查看更多>>摘要：To better understand the mechanisms of thermally aged and embrittled reactor pressure vessel steels, the hardening behavior and microstructure evolution of different Fe-Cu and Fe-Cu-Ni model alloys were investigated using transmission electron microscopy (TEM) and small-angle neutron scattering (SANS) analysis methods. The effects of different aging conditions and alloy composition on the formation and growth of Cu-enriched precipitates were discussed. The results showed that the hardness of the Fe-Cu model alloys with low copper content continuously increased with aging, increasing to peak hardness and then decreasing for the Fe-Cu model alloys with high copper content. The aging time to reach peak hardness was greatly reduced due to the addition of Ni in the Fe-Cu model alloys. The TEM results showed that with increased aging temperature and copper content, the Cu-enriched precipitates with a B2 structure precipitated in the ferritic matrix following a direct relationship. Therefore, Ni addition led to relatively higher precipitation nucleation and growth, and the size distribution of the Cu-enriched precipitates derived from the SANS was in overall good agreement with the TEM observations for 6-25 nm regions. Therefore, small-angle scattering could provide better details when analyzing smaller, nano-sized precipitates.(c) 2022 Elsevier B.V. All rights reserved.
查看更多>>摘要：Using the first-principles calculations and object kinetic Monte Carlo (OKMC) methods, we systematically investigate the influence of tantalum (Ta) on the behaviors of rhenium (Re) in tungsten (W) without and with irradiation defects, i.e., vacancies and self-interstitial atoms (SIAs). Due to the attractive interac-tion of Ta-Re pair, the Ta atoms energetically prefer to occupy the sites inside the pure Re clusters and Re-vacancy complexes. Conversely, there are strong repulsive interactions between Ta and large Re-SIA complexes in W. Therefore, the Re-rich clusters in irradiated W-Re-Ta alloys are largely composed of Re -SIA complexes, because of the experimental observation that the Ta atoms are ejected by the clusters. More importantly, the Ta addition reduces not only the mobility of SIA and interstitial Re but also the stability of interstitial-type Re clusters, and thus hinders the formation of Re-rich clusters in W both ki-netically and thermodynamically. This is confirmed by the OKMC simulations, in which the presence of Ta significantly decreases the average size, total volume and density of Re-rich clusters in irradiated W. These results are in good agreement with the experiments. Consequently, the present study will deepen the mechanistic understanding of the role of Ta on the aggregation of Re atoms in W after irradiation and provide a valuable reference for the development of advanced W-based materials.(c) 2022 Elsevier B.V. All rights reserved.
Christian, Jonathan H.Foley, Bryan J.Ciprian, ElodiaDick, Don D....
9页
查看更多>>摘要：For over 80 years, plutonium dioxide has been routinely produced via thermal decomposition of hydrated plutonium(IV) oxalate. Despite the longstanding utility of this process, the chemical structures of starting materials and intermediates produced during this thermal conversion remain ill-defined. To help resolve this uncertainty, we measured high-resolution Raman and infrared spectra of Pu(C2O4)(2)center dot 6H(2)O that was heated to 25, 100, 220, 250, 350, and 450 degrees C in air. Our measurements show that Pu(C2O4)(2)center dot 6H(2)O has a rich vibrational spectrum with at least 15 Raman bands between 180 cm(-1) and 1900 cm(-1) and 9 infrared bands between 800 cm(-1) and 4000 cm(-1). As Pu(C2O4)(2)center dot 6H(2)O is heated, water is liberated, and the oxalate ligand decomposes to produce plutonium oxycarbide species. When heated to 350 degrees C or higher, vibrational spectra are consistent with PuO2 with some residual carbon-containing species. Full vibrational spectra, powder X-ray diffraction, and scanning electron microscopy measurements of Pu(C2O4)(2)center dot 6H(2)O and its thermal degradation products are presented herein along with approximate assignments for observed spectral bands. These data can be used to validate and potentially improve existing computational models that describe the chemical structure of compounds produced during thermal degradation of plutonium (IV) oxalate. Given the utility of plutonium (IV) oxalate in synthesizing plutonium dioxide, these results are expected to provide value in the fields of nuclear fuel processing, nuclear nonproliferation, and nuclear forensics. (C) 2022 Elsevier B.V. All rights reserved.
查看更多>>摘要：The irradiation-induced defects in stainless steel internal components of pressurized water reactors combined with hydrogen uptake during the oxidation process could be a key parameter in the mechanism for Irradiation-Assisted Stress Corrosion Cracking (IASCC). A heat-treated 316 L SS containing a low amount of defects was Fe3+ ions-implanted; irradiation-induced defects types and depth distributions were characterized by Transmission electron microscopy (TEM). Deuterium was then inserted in the specimens either by cathodic charging or by sample exposure to deuterated primary water. Secondary Ions Mass Spectrometry - SIMS - permitted to access the deuterium distribution at the implanted stainless steel surface. A finite difference numerical solver accounting for hydrogen diffusion/trapping coupling was used to simulate the hydrogen transport in the implanted material, taken into consideration the specific heterogeneous character of the irradiation-induced defects distribution in matter. Taking as input data the experimental defects distribution associated with Frank loops or voids, the main trapping sites for hydrogen were assigned to voids, not Frank loops. Such numerical approach was able to deal accurately with the problem of hydrogen transport in a heterogeneous material as well as to differentiate two potential trap sites contributions to the experimental deuterium distribution. In addition, according to results obtained after primary water exposure, trapping at voids was still effective at 320 degrees C, signature of a high binding energy of hydrogen in voids. (C) 2022 Elsevier B.V. All rights reserved.
查看更多>>摘要：Samples of uranium diboride were sintered in inert, reducing and vacuum conditions with and without the addition of powdered carbon to green pellets. Carbon has been shown to improve the sintering of diboride materials via the formation of gaseous phases at lower temperatures than those at which grain coarsening occurs. The addition of 0.5 wt.% carbon was seen to improve sample density in both vacuum and reducing conditions, while an increase in carbon content to 1 wt.% carbon decreased the density achieved in reducing atmosphere and had a negligible effect in vacuum.(c) 2022 Elsevier B.V. All rights reserved.
Kocevski, VanchoCooper, Michael W. D.Claisse, Antoine J.Andersson, David A....
12页
查看更多>>摘要：Atomic-scale modeling of thermophysical and defect properties of uranium mononitride (UN) plays an important role in establishing a better understanding and improved models of UN fuel performance. Having an accurate interatomic potential is crucial for generating reliable data at finite temperatures using molecular dynamic simulations. We report a new interatomic potential for UN, based on a combination of many-body and pairwise interactions, a simple form that we later show could be easily adapted to include Xe-U and Xe-N interactions, i.e., generating a U-N-Xe interatomic potential. The potential was fitted to experimental thermal expansion and single crystal elastic constants, as well as Frenkel, Schottky, anti-Schottky, and antisite pair reaction energies from density functional theory (DFT) calculations. Using the potential, we successfully reproduced experimental lattice parameters, thermal expansion, single crystal elastic constants, and temperature dependent heat capacity. The potential also performs reasonably well in reproducing the energy of the aforementioned stoichiometric defect reactions and defect migration barriers calculated using DFT. However, the potential underestimates the energy difference between the tetrahedral and dumbbell uranium interstitials, and a more complex potential form might be needed to overcome this issue. The potential was also used to predict UN single crystal elastic constants and elastic properties at different temperatures, showing that UN becomes softer and more compressible with increasing temperature. We also compare our potential against literature data from previous empirical potentials, demonstrating similar or better behavior depending on the property of interest. To enable the simulation of Xe in UN a Buckingham potential has been fitted to DFT-derived Xe incorporation energies. The potential was then used to determine the activation energy for Xe diffusion due to various Xe-containing defects, with { Xe U : V U } exhibiting the lowest activation energy. (c) 2022 Elsevier B.V. All rights reserved.
查看更多>>摘要：Process conditions for the fabrication of porous uranium oxide microspheres prepared via internal gelation were assessed. To improve conditions for the application of infiltration, microstructural parameters such as density, porosity and specific surface area were assessed. Specifically, the effect of calcination temperature and the use of pore-formers was studied. Accessible porosity levels around 20% were obtained after calcination at 773 K or 823 K, without the use of a pore-former. As a novel application, starch was used as a low-temperature, burnable pore-former, and its effect was compared to that of graphite. Accessible porosity levels increased to 34% after calcination due to the use of starch, whereas the application of graphite was discarded because it requires too elevated calcination temperatures. A subset of porous uranium oxide microspheres was infiltrated with neodymium nitrate solution as a surrogate for americium nitrate. Very good agreement between targeted and actual Nd content was observed after sintering of the microspheres, and a maximum concentration of y = 25 mol% (U1-yNdyO2-x) could be reached. (C) 2022 Elsevier B.V. All rights reserved.
查看更多>>摘要：Uranium mononitride (UN) is an attractive fuel for a range of reactors, including, with adequate protection against reaction with water, for large scale and small modular light water reactors. The study of the speciation of fission products (Fps) in spent UN fuel is required to understand their properties such as phase stability and retention during long term storage and disposal. The present study reviews and ap-plies Hume -Rothery rules to predict this speciation. This law provides an estimate of the way UN may form solid solutions with actinide (An) and fission product mononitrides from Beginning of Life (BoL) to End of Life (EoL). Composition at EoL is estimated for a high burnup fuel (60 MW d kg(-1)) using the FISPIN fuel inventory code. Many Fps are trivalent (Ln, Y, Zr, Nb) and are expected to recrystallize in face centred cubic (fcc) solid solutions with UN. Other Fps are divalent (Ba, Sr) and monovalent (Cs, Rb) and some elements are non-valent (Mo, Tc, Ru, Rh, Pd) as well as noble gases (Xe, Kr), and halides (I, Br) which may form nano-precipitates. Actinides formed by neutron capture of U-238 are all An(3+) which are also expected and found in fcc solid solution with UN. The spent fuel is consequently formed of a large fraction of solid soluble nitrides (U, An, Ln, Y, Zr, Nb)N forming a single rather homogeneous phase according to the Hume-Rothery rules. This phase will also follow Vegard's law. Because of the low temperature of the fuel, the Fps are expected to be soluble or nano-dispersed precipitates. Microscopic precipitated phases are not expected.(c) 2022 Published by Elsevier B.V.
查看更多>>摘要：The role of oxygen in the electrolytic reduction of uranium oxide in molten salt has received increased attention in recent years. In contrast to solubility, few studies were reported on the kinetic property of oxygen in LiCl. In this work, diffusion behaviors of oxygen anion in the bulk salt and oxide cathode were both studied. A gold working electrode was used to conduct corresponding voltammetry studies in LiCl-rich melt at 923 K. The obtained diffusion coefficient increased when another salt (KCl or LiF) was introduced to LiCl. A model based on ohmic, concentration and electrochemical polarizations was derived and used to describe the electro-deoxidation of uranium oxide in molten salt. By analyzing experimen-tal current-time curves obtained from potentiostatic electrolysis, parameters such as exchange current density, conductivity of porous uranium filled with electrolyte and diffusion coefficient of oxygen anion in porous uranium were calculated. Taken together, the behavior of oxygen in the electro-deoxidation of uranium oxide in LiCl-rich melt can be better understood.(c) 2022 Elsevier B.V. All rights reserved.
查看更多>>摘要：In uranium-zirconium (U-Zr) based metallic fuels, different phases can form at different compositions and temperatures. Typically, lamellar delta-UZr2 and alpha-U phases are the dominant microstructures in U-rich U-Zr alloys at temperatures below 880 K. In this work, a finite element method based mesoscale modeling technique is used to calculate the effective thermal conductivities of such heterogeneous microstructures, using the thermal conductivities of two individual phases and their interphase thermal resistance (Kapitza resistance) as input parameters. The Kapitza resistance between delta-UZr2 and alpha-U is determined at different temperatures, which shows an approximately T-3 dependence in the temperature range be-tween 300 and 800 K. In addition, the Kapitza resistance exhibits a strong dependence on the aspect ratio of the delta-UZr2 phase. An analytical model is therefore developed to quantify the effects of both temperature and delta-UZr2 aspect ratio on the Kapitza resistance. Using this newly developed Kapitza resistance model, the effective thermal conductivities of a number of delta-UZr2 + alpha-U heterogeneous microstructures in U-Zr alloys, including non-lamellar microstructures, can be estimated accurately. (C) 2022 Elsevier B.V. All rights reserved.
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