Research progress of rare earth composite shielding materials

With the rapid development of nuclear technology,nuclear protection has received extensive attention.As an important strategic resource,rare earth plays an important role in the field of nuclear shielding materials.In this review,the shielding principles of rare earth materials are first introduced.According to the type of matrix,the characteristics and current research status of metal-based,inorganic nonmetallic-based and polymer-based rare earth shielding materials are reviewed.Meanwhile,the f...     

Dynamic Precipitation of Laves Phase in FeCrAl Alloy with Zr Addition

FeCrAl alloy is one of potential candidates for accident-tolerant-fuel (ATF)-cladding materials due to its excellent oxidation and corrosion resistance at accident temperature, combining good mechanical properties at service temperature. Alloying strategy is an important way for improving comprehensive properties of FeCrAl alloy through the precipitation of fine Laves phase. Zr alloying can stabilize the Laves phase due to its lower diffusion coefficient and solubility in body-centered-cubic ferrite matrix. Herein, it is found that Zr addition changes the dynamic precipitation features of Laves phase in FeCrAl alloy during high-temperature deformation, from only one type of Fe₂M (M = Nb, Mo, Ta) Laves phase to Fe₂Zr combining Fe₂M-type Laves phase. The Fe₂Zr-type Laves phase precipitates dynamically first, and the interface precipitates between which with ferrite matrix creates more nucleation sites for subsequent precipitation of Fe₂M Laves phase. The results can be possibly applied for alloy design and microstructure tailoring in series of FeCrAl alloys used for ATF cladding in the near future.     

Molecular dynamics studies of hydrogen diffusion in tungsten at elevated temperature: Concentration dependence and defect effects

Influence of hydrogen concentration and defects introduced by neutron irradiation on hydrogen diffusion in tungsten has been investigated by molecular dynamics simulation at elevated temperatures. Hydrogen diffusion is shown to be significantly restrained at high concentrations due to spontaneous formation of platelet-like hydrogen clusters. For neutron irradiation defects, self-interstitials, mono-vacancies and vacancy clusters are considered. By clustering and acting as dislocation loops, self-interstitials show considerable trapping effects on hydrogen, leading to the suppression of hydrogen effective diffusion and the change of diffusion model in which hydrogen mainly diffuses along dislocation lines instead of hopping between tetrahedral interstitial sites. Moreover, an equation connecting hydrogen diffusion parameters and the total length of dislocation loops is empirically established. Different influences of mono-vacancies and vacancy clusters on hydrogen diffusion have been carefully identified. With the same vacancy concentration, hydrogen diffusivity is lower with mono-vacancies than that with vacancy clusters because more isolated trapping sites are provided by mono-vacancies. This work is not only helpful for understanding the synergistic effects of neutron irradiation and plasma interaction, but also potentially applicable for larger scale simulations as input data.     

Mechanical and optical property assessment of irradiated SiC with displaced atoms

Explicit method to emulate neutron radiation effects on key ceramic materials is lacking, with most literatures stressing direct comparison between radiation parameters while underestimating the defect-property correlation. Herein, the evolutionary correlation between defects of displaced atoms and properties in 6H-SiC was systematically investigated, aiming to found the basis for performance assessment of neutron irradiated material. The point defect accumulation, defect cluster growth, and homogeneous crystalline-amorphous (c-a) transition were recognized, with different stages of Young’s modulus reduction and optical absorption enhancement accordingly triggered. Specifically, the relaxation process accompanying the c-a transition was believed to induce the final reduction stage of Young’s modulus and account for the conflict between existing research results. Average lattice disorder, regarded as the average density of displaced atoms, was proved to be the success reference in the property assessment approach, paving the way for the extension into high temperature radiation where more defect types coexist.     

Structural evolution and mechanical properties of Cansas-III SiC fibers after thermal treatment up to 1700 °C

The effects of thermal treatment on the Cansas-Ⅲ SiC fibers were investigated via heating at temperatures from 900 to 1700 ℃ for 1–5 h in argon atmosphere. The composition and morphology of the SiC fibers were characterized and the tensile strength of the SiC fiber bundles was analyzed via two-parameter Weibull distribution analysis. The results showed that the thermal treatment has negligible influence on the microstructure of the SiC fibers at temperatures ≤ 1100 ℃. At temperatures ≥ 1300 ℃, the surface of the fibers became rough with some visible particles. Particularly, at 1700 °C, numbers of holes appeared. With the increasing of heating temperature and holding time, the average tensile strength of the SiC fibers decreased gradually from 1.81 to 1.01 GPa. The decreasing of tensile strength can be attributed to the increase of critical defect sizes, grain growth and phase transformation (β→α) of SiC.     

In Steam Short-Time Oxidation Kinetics of FeCrAl Alloys

In order to investigate the oxidation process of FeCrAl alloy developed by NPIC under simulated LOCA conditions, three experimental groups of alloys were exposed to the steam atmosphere and heated to test temperature (550, 1000, and 1200 °C), respectively, and then maintained at the temperature for 4 hours. The oxidation kinetics of alloys were obtained with a high-precision synchronous thermal analyzer, and the oxide film was investigated by XPS, XRD, and SEM technologies. The results showed that the FeCrAl alloy still retains good oxidation resistance under 1200 °C steam atmosphere. The oxidation process of alloy at 1200 °C can be described into six stages.     

Microstructure response of Amosic-3 SiC/SiC composites under self-ion irradiation

Amosic-3 SiC/SiC composites were irradiated at 300 °C using 6 MeV Si ions to peak doses of 13 and 55 displacements per atom (dpa). The loss of amorphous carbon packets and the growth of SiC grains were simultaneously observed in Amosic-3 SiC fibers, using a combination of transmission electron microscopy (TEM) and Raman spectroscopy. A mechanism based on the grain growth theory was proposed to expound the relationship between the loss of carbon packets and the growth of SiC grains. Small and curved SiC grains can absorb surrounding carbon packets to grow themselves; at some point, these grains further grow at the expense of adjacent small SiC grains until their grain boundary became straight. TEM images were found to support the above mechanism.     

Influence of thermal effect on dislocation loop evolution in Fe+-irradiated CeO2 during in-situ annealing

Cerium dioxide (CeO₂) is widely used as a surrogate fuel for uranium dioxide (UO₂) because of their same cubic fluorite crystal structure, nearly identical lattice parameters and similar physical properties. The degree of irradiation damage is related not only to the irradiation dose but also to the thermal effect. The influence of the thermal effect on the microstructure of Fe⁺-irradiated CeO₂ was investigated using in-situ transmission electron microscopy analysis during annealing. Shrinkage of the dislocation loops was observed for the first time in the irradiated CeO₂ foil. The threshold size determining whether the dislocation loop shrank was found to be approximately 19 nm through experimental measurements. When the loop size was less than this value, the dislocation loop shrank or even disappeared as the annealing time increased. Moreover, the smaller the loop size, the larger the shrinkage. The shrinkage rate measured in the in-situ annealing experiment was approximately 0.08 nm/s, which was almost consistent with the theoretical calculation (about 0.11 nm/s). However, when the loop size was larger than 19 nm, the loop grew rapidly at the beginning of annealing and then remained stable.     

Influence of Cold-Rolling Reduction on Microstructure and Tensile Properties of Nuclear FeCrAl Alloy with Low Cr and Nb Contents

FeCrAl alloy is one of the most promising candidates as an accident-tolerant fuel (ATF) cladding material. Herein, the influence of cold-rolling (CR) reduction on microstructure and tensile properties of the as-annealed FeCrAl alloys, with low Cr and Nb contents, is systematically examined. With the increase in CR reduction, the grain size of FeCrAl alloy is obviously refined after annealing because the increase in stored deformation energy leads to enhanced recrystallization. However, the large CR reductions result in a severe mixed-grain microstructure, significantly reducing the uniform deformability of the FeCrAl alloy. The dislocation density of the as-annealed FeCrAl alloy decreases with the increase in CR reduction, except for the excessive CR reduction of 50%. Moreover, the Laves phases are crushed and dissolved during CR and annealing, as well as large amounts of refined Laves phases are found after large CR reductions. The pinning effect of the Laves phases can significantly improve the strength of FeCrAl alloy. Accordingly, the strengthening mechanisms of FeCrAl alloy consist of fine-grain strengthening, dislocation strengthening and precipitation strengthening. Finally, the FeCrAl alloy, with a CR reduction of 30%, achieves optimal tensile properties. This study can provide theoretical guidance for the industrial production of the FeCrAl alloy.