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- 2019
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Tidskriftsartikel (refereegranskat)
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| 2. |
- Bathellier, Didier, et al.
(författare)
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Effect of cationic chemical disorder on defect formation energies in uranium-plutonium mixed oxides
- 2022
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 132:17, s. 175103-
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Tidskriftsartikel (refereegranskat)abstract
- At the atomic scale, uranium-plutonium mixed oxides (U,Pu)O-2 are characterized by cationic chemical disorder, which entails that U and Pu cations are randomly distributed on the cation sublattice. In the present work, we study the impact of disorder on point defect formation energies in (U,Pu)O-2 using interatomic-potential and density functional theory (DFT + U) calculations. We focus on bound Schottky defects (BSD) that are among the most stable defects in these oxides. As a first step, we estimate the distance R-D around the BSD up to which the local chemical environment significantly affects their formation energy. To this end, we propose an original procedure in which the formation energy is computed for several supercells at varying levels of disorder. We conclude that the first three cation shells around the BSD have a non-negligible influence on their formation energy (R-D similar or equal to 7.0 angstrom). We apply then a systematic approach to compute the BSD formation energies for all the possible cation configurations on the first and second nearest neighbor shells around the BSD. We show that the formation energy can range in an interval of 0.97eV, depending on the relative amount of U and Pu neighboring cations. Based on these results, we propose an interaction model that describes the effect of nominal and local composition on the BSD formation energy. Finally, the DFT + U benchmark calculations show a satisfactory agreement for configurations characterized by a U-rich local environment and a larger mismatch in the case of a Pu-rich one. In summary, this work provides valuable insights on the properties of BSD defects in (U,Pu)O-2 and can represent a valid strategy to study point defect properties in disordered compounds.
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| 3. |
- Castin, N., et al.
(författare)
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Advanced atomistic models for radiation damage in Fe-based alloys : Contributions and future perspectives from artificial neural networks
- 2018
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Ingår i: Computational materials science. - : Elsevier. - 0927-0256 .- 1879-0801. ; 148, s. 116-130
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Tidskriftsartikel (refereegranskat)abstract
- Machine learning, and more specifically artificial neural networks (ANN), are powerful and flexible numerical tools that can lead to significant improvements in many materials modelling techniques. This paper provides a review of the efforts made so far to describe the effects of irradiation in Fe-based and W-based alloys, in a multiscale modelling framework. ANN were successfully used as innovative parametrization tools in these models, thereby greatly enhancing their physical accuracy and capability to accomplish increasingly challenging goals. In the provided examples, the main goal of ANN is to predict how the chemical complexity of local atomic configurations, and/or specific strain fields, influence the activation energy of selected thermally-activated events. This is most often a more efficient approach with respect to previous computationally heavy methods. In a future perspective, similar schemes can be potentially used to calculate other quantities than activation energies. They can thus transfer atomic-scale properties to higher-scale simulations, providing a proper bridging across scales, and hence contributing to the achievement of accurate and reliable multiscale models.
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| 4. |
- Castin, N., et al.
(författare)
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The dominant mechanisms for the formation of solute-rich clusters in low-Cu steels under irradiation
- 2020
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Ingår i: Materials Today Energy. - : Elsevier BV. - 2468-6069. ; 17
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Tidskriftsartikel (refereegranskat)abstract
- The formation of nano-sized, coherent, solute-rich clusters (NSRC) is known to be an important factor causing the degradation of the macroscopic properties of steels under irradiation. The mechanisms driving their formation are still debated. This work focuses on low-Cu reactor pressure vessel (RPV) steels, where solute species are generally not expected to precipitate. We rationalize the processes that take place at the nanometer scale under irradiation, relying on the latest theoretical and experimental evidence on atomic-level diffusion and transport processes. These are compiled in a new model, based on the object kinetic Monte Carlo (OKMC) technique. We evaluate the relevance of the underlying physical assumptions by applying the model to a large variety of irradiation experiments. Our model predictions are compared with new experimental data obtained with atom probe tomography and small angle neutron scattering, complemented with information from the literature. The results of this study reveal that the role of immobilized self-interstitial atoms (SIA) loops dominates the nucleation process of NSRC.
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| 5. |
- Castin, N., et al.
(författare)
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The effect of rhenium on the diffusion of small interstitial clusters in tungsten
- 2020
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Ingår i: Computational materials science. - : ELSEVIER. - 0927-0256 .- 1879-0801. ; 177
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Tidskriftsartikel (refereegranskat)abstract
- In this work, we use atomistic simulations to investigate the mobility and stability of self-interstitial atom (SIA) clusters of size 1-5 in W-Re alloys. We apply molecular statics and molecular dynamics (MD) simulations to determine the dimensionality of diffusion of the clusters, the activation energy of translation and rotation, and the energy of dissociation. The results show a strong effect of Re on the diffusion properties of SIA clusters, but not on its stability. The diffusion mechanism of the single SIA changes from 1-D migration with on-site rotations to full 3-D diffusion on the MD time and length scale due to the addition of Re. Further, the mobility of the SIA clusters is greatly reduced by the addition of Re. The obtained results can be readily used to parameterize coarse grain models such as object kinetic Monte Carlo and rate theory models.
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| 6. |
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| 7. |
- Chiapetto, Monica, et al.
(författare)
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Nanostructure evolution of neutron-irradiated reactor pressure vessel steels: Revised Object kinetic Monte Carlo model
- 2017
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Ingår i: Nuclear Instruments and Methods in Physics Research Section B. - : Elsevier. - 0168-583X .- 1872-9584. ; 393, s. 105-109
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Tidskriftsartikel (refereegranskat)abstract
- This work presents a revised set of parameters to be used in an Object kinetic Monte Carlo model to simulate the microstructure evolution under neutron irradiation of reactor pressure vessel steels at the operational temperature of light water reactors (∼300 °C). Within a “grey-alloy” approach, a more physical description than in a previous work is used to translate the effect of Mn and Ni solute atoms on the defect cluster diffusivity reduction. The slowing down of self-interstitial clusters, due to the interaction between solutes and crowdions in Fe is now parameterized using binding energies from the latest DFT calculations and the solute concentration in the matrix from atom-probe experiments. The mobility of vacancy clusters in the presence of Mn and Ni solute atoms was also modified on the basis of recent DFT results, thereby removing some previous approximations. The same set of parameters was seen to predict the correct microstructure evolution for two different types of alloys, under very different irradiation conditions: an Fe-C-MnNi model alloy, neutron irradiated at a relatively high flux, and a high-Mn, high-Ni RPV steel from the Swedish Ringhals reactor surveillance program. In both cases, the predicted self-interstitial loop density matches the experimental solute cluster density, further corroborating the surmise that the MnNi-rich nanofeatures form by solute enrichment of immobilized small interstitial loops, which are invisible to the electron microscope.
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| 8. |
- Cozza, Vittoria, et al.
(författare)
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Spatio-Temporal Keyword Queries in Social Networks
- 2013
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Ingår i: 17th East-European Conference on Advances in Databases and Information Systems (ADBIS). - Berlin, Heidelberg : Springer Berlin Heidelberg. ; , s. 70-83
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Konferensbidrag (refereegranskat)abstract
- Due to the large amount of social network data produced at an ever growing speed and their complex nature, recent works have addressed the problem of efficiently querying such data according to social, temporal or spatial dimensions. In this work we propose a data model that keeps into account all these dimensions and we compare different approaches for efficient query execution on a large real dataset using standard relational technologies.
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| 9. |
- Huang, Liangzhao, et al.
(författare)
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Impact of the local microstructure fluctuations on radiation-induced segregation in dilute Fe-Ni and Ni-Ti model alloys : A combined modeling and experimental analysis
- 2022
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 225
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Tidskriftsartikel (refereegranskat)abstract
- From a systematic atom probe tomography (APT) characterization of the radiation-induced segregation (RIS) in dilute Fe-Ni and Ni-Ti model alloys, we highlight fluctuations of the solute local concentration up to the scale of the APT specimens. We deduce the RIS at dislocation loops from a solute diffusion equation, that is solved at steady state, within the Voronoi's volume occupied by a single loop. From a statistical sampling of the Voronoi's volume and the dislocation loop radius modeled after the characterization of the microstructure by transmission electron microscopy, we provide the full RIS distribution. The present statistical approach of RIS demonstrates that the fluctuation of local solute concentrations in Fe-Ni and Ni-Ti mainly results from the dispersion in size and density of the dislocation loop population. Besides, we highlight the impact of the post-treatment parameters used in the APT protocol on the extracted RIS profiles. In Ni-Ti alloys, the simulated Ti-depletion profiles are in very good agreement with the measured ones. Furthermore, the dispersion of the loop radius and density is shown to play a critical role on the fluctuations of the Ti local concentration. In Fe-Ni, the identification of discrepancies between the simulated Ni-enrichment profiles and the measured ones provides a signature of additional operating mechanisms of the solute redistribution, such as radiation-induced precipitation.
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| 10. |
- Lange, Dale J., et al.
(författare)
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Pyrimethamine Significantly Lowers Cerebrospinal Fluid Cu/Zn Superoxide Dismutase in Amyotrophic Lateral Sclerosis Patients with SOD1 Mutations
- 2017
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Ingår i: Annals of Neurology. - : John Wiley & Sons. - 0364-5134 .- 1531-8249. ; 81:6, s. 837-848
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Tidskriftsartikel (refereegranskat)abstract
- Objective: Cu/Zn superoxide dismutase (SOD1) reduction prolongs survival in SOD1-transgenic animal models. Pyrimethamine produces dose-dependent SOD1 reduction in cell culture systems. A previous phase 1 trial showed pyrimethamine lowers SOD1 levels in leukocytes in patients with SOD1 mutations. This study investigated whether pyrimethamine lowered SOD1 levels in the cerebrospinal fluid (CSF) in patients carrying SOD1 mutations linked to familial amyotrophic lateral sclerosis (fALS/SOD1). Methods: A multicenter (5 sites), open-label, 9-month-duration, dose-ranging study was undertaken to determine the safety and efficacy of pyrimethamine to lower SOD1 levels in the CSF in fALS/SOD1. All participants underwent 3 lumbar punctures, blood draw, clinical assessment of strength, motor function, quality of life, and adverse effect assessments. SOD1 levels were measured in erythrocytes and CSF. Pyrimethamine was measured in plasma and CSF. Appel ALS score, ALS Functional Rating Scale-Revised, and McGill Quality of Life Single-Item Scale were measured at screening, visit 6, and visit 9. Results: We enrolled 32 patients; 24 completed 6 visits (18 weeks), and 21 completed all study visits. A linear mixed effects model showed a significant reduction in CSF SOD1 at visit 6 (p<0.001) with a mean reduction of 13.5% (95% confidence interval [CI] 58.4-18.5) and at visit 9 (p<0.001) with a mean reduction of 10.5% (95% CI55.2-15.8). Interpretation: Pyrimethamine is safe and well tolerated in ALS. Pyrimethamine is capable of producing a significant reduction in total CSF SOD1 protein content in patients with ALS caused by different SOD1 mutations. Further long-term studies are warranted to assess clinical efficacy.
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