Samir El Shawish

Jožef Stefan Institute | IJS · Reactor Engineering Division

A simple micromechanical model of polycrystalline materials is proposed, which enables us to swiftly produce grain-boundary-stress distributions induced by the uniform external loading (in the elastic strain regime). Such statistical knowledge of local stresses is a necessary prerequisite to assess the probability for intergranular cracking initiat...

Intergranular normal stresses (INS) are critical in the initiation and evolution of grain boundary damage in polycrystalline materials. To model the effects of such microstructural damage on a macroscopic scale, knowledge of INS is usually required statistically at each representative volume element subjected to various loading conditions. However,...

In this work, we propose a new crystal plasticity finite element model to simulate the mechanical behavior of austenitic stainless steel subjected to operational conditions typical for internal structures of light water reactors. In particular, we address the coupling of (i) hydrogen and (ii) irradiation defects (Frank loops) with mobile dislocatio...

Intergranular normal stresses (INS) are critical in the initiation and evolution of grain boundary damage in polycrystalline materials. To model the effects of such microstructural damage on a macroscopic scale, knowledge of INS is usually required statistically at each representative volume element subjected to various loading conditions. However,...

Dynamics of the Taylor bubble interface in the vertical counter-current flow was analyzed with videos at 100 to 800 frames per second. Taylor bubbles in air-water mixture were studied on time intervals of up to several minutes in stagnant conditions, where buoyancy is dynamically balanced by the inertial bubble drag in the downward turbulent flow....

A simple analytical model of intergranular normal stresses is proposed for a general elastic polycrystalline material with arbitrary shaped and randomly oriented grains under uniform loading. The model provides algebraic expressions for the local grain-boundary-normal stress and the corresponding uncertainties, as a function of the grain-boundary t...

An initial concept for the plasma diagnostic and control (D&C) system has been developed as part of European studies towards the development of a demonstration tokamak fusion reactor (DEMO). The main objective is to develop a feasible, integrated concept design of the DEMO D&C system that can provide reliable plasma control and high performance (el...

Detachment control in DEMO is a fundamental requirement to prevent damage to the plasma facing components. Thermo-currents flowing through the plasma facing components of the divertor cassette are driven by the thermoelectric voltage generated by the plasma temperature difference between the inner and outer target plates. Shunt and Rogowski coil me...

A finite element analysis of intergranular normal stresses is performed in order to identify a possible statistical correlation between the intergranular normal stresses and the corresponding grain boundary type within a polycrystalline aggregate. Elastic continuum grains of cubic lattice symmetry are assumed in the analysis. Meaningful results are...

Irradiation Assisted Stress Corrosion Cracking (IASCC) is a material degradation phenomenon affecting austenitic stainless steels used in nuclear Pressurized Water Reactors (PWR), leading to the initiation and propagation of intergranular cracks. Such phenomenon belongs to the broader class of InterGranular Stress Corrosion Cracking (IGSCC). A micr...

Irradiation Assisted Stress Corrosion Cracking (IASCC) is a material degradation phenomenon affecting austenitic stainless steels used in nuclear Pressurized Water Reactors (PWR), leading to the initiation and propagation of intergranular cracks. Such phenomenon belongs to the broader class of InterGranular Stress Corrosion Cracking (IGSCC). A micr...

A recently proposed isolated-target design solution for the DEMO divertor introduces a shunt resistor between the target plate and the cassette body, which are otherwise electrically isolated from each other. Such a design serves as a diagnostic tool for measuring thermo-currents (voltages) through a shunt to control plasma detachment. Ideally, lar...

The design activities of an insulated Plasma Facing Components-Cassette Body (PFCs-CB) support has been carried out under the pre-conceptual design phase for Eurofusion-DEMO Work Package DIV-1 "Divertor Cassette Design and Integration" - Eurofusion Power Plant Physics & Technology (PPPT) program. The Eurofusion-DEMO divertor is a key in-vessel comp...

We compare two full-field approaches – a crystal plasticity finite element method (CP-FEM) and crystal plasticity fast Fourier transform-based (CP-FFT) method – for a specific crystal plasticity law introduced for neutron-irradiated austenitic stainless steel SA304L currently used in nuclear reactor vessel internals. This particular law is employed...

The plasma diagnostic and control (D&C) system for a future tokamak demonstration fusion reactor (DEMO) will have to provide reliable operation near technical and physics limits, while its front-end components will be subject to strong adverse effects within the nuclear and high temperature plasma environment. The ongoing developments for the ITER...

Global plasma instabilities such as plasma disruptions and vertical displacement events (VDEs) can generate huge transient electrical currents in the conductive parts of the divertor. The interaction of the currents with the magnetic field induces Lorentz force loads. Particularly, the force induced by a VDE can impose a critical impact on the dive...

From a general point of view, InterGranular Stress-Corrosion Cracking (IGSCC) results from the interplay between mechanical loading and grain boundaries opening. The former leads to intergranular stresses in polycrystalline aggregates, the latter being either stress-accelerated or stress-induced. This work aims at obtaining intergranular normal str...

The influence of the power spectral density (PSD) profile of fluid temperature fluctuations on thermal fatigue predictions of pipes is analysed in this paper. The fatigue assessment employs the improved spectral method for the generation of synthetic temperature histories, simplified one-dimensional pipe model and codified rules for fatigue design...

This work concerns a comparison between Finite Element Method (FEM) simulations and Fast Fourier Transforms (FFT) based simulations on an irradiated austenitic stainless steel (SA304L) currently used as material for internals in nuclear reactors. The main objective was to model the elasto-viscoplastic behavior of this material with 3-dimensional nu...

The turbulent mixing of fluids at different temperatures in T-junctions may be the cause of thermal fatigue damage in the surrounding pipes. In the nuclear industry, primary water leakages have occurred in the safety related piping due to this fluid phenomenon and, currently, there are no generally accepted design rules to avoid the thermal fatigue...

Crystal plasticity finite element models have been extensively used to simulate various aspects of polycrystalline deformations. A common weakness of practically all models lies in a relatively large number of constitutive modeling parameters that, in principle, would require dedicated measurements on proper length scales in order to perform reliab...

Large sets of fluid temperature histories and a recently proposed thermal fatigue assessment procedure are employed in this paper to deliver more accurate statistics of predicted lives of pipes and their uncertainties under turbulent fluid mixing circumstances. The wide variety of synthetic fluid temperatures, generated with an improved spectral me...

The turbulent mixing of fluids at different temperatures is a well-recognized source of thermal fatigue in the safety related piping of nuclear power plants. The fluid temperature fluctuations at the fluid-wall interface, caused by the turbulent mixing, induce stress fluctuations in the pipe, which may lead, in some circumstances, to fatigue and su...

In order to predict InterGranular Stress Corrosion Cracking (IGSCC) of post-irradiated austenitic stainless steel in Light Water Reactor (LWR) environment, reliable predictions of intergranular stresses are required. Finite elements simulations have been performed on realistic polycrystalline aggregate with recently proposed physically-based crysta...

This paper proposes improved thermal fatigue assessment of pipes subjected to turbulent fluid mixing using an improved spectral loading approach. The fluid temperature histories are generated synthetically from the spatially incomplete experimental or very expensive computational data, preserving consistency with the first two statistical moments a...

The evolution of grain misorientations (also called orientation gradients) at and close to the free surface of polycrystalline aggregates under tensile straining is analyzed within the crystal plasticity finite element model. The grain misorientation is defined as an angle of crystal orientation change with respect to the average grain orientation....

A micro-scale crystal plasticity model, recently developed by CEA, is used to simulate a nonlinear mechanical response of type 304L austenitic stainless steel subjected to neutron irradiation. The model is implemented in codes ABAQUS and Cast3M to perform finit element simulations of the polycrystalline aggregates loaded in tension. Grain topology...

Thermal fatigue assessment of pipes due to turbulent fluid mixing in T-junctions is a rather difficult task because of the existing uncertainties and variability of induced thermal stresses. In these cases, thermal stresses arise on three-dimensional pipe structures due to complex thermal loads, known as thermal striping, acting at the fluid-wall i...

There is a need to perform three-dimensional mechanical analyses of pipes, subjected to complex thermo-mechanical loadings such as the ones evolving from turbulent fluid mixing in a T-junction. A novel approach is proposed in this paper for fast and reliable generation of random thermal loads at the pipe surface. The resultant continuous and time-d...

We propose and demonstrate several tests to estimate the performance of the cohesive elements in ABAQUS for modeling grain boundaries in complex spatial structures such as polycrystalline aggregates. The performance of the cohesive elements is checked by comparing the computed stresses with the theoretically predicted values for a homogeneous mater...

Stainless steel is a commonly used material in safety-important components of nuclear power plants. In order to study degradation mechanisms in stainless steels, like crack initiation and propagation, it is important to characterize the degree of plastic strain on microstructural level. One way to estimate local plastic strain is by measuring local...

Thermal fatigue is a structural damage of materials induced by the cyclic thermal loads that are frequently generated by the changes of fluid temperature inside of pipes. Among the thermal fatigue assessment methods we find the one-dimensional (1D) approach. Thermal, mechanical and fatigue analyses are performed for the pipe wall assuming that the...

We propose a simple anisotropic elasto-plastic finite element model to quantify the deformation heterogeneity locally within the grains of a polycrystal under plastic deformation. The model is calibrated against the electron backscatter diffraction measurements on stainless steel under uniaxial tensile loading. The spatial model of the specimen is...

We investigate - with Monte Carlo computer simulations - the phase behaviour of dimeric colloidal molecules on periodic substrates with square symmetry. The molecules are formed in a two-dimensional suspension of like charged colloids subject to periodic external confinement, which can be experimentally realized by optical methods. We study the evo...

Dynamics of adhesion of single liposome at the charged mercury interface is analyzed through its amperometric signal using a reaction kinetics model and a mechanical model. We present analytical solutions of the reaction kinetics model for decoupling and identifying temporal evolution of three distinct states: i) the initial state corresponding to...

We investigate the Periodic Anderson model in the regime of itinerant ferromagnetism. We compare Quantum Monte Carlo (QMC) results with results obtained using the Gutzwiller approximation (GA). As expected, the energy of the paramagnetic state is overestimated by the GA in comparison with QMC results; however, the partially saturated ferromagnetic...

We theoretically and numerically studied ordering of "colloidal ionic clusters" on periodic substrate potentials such as those generated by optical trapping. Each cluster consists of three charged spherical colloids: two negatively and one positively charged. The substrate is a square or rectangular array of traps, each confining one such cluster....

A vast range of both living and inanimate planar cellular partitions obeys universal empirical laws describing their structure. To better understand this observation, we analyze the morphometric parameters of a sizeable set of experimental data that includes animal and plant tissues, patterns in desiccated starch slurry, suprafroth in type-I superc...

We study the electron spin resonance (ESR) of low-dimensional spin systems at high temperature, and test the Kubo-Tomita theory of exchange narrowing. In finite-size systems (molecular magnets), we found a double-peak resonance which strongly differs from the usual Lorentzian. For infinite systems, we have predictions for the linewidth and lineshap...

A two-dimensional colloidal suspension subject to a periodic substrate evolves into a colloidal molecular crystal under situations of strong confinement. We focus on the long-range orientational order thereby emerging, in the ground state. We study by simulations the situations where in each trap lies a pair of identical colloids, or alternatively...

En-face cross-section of simple biological tissues such as planar epithelia resembles polygons that tile the plane. We show that the structure of such tissues can be explained by an equilibrium model where energy degenerate polygons in an entropy-maximizing tiling are described by a single geometric parameter reduced area a measuring their roundedn...

We report unexpected behaviour in a family of Cu spin- 1/2 systems, in which an apparent gap in the low energy magneto-optical absorption spectrum opens at low temperature. This previously unreported collective phenomenon arises at temperatures where the energy of the dominant exchange interaction exceeds the thermal energy. Simulations of the obse...

SrCu2(BO3)2 is a quasi-2D quantum spin system known to possess a collective singlet ground state and a realization of the Shastry-Suthrland model. One aspect of the study of SrCu2(BO3)2 for which there is a little information is the influence of impurities on the nature of the singlet ground state. There is much interest in such studies due to the...

Pulsed-field magnetization experiments (up to 85 T) and electron paramagnetic resonance (EPR) experiments (10-110 GHz) are reported on a family of organic Cu-based two-dimensional (2D) Heisenberg magnets. The low-T M(H) relationship is concave, with a sharp transition to a saturation value at a critical field Hc. Monte-Carlo simulations including a...

Two dimensional suspensions of spherical colloids subject to periodic external fields exhibit a rich variety of molecular crystalline phases. We study in simulations the ground state configurations of dimeric and trimeric systems, that are realized on square and triangular lattices, when either two or three macroions are trapped in each external po...

The dc magnetization and the electron spin resonance (ESR) measurements have been performed on alpha-NaMnO2 polycrystalline sample, a quantum spin system on a frustrated two-dimensional (2D) triangular lattice with spatially anisotropic Heisenberg exchange. The former measurements reveal a realization of the high-spin state (S=2) on magnetic Mn3+ s...

We show that temperature and magnetic field properties of the entanglement between spins on the two-dimensional Shastry-Sutherland lattice can be qualitatively described by analytical results for a qubit tetramer. Exact diagonalization of clusters with up to 20 sites reveals that the regime of fully entangled neighboring pairs coincides with the re...

High resolution neutron scattering measurements on a single crystal of SrCu(2-x)Mgx(BO3)2 with x approximately 0.05 reveal the presence of new spin excitations within the gap of this quasi-two-dimensional, singlet ground state system. The application of a magnetic field induces Zeeman-split states associated with S=1/2 unpaired spins which are anti...

We employ finite-temperature Lanczos technique on finite clusters to compute dynamical spin structure factor of Shastry–Sutherland model, which describes quasi-two-dimensional dimer spin-liquid SrCu2(BO3)2 compound. Unusual temperature dependence of calculated spectra is in agreement with inelastic neutron scattering measurements. Introduction of n...

Using variational algorithm on an infinite Shastry-Sutherland (SS) lattice we show that the introduction of a static nonmagnetic impurity into a dimerized ground state leads to a formation of a small, localized spin polaron surrounding the impurity site. Due to a particular symmetry of the SS lattice, the polaron is extremely anisotropic with a sho...

We present an experimental and theoretical study of the magnetically frustrated spin system in pure and substitutionally disordered compounds from the Cu2Te2O5X2 family of quantum magnets. Experimental magnetic susceptibilities and specific heats were analyzed simultaneously using models of (i) isolated tetrahedra of four antiferromagnetically coup...

We present an experimental and theoretical study of the magnetically frustrated spin system in pure and substitutionally disordered compounds from the Cu2Te2O5X2 family of quantum magnets. Experimental magnetic susceptibilities and specific heats were analyzed simultaneously using models of (i) isolated tetrahedra of four antiferromagnetically coup...

Using finite temperature Lanczos technique on finite clusters we calculate dynamical spin structure factor of the quasi-two-dimensional dimer spin liquid SrCu2(BO3)2 as a function of wave vector and temperature. At low temperatures the peak belonging to the lowest spin excitations is split due to spin anisotropy, in accord with the experimental dat...

We report measurements of the specific heat of the quantum spin liquid system SrCu2(BO3)2 in continuous magnetic fields H of up to 33T. The specific heat data, when combined with a finite temperature Lanczos diagonalization of the Shastry-Sutherland Hamiltonian, indicate the presence of a nearest neighbor Dzyaloshinsky-Moriya (DM) interaction that...

We report measurements of the specific heat of them quantum spin liquid system SrCu2(BO3)(2) in continuous magnetic fields H of up to 33 T. The specific heat data, when combined with a finite temperature Lanczos diagonalization of the Shastry-Sutherland Hamiltonian, indicate the presence of a nearest neighbor Dzyaloshinsky-Moriya (DM) interaction t...

We calculate the electron spin resonance (ESR) spectra of the quasi-two-dimensional dimer spin liquid SrCu2(BO3)2 as a function of magnetic field B. Using the standard Lanczos method, we solve a Shastry-Sutherland Hamiltonian with additional Dzyaloshinsky-Moriya (DM) terms which are crucial to explain different qualitative aspects of the ESR spectr...

The thermodynamic properties: specific heat and magnetization are studied as a function of temperature, doping, and interlevel spacing within the two-dimensional extended Falicov–Kimball model for spinless fermions. It was recently shown that the strong coupling limit of the above model possesses electronically driven ferroelectric order. Thermodyn...

We analyze the high-temperature conductivity in one-dimensional integrable models of interacting fermions: the t-V model (anisotropic Heisenberg spin chain) and the Hubbard model, at half-filling in the regime corresponding to insulating ground state. A microcanonical Lanczos method study for finite size systems reveals anomalously large finite-siz...

The thermodynamic properties of the extended Falicov-Kimball model (FKM) for spinless fermions are studied as a function of temperature, doping, and interlevel spacing on the two-dimensional lattice. Thermodynamic quantities are calculated using the finite-temperature Lanczos method with additional phase averaging for a system of 4×4 sites. Changes...

We report measurements of the specific heat of the quantum spin liquid system SrCu2(BO3)2 in continuous magnetic fields H of up to 33 T. The specific heat vs temperature at zero field shows an anomaly at 8 K, marking the opening of a gap in the spin singlet excitations. At fields H~12 T, we clearly see a second anomaly that shifts to lower temperat...

We show how to generalise the zero temperature Lanczos method for calculating dynamical correlation functions to finite temperatures. The key is the microcanonical ensemble, which allows us to replace the involved canonical ensemble with a single appropriately chosen state; in the thermodynamic limit it provides the same physics as the canonical en...

The properties of the polaron and bipolaron are explored in the one-dimensional Jahn-Teller model with dynamical quantum phonons. The ground-state properties of the polaron and bipolaron are computed using a recently developed variational method. Dynamical properties of the ground state of a polaron are investigated by calculating the optical condu...