipitates emerged in 6061 alloy after neutron irradiation. Moreover, radiation-induced segregation was also detected along the grain boundaries. After tensile testing, the pure aluminum showed a lower yield strength and larger uniform elongation than 6061 alloy, where a moderate uniform elongation was still reserved after irradiation to 4.2dpa. The severely reduced ductility in 6061 alloy is supposed to be caused by the transmutation product silicon which contributes to the increase of dispersed multiple precipitates after irradiation. Our results indicate that precipitates especially Mg2Si play a decisive role in void suppression, but are not suitable in diminishing the radiation hardening.

The lattice structure and elastic properties of Dy2Hf2O7 at 0 K and 0-40 GPa are calculated by using firstprinciples methods. The results show that the calculated structural parameters are in good agreement with the experimental values. The elastic constants increase with the increase of pressure, and all the elastic constants satisfy the Born criterion, that is, the Dy2Hf2O7 crystal is kinetically stable within the range of the calculated conditions. In addition, the thermodynamic properties of Dy2Hf2O7 crystal at different temperatures (0-2000 K) and pressures (0-40 GPa) are calculated by using the quasi-harmonic Debye model, and the temperature-pressure dependence of thermodynamic properties, such as the heat capacity, coefficient of thermal expansion, and Debye temperatures, is obtained and compared with the experimental values. We also obtain the melting point and hardness of Dy2Hf2O7 compound by using empirical formulas in the literature.

随SPRR-300研究堆约30 a的长时间运行,位于活性区附近的石墨箱体经历了长期的中子辐照.在长期服役的石墨箱体上取样,研究了其热学、力学以及微观结构变化,并与商用IG110,NG-CT-10石墨进行了对比.研究结果表明,经长时间低剂量率的中子辐照后,SPRR-300堆内随堆辐照石墨的晶格中出现了明显的辐照损伤缺陷,这些缺陷主要为位错环、层错、孔洞和微裂纹等,并出现了一定程度的非晶化.这些辐照损伤缺陷直接或间接地引起了石墨热学、力学性能的变化,主要表现为热膨胀系数、热扩散系数、抗压强度和抗弯强度的下降以及弯曲弹性模量的上升.

The uranium zirconium (U-Zr) metallic nuclear fuel has long been considered as a promising candidate fuel type for fast breeder reactors. One of the key performance issues of the U-10Zr metallic fuel is its rather strong swelling, which would lead to significant fuel cladding mechanical interaction (FCMI) at relatively high fuel burn-ups. It is therefore imperative to understand the fuel swelling behaviors and the underlying physics mechanisms. Recently published experimental results of U-10Zr fuel swelling behaviors in a low temperature regime (400-600K) showed perceivable swelling of the in-pile irradiated fuel. However, the underlying physics mechanism behind this swelling is not clear. In this work, we performed in-depth analyses on the microstructural features of the irradiated fuel with metallography and conducted a finite-element based simulation on a computational platform, COMSOL, to determine the hydrostatic stress in the irradiated fuel. The spherical objects in the metallography result have been identified as cavities with relatively low fission gas pressure instead of equilibrium gas bubbles. A rate theory code based on the cavitational void swelling model has been developed and the fuel swelling behaviors of U-10Zr fuel in the low temperature regime have been assessed. The simulation results suggest a much lower migration energy for vacancy diffusion in the metallic fuel compared to what is currently used, and a new set of key parameters for the rate theory model were determined.

The structure, electronic, elasticity and thermodynamic properties, phonon spectra of Be12V alloy under high temperatures and pressure are investigated by using first-principles calculations based on pseudo-potential planewave density functional theory method within using the generalized gradient approximation (GGA) and quasiharmonic Debye model. The results show that the calculated equilibrium structural parameters of Be12V are in good agreement with the experiments. Under high pressure, the elastic constant of Be12V alloy increases with the increase of pressure. The all elastic constants satisfy the Born's mechanical criterion, indicating that Be12V crystal is dynamic stable under the applied pressure (0 GPa similar to 50 GPa), and the pressure has little effect on the electronic properties of Be12V. The phonon spectra and phonon density of states under pressure from 0 GPa to 50 GPa are presented for the Be12V and discussed the effects of pressure on some properties. By the quasi-harmonic Debye model, the thermodynamic properties of Be12V under high temperature and pressure were predicted, and the relationship of heat capacity, thermal expansion coefficient with temperature and pressure was obtained in the range of 0-1300 K and 0-20 GPa, respectively.

The elastic and thermodynamic properties of tetragonal Be12Ti under high temperature and pressure are investigated by first-principles calculations based on pseudopotential plane-wave density functional theory (DFT) within the generalized gradient approximation (GGA) and quasi-harmonic approximation (QHA). The calculated lattice parameters and bulk modulus are in good agreement with the available experimental data. The calculated elastic constants of Be12Ti increase monotonously with increasing pressure, and the elastic stability criterion and the phonon dispersion calculation show that the Be12Ti crystal satisfies the mechanical and dynamic stability under applied pressure (0-100 GPa). The related mechanical properties such as bulk modulus (B), shear modulus (G), Young's modulus (E), and Poisson's ratio (ν) are also studied for polycrystalline of Be12Ti; the calculated B/G value shows that Be12Ti behaves in a brittle manner, and higher pressure can significantly improve the brittleness of Be12Ti. The elastic anisotropy is demonstrated by the elastic anisotropy factors. The direction-dependent Young's modulus and bulk modulus of Be12Ti are dealt with in detail under pressure from 0 GPa to 100 GPa. The pressure and temperature dependencies of the relative volume, the bulk modulus, the elastic constants, the heat capacity and the thermal expansion coefficient, as well as the entropy are obtained and discussed using the quasi-harmonic approximation in the ranges of temperature 0-1600 K and pressure 0-100 GPa.

High-energy (up to 80 keV) displacement cascades in hcp-Zr are studied by classical Molecular Dynamics (MD) simulations. The statistics of defect production are reported and the formation of subcascades and defect clusters are analyzed. We find that the probability of subcascade formation increases along with the incident energy, and already reaches 100% at 80 keV. Our simulations also reveal that high-energy cascades could significantly promote the formation of defect clusters, sometimes even directly create experimental-scale (around 3 nm) vacancy clusters. Our research provides basic knowledge of highenergy displacement cascades in hcp-Zr, and offers a possible explanation for the low-dose irradiation experiments of Zr-based alloys. (C) 2018 Elsevier B.V. All rights reserved.

Understanding the mechanical properties of structural materials under an irradiation environment stands as a major challenge for developing advanced nuclear systems. In this paper, the mechanical properties, including the Young's modulus, yield stress, and hardness, of SiC samples irradiated by neutron and heavy ions (3 MeV C and Si ions) have been investigated by nanoindentation measurement. When the load-depth curves were analyzed by the widely used Oliver-Pharr method, for the samples irradiated at the same displacement damage level (0.1 and 0.2 dpa), both the elastic modulus and hardness of the ion irradiated SiC are significantly higher compared with those of neutron irradiated samples. The discrepancy is mainly attributed to the irradiation induced surface lateral stress in ion irradiated samples, which cannot be taken into account for the Oliver-Pharr method. After carefully considering this effect by the finite element method in simulating the load-depth curves, both the Young's modulus and yield stress of ion irradiated samples agree well with those of neutron irradiated samples. This study reveals that by the combined method of nanoindentation and finite element, the mechanical properties, including the Young's modulus, yield stress and hardness, for neutron irradiated SiC can be reasonably evaluated by MeV heavy ion irradiation.

Transmission electron microscopy (TEM) and Secondary Ion Mass Spectroscopy (SIMS) were used to investigate the microstructural and chemical evolution in a zirconium alloy processed by high pressure torsion (HPT) under 4 GPa at room temperature. Results showed that the grain sizes were significantly reduced and no alpha to omega phase transformation was detected after the HPT processing. A new phenomenon of which the Cr atoms diffused from the second phase particles (SPPs) to the zirconium matrix was confirmed. Further TEM analysis indicated that a large number of dislocations were generated especially in the vicinity of the SPPs. The possible mechanisms for the absence of the a to to phase transformation and the Cr depletion of the SPPs were discussed. (C) 2017 Elsevier B.V. All rights reserved.

A multiscale sequence of simulation should be used to predict properties of materials under irradiation. Binary collision theory and molecular dynamics (MDs) method are commonly used to characterize the displacement cascades induced by neutrons in a material. In order to reduce the clock time spent for the MD simulation of damages induced by high-energy primary knock-on atoms (PKAs), the damage zones were split into sub-cascade according to the sub-cascade formation criteria. Two well-known codes, Geant4 and TRIM, were used to simulate high-energy PKA-induced cascades in B4C and then produce the secondary knock-on atom (SKA) energy spectrum. It has been found that both high-energy primary knock-on B and C atoms move a long range in the boron carbide. These atoms produce sub-cascades at the tip of trajectory. The energy received by most of the SKAs is <10keV, which can be used as input to reduce the clock time spent for MD simulation.

Irradiation effects of neutron and 3 MeV C+, Si+ in 6H-SiC were investigated by Raman spectroscopy and high-resolution XRD. The total disorder values of neutron irradiated SiC agree well with that of samples irradiated by ions at the same doses respectively. On the other hand, high-resolution XRD results shows that the lattice strain rate caused by neutron irradiation is 6.8%/dpa, while it is only 2.6%/dpa and 4.2%/dpa for Si+ and C+ irradiations respectively. Our results illustrate that the total disorder in neutron irradiated SiC can be accurately simulated by MeV Si+ or C+ irradiations at the same dose, but for the lattice strain and strain-related properties like surface hardness, the depth profile of irradiation damages induced by energetic ions must be considered. This research will contribute to a better understanding of the difference in irradiation effects between neutron and heavy ions. (C) 2016 Elsevier B.V. All rights reserved.

Ultra-cold liquid hydrogen was used as neutron moderator at CMRR cold neutron source .Low tem-perature helium gas worked as heat transmission medium for hydrogen .In this paper , low reactor power would affect hydrogen liquefaction process or not was investigated experimentally .Liquid hydrogen vaporization process parameters characteristics at different reactor conditions were demonstrated .What's more, liquid hydrogen spon-taneous evaporation process were tested after different reactor power shutdown also .The results have shown that hydrogen liquefaction process was more smoothly than the vaporization process .Hydrogen pressure jump ap-peared occasionally in the process of vaporization .High strength heat radiation would accelerate the evaporation of liquid hydrogen .

The effects of pressure on the structural and elastic properties of Be12Ti were investigated by the generalized gradient approximation (GGA) with a Perdew–Burke–Ernzerhof (PBE) exchange-correlation function using density-functional theory.

Studies on thermodynamic and elastic properties of hexagonal ZnO (wurtzite structure) under high temperature have not been reported usually from no matter experimental or theoretic methods. In this work, we study these properties by ab-initio together with quasi-harmonic Debye model. The value of Cv tends to the Petit and Dulong limit at high temperature under any pressure, 49.73 J/mol K. And Cv is greatly limited by pressure at intermediate temperatures. Nevertheless, the limit effect on Cv caused by pressure is not obvious under low as well as very high temperature. The thermal expansions along a or c axis are almost same under temperature, which increase with temperature like a parabola. C11, C33, C12 and C13 decrease with temperature a little, which means that mechanics properties are weakened respectively. Crown Copyright (C) 2014 Published by Elsevier B.V. All rights reserved.

The elastic and thermodynamic properties of ZrMo2 under high temperature and pressure are investigated by first-principles calculations based on pseudopotential plane-wave density functional theory (DFT) within the generalized gradient approximation (GGA) and quasi-harmonic Debye model. The calculated lattice parameters are in good agreement with the available experimental data. The calculated elastic constants of ZrMo2 increase monotonically with increasing pressure, and the relationship between the elastic constants and pressure show that ZrMo2 satisfies the mechanical stability criteria under applied pressure (0-65 GPa). The related mechanical properties such as bulk modulus (B), shear modulus (G), Young's modulus (E), and Poisson's ratio ( v) are also studied for polycrystalline of ZrMo2. The calculated BIG value shows that ZrMo2 behaves in a ductile manner, and higher pressure can significantly improve the ductility of ZrMo2. The pressure and temperature dependencies of the relative volume, the bulk modulus, the elastic constants, the heat capacity and the thermal expansion coefficient, as well as the Gruneisen parameters are obtained and discussed by the quasi-harmonic Debye model in the ranges of 0-1800 K and 0-65 GPa. (C) 2014 Elsevier B.V. All rights reserved.

14.5K cryogenic Helium is the refrigerating medium of the Cold Neutron Source ( CNS) Facility.Hy-drogen is liquefied from normal atmospheric temperature by the operation of CNS Refrigeration Cryogenic System (RCS).Liquid hydrogen overflows the Moderator Chamber .Thermal neutrons around liquid hydrogen deliver their energy to liquid hydrogen and then they become cold neutrons .Cold neutrons travel along the neutron guide tube and then arrive to the neutron spectrometers in the scattering hall .Stably operating of all CNS sub-systems is the base for obtaining cold neutron .And RCS debugging is the most complex one in all works .Trou-ble and problem solving during the commissioning operation of RCS is the primary coverage of this paper .

NbH2 hydride is an important material in hydrogen storage materials. However, until now there have been no experimental and theoretical data on the elastic and thermodynamic properties. The structure and thermodynamic properties of cubic-NbH2 under high temperatures and pressure are investigated by first-principles study based on the pseudo-potential plane-wave density functional theory method using the generalized gradient approximation and quasi-harmonic Debye model. The results show that the calculated structural parameters of NbH2 are in good agreement with the available experimental results and other theoretical data. The obtained elastic constants satisfy the requirement for mechanical stability, indicating that the NbH2 crystal is stable in the investigated pressure and temperature ranges. Through the quasi-harmonic Debye model, in which the phononic effects are considered, the thermodynamic properties of NbH2, such as the thermal expansion coefficient and the heat capacity dependence of temperature and pressure in the ranges 0–1100 K and 0–70 GPa, are also obtained, respectively.

Structure and elastic property of ZrV2 under high pressure are investigated with first-principles calculations based on plane-wave pseudo-potetial in the framework of density functional theory within generalized gradient approximation(GGA).With a quasi-harmonic Debye model,in which phonon effects are considered,we calculated thermodynamic properties of ZrV2 in a pressure range from 0 to 20 GPa and temperature range from 0 to 1 200 K.Pressure dependence of elastic constants,bulk modulus and heat capacity,and thermal expansion with pressure and temperature are presented.It shows that calculated lattice parameters of ZrV2 are in good agreement with existing experimental data and other theoretical results.Elastic constants,Debye temperature and bulk modulus increases with increasing pressure.Relative volume,heat capacity decreases with increasing pressure.Temperature effect is weaker than pressure effect in thermal expansion of ZrV2 under high pressures.