Microstructure evolution of irradiated tungsten: Crystal effects in He and H implantation as modelled in the Binary Collision Approximation M. Hou a, * , C.J. Ortiz b , C.S. Becquart c , C. Domain d , U. Sarkar e , A. Debacker c a Physique des Solides Irradiés et des Nanostructures CP234, Université Libre de Bruxelles, Bd du Triomphe, B-1050 Brussels, Belgium b Laboratorio Nacional de Fusión por Confinamiento Magnético, CIEMAT, E-28040 Madrid, Spain c Unité Matériaux Et Transformations (UMET), UMR 8207, Université de Lille 1, F-59655 Villeneuve d’Ascq Cedex, France d EDF-R&D Département MMC, Les Renardières, F-77818 Moret sur Loing Cedex, France e Physics Department, Assam University, Silchar, India article info Article history: Received 5 March 2010 Accepted 1 June 2010 abstract It is important to develop an understanding of the evolution of W microstructure the conditions the International Thermonuclear Experimental Reactor (ITER) as well as the DEMOnstration Power Plan (DEMO), and modelling techniques can be very helpful. In this paper, the Binary Collision Approximation of Molecular Dynamics as implemented in the Marlowe code is used to model the slowing down of atomic helium and hydrogen on tungsten in the 1–100 keV range. The computed helium and Frenkel Pairs (FP) distributions are then used as input for the simulation of isochronal annealing experiments with an Object Kinetic Monte Carlo (OKMC) model. Parameterisation is discussed in a companion paper to this one. To model inelastic energy losses beyond the Lindhard regime, a new module has been implemented in the Marlowe code which is presented here, along with a discussion on various parameters of the model important in the modelling of channelled trajectories. For a given total inelastic stopping cross section, large differences in low energy channelling ranges are identified depending on whether inelastic energy loss is considered to be purely continuous or to also occur during the atomic collisions. In polycrystals, the channelling probability is shown to be significant over the whole range of slowing down energies considered. Channelling together with short replacement sequences has the effect of reducing the FP pro- duction efficiency by more than a factor two in polycrystalline as compared with an hypothetical struc- tureless tungsten. This has a crucial effect on the helium isochronal desorption spectra predicted by the OKMC simulations. Those predicted with structureless tungsten are at variance with experiment, due to the overestimation of He trapping on the radiation induced defects. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction One of the main issues of nuclear fusion technology is the inter- action between the excited plasma and the wall in which it is con- fined. This wall is irradiated by an intense current of various species produced by fusion reactions. These species are essentially light ions and 14 MeV neutrons. The consequences are the back- scattering of a fraction of the ions, neutralised at the wall surface and the sputtering of surface atoms, also mainly in a neutral state, that reduce the quality of the plasma. At the same time, partial re- deposition together with radiation-induced surface segregation modify the surface chemical composition and thereby the plasma– wall interaction properties. These effects are particularly crucial in specific areas of the wall, such as the divertor in tokamaks sub- jected to particularly intense irradiation. Another major conse- quence of this high dose and high flux irradiation is the degradation in the long term of the mechanical properties of the wall material, because of damage generated by the implanted ions and neutrons. This paper focuses on hydrogen and helium penetration as well as related atomic displacement, while a companion paper dis- cusses the parameterisation for fusion conditions of an Object Ki- netic Monte Carlo (OKMC) code to predict the microstructural evolution of irradiated tungsten up to the mesoscopic scale [1]. These are part of a broad effort to set up predictive models of reac- tor materials under irradiation covering the range of scales from the atomic level to macroscopic [2–7]. More specifically, the pur- pose of the present paper is to provide this information using the Binary Collision Approximation to classical molecular dynamics, taking into account the polycrystalline nature of wall materials as realistically as possible. As will be shown in the last sections, the anisotropic nature of polycrystals at the atomic scale has a marked impact on mesoscopic model predictions. 0022-3115/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2010.06.004 * Corresponding author. Tel.: +32 2 6505735; fax: +32 2 6505227. E-mail address: mhou@ulb.ac.be (M. Hou). Journal of Nuclear Materials 403 (2010) 89–100 Contents lists available at ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat