For many centuries, the study of the Sun has been an important testbed for understanding stars that are further away. One of the first astronomical observations Galileo Galilei made in 1612 with the newly invented telescope concerned the sunspots, and in 1814, Joseph von Fraunhofer employed his new spectroscope to discover the absorption lines in the solar spectrum that are now named after him. Even though more refined and new modes of observation are now available than in the days of Galileo and Fraunhofer, the study of the Sun is still high on the agenda of contemporary science, due to three guiding interests. The first is connected to the ages-old human striving to understand the structure of the larger world surrounding us. Modern telescopes, some of them even based outside the Earth’s atmosphere in space, have succeeded in observing astronomical objects that are billions of lightyears away. However, for practical reasons precision data that are important for understanding stars can still only be gained from the Sun. In a sense, the observations of far-away astronomical objects thus call for a more precise study of the closeby, of the Sun, for their interpretation. The second interest stems from the human desire to understand the essence of the world, in particular the elementary particles of which it consists. Large accelerators have been constructed to produce and collide these particles. However, man-made machines can never be as luminous as the Sun when it comes to producing particles. Solar neutrinos have thus served not only as an astronomical tool to understand the Sun’s inner workings, but their behavior on the way from the Sun to the Earth is also being studied with the aim to understand their nature and interactions. The third interest is strictly connected to life on Earth. A multitude of research has shown that even relatively slight changes in the Earth’s climate may strongly affect the living conditions in a number of densely populated areas, mainly near the ocean shore and in arid regions. Thus, great effort is expended on the study of greenhouse gases in the Earth’s atmosphere. Also the Sun, via the solar irradiance and via the effects of the so-called solar wind of magnetic particles on the Earth’s atmosphere, may affect the climate. There is no proof linking solar effects to short-term changes in the Earth’s climate. However, such effects cannot be excluded, either, making it necessary to study the Sun. The experiments summarized in the present work contribute to the present-day study of our Sun by repeating, in the laboratory, some of the nuclear processes that take place in the core of the Sun. They aim to improve the precision of the nuclear cross section data that lay the foundation of the model of the nuclear reactions generating energy and producing neutrinos in the Sun. In order to reach this goal, low-energy nuclear physics experiments are performed. Wherever possible, the data are taken in a low-background, underground environment. There is only one underground accelerator facility in the world, the Laboratory Underground for Nuclear Astrophysics (LUNA) 0.4MV accelerator in the Gran Sasso laboratory in Italy. Much of the research described here is based on experiments at LUNA. Background and feasibility studies shown here lay the base for future, higher-energy underground accelerators. Finally, it is shown that such a device can even be placed in a shallow-underground facility such as the Dresden Felsenkeller without great loss of sensitivity.

Ein neues, verallgemeinertes Modell für Blasenkoaleszenz und –zerfall wurde entwickelt. Es basiert auf physikalischen Überlegungen und berücksichtigt verschiedene Mechanismen, die zu Blasenkoaleszenz und –zerfall führen können. In einer ausführlichen Literaturrecherche wurden zunächst die verfügbaren Modelle zusammengestellt und analysiert. Es zeigte sich, dass viele widersprüchliche Modelle veröffentlicht wurden. Keins dieser Modelle erlaubt die Vorhersage der Entwicklung der Blasengrößenverteilungen entlang einer Rohrströmung für einen breiten Bereich an Kombinationen von Volumenströmen der Gas- und der Flüssigphase. Das neue Modell wurde ausführlich in einem vereinfachten Testsolver untersucht. Dieser erfasst zwar nicht alle Einzelheiten einer sich entlang des Rohres entwickelten Strömungen, erlaubt aber im Gegensatz zu den CFD-Simulationen eine Vielzahl von Variationsrechnungen zur Untersuchung des Einflusses einzelner Größen und Modelle. Koaleszenz und Zerfall kann nicht getrennt von anderen Phänomenen und Modellen, die diese widerspiegeln, betrachtet werden. Es bestehen enge Wechselwirkungen mit der Turbulenz der Flüssigphase und dem Impulsaustausch zwischen den Phasen. Da die Dissipationsrate der turbulenten kinetischen Energie ein direkter Eingangsparameter für das neue Modell ist, wurde die Turbulenzmodellierung besonders genau untersucht. Zur Validierung des Modells wurde eine TOPFLOW-Experimentalserie zur Luft-Wasser-Strömungen in einem 8 m langen DN200-Rohr genutzt. Die Daten zeichnen sich durch eine hohe Qualität aus und wurden im Rahmen des TOPFLOW-IIVorhabens mit dem Ziel eine Grundlage für die hier vorgestellten Arbeiten zu liefern, gewonnen. Die Vorhersage der Entwicklung der Blasengrößenverteilung entlang des Rohrs konnte im Vergleich zu den bisherigen Standardmodellen für Blasenkoaleszenz und -zerfall in CFX deutlich verbessert werden. Einige quantitative Abweichungen bleiben aber bestehen. Die vollständigen Modellgleichungen sowie eine Implementierung über „User-FORTRAN“ in CFX stehen zur Verfügung und können für weitere Arbeiten zur Simulation polydisperser Blasenströmungen genutzt werden.

Umströmte Rohr- bzw. Stabbündel sind als Übertrager von Wärmeenergie in einem breiten Spektrum von Anwendungsgebieten zu finden. Beispiele sind Heizkörper, Kühlaggregate, Heizpatronen, industrielle Wärmetauscher und Brennelemente in Kernreaktoren. Für jede dieser Anwendungen besteht die Anforderung, die Wärmeübertragung an den Wärmeübertragerflächen zu optimieren. Dabei besteht eine enge Kopplung zwischen Wärmetransport und Strömungsstruktur. Eine besonders effiziente Form der Wärmeübertragung ist die Verdampfung. Diese wird unter anderem bei Brennelementen in Druckwasserreaktoren genutzt. Hier siedet das Kühlwasser an der Brennstaboberfläche. Durch Kondensation der Dampfblasen in der unterkühlten Kernströmung wird die Wärme dann effizient in die Flüssigphase übertragen. Durch die hohe Verdampfungsenthalpie des Wassers wird beim Strömungssieden ein viel höherer Wärmestrom in das Kühlwasser übertragen, als bei rein einphasig-konvektivem Wärmetransport. Sicherheitstechnisch relevant für Brennelemente in Leichtwasserreaktoren ist der Übergang vom Blasensieden zum Filmsieden (kritischer Wärmestrom). Dieser muss unter allen Umständen vermieden werden, um die Integrität der Brennstabhüllen zu gewährleisten, die bei Überschreiten der kritischen Heizflächenbelastung aufschmelzen bzw. reißen können. Aus diesem Grund werden im Rahmen eines vom Bundesministerium für Bildung und Forschung geförderten Projektes (Förderkennzeichen 02NUK010A) numerische Strömungsberechnungsmodelle entwickelt, die bei der Beschreibung und numerischen Behandlung der Siedephänomene helfen sollen. Zur Validierung dieser Modelle anhand von Experimenten wurde ein Strömungskanal konstruiert, in dem ein vertikales Stabbündel von einem Kältemittel (RC318) aufwärtig durchströmt wird. Der Versuchsstand ist so konzipiert, dass ein optischer und messtechnischer Zugang zu den umströmten Einbauten gegeben ist. Damit sind Messungen in Zweiphasenströmungen ebenso möglich, wie Untersuchungen zur einphasigen Durchströmung. Für später erfolgende Zweiphasen-Experimente mit Stabbeheizung wurden zunächst Voruntersuchungen zur einphasigen Durchströmungen durchgeführt, welche insbesondere Aufschluss über die Homogenität der Strömung in den Unterkanälen sowie die Existenz von Querströmungen geben sollten. Als Messverfahren dafür wurde die Particle Image Velocimetry (PIV) ausgewählt, welche es ermöglicht, zweidimensionale Strömungsfelder aufzuzeichnen. Die experimentellen Studien erfolgten am Optical Multi-Phase Flow Research Laboratory des Nuclear Engineering Department der Texas A&M University in College Station, USA. Die Untersuchungen wurden für drei Volumenstromraten durchgeführt. Der vorliegende Bericht umfasst die Beschreibung des Versuchsstandes und der Messmethodik, eine Vorstellung des Auswerteverfahrens und relevanter Ergebnisse sowie eine Fehlerbetrachtung.

A new method of treating spectral history effects in reactor core calculations was developed and verified in this dissertation. The nature of history effects is a dependence of fuel properties not only on the burnup, but also on the local spectral conditions during burnup. The basic idea of the proposed method is the use of the plutonium-239 concentration as the spectral history indicator. The method was implemented in the reactor dynamics code DYN3D and provides a correction for nodal cross sections according to the local spectral history. A verification of the new method was performed by single-assembly calculations in comparison with results of the lattice code HELIOS. The application of plutonium-based history correction significantly improves the cross section estimation accuracy both for UOX and MOX fuel, with quadratic and hexagonal geometry. The new method was applied to evaluate the influence of history effects on full-core calculation results. Analysis of a PWR equilibrium fuel cycle has shown a significant effect on the axial power distribution during a whole cycle, which causes axial temperature and burnup redistributions. The observed neutron flux redistribution improves neutron economy, so the fuel cycle is longer than in calculations without history corrections. Analyses of hypothetical control rod ejection accidents have shown a minor influence of history effects on the transient course and safety relevant parameters. / Eine neue Methode zur Modellierung der Spektralgeschichte als Bestandteil von Kernreaktorberechnungen wurde in dieser Dissertation entwickelt und verifiziert. Die spektrale Abbrandgeschichte hat praktische Bedeutung für die Brennstoffeigenschaften, die nicht nur von der Höhe des Abbrandwertes, sondern auch vom lokalen Neutronenspektrum während des Abbrandprozesses abhängen. Die Grundidee der vorgeschlagenen Methode besteht in der Nutzung der lokalen Plutonium-239-Konzentration als quantitativen Indikator für die spektrale Abbrandgeschichte. Die Methode wurde in das Reaktordynamikprogramm DYN3D implementiert; sie gewährleistet eine Korrektur der nodalen Wirkungsquerschnitte gemäß der lokalen spektralen Abbrandgeschichte. Eine Verifikation der neuen Methode wurde mit Einzelbrennelementberechnungen im Vergleich zu Ergebnissen des Zellabbrandprogramms HELIOS durchgeführt. Die Korrektur auf der Basis der Plutoniumkonzentration verbessert die Genauigkeit der Wirkungsquerschnitte signifikant, sowohl für UOX als auch für MOX, in quadratischer und hexagonaler Geometrie. Die neue Methode wurde für die Bestimmung des Einflusses der spektralen Abbrandgeschichte auf die Modellierung ganzer Reaktorkerne angewandt. Die Analyse eines DWRGleichgewichtszyklus zeigt eine signifikante Auswirkung auf die axiale Leistungsverteilung während eines ganzen Zyklus. Über die axiale Temperaturverteilung (Rückkopplung) entsteht wiederum eine Rückwirkung auf die Abbrandverteilung selbst. Die beobachtete modifizierte Neutronenflussverteilung verbessert die Neutronenökonomie, sodass der Brennstoffzyklus länger wird, verglichen mit Berechnungen ohne Berücksichtigung der Abbrandgeschichte. Analysen von hypothetischen Stabauswurfszenarien ergaben einen nur geringen Einfluss der Abbrandgeschichte auf Transientenverlauf und sicherheitsrelevante Parameter.

Im Rahmen der Arbeit wurden Experimente aus den Jahren 2008/09 für die Kerne 86Kr und 136Ba analysiert. Zur Auswertung mussten neben Photonenfluss- und Effizienzbestimmung auch Simulationen durchgeführt werden, welche die experimentellen Bedingungen widerspiegeln. Nicht am Kern gestreute Ereignisse und Detektorantwortfunktionen wurden mit Hilfe des Programmpaketes GEANT4 simuliert, um in den gemessenen Daten berücksichtigt zu werden. Daraus zeigt sich, dass neben diskreten Energiezuständen auch ein beachtlicher Teil des ermittelten Anregungsquerschnitts in einer Art Quasikontinuum aus unauflösbaren Peaks liegt. Die ermittelten Wirkungsquerschnitte werden mit Hilfe eines statistischen Ansatzes auf Verzweigung in mögliche Zerfallskanäle und auf Fütterung durch Zustände höherer Energie korrigiert.

Ion beam therapy (IBT) is a promising treatment option in radiotherapy. The characteristic physical and biological properties of light ion beams allow for the delivery of highly tumour conformal dose distributions. Related to the sparing of surrounding healthy tissue and nearby organs at risk, it is feasible to escalate the dose in the tumour volume to reach higher tumour control and survival rates. Remarkable clinical outcome was achieved with IBT for radio-resistant, deep-seated, static and well fixated tumour entities. Presumably, more patients could benefit from the advantages of IBT if it would be available for more frequent tumour sites. Those located in the thorax and upper abdominal region are commonly subjected to intra-fractional, respiration related motion. Different motion compensated dose delivery techniques have been developed for active field shaping with scanned pencil beams and are at least available under experimental conditions at the GSI Helmholtzzentrum für Schwerionenforschung (GSI) in Darmstadt, Germany. High standards for quality assurance are required in IBT to ensure a safe and precise dose application. Both underdosage in the tumour and overdosage in the normal tissue might endanger the treatment success. Since minor unexpected anatomical changes e.g. related to patient mispositioning, tumour shrinkage or tissue swelling could already lead to remarkable deviations between planned and delivered dose distribution, a valuable dose monitoring system is desired for IBT. So far, positron emission tomography (PET) is the only in vivo, in situ and non-invasive qualitative dose monitoring method applied under clinical conditions. It makes use of the tissue autoactivation by nuclear fragmentation reactions occurring along the beam path. Among others, +-emitting nuclides are generated and decay according to their half-life under the emission of a positron. The subsequent positron-electron annihilation creates two 511 keV photons which are emitted in opposite direction and can be detected as coincidence event by a dedicated PET scanner. The induced three-dimensional (3D) +- activity distribution in the patient can be reconstructed from the measured coincidences. Conclusions about the delivered dose distribution can be drawn indirectly from a comparison between two +-activity distributions: the measured one and an expected one generated by a Monte-Carlo simulation. This workflow has been proven to be valuable for the dose monitoring in IBT when it was applied for about 440 patients, mainly suffering from deep-seated head and neck tumours that have been treated with 12C ions at GSI. In the presence of intra-fractional target motion, the conventional 3D PET data processing will result in an inaccurate representation of the +-activity distribution in the patient. Fourdimensional, time-resolved (4D) reconstruction algorithms adapted to the special geometry of in-beam PET scanners allow to compensate for the motion related blurring artefacts. Within this thesis, a 4D maximum likelihood expectation maximization (MLEM) reconstruction algorithm has been implemented for the double-head scanner Bastei installed at GSI. The proper functionality of the algorithm and its superior performance in terms of suppressing motion related blurring artefacts compared to an already applied co-registration approach has been demonstrated by a comparative simulation study and by dedicated measurements with moving radioactive sources and irradiated targets. Dedicated phantoms mainly made up of polymethyl methacrylate (PMMA) and a motion table for regular one-dimensional (1D) motion patterns have been designed and manufactured for the experiments. Furthermore, the general applicability of the 4D MLEM algorithm for more complex motion patterns has been demonstrated by the successful reduction of motion artefacts from a measurement with rotating (two-dimensional moving) radioactive sources. For 1D cos2 and cos4 motion, it has been clearly illustrated by systematic point source measurements that the motion influence can be better compensated with the same number of motion phases if amplitudesorted instead of time-sorted phases are utilized. In any case, with an appropriate parameter selection to obtain a mean residual motion per phase of about half of the size of a PET crystal size, acceptable results have been achieved. Additionally, it has been validated that the 4D MLEM algorithm allows to reliably access the relevant parameters (particle range and lateral field position and gradients) for a dose verification in intra-fractionally moving targets even from the intrinsically low counting statistics of IBT-PET data. To evaluate the measured +-activity distribution, it should be compared to a simulated one that is expected from the moving target irradiation. Thus, a 4D version of the simulation software is required. It has to emulate the generation of +-emitters under consideration of the intra-fractional motion, their decay at motion state dependent coordinates and to create listmode data streams from the simulated coincidences. Such a revised and extended version that has been compiled for the special geometry of the Bastei PET scanner is presented within this thesis. The therapy control system provides information about the exact progress of the motion compensated dose delivery. This information and the intra-fractional target motion needs to be taken into account for simulating realistic +-activity distributions. A dedicated preclinical phantom simulation study has been performed to demonstrate the correct functionality of the 4D simulation program and the necessity of the additional, motionrelated input parameters. Different to the data evaluation for static targets, additional effort is required to avoid a potential misleading interpretation of the 4D measured and simulated +-activity distributions in the presence of deficient motion mitigation or data processing. It is presented that in the presence of treatment errors the results from the simulation might be in accordance to the measurement although the planned and delivered dose distribution are different. In contrast to that, deviations may occur between both distributions which are not related to anatomical changes but to deficient 4D data processing. Recommendations are given in this thesis to optimize the 4D IBT-PET workflow and to prevent the observer from a mis-interpretation of the dose monitoring data. In summary, the thesis contributes on a large scale to a potential future application of the IBT-PET monitoring for intra-fractionally moving target volumes by providing the required reconstruction and simulation algorithms. Systematic examinations with more realistic, multi-directional and irregular motion patterns are required for further improvements. For a final rating of the expectable benefit from a 4D IBT-PET dose monitoring, future investigations should include real treatment plans, breathing curves and 4D patient CT images.

THEREDA (Thermodynamic Reference Database) is a collaborative project, which has been addressed this challenge. The partners are Helmholtz-Zentrum Dresden-Rossendorf, Karlsruhe Institute of Technology (KIT-INE), Gesellschaft für Anlagen- und Reaktorsicherheit Braunschweig mbH (GRS), TU Bergakademie Freiberg (TUBAF) and AF-Consult Switzerland AG (Baden, Switzerland). The aim of the project is the establishment of a consistent and quality assured database for all safety relevant elements, temperature and pressure ranges, with its focus on saline systems. This implied the use of the Pitzer approach to compute activity coefficients suitable for such conditions. Data access is possible via commonly available internet browsers under the address http://www.thereda.de. One part of the project - the data collection and evaluation for uranium – was a task of the Helmholtz-Zentrum Dresden-Rossendorf. The aquatic chemistry and thermodynamics of U(VI) and U(IV) is of great importance for geochemical modelling in repository-relevant systems. The OECD/NEA Thermochemical Database (NEA TDB) compilation is the major source for thermodynamic data of the aqueous and solid uranium species, even though this data selection does not utilize the Pitzer model for the ionic strength effect correction. As a result of the very stringent quality demands, the NEA TDB is rather restrictive and therefore incomplete for extensive modelling calculations of real systems. Therefore, the THEREDA compilation includes additional thermodynamic data of solid secondary phases formed in the waste material, the backfill and the host rock, though falling into quality assessment (QA) categories of lower accuracy. The data review process prefers log K values from solubility experiments (if available) to those calculated from thermochemical data.

Direct Contact Condensation between steam and water as well as bubble entrainment below the water surface play an important role in different accident scenarios for light water reactors. One example is the emergency core cooling water injection into a two-phase mixture. It has to be considered for example to evaluate potential pressurized thermal shock phenomena. This report documents experiments conducted in flat basin inside the TOPFLOW pressure chamber aiming on the generation of a database useful for CFD model development and validation. It comprises 3 different setups: condensation at a stratified flow of sub-cooled water, condensation at a sub-cooled water jet and a combination of both phenomena with steam bubble entrainment. The documentation includes all details on the experimental set up, on experimental conditions (experimental matrices), on the conduction of the experiments, on measuring techniques used and on data evaluation procedures. In addition, selected results are presented.

The advantages of contemporary particle injectors are high bunch charges and good beam quality in the case of normal conducting RF guns and increased repetition rates in the one of DC injectors. The technological edge of the concept of superconducting radio frequency injectors is to combine the strengths of both these sides. As many future accelerator concepts, such as energy recovery linacs, high power free electron lasers and certain collider designs, demand particle sources with high bunch charges and high repetition rates combined, applying the superconductivity of the accelerator modules to the injector itself is the next logical step. However, emittance compensation — the cornerstone for high beam quality — in case of a superconducting injector is much more challenging than in the normal conducting one. The use of simple electromagnets generating a solenoid field around the gun’s resonator interferes with its superconducting state. Hence, it requires novel and sophisticated techniques to maintain the high energy gain inside the gun cavity, while at the same time alleviating the detrimental fast transverse emittance growth of the bunch. In the case of the ELBE accelerator at the Helmholtz-Zentrum Dresden-Rossendorf, a superconducting electron accelerator provides beam for several independent beamlines in continuous wave mode. The applications include IR to THz free electron lasers, neutron and positron generation, to Thompson backscattering with an inhouse TW laser, and hence, call for a flexible CW injector. Therefore, the development of a 3.5 cell superconducting electron gun was initiated in 1997. The focus of this thesis lies on three approaches of transverse emittance compensation for this photoinjector: RF focusing, the installation of a superconducting solenoid close to the cavity’s exit, and the introduction of a transverse electrical mode of the RF field in the resonator. All three methods are described in theory, examined by numerical simulation, and experimentally reviewed in the particular case of the ELBE SRF Gun II at HZDR and a copy of its niobium resonator at Thomas Jefferson National Laboratory, Newport News, VA, USA.

Laser plasma wakefield accelerators have seen tremendous progress in the last years, now capable of producing electron beams in the GeV energy range. The inherent few-femtoseconds short bunch duration of these accelerators leads to ultra-high peak-currents. Reducing the energy spread found in these accelerators, while scaling their output to hundreds of kiloampere peak current would stimulate the next generation of radiation sources covering high-field THz, high-brightness X-ray and -ray sources, compact free-electron lasers and laboratory-size beam-driven plasma accelerators. At such high currents, an accelerator operates in the beam loaded regime where the accelerating field is strongly modified by the self-fields of the injected bunch, potentially deteriorating key beam parameters. However, if appropriately controlled, the beam loading effect can be employed to improve the accelerator’s performance, specifically to reduce the energy spread. In this thesis the beam-loading effect is systematically studied at a quasi-monoenergetic nanocoulomb-class laser wakefield accelerator. For this purpose, a tailored scheme of the self-truncated ionisation injection process is introduced for the non-linear bubble regime. This scheme facilitates stable and tunable injection of high-charge electron bunches within a short and limited time-frame, ensuring low energy spread right after injection. Employing a three millimetres gas-jet acceleration medium and a moderate 150 TW short pulse laser system as driver, unprecedented charges of up to 0.5 nC within a quasi-monoenergetic peak and energies of ~0.5 GeV are achieved. Studying the beam loading mechanism, it is demonstrated that at the optimal loading condition, i.e. at a specific amount of injected charge, performance of the accelerator is optimised with a minimisation of the energy spread. At a relative energy spread of only 15%, the associated peak current is around 10 kA, while scaling this scheme to operate with a petawatt driver laser promises peak-currents up to 100 kA.