[en] In this thesis, the behaviour of a material situated in a fusion reactor was studied using molecular dynamics simulations. Simulations of processes in the next generation fusion reactor ITER include the reactor materials beryllium, carbon and tungsten as well as the plasma hydrogen isotopes. This means that interaction models, i.e. interatomic potentials, for this complicated quaternary system are needed. The task of finding such potentials is nonetheless nearly at its end, since models for the beryllium-carbon-hydrogen interactions were constructed in this thesis and as a continuation of that work, a beryllium-tungsten model is under development. These potentials are combinable with the earlier tungsten-carbon-hydrogen ones. The potentials were used to explain the chemical sputtering of beryllium due to deuterium plasma exposure. During experiments, a large fraction of the sputtered beryllium atoms were observed to be released as BeD molecules, and the simulations identified the swift chemical sputtering mechanism, previously not believed to be important in metals, as the underlying mechanism. Radiation damage in the reactor structural materials vanadium, iron and iron chromium, as well as in the wall material tungsten and the mixed alloy tungsten carbide, was also studied in this thesis. Interatomic potentials for vanadium, tungsten and iron were modified to be better suited for simulating collision cascades that are formed during particle irradiation, and the potential features affecting the resulting primary damage were identified. Including the often neglected electronic effects in the simulations was also shown to have an impact on the damage. With proper tuning of the electronphonon interaction strength, experimentally measured quantities related to ion-beam mixing in iron could be reproduced. The damage in tungsten carbide alloys showed elemental asymmetry, as the major part of the damage consisted of carbon defects. On the other hand, modelling the damage in the iron chromium alloy, essentially representing steel, showed that small additions of chromium do not noticeably affect the primary damage in iron. Since a complete assessment of the response of a material in a future full-scale fusion reactor is not achievable using only experimental techniques, molecular dynamics simulations are of vital help. This thesis has not only provided insight into complicated reactor processes and improved current methods, but also offered tools for further simulations. It is therefore an important step towards making fusion energy more than a future goal. (orig.)

[en] Since beryllium is a strong candidate for the main plasma-facing material in future fusion reactors, its sputtering behaviour plays an important role in predicting the reactor’s life-time. Consensus about the actual sputtering yields has not yet been achieved, as observations are influenced by experimental method and/or studied sample. In this work, the beryllium sputtering due to deuterium and beryllium self-bombardment is analyzed using molecular dynamics simulations. The main methodological aspects that influence the outcome, such as flux and fluence of the bombardment, are highlighted, and it is shown that the simulated yields also depend on the sample structure and deuterium content

[en] We study the initial state of irradiation damage in WC, an alloy with a large mass difference between the constituents, using molecular dynamics computer simulations. We find that a vast majority of the resulting isolated defects are carbon. Moreover, an in-cascade defect recombination effect similar to that in metals is observed. Both effects are shown to be related to the high formation energy of W defects

[en] Stainless steels found in real-world applications usually have some C content in the base Fe–Cr alloy, resulting in hard and dislocation-pinning carbides—Fe₃C (cementite) and Cr₂₃C₆—being present in the finished steel product. The higher complexity of the steel microstructure has implications, for example, for the elastic properties and the evolution of defects such as Frenkel pairs and dislocations. This makes it necessary to re-evaluate the effects of basic radiation phenomena and not simply to rely on results obtained from purely metallic Fe–Cr alloys. In this report, an analytical interatomic potential parameterization in the Abell–Brenner–Tersoff form for the entire Fe–Cr–C system is presented to enable such calculations. The potential reproduces, for example, the lattice parameter(s), formation energies and elastic properties of the principal Fe and Cr carbides (Fe₃C, Fe₅C₂, Fe₇C₃, Cr₃C₂, Cr₇C₃, Cr₂₃C₆), the Fe–Cr mixing energy curve, formation energies of simple C point defects in Fe and Cr, and the martensite lattice anisotropy, with fair to excellent agreement with empirical results. Tests of the predictive power of the potential show, for example, that Fe–Cr nanowires and bulk samples become elastically stiffer with increasing Cr and C concentrations. High-concentration nanowires also fracture at shorter relative elongations than wires made of pure Fe. Also, tests with Fe₃C inclusions show that these act as obstacles for edge dislocations moving through otherwise pure Fe. (paper)

[en] Understanding of sputtering by ion bombardment is needed in a wide range of applications. In fusion reactors, ion impacts originating from a hydrogen-isotope-rich plasma will lead, among other effects, to sputtering of the wall material. To study the effect of plasma impurities on the sputtering of the wall mixed material tungsten carbide molecular dynamics simulations were carried out. Simulations of cumulative D cobombardment with C, W, He, Ne or Ar impurities on crystalline tungsten carbide were performed in the energy range 100-300 eV. The sputtering yields obtained at low fluences were compared to steady state SDTrimSP yields. During bombardment single C atom sputtering was preferentially observed. We also detected significant W_xC_y molecule sputtering. We found that this molecule sputtering mechanism is of physical origin.

[en] Considerable quantitative uncertainty has remained regarding the amount and structure of defects produced in molecular dynamics simulations of collision cascades in Fe. The problem is most likely related to the description of interstitial energetics in the interatomic potentials. Three potentials have recently been developed for Fe, which, even though they have different physical motivations and functional forms, describe the interstitial energetics well. Using these potentials, we simulate recoil collision cascades in Fe in the recoil energy range 0.5-20 keV. Prior to the cascade simulations a realistic high-energy repulsive part was added to two of the potentials, adjusting the fit to reproduce the experimentally obtained threshold displacement energies. The results show that the total Frenkel pair production, as predicted by the three potentials, is the same within the statistical uncertainty, but also that some differences remain in the fraction of clustered defects. However, these differences are smaller than those predicted by previous potentials

[en] The interaction of fusion reactor plasma with the material of the first wall involves a complex multitude of interlinked physical and chemical effects. Hence, modern theoretical treatment of it relies to a large extent on multiscale modelling, i.e. using different kinds of simulation approaches suitable for different length and time scales in connection with each other. In this review article, we overview briefly the physics and chemistry of plasma–wall interactions in tokamak-like fusion reactors, and present some of the most commonly used material simulation approaches relevant for the topic. We also give summaries of recent multiscale modelling studies of the effects of fusion plasma on the modification of the materials of the first wall, especially on swift chemical sputtering, mixed material formation and hydrogen isotope retention in tungsten. (paper)

[en] A Monte-Carlo code called SURO has been developed to study the influence of surface roughness on the impurity deposition characteristic in fusion experiments. SURO uses the test particle approach to describe the impact of background plasma and the deposition of impurity particles on a sinusoidal surface. The local impact angle and dynamic change of surface roughness as well as surface concentrations of different species due to erosion and deposition are taken into account. Coupled with 3D Monte-Carlo code ERO, SURO was used to study the impact of surface roughness on "1"3C deposition in "1"3CH_4 injection experiments in TEXTOR. The simulations showed that the amount of net deposited "1"3C species increases with surface roughness. Parameter studies with varying "1"2C and "1"3C fluxes were performed to gain insight into impurity deposition characteristic on the rough surface. Calculations of the exposure time needed for surface smoothing for TEXTOR and ITER were also carried out for different scenarios. (author)

[en] As a step in the process of assessing the reliability of interatomic potentials for iron, we compare experimental measurements of ion beam mixing with values obtained from molecular dynamics simulations. We include the electron-phonon coupling (EPC) model by Hou et al. [Q. Hou, M. Hou, L. Bardotti, B. Prevel, P. Melinon, A. Perez, Phys. Rev. B 62 (2000) 2825] in the simulations and consider a range of coupling strenghts. Three different iron interatomic potentials are used. We discuss the effect of the coupling on the primary damage and how the damage is influenced by different velocity minima for applying electron stopping.

[en] While covalently bonded materials such as carbon are well known to be eroded by chemical sputtering when exposed to plasmas or low-energy ion irradiation, pure metals have been believed to sputter only physically. The erosion of Be when subject to D bombardment was in this work measured at the PISCES-B facility and modelled with molecular dynamics simulations. During the experiments, a chemical effect was observed, since a fraction of the eroded Be was in the form of BeD molecules. This fraction decreased with increasing ion energy. The same trend was seen in the simulations and was explained by the swift chemical sputtering mechanism, showing that pure metals can, indeed, be sputtered chemically. D ions of only 7 eV can erode Be through this mechanism.