查看更多>>摘要：Voids are common radiation-induced defects in nuclear materials and produce detrimental effects on mechanical properties. The influence of a pre-existing void on mechanical behavior has been investigated in single crystals, but the effects of void located at bimetal interfaces on the dislocation nucleation mechanisms and associated mechanical performance have not been fully studied. Using Cu/Nb metallic nanolayered composites (MNCs) as a model system, with atomic-scale simulations, we report on the influence of a pre-existing void at the Cu/Nb interface on dislocation nucleation and deformation behavior. Compared with the void-free system, the size, location and shape of the voids can influence dislocation behavior significantly, such as the initial nucleation site, the critical nucleation stress, and the preferred slip system. As the size of the faceted void increases from 3 to 4 nm, the dominant mechanism of dislocation nucleation changes from a misfit dislocation-assisted localized shear to a void edge-assisted Frank-Read-like mechanism. Also, the location of the faceted void can change the coupled strain field between the void and the misfit dislocation, ultimately affecting the preferred slip system selection. In contrast, the spherical void cannot effectively activate the Frank-Read-like dislocation source due to the absence of edges. This work reveals the effects of void morphology on dislocation behavior, and provides an explanation for co-dominated plasticity of bimetal interface and void.
查看更多>>摘要：A side effect of high level waste vitrification at 1100 °C is the volatilization of radioactive cesium (Cs), recaptured further on in the process as a solid aerosol by a dust scrubber. Feedback from industry suggests that Cs combines with technetium (Tc) – simulated here by rhenium (Re), chosen for its similar behavior – to form volatile species. This work characterized Cs volatilization and aerosol formation. The first experiment involved elaborating and analyzing a simplified glass enriched with Cs and Re, which reinforce the volatilization phenomena. The results showed that a demixed liquid phase rich in Cs, Re, Mo, and Na formed in the liquid melt, which is reminiscent of a molybdic phase evoked in the literature. Moreover, analyzing the condensed gasses led to the conclusion that the alkalis and Re volatilize congruently at around 700 °C. In the second experiment, a series of measurements of the gas phase characterized aerosols throughout the process. The size distribution results showed that the particles formed in the furnace were mostly submicronic, and that the aerosols in the calciner and the dust scrubber included nanoparticles and micronic particles. SEM-EDS characterization revealed the composition of a spherical particle from the furnace made of two nested phases: one rich in Cs-Re-O, and the other rich in Na-Mo-O. The analyses performed showed that Cs and Re could be found in every particle sampled throughout the process, and that the micronic particles might originate from the aggregation of submicronic particles because of composition similarities. Thus, the observations led to a better understanding of the behavior of the elements of interest in both the melt and the gas phase.
查看更多>>摘要：This work presents a thorough characterization of the texture development during the initial step of a cold pilgering process of a Zircaloy-4 TREX tube, on which the axial cross section is reduced by 80% over a 40cm length with an overall Q-ratio of 2.34, followed by a recrystallization thermal treatment. Texture across a tapered specimen representing this working zone was characterized by neutron diffraction and high energy synchrotron x-ray diffraction at 28 positions (4 locations along its length and 7 locations across its thickness with 0.3 mm spatial resolution), and on the deformed end after the thermal treatment. Results show that the starting material (TREX tube) presents a texture gradient through its wall thickness, with the c poles aligned to the hoop direction at the outer surface (with radial and hoop Kearns factors: fR=0.36, fH= 0.50), and gradually rotating around the axial direction to become a quasi-radial orientation (～35o out of the radial direction) at the inner surface (fR=0.43, fH=0.35). The overall texture becomes more radial due to the cold pilgering, but the texture gradient remains during the whole deformation process, being always more radial at the inner surface, (fR=0.53, fH=0.37 at 80%) than at the outer surface (fR=0.41, fH=0.50at 80%). The most remarkable changes in texture produced by the rolling process is a 30o rotation of the grains around their c-axis, producing a marked increase of the <112ˉ0> // axial fiber texture at expenses of the original <101ˉ0> // axial fiber texture observed in the TREX tube. The observed changes in texture during deformation were correctly explained by visco-plastic self-consistent (VPSC) modelling. This was achieved after the inclusion of the basal <a> slip system, which plays a central role to explain the 30o rotation around the c-axis during deformation. The texture evolution was correctly described after the inclusion of a drag effect to grain rotation due to the interaction with neighbouring grains. The through thickness texture differences are greatly reduced by the recrystallization thermal treatment, with similar Kearns factors at the inner surface (fR=0.50,fH=0.28) and the outer surface (fR=0.48, fH=0.39). The predominance of the <101ˉ0> // axial fiber texture is recovered after recrystallization.
查看更多>>摘要：SiOC polymer-derived ceramics (PDCs) are commonly used materials for advanced structural and functional applications. Especially, when in the form of fibers, they are used in the SiCf/SiC composites that is being considered an option for fuel cladding components in the current and next-generation light water reactors. But the radiation effects of the SiOC PDCs have been rarely reported (mostly focused on SiC). Here we prepared SiOC nanocomposites consisting of β-SiC nanocrystals and turbostratic graphite embedded in amorphous SiOC matrix, and irradiated them at room temperature with 4 MeV Kr ions to different damage levels. The irradiation-induced changes in the microstructures, surface morphologies, and mechanical properties were studied. It is indicated much severer irradiation condition leads to higher rearrangement degree of Si-containing tetrahedral units in the amorphous SiOC matrix, higher disordering degree of the turbostratic graphite, but crystallinity of the β-SiC is preserved. No obvious change is detected in the macro properties such as surface morphologies and mechanical properties (hardness and modulus). This study indicates the potential of SiOC PDCs to radiation damage.
查看更多>>摘要：The authors studied radiation-induced structural-phase transformations that had occurred in EP823 steel of industrial application under conditions of its high-dose neutron irradiation in a fast fission reactor. The method of transmission M?ssbauer spectroscopy with resonance detection and transmission electron microscopy was utilized. It has been observed that in ferritic-martensitic steel, under the neutron irradiation with doses above 50 dpa at temperatures of 570–660 °C, the decomposition of a BCC solid solution together with the release of the alloying elements chromium, molybdenum, and carbon from the metal matrix occurs. It is accompanied by the formation of an intermetallic χ phase and carbides of the Me23C6 type. The authors revealed the formation of the vacancy-clustered and supposedly gas-filled pores in the structure after irradiation.
查看更多>>摘要：Tristructural isotropic (TRISO) fuel particles have been primarily developed for high-temperature gas-cooled nuclear reactors and can be subjected to oxidizing environments for extended periods in an off-normal accident scenario. Surrogate TRISO fuel particles were oxidized in air at 1,000 or 1,100 °C for up to 120 h. The oxide scale morphology and thickness were studied via scanning electron microscopy, focused ion beam, and atomic force microscopy. TRISO particles oxidized at 1,100 °C exhibited a highly crystalline oxide scale, which led to significant cracking and irregularly shaped closed porosity, whereas those oxidized at 1,000 °C possessed a primarily amorphous oxide scale, which contained small, rounded internal pores and no larger defects. The observed phenomena deviated from the expected behavior based on models for oxide growth on flat-plate and fiber SiC. The oxidation kinetics of TRISO fuel particles in high-temperature air were investigated without mechanically deforming the surface and were analyzed with respect to oxide morphology.
查看更多>>摘要：In-vessel retention (IVR) is an effective strategy of ensuring the structural integrity of RPV under severe accidents. In the IVR condition, due to the geometric discontinuity of pressure vessel, multiaxial creep is the main form of failure of RPV. Moreover, 16MND5 steel undergoes a phase transition from bainite to austenite. In order to study the multiaxial creep failure mechanism, creep tests on different notched bar are carried out at 700℃. The influence of grain boundary precipitation on creep behaviors was studied by micro-structural observation. Furthermore, based on the Kachanov-Robotnov continuous damage model (CDM), the term of activation energy Q is adopted in the newly-developed model as the correction parameter that is used to study the damage evolution and multiaxial creep behaviors of 16NND5 steel. Finally, the good agreement is achieved between theoretical prediction and experimental results.
查看更多>>摘要：Lacunar apatite host, NaPb4(PO4)3was converted into a ceramic wasteform with the incorporation of Cs and Sr ions. The systematic substitution yielded the wasteform composition Na0.9Cs0.1Pb3Sr(PO4)3 with 8.52 wt% of Strontium and 1.29 wt% of caesium. The wasteform was subjected to continuous heating in the temperature range 298–973 K and aqueous flow to study the thermal stability and chemical durability respectively. The wasteforms response was studied by powder X-Ray diffraction analysis (Room temperature and High temperature), electron microscopic & elemental analysis and normalized mass loss (NLi) & leach rate (LRi) calculations. The average thermal expansion coefficient (αavg) value of 14.4 ± 1.1 × 10?6 K?1 and the leach rates of immobilised Cs, LRCs (10?6–10?8 gm?2d?1) and Sr, LRSr (10?3–10?4 gm?2d?1) demonstrate the structural integrity of the phase. The anisotropic nature of thermal expansion and the mechanistic feature of leaching are discussed.
查看更多>>摘要：Structural and electronic properties of c-Fe2Zr, t-FeZr2 and o-FeZr3 intermetallic phases in the presence of s-, p- and d-block fission metals (FMs), viz., Rb, Sr, In, Sn, Sb, Te, Cs, Ba, Y, Nb, Mo, Tc, Ru, Rh, Pd, Ag and Cd are studied using density functional theory (DFT). The mono-vacancy and di-vacancy formation energies of Fe and Zr in these intermetallics are also calculated. It has been found that s-block FMs are less weakly bounded to the intermetallics than the p? and d?block FMs. The solution energies of p? and d?block fission atoms are either slightly positive or negative. The high solution energies of s-block FMs suggest that these FMs are highly insoluble in the Fe-Zr intermetallics. The solution energy hierarchy of the FMs follow the trend as Cs > Rb > Ba > Sr > Ag > Cd > Y > Mo > Nb > In > Te > Sn > Tc > Sb > Pd > Ru > Rh. We also report the site preferences of these FMs in Fe-Zr intermetallics based on their incorporation/solution energies. The electronic property analysis indicate a predominant metallic bonding with small covalent and ionic contributions. Finally, these intermetallics have been compared in terms of their potentials to accommodate these FMs.
查看更多>>摘要：The nuclear industry has growing interest in replacing forgings with structural components fabricated by powder metallurgy with hot isostatic pressing (PM-HIP), owing to their chemical homogeneity, uniform grain structure, and near-net shape production. This study compares the ion irradiation response of PM-HIP and forged Alloy 625, over 50 and 100 dpa, 400 °C and 500 °C. Microstructure is characterized using down-zone bright-field scanning transmission electron microscopy (DZBFSTEM), and hardening is characterized using nanoindentation. PM-HIP Alloy 625 has a lower initial dislocation line density, resulting in a more rapid onset of dislocation loop growth and unfaulting than the forged material. But the total defect population (i.e. loop line length plus dislocation density) is insensitive to fabrication method. This finding shows promise for the eventual qualification of PM-HIP alloys for nuclear applications.
NSTL
Elsevier