Nasr M. Ghoniem

Operating temperature windows for fusion reactor structural materials

Abstract A critical analysis is presented of the operating temperature windows for nine candidate fusion reactor structural materials: four reduced-activation structural materials (oxide-dispersion-strengthened and ferritic/martensitic steels containing 8–12%Cr, V–4Cr–4Ti, and SiC/SiC composites), copper-base alloys (CuNiBe), tantalum-base alloys (e.g. Ta–8W–2Hf), niobium alloys (Nb–1Zr), and molybdenum and tungsten alloys. The results are compared with the operating temperature limits for Type 316 austenitic stainless steel. Several factors define the allowable operating temperature window for structural alloys in a fusion reactor. The lower operating temperature limit in all body-centered cubic (BCC) and most face-centered cubic (FCC) alloys is determined by radiation embrittlement (decrease in fracture toughness), which is generally most pronounced for irradiation temperatures below ∼0.3 T M where T M is the melting temperature. The lower operating temperature limit for SiC/SiC composites will likely be determined by radiation-induced thermal conductivity degradation, which becomes more pronounced in ceramics with decreasing temperature. The upper operating temperature limit of structural materials is determined by one of four factors, all of which become more pronounced with increasing exposure time: (1) thermal creep (grain boundary sliding or matrix diffusional creep); (2) high temperature He embrittlement of grain boundaries; (3) cavity swelling (particularly important for SiC and Cu alloys); or (4) coolant compatibility/corrosion issues. In many cases, the upper temperature limit will be determined by coolant corrosion/compatibility rather than by thermal creep or radiation effects. The compatibility of the structural materials with Li, Pb–Li, Sn–Li, He and Flibe (Li 2 BeF 4 ) coolants is summarized.

On the exploration of innovative concepts for fusion chamber technology

Abstract This study, called APEX, is exploring novel concepts for fusion chamber technology that can substantially improve the attractiveness of fusion energy systems. The emphasis of the study is on fundamental understanding and advancing the underlying engineering sciences, integration of the physics and engineering requirements, and enhancing innovation for the chamber technology components surrounding the plasma. The chamber technology goals in APEX include: (1) high power density capability with neutron wall load >10 MW/m 2 and surface heat flux >2 MW/m 2 , (2) high power conversion efficiency (>40%), (3) high availability, and (4) simple technological and material constraints. Two classes of innovative concepts have emerged that offer great promise and deserve further research and development. The first class seeks to eliminate the solid “bare” first wall by flowing liquids facing the plasma. This liquid wall idea evolved during the APEX study into a number of concepts based on: (a) using liquid metals (Li or Sn–Li) or a molten salt (Flibe) as the working liquid, (b) utilizing electromagnetic, inertial and/or other types of forces to restrain the liquid against a backing wall and control the hydrodynamic flow configurations, and (c) employing a thin (∼2 cm) or thick (∼40 cm) liquid layer to remove the surface heat flux and attenuate the neutrons. These liquid wall concepts have some common features but also have widely different issues and merits. Some of the attractive features of liquid walls include the potential for: (1) high power density capability; (2) higher plasma β and stable physics regimes if liquid metals are used; (3) increased disruption survivability; (4) reduced volume of radioactive waste; (5) reduced radiation damage in structural materials; and (6) higher availability. Analyses show that not all of these potential advantages may be realized simultaneously in a single concept. However, the realization of only a subset of these advantages will result in remarkable progress toward attractive fusion energy systems. Of the many scientific and engineering issues for liquid walls, the most important are: (1) plasma–liquid interactions including both plasma–liquid surface and liquid wall–bulk plasma interactions; (2) hydrodynamic flow configuration control in complex geometries including penetrations; and (3) heat transfer at free surface and temperature control. The second class of concepts focuses on ideas for extending the capabilities, particularly the power density and operating temperature limits, of solid first walls. The most promising idea, called EVOLVE, is based on the use of a high-temperature refractory alloy (e.g. W–5% Re) with an innovative cooling scheme based on the use of the heat of vaporization of lithium. Calculations show that an evaporative system with Li at ∼1 200°C can remove the goal heat loads and result in a high power conversion efficiency. The vapor operating pressure is low, resulting in a very low operating stress in the structure. In addition, the lithium flow rate is about a factor of ten lower than that required for traditional self-cooled first wall/blanket concepts. Therefore, insulator coatings are not required. Key issues for EVOLVE include: (1) two-phase heat transfer and transport including MHD effects; (2) feasibility of fabricating entire blanket segments of W alloys; and (3) the effect of neutron irradiation on W.

Multiscale modelling of nanomechanics and micromechanics: an overview

Recent advances in analytical and computational modelling frameworks to describe the mechanics of materials on scales ranging from the atomistic, through the microstructure or transitional, and up to the continuum are reviewed. It is shown that multiscale modelling of materials approaches relies on a systematic reduction in the degrees of freedom on the natural length scales that can be identified in the material. Connections between such scales are currently achieved either by a parametrization or by a ‘zoom-out’ or ‘coarse-graining’ procedure. Issues related to the links between the atomistic scale, nanoscale, microscale and macroscale are discussed, and the parameters required for the information to be transferred between one scale and an upper scale are identified. It is also shown that seamless coupling between length scales has not yet been achieved as a result of two main challenges: firstly, the computational complexity of seamlessly coupled simulations via the coarse-graining approach and, secondly, the inherent difficulty in dealing with system evolution stemming from time scaling, which does not permit coarse graining over temporal events. Starting from the Born–Oppenheimer adiabatic approximation, the problem of solving quantum mechanics equations of motion is first reviewed, with successful applications in the mechanics of nanosystems. Atomic simulation methods (e.g. molecular dynamics, Langevin dynamics and the kinetic Monte Carlo method) and their applications at the nanoscale are then discussed. The role played by dislocation dynamics and statistical mechanics methods in understanding microstructure self-organization, heterogeneous plastic deformation, material instabilities and failure phenomena is also discussed. Finally, we review the main continuum-mechanics-based framework used today to describe the nonlinear deformation behaviour of materials at the local (e.g. single phase or grain level) and macroscopic (e.g. polycrystal) scales. Emphasis is placed on recent progress made in crystal plasticity, strain gradient plasticity and homogenization techniques to link deformation phenomena simultaneously occurring at different scales in the material microstructure with its macroscopic behaviour. In view of this wide range of descriptions of material phenomena involved, the main theoretical and computational difficulties and challenges are critically assessed.

Molecular dynamics determination of defect energetics in beta -SiC using three representative empirical potentials

The determination of formation and migration energies of point and clustered defects in SiC is of critical importance to a proper understanding of atomic phenomena in a wide range of applications. We present here calculations of formation and migration energies of a number of point and clustered defect configurations. A newly developed set of parameters for the modified embedded-atom method (MEAM) is presented. Detailed molecular dynamics calculations of defect energetics using three representative potentials, namely the Pearson potential, the Tersoff potential and the MEAM, have been performed. Results of the calculations are compared to first-principles calculations and to available experimental data. The results are analysed in terms of developing a consistent empirical interatomic potential and are used to discuss various atomic migration processes.

On dislocation interaction with radiation-induced defect clusters and plastic flow localization in fcc metals

Abstract Plastic instability associated with formation of narrow flow channels results from dislocation pinning–unpinning by defect clusters. We investigate the dynamics of dislocation interaction with radiation-induced defect clusters, and specifically with, firstly, sessile self-interstitial atom clusters in dislocation decorations and, secondly, stacking-fault tetrahedra (SFTs) in the matrix. It is shown that the critical stress to free trapped dislocations from pinning atmospheres can be a factor of two smaller than values obtained on the basis of rigid dislocation interactions. The unpinning mechanism is a consequence of the growth of morphological instabilities on the dislocation line. Dislocation sources are activated in spatial regions of low SFT density, where their destruction by glide dislocations leads to subsequent growth of localized plasticity in dislocation channels. We show that removal of SFTs is associated with simultaneous dislocation glide and climb. Jogs of atomic dimensions are formed when a fraction of SFT vacancies are absorbed by pipe diffusion. The width of a flow channel is explained in terms of two length scales: the size of an individual SFT, and the dislocation source-to-boundary distance (d of the order of micrometres). While dislocation segments climb by a few atomic planes with each SFT destruction event, d determines the total number of such events. Numerically computed channel widths (about 70–150nm), and the magnitude of radiation hardening in copper are consistent with experimental observations.

Theory of helium transport and clustering in materials under irradiation

A theoretical model is developed to describe helium transport and clustering during irradiation. Diffusional reactions of helium with vacancies and vacancy clusters, with extended sinks for helium absorption, and direct reactions with displacement producing particles are included. The full description developed is employed in numerical computations. A simpler description is also developed in the limits where certain reactions are unimportant. Analytical expressions for the effective diffusion coeficient of helium are derived. Regimes of the parameter space of dose rate, temperature, helium generation rate, sink strength and other important conditions, where the effective diffusion coefficient is dictated by three different physical processes, are defined. The result is determined by the dominant release mechanism of helium bound in vacancies-thermal detrapping, replacement by the self-interstitial or direct displacement. Results from the full computations and the analytical expressions are compared.

A parallel algorithm for 3D dislocation dynamics

Dislocation dynamics (DD), a discrete dynamic simulation method in which dislocations are the fundamental entities, is a powerful tool for investigation of plasticity, deformation and fracture of materials at the micron length scale. However, severe computational difficulties arising from complex, long-range interactions between these curvilinear line defects limit the application of DD in the study of large-scale plastic deformation. We present here the development of a parallel algorithm for accelerated computer simulations of DD. By representing dislocations as a 3D set of dislocation particles, we show here that the problem of an interacting ensemble of dislocations can be converted to a problem of a particle ensemble, interacting with a long-range force field. A grid using binary space partitioning is constructed to keep track of node connectivity across domains. We demonstrate the computational efficiency of the parallel micro-plasticity code and discuss how O(N) methods map naturally onto the parallel data structure. Finally, we present results from applications of the parallel code to deformation in single crystal fcc metals.

The investigation of high intensity laser driven micro neutron sources for fusion materials research at high fluence

The application of fast pulse, high intensity lasers to drive low cost DT point neutron sources for fusion materials testing at high ux/uence is investigated. At present, high power bench- top lasers with intensities of 10 18 W=cm 2 are routinely employed and systems capable of10 21 W=cm 2 are becoming available. These potentially oer sucient energy density for ecient neutron production in DT targets with dimensions of around 100 m. Two dierent target concepts are analysed | a hot ion, beam{target system and an exploding pusher target system | and neutron emission rates are evaluated as a function of laser and target conditions. Compared with conventional beam{target neutron sources with steady state liquid cooling, the driver energy here is removed by sacricial vaporization of a small target spot. The resulting small source volumes oer the potential for a low cost, high ux source of 14 MeV neutrons at close coupled, micro (1 mm) test specimens. In particular, it is shown that a laser driven target with100 J/pulse at 100 Hz (i.e.10 kW average power) and laser irradiances in the range I 2 10 17 10 19 W m 2 =cm 2 could produce primary, uncollided neutron uxes at the test specimen in the 10 14 10 15 nc m 2 s 2 range. While focusing on the laser{plasma interaction physics and resulting neutron production, the materials science required to validate computational damage models utilizing 100 dpa irradiation of such specimens is also examined; this may provide a multiscale predictive capability for the behaviour of engineering scale components in fusion reactor applications.

Interaction and accumulation of glissile defect clusters near dislocations

Abstract Accumulation of nano-size prismatic defect clusters near slip-dislocations results from their mutual elastic interaction. We present here 3-D isotropic elasticity calculations for the interaction energy between radiation-induced nano-size prismatic loops and grown-in dislocation loops. The current treatment extends the work of Trinkaus et al. in two respects. First, a computational method for full 3-D analysis of interaction energies in bcc Fe and fcc Cu is developed. Second, the theoretical method of Kroupa is computationally implemented for rigorous calculations of force, torque and induced surface energy on defect clusters. It is shown that small clusters are trapped within a zone of ∼10 nm in bcc Fe, and ∼20 nm in fcc Cu at room temperature, in rough agreement with experimental observations. Clusters can be absorbed in the core of grown-in dislocations because of unbalanced moments, which provide sufficient energy for rotation of their Burgers vectors in a zone of 2–3 nm in Fe. Near the dislocation core (within a few nanometers), sessile defect clusters in Cu are shown to convert to a glissile configuration.

Dislocation motion in anisotropic multilayer materials

Line integral forms for the elastic field of dislocations in anisotropic, multilayer materials are developed and utilized in Parametric Dislocation Dynamics (PDD) computer simulations. Developed equations account for interface image forces on dislocations as a result of elastic modulus mismatch between adjacent layers. The method is applied to study dislocation motion in multilayer thin films. The operation of dislocation sources, dislocation pileups, confined layer slip (CLS), and the loss of layer confinement are demonstrated for a duplex Cu/Ni system. The strength of a thin film of alternating nanolayers is shown to increase with decreasing layer thickness, and that the maximum strength is determined by the Koehler barrier in the absence of coherency strains. For alternating Cu/Ni nanolayers, the dependence of the strength on the duplex layer thickness is found to be consistent with experimental results, down to a layer thickness of ≈10nm.