C. Domain

Influence of the interatomic potentials on molecular dynamics simulations of displacement cascades

Abstract Molecular dynamics (MD) is a powerful tool to study the displacement cascades initiated by the neutrons when they interact with matter. Key components of this technique are the interatomic potentials which model the binding of the different constitutive atoms. There exist many interatomic potentials dedicated to α-Fe and we have tested three of them for the study of radiation damage. We have found that the primary damage is potential sensitive. From our study, it appears that some characteristics of the potentials, not always considered, can be correlated to the type of damage produced by displacement cascades. The repulsive part of the potential has a strong influence on the cascade morphology. Moreover, equilibrium properties such as the atoms mean square displacements, the vacancy migration and vacancy–vacancy binding energies also appear to have some influence and should be investigated carefully when simulating radiation damage. It is therefore very important to use extreme care when trying to obtain quantitative results from MD simulations.

The role of Cu in displacement cascades examined by molecular dynamics

Abstract Molecular dynamics simulations of displacement cascades in pure Fe, Fe–0.2 at.% Cu and Fe–2 at.% Cu have been done with different primary knocked-on atom (PKA) energies. The typical defects (vacancies and interstitials) appear during the cascade process. Most of the defects recombine in the first few picoseconds. The presence of Cu atoms does not seem to influence the primary damage in the time span covered by MD. Neither Cu precipitates, nor dilute atmospheres have been observed to form in the course of the MD simulations. However, a tendency to form mixed objects (vacancies or interstitials gathered with Cu atoms) is noticed in the Fe–2 at.% Cu. Our feeling is that the numerous vacancies left at the end of the cascade recombination phase will interact with the Cu atoms and will act as attractive centers. The role of the interstitials is less clear for the moment.

Ab initio calculation of intrinsic point defects in CuInSe2

Abstract Preliminary ab initio calculation of different point defects energy and electronic density of states have been performed on the prototype chalcopyrite semiconductor CuInSe 2 . The simulation method used is based on the density functional theory within the framework of pseudo-potentials and plane waves basis. The isolated neutral defects considered are: V Cu , V In , V Se , Cu i , In i , In Cu , Cu In and the complex defects are 2Cu i +Cu In , In Cu +Cu In and 2V Cu +In Cu , some of which being computed for the first time by advanced ab initio techniques. In agreement with previous results, we show that some point defects (such as V Cu ) and pair defects (2V Cu +In Cu ) have very low formation energies. Some energies of formation were found significantly lower than previous estimations. The comparison of the formation energies with the exchange correlation (LDA or GGA) is discussed. The perturbation induced by the presence of some of these ideal defects on the density of states is also presented.

Massively parallel molecular dynamics simulations with EAM potentials

Abstract Molecular dynamics of cascades in pure iron and iron–copper alloys using embedded atom method type of interatomic potentials are presented. Reliable simulations of radiation damage at the atomic scale with high energy Primary Knocked Atoms (PKA) need systems with large numbers of particles and very long computational time. To perform the simulation in a reasonable amount of time high-performance computer systems such as massively parallel machines need to be used. This paper presents the parallelisation strategy applied to a serial classical Molecular Dynamics code: DYMOKA. The original sequential Fortran code CDCMD from the University of Connecticut was first improved algorithmically by applying a link cell method for the neighbour list construction of the Verlet list, resulting in a fully linear algorithm. The parallelisation strategy adopted is a multidimensional domain decomposition of the simulation box using a link cell method and a Verlet list method for each subdomain independently. The p...

Simulation of Irradiation Effects in Reactor Pressure Vessel Steels: the Reactor for Virtual Experiments (REVE) Project

Components of commercial nuclear reactors are subjected to neutron bombardments that can modify their mechanical properties. Prediction of in-service and post-service behaviors generally requires irradiation in so-called “test reactors” as well as subsequent mechanical testing in specialized hot cell facilities. However, the use of these research facilities is becoming more problematic, in particular due to increasing costs and decreasing availability. One way of partially mitigating these problems is to complement the empirical approach by developing tools for numerical simulation of irradiation effects in materials. The development of such tools is clearly an ambitious task that will require a long-term international collaborative effort. In this paper, we present an outline of the Reactor for Virtual Experiments (REVE) project, a collaborative European and American effort aimed at developing quantitative simulations of irradiation effects in materials. The first demonstration phase of REVE will target embrittlement of reactor pressure vessel (RPV) steels, since the effects and mechanisms of irradiation damage in this material are relatively well understood and many modeling tools have been developed or are under development in this field. As for any experiment, the input variables of the REVE simulation will be the neutron spectrum, time and temperature of irradiation, the alloy composition (e.g., Cu, Ni, Mn, and C contents) and microstructure and the unirradiated mechanical properties. The simulations will predict the irradiation-induced increases of yield stress and Charpy transition temperature as well as the decrease of toughness due to the concomitant evolution of the microstructure.

Comparison between three complementary approaches to simulate ‘ large ’ fluence irradiation: application to electron irradiation of thin foils

A comparison between the predictions of different models is tempted on the simple case of a thin foil irradiated with electrons. The agreement and differences between the Monte-Carlo approaches and the cluster dynamics enlighten the necessity, for any future development, of settling down on a firm basis the emission rates from defect clusters. Despite its simplicity, this simple irradiation test shows that vacancy clusters nucleation is favoured in the self interstitial atom denuded zones along the surfaces, promoting the possible growth of particularly large cavities in this very region at later stages.

Kinetic Monte Carlo Simulations of Fecu Alloys

The steel vessels of pressurized water reactors are embrittled by neutron irradiation. It is well known that copper atoms play an important role in the embrittlement and that different Cu-containing defects such as Cu-rich clusters (sometimes called atmospheres), Cu precipitates and Cu-vacancy complexes have been identified experimentally. It is still difficult to link the formation of these defects to the primary damage resulting from the neutron inducing displacement cascades. Therefore, the authors investigate the evolution of the primary damage in FeCu alloys using kinetic Monte Carlo simulations based on a vacancy diffusion mechanism. The calculations rely on adapted, phenomenological, n-body potentials that satisfactorily reproduce properties of FeCu. At room temperature, experimentally identified defects such as Cu-vacancy complexes (one Cu atom bound to three of four vacancies) form in the courses of the simulations. Furthermore, it appears that complex defects such as a Cu atom linked to two vacancies are very mobile and are responsible for the formation of small Cu clusters.

Modelling of Radiation Damage in Fe-Cr Alloys

High-Cr ferritic/martensitic steels are being considered as structural materials for a large number of future nuclear applications, from fusion to accelerator-driven systems and GenIV reactors. Fe-Cr alloys can be used as model materials to investigate some of the mechanisms governing their microstructure evolution under irradiation and its correlation to changes in their macroscopic properties. Focusing on these alloys, we show an example of how the integration of computer simulation and theoretical models can provide keys for the interpretation of a host of relevant experimental observations. In particular we show that proper accounting for two basic features of these alloys, namely, the existence of a fairly strong attractive interaction between self-interstitials and Cr atoms and of a mixing enthalpy that changes sign from negative to positive around 8 to 10 % Cr, is a necessary and, to a certain extent, sufficient condition to rationalize and understand their behavior under irradiation. These features have been revealed by ab initio calculations, are supported by experimental evidence, and have been adequately transferred into advanced empirical interatomic potentials, which have been and are being used for the simulation of damage production, defect behavior, and phase transformation in these alloys. The results of the simulations have been and are being used to parameterize models capable of extending the description of radiation effects to scales beyond the reach of molecular dynamics. The present paper intends to highlight the most important achievements and results of this research activity.

Short- and long-range orders in Fe–Cr: A Monte Carlo study

Atomistic kinetic Monte Carlo simulations based on the two-band semiempirical cohesive model for Fe-Cr have revealed a body centered tetragonal Fe(14)Cr ordered compound at very low temperatures. D ...

Computer simulations study of iron–copper alloy

Abstract The vessel steels of pressurised water reactors are embrittled by neutron irradiation. The changes of mechanical properties are commonly supposed to result from the formation of point defects, dislocation loops, voids and Cu-rich precipitates. The composition of such precipitates, specially the existence of vacancies, is not accessible through experiments. It is suggested that two mechanisms promote the formation of Cu-rich features in pressure vessel steels during neutron irradiation. When the Cu content is lower than 0.1%, solute-rich atmospheres can be formed in displacement cascades. For higher Cu contents, in addition to the latter phenomenon, a mechanism of accelerated precipitation can also induce the formation of Cu-rich clusters or precipitates. In order to understand all the mechanisms, Molecular Dynamics and Monte Carlo simulations have been carried out in pure Fe, and Fe–Cu alloys. Inter-atomic potentials of the Embedded Atom Method type, found in literature for pure Fe and pure Cu we...