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A quantitative analysis of an in situ Xe ion implantation experiment on single crystal molybdenum via a molecular dynamics informed kinetic rate theory simulation

期刊: JOURNAL OF NUCLEAR MATERIALS, 2020; 540 ()

Kinetic rate theory is a mature method that has long been used to model fission gas behaviors in nuclear fuels. However, uncertainties remaining in the key parameters of the kinetic rate theory models often lead to doubts in the accuracy of this method. In this work, the results of an in situ Xe ion implantation experiment at the IVEM facility were interpreted via a Molecular Dynamics (MD) informed kinetic rate theory simulation. The complexity of the rate theory model is significantly reduced according to some key experimental information. The MD method was used to supply the irradiation-enhanced Xe diffusion coefficient to the rate theory model. The bubble nucleation factor and the bubble resolution coefficient were also determined. A parametric study was performed to gauge the sensitivity of the calculation results to the distribution of the irradiation-enhanced Xe diffusion coefficient, the value of the bubble nucleation factor, and the resolution coefficient. It was shown that the calculated bubble size distribution is highly sensitive to all these parameters. (C) 2020 Elsevier B.V. All rights reserved.

Morphological survey of U-2Nb alloy under isothermal transformation

期刊: JOURNAL OF NUCLEAR MATERIALS, 2020; 540 ()

The morphology of isothermally transformed products of U-2Nb alloy in the early stage of phase transformation was systematically investigated in the temperature range of 450-635 degrees C. Two kinds of products were identified corresponding to different transformation mechanisms. The first kind is acicular-like under Light Optical Microscopy (LOM). Careful examination demonstrated that each acicular structure contains many parallel units in lath or plate shape. Such structure has transformed through monotectoid reaction, which occurs at temperature between 550 and 635 degrees C in current work. Starting from 550 degrees C, isothermal holding at lower temperature results in another kind of structure, which is blocky or nodular-like under LOM. Higher magnification reveals that the structure is lamellar. Nucleation and growth characteristics of these two kinds of structure were discussed based on observations in morphological length scale. Similarities with acicular structures observed in other well understood alloys, e.g. Widmanstatten and bainitic ferrite in Fe-C system, were identified. On the other hand, the lamellar structure shows similarities to products transformed through discontinuous precipitation as has been well established in many binary systems with substitutional solute, e.g. Al-Zn system. (C) 2020 Published by Elsevier B.V.

Theoretical investigations on structural and thermo-mechanical properties of layered ternary carbide Th-Al-C systems

期刊: JOURNAL OF NUCLEAR MATERIALS, 2020; 540 ()

With the concept of accident tolerant fuel systems being proposed, the new generation of nuclear fuels have attracted wide attention. In this work, a novel type of layered ternary carbides Th-Al-C are found by first-principles calculations via the density functional theory and the structures, electronic structures, thermal and mechanical properties are predicted. The calculation results show that all the Th-Al-C materials inspected are elastically and dynamically stable. From the decomposition reaction analysis, the ThAl3C3 phase is thermodynamically stable and Th2Al3C4 might also be obtained in the laboratory. These compounds have good elastic moduli and thermal properties. Th2AlC is a ductile material whereas others are brittle. The shear modulus and Debye temperature of Th-Al-C series decreases as the thickness of Th -C or Al-C slabs increases. The lattice thermal conductivity of the Th-Al-C system increases in the sequence of Th3Al4C6, Th2AlC, Th2Al3C5, Th2Al4C5, Th3Al3C5, Th4Al3C6, Th2Al3C4 and ThAl3C3. Moreover, both the ThAl3C3 and Th2Al3C4 not only have satisfactory mechanical properties, but also exhibit excellent thermal properties, which are comparable to those of Zr/Hf-Al-C compounds. The current theoretical investigations may promote the exploration of the novel layered ternary actinide carbides and may provide a new clue for finding high performance nuclear fuels. (C) 2020 Published by Elsevier B.V.

Microwave-sintering preparation, phase evolution and chemical stability of Na1-2xSrxZr2(PO4)(3) ceramics for immobilizing simulated radionuclides

期刊: JOURNAL OF NUCLEAR MATERIALS, 2020; 540 ()

Sodium zirconium phosphate (NaZr2(PO4)(3), NZP) family ceramics were considered as a potential host matrix for the immobilization of nuclear waste. In this study, the dense Sr-incorporated NZP ceramics with the formula of Na1-2xSrxZr2(PO4)(3) (x = 0, 0.1, 0.2, 0.25, 0.3, 0.4, and 0.5) were fabricated by microwave-assisted solid-state reaction method. The phase evolution, densification behavior and chemical stability of the as-prepared samples were systematically investigated by XRD, Rietveld refinement, Raman, SEM-EDS and PCT method. The results demonstrated that Na in the NZP lattice could be substituted by the simulated fission product Sr, and the phase composition gradually changed from NZP to Sr0.5Zr2(PO4)(3) (SrZP) with the increase of Sr content. The samples presented high densification and uniform elements distribution after microwave sintering at 1100 degrees C for 2 h. Moreover, the as-prepared samples exhibited the superior chemical stability. The normalized elemental leaching rate of Sr, Na, P, and Zr were 10(-3), 10(-4), 10(-3), and <10(-7) g.m(-2).d(-1), respectively. (C) 2020 Elsevier B.V. All rights reserved.

Evolution of microstructure and texture of moderately warm-rolled pure tungsten during annealing at 1300 degrees C

期刊: JOURNAL OF NUCLEAR MATERIALS, 2020; 540 ()

The mechanical behavior, microstructure and texture evolution were investigated during isothermal annealing at 1300 degrees C of pure tungsten moderately warm-rolled to 67% thickness reduction. The degradation of the mechanical properties is characterized by hardness testing. The microstructure and texture evolution during heat treatment were characterized by Electron Backscatter Diffraction. During annealing of the moderately warm-rolled tungsten, recrystallization occurred first, quickly followed by relatively slow grain growth. The recrystallized volume fractions determined from hardness measurements and microstructural characterization were essentially the same. The evolution of the grain sizes during recrystallization was analyzed independently for deformed and recrystallized grains. Quantitative texture analysis showed that the overall texture strength is enhanced after recrystallization. As recrystallization strongly affects the mechanical properties of tungsten, such insights in the annealing behavior of warm-rolled tungsten plates are valuable for an understanding of their performance as potential plasma-facing materials in future fusion reactors. (C) 2020 Elsevier B.V. All rights reserved.

In-situ studies on the mechanical properties of He ion irradiated nanotwinned Ag

期刊: JOURNAL OF NUCLEAR MATERIALS, 2020; 540 ()

Recent studies show that twin boundaries are effective defect sinks in eliminating radiation-induced defects. However, the influence of radiation-induced defects on the mechanical behavior of nano-twinned (NT) materials is less well understood. In this study, we investigate the mechanical properties of He ion irradiated NT Ag. In-situ micropillar compression tests show that deformation induces prominent detwinning in as-deposited NT Ag, whereas He ion irradiation alleviates detwinning during deformation of NT Ag. The radiation hardening mechanism and the influence of He bubbles on the deformation behavior of NT Ag are discussed. (C) 2020 Elsevier B.V. All rights reserved.

An AIMD plus U simulation of low-energy displacement events in UO2

期刊: JOURNAL OF NUCLEAR MATERIALS, 2020; 540 ()

In UO2, due to the existence of U 5f electrons, the strong-correlation effects are significant, and correction of on-site Coulomb interaction is necessary. For strongly-correlated electronic system like UO2, however, it is a challenge to consider such correction during simulation of their dynamic processes under radiation. In this study, a new method, i.e., AIMD + U, in which ab intro molecular dynamics method and correction of on-site Coulomb interaction between U 5f electrons are combined, is employed to investigate the response of UO2 to low energy radiation. The threshold displacement energies for O and U atoms have been determined, and defect creation and distribution have been provided. New mechanism for defect generation, and novel defect types, including uranium (oxygen) dumbbell interstitials and uranium antisite defects, are revealed from this study. (C) 2020 Elsevier B.V. All rights reserved.

B2O3 -assisted low-temperature crystallization of pollucite structures and their potential applications in Cs+ immobilization

期刊: JOURNAL OF NUCLEAR MATERIALS, 2020; 540 ()

Pollucite structures present remarkable chemical stability and good corrosion resistance against the leaching medium, which renders them promising for the permanent disposal of radioactive Cs-137. However, it is still challenging to prepare pollucite with dense structures and high immobilization performance at low temperature so far. Additionally, the fundamental understanding of the mechanism underlying the leaching behaviors of Cs+ from pollucite remains largely unexplored. In this paper, we realize the low-temperature production of pollucite by tailoring the nature of geopolymer precursors and employing B2O3 additives. It has been shown that the introduction of B2O3 not only significantly reduces the crystallization temperature of pollucite, but also improves the immobilization performance of Cs through encapsulation effect. Differential scanning calorimetry and X-ray diffraction confirm the formation of crystalline pollucite from amorphous geopolymer at similar to 700 degrees C in the presence of 7.5 wt% B2O3. By further increasing the treatment temperature to 1000 degrees C, pollucite grains can be well-encapsulated by glass phase, exhibiting leaching rates similar to 2 order lower than their unencapsulated counterparts. Moreover, distinct leaching mechanisms that govern the leaching behaviors of Cs+ from specimens being treated at different temperatures are unveiled. We anticipate that the proposed scheme facilitates the low-temperature production of high-quality pollucite structures with promising applications in the immobilization of hazardous wastes. (C) 2020 Elsevier B.V. All rights reserved.

Age-hardening mechanisms of 15Cr ODS ferritic steels with 5, 7 and 9 wt.% Al at 475 degrees C for 9000 h

期刊: JOURNAL OF NUCLEAR MATERIALS, 2020; 540 ()

Thermal ageing behaviors of 15 wt.% Cr-oxide dispersion strengthened (ODS) steels with the contents of Al of 5, 7 and 9 wt.%, which were isothermally aged at 475 degrees C for 9000 h, have been investigated by Vickers hardness measurement, tensile test and atom probe tomography (APT) to correlate the age-hardening with the evolution of nanometer-scale structures for understanding the age-hardening mechanisms. The age-hardening of 15Cr ODS steel is attributed to alpha-alpha' phase separation. The age-hardening of 15Cr-5Al ODS steel is considerably higher, relative to 15Cr, 15Cr-7Al and 15Cr-9Al ODS steels, which is due to the strengthening by both Cr-enriched alpha' phases and (Ti, Al)-enriched beta' phases formed in 15Cr-5Al ODS steel. In 15Cr-7Al and 15Cr-9Al ODS steels, however, the age-hardening is only attributed to the beta' phase strengthening because alpha-alpha' phase separation is completely suppressed in the ODS steels when the content of Al is >= 7 wt.%. In all the Al-added ODS steels, fairly large amounts of beta' phases with crystallography closely related to Heusler-type Fe2AlTi precipitated during the thermal ageing. The strengthening by the beta' phases becomes more significant with the increasing of Al content from 5 wt.% to 9 wt.%. Core/shell structured oxide particles were observed only when alpha-alpha' phase separation occurred, indicating that the formation of such core/shell structure might be closely associated with the process of alpha-alpha' phase separation. The age-hardening mechanisms of the ODS steels with different Al-contents were proposed based on the dislocation barrier models where the amounts of age-hardening were correlated with the nanostructures characterized by APT. The theoretically predicted values of the age-hardening of the FeCrAl-ODS steels agreed well with the values experimentally determined. (C) 2020 Elsevier B.V. All rights reserved.

Void-shape dependent hardening model in irradiated face-centered-cubic metals

期刊: JOURNAL OF NUCLEAR MATERIALS, 2020; 540 ()

The shape of irradiation-induced void at high temperatures and irradiation doses in metal materials is usually the complex-shaped polyhedron, to strongly affect the mechanical properties and service performances. Here, considering the three-dimensional geometric configuration of polyhedron void, we develop a model to calculate critical resolved shear stress for the interaction between dislocation and polyhedral void. This model can be used into the crystal plasticity theoretical framework, to predict the void-shape-dependent hardening behavior under different irradiation doses, times, and temperatures in the irradiated face-centered-cubic metals. The effect of the spatial interaction between polyhedron void and sliding dislocation has been considered by a probability-dependent void-strengthening mechanism, for the accurate prediction of the yield stress. The feasibility is verified by comparing the experimental data of the irradiated polycrystalline Cu and irradiated 304 stainless steel with the numerical result calculated from the current model. The contribution of cubic void on hardening is higher than that of octahedral and cuboctahedral voids, owing to the difference of the cross-sectional shape caused by the dislocation-bypassing-void process, where the triangle section provides a higher critical resolved shear stress than the hexagon section. The hardening contribution of the cubic void increases firstly, and then decreases with the increasing void volume fraction, revealing the hardening-to-softening transformation. In the irradiated polycrystalline Ni, the yield stress increases with increasing radiation dose and time, and decreases with increasing irradiation temperature, to provide the accurate irradiation hardening predictions. The current model establishes a theoretical approach to capture the hardening behavior in the irradiated metals containing polyhedron void, for predicting the yield strength of metal materials under the changeable irradiated environments. (C) 2020 Elsevier B.V. All rights reserved.

The stability of gamma ' precipitates in a multi-component FeCoNiCrTi0.2 alloy under elevated-temperature irradiation

期刊: JOURNAL OF NUCLEAR MATERIALS, 2020; 540 ()

Precipitation hardening by gamma' precipitates is an effective strengthening mechanism for designing novel high-entropy alloys (HEAs). The stability of these gamma' precipitates under elevated temperature irradiation is a major concern for their application in nuclear industry. In the present study, a gamma'-strengthened FeCoNiCrTi0.2 HEA is irradiated using 6.4 MeV Fe3+ ions at 400 degrees C, 500 degrees C and 600 degrees C, respectively. After irradiation, transmission electron microscopy (TEM) and atom probe tomography (APT) has characterized the structural and compositional stability of the gamma' precipitates. The results show that the ordered L1(2) structure of gamma' precipitate is susceptible to the radiation damage, whereas their basic compositional features can sustain up to similar to 13 dpa at these temperatures. APT reveals that the dissolution of gamma' precipitates start from the surface of the precipitates at the early stage of irradiation. Dissolution and disordering of the precipitates are dependent on the irradiation dose and temperature. APT also shows that small Ni and Ti-rich clusters form after irradiation at elevated temperatures. The behavior of gamma' precipitates under irradiation is believed to be controlled by two competing factors, displacement damage and radiation enhanced diffusion. Both can influence the radiation induced disordering/dissolution and rep recipitation. (C) 2020 Published by Elsevier B.V.

Irradiation hardening of pure vanadium and vanadium alloys due to helium ion implantation and displacement damage

期刊: JOURNAL OF NUCLEAR MATERIALS, 2020; 540 ()

Pure V and V-5Cr-5Ti were irradiated by He and high-energy heavy ions to various doses. Nano-indentation was used to characterize the hardness change induced by ion irradiation. Under high-energy heavy ions irradiation, the hardness of pure V increased rapidly and saturated at very low damage level (0.03 dpa), the hardness of V-5Cr-5Ti increased slowly and tended to saturate at the damage higher than 0.4 dpa. In contrast to high-energy heavy-ion irradiation, more significant hardening occurred in both pure V and V-5Cr-5Ti under He ion irradiation. The significant hardening induced by He ion irradiation at the low dose was probably from the coarsening of dislocation loops due to the existence of He atoms in materials. TEM results revealed that He bubbles with high density have been formed in V-5Cr-5Ti at the high dose. The estimation based on the dispersion barrier hardening model suggested that He bubbles exhibited as weak obstacles due to the limitation of size, the major contribution to hardening was considered to be from dislocation loops. (C) 2020 Elsevier B.V. All rights reserved.

Modelling of deuterium retention and outgassing in self-damaged tungsten under low-energy atomic D flux irradiation: The effects of surface processes

期刊: JOURNAL OF NUCLEAR MATERIALS, 2020; 540 ()

Deuterium (D) outgassing and retention in self-damaged tungsten (W) during low-energy D atom irradiation are systematically studied using an upgraded version of the Hydrogen Isotope Inventory Processes Code (HIIPC) with the implementation of a surface module. The surface module due to the existence of chemisorption site is mainly characterized by the surface-energy barriers of D adsorption and absorption. The simulated total D amount is in good agreement with the experimental measurement. The impact of surface dynamics on the surface retention and total bulk retention is investigated. The simulation shows that total bulk retention first increases and then decreases with the increment of material temperature (T) due to surface effects. With the increment of surface energy barriers, the saturation of surface retention is observed. With low energy barrier, surface process shows a dominant role in bulk D retention. The effect of surface barriers on D outgassing is also studied. The results suggest that variations of surface energy barriers have a significant influence on D outgassing, which is linked to fuel recycling. Simulation reveals that fuel recycling greatly depends on T and irradiation flux before surface saturation, and total bulk retention shows a remarkable dependence on the permeation barrier. Saturation of total bulk retention is observed with low permeation barrier, whereas high permeation barrier could decrease the uptake of D due to the buildup of strong inhibition on surface, thus remarkably reduce total bulk retention. (C) 2020 Elsevier B.V. All rights reserved.

Effects of carbon doping on irradiation resistance of Fe38Mn40Ni11Al4Cr7 high entropy alloys

期刊: JOURNAL OF NUCLEAR MATERIALS, 2020; 540 ()

High-entropy alloys (HEAs) have become newly emerging candidates as structural materials of advanced fission reactor because of their excellent mechanical properties and irradiation resistance. Recently, carbon doped HEAs exhibited improved mechanical properties, such as yield strength and elongation. However, the effects of carbon doping on the irradiation resistance of HEAs need further investigation. Here, the irradiation-induced defects and irradiation hardening of Fe38Mn40Ni11Al4Cr7 HEA with different carbon contents were investigated by using 5 MeV Xe23+ heavy-ion irradiation at room temperature, and multiple characterization methods were used to provide the essential evidences. Results showed that the carbon doped samples exhibited smaller-sized dislocation loops and significantly lower hardening rate than those of undoped samples. The reason is attributed to two aspects: Firstly, interstitial carbon would significantly increase lattice distortion and migration energy of self-interstitial atoms, thereby inhibiting the formation of defects. Secondly, carbon atoms would act as obstacles that hindered the evolution of defects. Consequently, our study indicated the potential of using carbon doped HEAs as irradiation-resistant materials. (C) 2020 Elsevier B.V. All rights reserved.

Impact of microstructural properties on hardness of tungsten heavy alloy evaluated by stand-off LIBS after PSI plasma irradiation

期刊: JOURNAL OF NUCLEAR MATERIALS, 2020; 540 ()

The key issue in tokamak is change of physical properties of plasma facing materials, such as tungsten due to extreme conditions and long pulse operation. Practically, it is difficult to analyze hardness changes online through conventional methods. Hence, an online monitoring technique is much desired which measures hardness changes in fusion devices. In this study, laser induced breakdown spectroscopy (LIBS) is used as in-situ monitoring tool to measure hardness of tungsten heavy alloy (97W-2Ni-1Fe) samples after exposure to different irradiation of plasma ranging from 0.108 to 1.00 MW/m(2). The exposure of PSI plasma beam is a key factor which give rise to pronounced change in crystallographic estimation, microstructural properties and surface hardness. The X-ray diffraction study revealed modification in the structure caused, by variation in the peak intensity, crystal size, dislocation line density and microstrains upon plasma irradiation. The change in crystallographic measurements and surface hardness are well correlated with irradiated PSI power densities. Moreover, the hardness influence on LIBS intensity of tungsten heavy alloy (WHA) was carried out. The LIBS calibration curves were formed by utilizing the ratio of ionic to atomic line intensities of tungsten versus the Vickers hardness. The ratio of ionic to atomic spectral emission line intensity and plasma electron temperature (T-e), significantly increases with hardness. The obtained Pearson correlation coefficient (R-2) values (in case of calibration curve and plasma temperature) in measuring hardness of all investigated samples have good approximation which indicate the preciseness of LIBS approach. The achieved results in the present study by in-situ LIBS system confirmed its potential ability to estimate the hardness of WHA more efficient than other conventional method. (C) 2020 Elsevier B.V. All rights reserved.

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