Browsing by Subject "Tungsten"

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  • Sand, A.E.; Byggmästar, J.; Zitting, A.; Nordlund, K. (2018)
    Most experimental work on radiation damage is performed to fairly high doses, where cascade overlap effects come into play, yet atomistic simulations of the primary radiation damage have mainly been performed in initially perfect lattice. Here, we investigate the primary damage produced by energetic ion or neutron impacts in bcc Fe and W. We model irradiation effects at high fluence through atomistic simulations of cascades in pre-damaged systems. The effects of overlap provide new insights into the processes governing the formation under irradiation of extended defects. We find that cascade overlap leads to an increase in the numbers of large clusters in Fe, while in W such an effect is not seen. A significant shift in the morphology of the primary damage is also observed, including the formation of complex defect structures that have not been previously reported in the literature. These defects are highly self-immobilized, shifting the damage away from the predominance of mobile 1/2〈111〉 loops towards more immobile initial configurations. In Fe, where cascade collapse is extremely rare in molecular dynamics simulations of individual cascades, we observe the formation of vacancy-type dislocation loops from cascade collapse as a result of cascade overlap.
  • Widdowson, Anna; Coad, Joseph Paul; Alves, Eduardo; Baron-Wiechec, A.; Catarino, Norberto; Corregidor, Victoria; Heinola, Kalle; Krat, Stepan; Makepeace, C.; Matthews, Guy F.; Mayer, Matej; Mizohata, Kenichiro; Sertoli, Marco (2019)
    Post mortem analysis shows that mid and high atomic number metallic impurities are present in deposits on JET plasma facing components with the highest amount of Ni and W, and therefore the largest sink, being found at the top of the inner divertor. Sources are defined as “continuous” or “specific”, in that “continuous” sources arise from ongoing erosion from plasma facing surfaces and “specific” are linked with specific events which decrease over time until they no longer act as a source. This contribution evaluates the sinks and estimates sources, and the balance gives an indication of the dominating processes. Charge exchange neutral erosion is found to be the main source of nickel, whereas erosion of divertor plasma facing components is the main source of tungsten. Specific sources are shown to have little influence over the global mid- and high-Z impurity concentrations in deposits.
  • Bonny, G.; Castin, N.; Bakaev, A.; Sand, A. E.; Terentyev, D. (2020)
    In recent years, a number of systematic investigations of high-energy collision cascades in tungsten employing advanced defect analysis tools have shown that interstitial clusters can form complex non-planar dislocation structures. These structures are sessile in nature and may potentially have a strong impact on the long-term evolution of the radiation microstructure. To clarify this aspect, we selected several representative primary damage states of cascades debris and performed annealing simulations using molecular dynamics (MD). We found that immobile complexes of non-planar dislocation structures (CDS) evolve into glissile and planar shaped 1/2 <1 1 1 > loops with an activation energy of similar to 1.5 eV. The CDS objects were implemented in an object kinetic Monte Carlo (OKMC) model accounting for the event of transformation into 1-D migrating loops, following the MD data. OKMC was then used to investigate the impact of the transformation event (and the associated activation energy) on the evolution of the microstructure.
  • Granberg, F.; Byggmastar, J.; Nordlund, K. (2021)
    Tungsten has been chosen as the plasma-facing wall material in fusion reactors, due to its high density and melting point. The wall material will not only be sputtered at the surface, but also damaged deep inside the material by energetic particles. We investigate the high-dose damage production and accumulation by computational means using molecular dynamics. We observe that the choice of interatomic potential drastically affects the evolution. The structure and stability of the obtained defect configurations are validated using a quantum-accurate Gaussian approximation potential. (c) 2021 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )
  • Lindblom, O.; Ahlgren, T.; Heinola, K. (2021)
    Solute hydrogen can cause many damaging processes in the lattices of metals, such as deformation of the material, which can take place in large scales through blistering and embrittlement. Especially in nuclear fusion applications, the trapped hydrogen isotope Tritium in the reactor wall materials can pose a radiological safety hazard. Techniques for hydrogen removal from metals usually require high temperatures. However, an efficient low temperature method to remove hydrogen is the so-called isotope exchange mechanism, where one isotope is being removed from the material by replacing it by another isotope introduced in the material. The atomic scale exchange mechanism of isotope exchange has not yet been determined. In this study we use molecular dynamics simulations to provide an atomic-scale explanation to the processes related to hydrogen isotope exchange in bulk materials. The results show that the lattice mono-vacancies and small vacancy clusters, usually produced in irradiation experiments, exhibit isotope exchange at low temperatures. The isotope exchange process should also be seen in other hydrogen trapping defects with similar trapping properties as vacancies.
  • Domínguez-Gutiérrez, F.J.; Byggmästar, J.; Nordlund, K.; Djurabekova, F.; von Toussaint, U. (2020)
    The analysis of the damage on plasma facing materials (PFM), due to their direct interaction with the plasma environment, is needed to build the next generation of nuclear fusion reactors. After systematic analyses of numerous materials over the last decades, tungsten has become the most promising candidate for a nuclear fusion reactor. In this work, we perform molecular dynamics (MD) simulations using a machine learned interatomic potential, based on the Gaussian Approximation Potential framework, to model better neutron bombardment mechanisms in pristine W lattices. The MD potential is trained to reproduce realistic short-range dynamics, the liquid phase, and the material recrystallization, which are important for collision cascades. The formation of point defects is quantified and classified by a descriptor vector (DV) based method, which is independent of the sample temperature and its constituents, requiring only modest computational resources. The locations of vacancies are calculated by the k-d-tree algorithm. The analysis of the damage in the W samples is compared to results obtained by Finnis–Sinclair and Tersoff–Ziegler–Biersack–Littmark potentials, at a sample temperature of 300 K and a primary knock-on atom (PKA) energy range of 0.5–10 keV, where a good agreement with the reported number of Frenkel pair is observed. Our results provide information about the advantages and limits of the machine learned MD simulations with respect to the standard ones. The formation of dumbbell and crowdion defects as a function of PKA energy were identified and distinguished by our DV method.
  • Castin, N.; Bakaev, A.; Bonny, G.; Sand, A. E.; Malerba, L.; Terentyev, D. (2017)
    We propose an object kinetic Monte Carlo (OKMC) model for describing the microstructural evolution in pure tungsten under neutron irradiation. We here focus on low doses ( under 1 dpa), and we neglect transmutation in first approximation. The emphasis is mainly centred on an adequate description of neutron irradiation, the subsequent introduction of primary defects, and their thermal diffusion properties. Besides grain boundaries and the dislocation network, our model includes the contribution of carbon impurities, which are shown to have a strong influence on the onset of void swelling. Our parametric study analyses the quality of our model in detail, and confronts its predictions with experimental microstructural observations with satisfactory agreement. We highlight the importance for an accurate determination of the dissolved carbon content in the tungsten matrix, and we advocate for an accurate description of atomic collision cascades, in light of the sensitivity of our results with respect to correlated recombination. (C) 2017 Published by Elsevier B.V.
  • Meluzova, D. S.; Babenko, P. Yu.; Shergin, A. P.; Nordlund, K.; Zinoviev, A. N. (2019)
    Particle reflection coefficients for scattering of hydrogen and deuterium atoms from amorphous beryllium, carbon and tungsten were obtained, which are of interest for thermonuclear reactor physics. For the case of deuterium scattering from tungsten the data were also calculated for polycrystalline and crystalline target. The calculations were carried out by two methods: by modeling the trajectories of the incident particles and by using the binary collision approximation. Interaction potentials between hydrogen and helium atoms and the selected materials were calculated in the scope of the density function theory using program DMol for choosing wave functions. The dependence of the reflection coefficient RN on the potential well depth was found. The results demonstrate a good agreement with the available experimental values.
  • Fellman, Aslak (Helsingin yliopisto, 2021)
    The plasma-facing materials of future fusion reactors are exposed to high doses of radiation. The characterization of the radiation damage is an essential part in the study of fusion relevant materi- als. Electron microscopy is one of the most important tools used for characterization of radiation damage, as it provides direct observations of the microstructure of materials. However, the char- acterization of defects from electron microscope images remains difficult. Simulated images can be used to bridge the gap between experimental results and models. In this thesis, scanning transmission electron microscope (STEM) images of radiation damage were simulated. Molecular dynamics simulations were employed in order to create defects in tungsten. STEM images were simulated based on the created systems using the multislice method. A data- base of images of h001i dislocation loops and defects produced from collision cascade simulations was generated. The simulated images provide insight into the observed contrast of the defect structures. Differences in the image contrast between vacancy and interstitial h001i dislocation loops were reported. In addition to this, the results were compared against experimental images and used in identification of a dislocation loop. The simulated images demonstrate that it is feasible to simulate STEM images of radiation damage produced from collision cascade simulations.
  • Valles, G.; Martin-Bragado, I.; Nordlund, K.; Lasa, A.; Björkas, C.; Safi, E.; Perlado, J. M.; Rivera, A. (2017)
    Recently, tungsten has been found to form a highly underdense nanostructured morphology ("W fuzz") when bombarded by an intense flux of He ions, but only in the temperature window 900-2000 K. Using object kinetic Monte Carlo simulations (pseudo-3D simulations) parameterized from first principles, we show that this temperature dependence can be understood based on He and point defect clustering, cluster growth, and detrapping reactions. At low temperatures (2300 K), all He is detrapped from clusters, preventing the formation of the large clusters that lead to fuzz growth in the intermediate temperature range. (C) 2017 Elsevier B.V. All rights reserved.
  • Bernard, E; Sakamoto, R; Hodille, E.; Kreter, A; Autissier, E; Barthe, M-F; Desgardin, P; Schwarz-Selinger, T; Burwitz, V; Feuillastre, S; Pieters, G; Rousseau, B; Iolavega, M; Bisson, R; Ghiorghiu, F; Corr, C; Thompson, M; Doerner, R; Markelj, S; Yamada, H; Yoshida, N; Grisolia, C (2019)
    Plasma-facing materials for next generation fusion devices, like ITER and DEMO, will be submitted to intense fluxes of light elements, notably He and H isotopes (HI). Our study focuses on tritium (T) retention on a wide range of W samples: first, different types of W materials were investigated to distinguish the impact of the pristine original structure on the retention, from W-coated samples to ITER-grade pure W samples submitted to various annealing and manufacturing procedures, along with monocrystalline W for reference. Then, He and He-D irradiated W samples were studied to investigate the impact on He-damages such as nano-bubbles (exposures in LHD or PSI-2) on T retention. We exposed all the samples to tritium gas-loading using a gentle technique preventing any introduction of new damage in the material. Tritium desorption is measured by Liquid Scintillation counting (LSC) at ambient and high temperatures (800 degrees C). The remaining T inventory is then measured by sample full dissolution and LSC. Results on T inventory on He exposed samples highlighted that in all cases, tritium desorption as a gas (HT) increases significantly due to the formation of He damages. Up to 1.8 times more T can be trapped in the material through a competition of various mechanisms, but the major part of the inventory desorbs at room temperature, and so will most likely not take part to the long-term trapped inventory for safety and operational perspectives. Unfortunately, investigation of "as received" industrial W (used for the making of plasma-facing materials) highlighted a strong impact of the pre existing defects on T retention: up to 2.5 times more T is trapped in "as received W" compared to annealed and polish W, and desorbs only at 800 degrees C, meaning ideal W material studies may underestimate T inventory for tokamak relevant conditions.
  • Zibrov, M.; Egger, W.; Heikinheimo, J.; Mayer, M.; Tuomisto, F. (2020)
    The thermal evolution of vacancies and vacancy clusters in tungsten (W) has been studied. W (100) single crystals were irradiated with 200 keV hydrogen (H) ions to a low damage level (5.8 x 10(-3) dpa) at 290 K and then annealed at temperatures in the range of 500-1800 K. The resulting defects were characterized by positron annihilation lifetime spectroscopy (PALS) and positron annihilation Doppler broadening spectroscopy (DBS). Annealing at 700 K resulted in the formation of clusters containing 10-15 vacancies, while at 800 K and higher temperatures clusters containing about 20 vacancies or more were formed. Reduction of the defect concentration likely accompanied by further coarsening of the clusters started at 1300 K and ended at 1800 K with the complete defect recovery. The determined cluster sizes at 700 K and 800 K were larger than the estimated minimum cluster sizes that are thermally stable at these temperatures, indicating that the migration and ensuing coalescence of small clusters plays an important role in cluster growth. (C) 2020 Max-Planck-Institut fur Plasmaphysik. Published by Elsevier B.V. All rights reserved.