Computer simulation of multi-elemental fusion reactor materials

Thermonuclear fusion is a sustainable energy solution, in which energy is produced using similar processes as in the sun. In this technology hydrogen isotopes are fused to gain energy and consequently to produce electricity. In a fusion reactor hydrogen isotopes are confined by magnetic fields as ionized gas, the plasma. Since the core plasma is millions of degrees hot, there are special needs for the plasma-facing materials. Moreover, in the plasma the fusion of hydrogen isotopes leads to the production of high energetic neutrons which sets demanding abilities for the structural materials of the reactor. This thesis investigates the irradiation response of materials to be used in future fusion reactors. Interactions of the plasma with the reactor wall leads to the removal of surface atoms, migration of them, and formation of co-deposited layers such as tungsten carbide. Sputtering of tungsten carbide and deuterium trapping in tungsten carbide was investigated in this thesis. As the second topic the primary interaction of the neutrons in the structural material steel was examined. As model materials for steel iron chromium and iron nickel were used. This study was performed theoretically by the means of computer simulations on the atomic level. In contrast to previous studies in the field, in which simulations were limited to pure elements, in this work more complex materials were used, i.e. they were multi-elemental including two or more atom species. The results of this thesis are in the microscale. One of the results is a catalogue of atom species, which were removed from tungsten carbide by the plasma. Another result is e.g. the atomic distributions of defects in iron chromium caused by the energetic neutrons. These microscopic results are used in data bases for multiscale modelling of fusion reactor materials, which has the aim to explain the macroscopic degradation in the materials. This thesis is therefore a relevant contribution to investigate the connection of microscopic and macroscopic radiation effects, which is one objective in fusion reactor materials research.Lämpöydinfuusio on kestävä energiaratkaisu, joka käyttää samoja prosesseja kuin auringossa. Tässä tekniikassa energia saadaan yhdistämällä vedyn isotooppeja ja tuottamalla siten sähköä. Fuusioreaktorissa vedyn isotoopit keskittyvät magneettikentässä ionisoituna kaasuna, plasmana. Koska ydinplasma on miljoonia asteita kuumaa, plasman suuntaiseen seinämateriaaliin on erityisiä vaatimuksia. Lisäksi vedyn isotooppien fuusio plasmassa johtaa korkeaenergisien neutronien tuotantoon, jotka asettavat korkeita vaatimuksia rakennusmateriaaleihin. Tämä väitöskirja tutkii materiaalien säteilytysvastetta, jotta niitä voitaisiin käyttää tulevaisuuden fuusioreaktoreissa. Väitöskirjan tutkimus tehtiin teoreettisesti tietokonesimulaatioiden avulla atomitasolla. Toisin kuin aiemmissa tutkimuksissa, tässä työssä käytettiin monimutkaisempia materiaaleja. Volframikarbidin tutkittiin ensiseinämämateriaalina, ja teräsrakenteide tutkimuksessa käytettiin sekä rautanikkeliä että rautakromia mallimateriaalina. Tulokset ovat mikroskooppiskaalassa, ja niitä käytetään pohjana monimittakaavamallinnuksessa jolla pyritään määrittämään makroskooppinen säteilyvaste. Tavoitteena on selvittää mikroskooppisen ja makroskooppisen säteilyvaikutuksien yhteydet

Chemical potential of quadrupolar two-centre Lennard-Jones fluids by gradual insertion

The gradual insertion method for direct calculation of the chemical potential by molecular simulation is applied in the NpT ensemble to different quadrupolar two-centre Lennard-Jones fluids at high density state points. The results agree well with Widom's test particle insertion but show at very high densities significantly smaller statistical uncertainties. The gradual insertion method, which is coupled here with preferential sampling, extends the density range where reliable information on the chemical potential can be obtained. Application details are reported

Chemical sputtering of Be due to D bombardment

Peer reviewe

Mechanisms of silicon sputtering and cluster formation explained by atomic level simulations

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/106088/1/jms3317.pd

Primary Radiation Damage Formation

The physical processes that give rise to changes in the microstructure, and the physical and mechanical properties of materials exposed to energetic particles are initiated by essentially elastic collisions between atoms in what has been called an atomic displacement cascade. The formation and evolution of this primary radiation damage mechanism are described to provide an overview of how stable defects are formed by displacement cascades, as well as the nature and morphology of the defects themselves. The impact of the primary variables cascade energy and irradiation temperature are discussed, along with a range of secondary factors that can influence damage formation

Verifikation von Anwendungen für Contiki basierende low-power eingebettete Systeme mit Hilfe von Software Model Checking

The main building blocks for the internet of things are connected embedded systems. Often these systems are also used in safety critical applications. Therefore, it is particularly important that these devices work according to their specification i.e. they behave as intended. Nowadays, even for simple devices embedded operating systems as Contiki are used to simplify application development and to increase portability between different hardware platforms. The main objective of this thesis is to present a methodology for the verification of software applications written for the operation system Contiki, taking the system hardware into account. Therefore, software model checking and especially bounded model checking [BCC⁺03] is used as a technique, which allows to formally verify software for embedded systems. For verifying the software against its specification, it is also necessary to build a model of the system hardware. Thereby, the difficulty is to create a model which is detailed enough to capture the hardware behavior so that the software performs correctly, while keeping the computation effort for the verification process manageable. In this work, the drivers which communicate with the hardware are therefore replaced with abstract models during the verification process. This enables the verification based on an abstract hardware platform independent of specific hardware. A special role within embedded systems play interrupts. Interrupts are used to save power and can also be used to react on external events. Current methods for verification of interrupt driven software are based on the interleaving model and partial order reduction to reduce the size of the verification problem. This thesis argues that this method is not sufficient for software, whose behavior relies on periodically occurring interrupts. Therefore, in this thesis, a new approach called periodic interrupt modeling is introduced. This approach can be applied automatically and reduces the number of incorrect verification results due to inaccurate modeling. In addition, properties can be proven that depend on the number of occurring interrupts. Using applications for the Contiki operating system, and based on a verification flow, the approaches toward interrupt modeling are compared.Vernetzte eingebettete Systeme bilden die Grundlage für das Internet of Things (IoT). Sie arbeiten häufig autark und in sicherheitskritischen Bereichen. Deshalb ist es wichtig sicherzustellen, dass die Systeme sich korrekt, also gemäß ihrer Spezifikation, verhalten. Zur Erstellung von Software für solche Systeme werden Betriebssysteme wie Contiki verwendet, welche die Programmierung von Anwendungen vereinfachen und die Portabilität zwischen verschiedenen Hardware-Plattformen ermöglichen. In dieser Dissertation wird eine Methodik zur Verifikation von Software-Anwendungen, unter Berücksichtigung der Hardwareumgebung, für eingebettete Systeme anhand des Betriebssystems Contiki vorgestellt. Es wird die Technik des Software Model Checking [CGP00] - und dabei insbesondere Bounded Model Checking [BCC⁺03] - verwendet, welche es ermöglicht die Software eines eingebetteten Systems formal zu verifizieren. Um die Verifikation durchzuführen, muss auch die Hardware des Systems modelliert werden. Die Herausforderung hierbei ist es, die Interaktionen der Software mit der Hardware hinreichend genau abzubilden und gleichzeitig den Aufwand für die Verifikation hinsichtlich des benötigten Rechenaufwands beherrschbar zu halten. In dieser Arbeit wird deshalb ein Ansatz verwendet, der die Treiber, die auf die Hardware zu- greifen durch Softwaremodelle für die Verifikation ersetzt. Dies ermöglicht es Anwendungen für Contiki mit Hilfe einer abstrakten Verifikationsumgebung zu überprüfen. Ein besonderer Aspekt bei eingebetteten Systemen ist die Verwendung von Interrupts, welche es ermöglichen Energie, einzusparen und auf externe Ereignisse zu reagieren. In bisherigen Ansätzen zur Verifikation werden Interrupts mit dem Interleaving Modell modelliert und das Verifikationsproblem mit Hilfe von Partial Order Reduction [CGP00, BK08] reduziert. Diese Arbeit zeigt, dass dieser Ansatz für die Verifikation von Anwendungen, welche periodische Interrupts verwenden, nicht ausreichend ist. Deshalb wird mit Periodic Interrupt Modelling ein neuer Ansatz zur Modellierung von Interrupts vorgestellt. Dieser Ansatz kann automatisiert angewendet werden und verringert die Anzahl von falschen Verifikationsergebnissen aufgrund ungenauer Modellierung. Zusätzlich ist es möglich Eigenschaften zu überprüfen, die von der Häufigkeit von Interrupt-Aufrufen abhängen. Anhand des in der Arbeit entwickelten Verifikationsablaufs werden Beispielprogramme für Contiki untersucht und die Ansätze zur Interruptmodellierung verglichen

MD simulations of low energy deuterium irradiation on W, W2_{2}C and surfaces

According to the present design beryllium (Be), tungsten (W) and carbon (C) will be the plasma facing materials in the ITER fusion reactor. Due to sputtering and subsequent re-deposition, mixing of these materials will occur. In this context, molecular dynamics simulations of cumulative, low energy and high flux D bombardment of pure W and tungsten carbides (WC, W2CW2C) were carried out. The retention and sputtering properties as well as the structural deformation were analysed and comparisons to SDTrimSP simulations were made. Almost no tungsten is sputtered in the energy range considered and the D backscattering is lower in pure tungsten than in any of the tungsten carbides. In WC and W2CW2C, the deuterium is mainly trapped forming small molecules, whereas mostly atomic D is present in pure W. The C sputtering increases with C content in the material, and shows a peak at the bombardment energy ∼50 eV, most likely due to the swift chemical sputtering mechanism. Pure W is seen to lose its crystallinity in the areas where D is present. After the D irradiation, the composition of both WC and W2CW2C is mostly W in the topmost layers, due to preferential sputtering of C, an amorphous D–C mixture underneath and an undisturbed lattice in the rest of the cell