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  • On the classification and quantification of crystal defects after energetic bombardment by machine learned molecular dynamics simulations
    Nucl. Mater. Energy (IF 2.025) Pub Date : 2019-12-20
    F.J. Domínguez-Gutiérrez; J. Byggmästar; K. Nordlund; F. Djurabekova; U. von Toussaint

    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.

    更新日期:2019-12-20
  • Dose rates and specific activities of copper based materials irradiated during the TT plasma operation at JET
    Nucl. Mater. Energy (IF 2.025) Pub Date : 2019-12-19
    Andrius Tidikas; Simona Breidokaite; Andrius Slavickas; Gediminas Stankunas

    Before deuterium-tritium (DTE2), a new tritium campaign (TT) with the objective to validate the activities, dose rates for selected materials and OLTIS container components, as resulting from neutron irradiation at Joint European Torus (JET), is planned. This paper describes the activation and dose rate calculation performed for copper based materials irradiated throughout the foreseen TT plasma operation. Preparatory experiments and calculations are in progress for TT campaign to measure neutron flux spectrum at the irradiation position using dosimetry foils. In addition, the materials to be considered for irradiation during the TT campaign of the OLTIS were identified and analyzed in this study: Al-Bronze and CuCrZr. The neutron-induced activities and dose rates at shutdown were calculated by activation inventory code FISPACT-2010 using the neutron fluxes and spectra provided by the detailed MCNP5 neutron transport calculations. After the irradiation, the activities and dose rates are calculated at several cooling times after the end of irradiation. Highest dose rates the end of irradiation are attributed to Cu-64 radionuclide. Co-58 and Co-60 are the most dominant radionuclides during intermediate and long cooling periods. Highest activities are ascribed to Cu-64 at the beginning and Ni-63 at the end of investigation time.

    更新日期:2019-12-19
  • Formation of beryllium-hydrogen ions in chemical sputtering from 20 to 420 eV
    Nucl. Mater. Energy (IF 2.025) Pub Date : 2019-12-19
    Lorenz Ballauf; Faro Hechenberger; Reinhard Stadlmayr; Timo Dittmar; Matthias Daxner; Samuel Zöttl; Friedrich Aumayr; Zdenek Herman; Paul Scheier

    The formation of BeD+ in collisions of deuterium molecular ions with a beryllium surface at hyperthermal energies is investigated, using a purpose-built tandem mass spectrometer setup, “SurfTOF”. In the manifold of products, hydrogen-containing molecules are of special interest in fusion devices. Our experiments show, that they can be formed in nearly all conditions comparable to plasma and wall temperature, with two mechanisms involved: First, BeD+ is formed directly in a reaction of the D2+ projectiles with the Be surface, which is the main aspect of this work. There was no evidence found, that this particular mechanism relies on D implanted in the bulk Be. BeD2+ was not detected. The second reaction pathway produces BeH+. It is efficiently formed when H2O molecules are adsorbed on the surface, much less abundant for hydrocarbon adsorbates. The yield of Be+, BeH+ and BeD+ products, is measured as a function of several parameters: Scanning the impact energy from 20 to 426 eV reveals that the molecular BeD+ is preferably produced at low energies. At 20 eV, its abundance is three times higher than that of pure Be+. When the surface temperature is raised from 298 to 673 K, sputtering yields increase two orders of magnitude for both, Be+ and BeD+.

    更新日期:2019-12-19
  • Nano-indentation of ion-irradiated nuclear structural materials: A review
    Nucl. Mater. Energy (IF 2.025) Pub Date : 2019-12-17
    Xiazi Xiao; Long Yu

    Ion irradiation offers a promising strategy to emulate the irradiation damage induced by energetic neutrons. However, the characterization of the mechanical properties of ion-irradiated materials is not easy considering the limited irradiation depth and inhomogeneous distribution of irradiation-induced defects. Over the last decades, nano-indentation has been recognized as a valid technique to investigate the localized mechanical responses at micro-scale. Therefore, the combination of ion irradiation and nano-indentation has been extensively developed in recent years to study the mechanical behaviors of nuclear structural materials with irradiation effect. In this review, current developments of experimental observations, numerical simulations and theoretical models are summarized concerning both microstructural evolution and macroscopic deformation. Corresponding analysis and discussion could help obtain a sophisticated comprehension of the fundamental deformation mechanisms resulting in mechanical degradation of ion-irradiated materials, and further assist the development of next-generation nuclear structural materials.

    更新日期:2019-12-18
  • Cavity formation and hardness change in He implanted EUROFER97 and EU-ODS EUROFER
    Nucl. Mater. Energy (IF 2.025) Pub Date : 2019-11-20
    M. Roldán, P. Fernández, J. Rams, A. Gómez-Herrero, M. Malo

    RAFM steels EUROFER97 and EU-ODS EUROFER samples have been implanted with He ions at 40 keV (ion penetration depth ~300 nm) at a dose of ~ 1 × 1015 ion/cm2 and different temperatures up to 550 °C. Post-irradiation examination of the samples has been performed using nanoindentation along with conventional and scanning transmission electron microscopy (CTEM/STEM in Annular Dark Field mode). The specimens were indented up to 500 nm by CSM method (Continuous Stiffness Measurement), in order to assess the changes in nanoindentation hardness values due to the irradiation. After indentation tests, lamella was extracted from each implanted sample which included at least one nanoindentation cross-section. The changes in hardness were correlated with any microstructure modification detected by TEM. A clear trend can be observed; the hardness values increased with irradiation temperature, as well as the He bubble nucleation and population density. A remarkable change in bubble distribution in EUROFER97 was found at the two highest irradiation temperatures (450 °C and 550 °C). In contrast, an increase in the cavities size was observed in EU-ODS EUROFER, but no new nucleation was observed for the same temperatures. These observations suggest that for these particular conditions, the cavity growth is enhanced, rather than new nucleation, which depends strongly on the material microstructure.

    更新日期:2019-12-11
  • Exploring SF- in-out asymmetry and detachment bifurcation in HL-2M with E × B by SOLPS
    Nucl. Mater. Energy (IF 2.025) Pub Date : 2019-11-30
    Hailong Du, Guoyao Zheng, Xavier Bonnin, Jiaxian Li, Lei Xue, Xuru Duan, Wenyu Huang

    In this paper, the SF- (with the second X-point located near the outer target surface) in-out asymmetry and detachment bifurcation in HL-2M were explored by the edge plasma code SOLPS5.1 including electric but neglecting magnetic drifts. The modeling results reveal that the radial electric drift component (Epol × B) can play a key role on inducing the SF- in-out asymmetry due to the effective carbon (C) screen capacity and high heat dissipation. The Epol × B drift can enhance the poloidal electric drift component (Er × B), and their synergistic effects can induce a marked in-out asymmetry. Besides, the results showed that the detachment bifurcation in SF- also could occur under given conditions. The presence of four factors (including E × B, C impurity, normal-Bt, and relatively small radial anomalous transport coefficients) forms the necessary conditions for producing the detachment bifurcation. The root reason for the detachment bifurcation formation is due to the strong interaction of Te, E×B, and C impurity radiation loss in the outer divertor region with normal-Bt direction. The detachment bifurcation could be removed or mitigated by weakening their interaction. Finally, the time-dependent detachment bifurcation process is simulated in this work to better understand its physical picture.

    更新日期:2019-12-11
  • Experimental investigation of the microstructural changes of tungsten monoblocks exposed to pulsed high heat loads
    Nucl. Mater. Energy (IF 2.025) Pub Date : 2019-11-30
    V. Shah, M.P.F.H.L. van Maris, J.A.W. van Dommelen, M.G.D. Geers
    更新日期:2019-12-11
  • Determination of retained tritium from ILW dust particles in JET
    Nucl. Mater. Energy (IF 2.025) Pub Date : 2019-04-30
    N. Ashikawa, Y. Torikai, N. Asakura, T. Otsuka, A. Widdowson, M. Rubel, M. Oyaizu, M. Hara, S. Masuzaki, K. Isobe, Y. Hatano, K. Heinola, A. Baron-Wiechec, S. Jachmich, T. Hayashi

    Quantitative tritium inventory in dust particles from campaigns in the JET tokamak with the carbon wall (2007-2009) and the ITER-like wall (ILW 2011-2012) were determined by the liquid scintillation counter and the full combustion method. A feature of this full combustion method is that dust particles were covered by a tin (Sn) which reached 2100 K during combustion under oxygen flow. The specific tritium inventory for samples from JET with carbon and with metal walls was measured and found to be similar. However, the total tritium inventory in dust particles from the ILW experiment was significantly smaller in comparison to the carbon wall due to the lower amount of dust particles generated in the presence of metal walls.

    更新日期:2019-12-11
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