The aim of this work was to investigate the precipitation and stability of nm-sized Ti oxides in vanadium-based alloys, a prime candidate material for future nuclear fusion reactors based on the magnetic confinement of the plasma. Fusion energy reproduces the nuclear reactions occurring in stars. It can potentially produce more energy than current nuclear fission power plants, and it is meant to be a solution to the clash of today's increasing energy demand with the continuous decrease of fossil-based energy sources, whose use is harmful for the environment. The operating conditions in a fusion reactor will be unprecedented in terms of ultra-high temperatures, stresses, radiation fields and very corrosive media. Only a limited number of materials may be able to withstand such combination of harsh environmental conditions, and vanadium-based alloys are among them. Recent research efforts have identified V-4Cr-4Ti as the most promising vanadium-based alloy for application in the first wall of future fusion nuclear reactors such as DEMO and beyond. The presence of TiO-type precipitates, containing relatively small amounts of C and N, strongly influences the final mechanical properties and radiation resistance of the alloy. Therefore, a thorough understanding of the precipitate structure and evolution at both relatively high temperatures and radiation dose levels is primordial to predict and optimise the final performance of the structural component in the fusion reactor. This thesis is written in alternative format and collects one article already published in Scripta Materialia, and two additional articles to be submitted to peer-review scientific journals. Atomic resolution imaging of the precipitates, coupled with chemical analysis, constitutes the main body of the first article: a novel intergrowth of the fcc Ti oxide in the bcc V matrix is revealed at the precipitate/matrix interface. The evolution of the vacancies present in the TiO precipitates above 400°C, together with the recovery of dislocations in the matrix and the formation of extra precipitates, is studied in the second article by positron annihilation spectroscopy and micro-hardness measurements. The formation of additional precipitates below 400°C induced by radiation is assessed in the third article using proton irradiation as a surrogate of neutron damage. The structure of those additional precipitates and of the dislocation loops induced by the proton bombardment is characterized by advanced analytical electron microscopy.

No description available.

In an age of shrinking research and development (RandD) budgets, sustaining big science and technology (SandT) projects is inevitably questioned by publics and policy makers. The fusion enterprise is an exemplar. The effort to develop a viable system to produce unlimited and environmentally benign electricity from fusion of hydrogen isotopes has been a goal for six decades and consumed vast financial and intellectual resources in North America, Europe, and Asia. In terms of prolonged duration and sustained resource investment, the endeavor has developed into a huge fusion enterprise. Yet, no practical system for the generation of electricity has yet been demonstrated. This is the paradox at the heart of the fusion enterprise. Why, despite unfulfilled visions and broken promises, has the grand fusion enterprise endured? How can such a long-term enterprise persist in a funding culture that largely works in short-term cycles? Adapting Sheila Jasanoff's thesis of "sociotechnical imaginaries", I examine the relationship of shared and contrasting visions, co-produced expressions of nature and society, and distinctpolitical cultures in the quest for viable fusion. A systematic cultural and technological comparison of three fusion ventures, the National Spherical Torus Experiment Upgrade, the International Thermonuclear Experimental Reactor (ITER), and Wendelstein-7X, exposes how these projects and the institutions they inhabit frame the goals, risks, and benefits of the fusion enterprise and sustain a common set of fusion imaginaries. Positioned within the Princeton Plasma Physics Laboratory in the United States, the international ITER Organization sited in France, and the Max Planck Institute for Plasma Physics in Germany, the three projects are prime examples of big science and technology. Rigorous research and analysis of these cases advance the thesis of the unfulfilled utopian vision of fusion energy that has endured for more than sixty years. / Doctor of Philosophy / In an age of shrinking research and development budgets, sustaining big science and technology projects is inevitably questioned by publics and policy makers. The fusion enterprise is an exemplar. The effort to develop a viable system to produce unlimited and environmentally benign electricity from fusion of hydrogen isotopes has been a goal for six decades and consumed vast financial and intellectual resources in North America, Europe, and Asia. In terms of prolonged duration and sustained resource investment, the endeavor has developed into a huge fusion enterprise. Yet, no practical system for the generation of electricity has yet been demonstrated. This is the paradox at the heart of the fusion enterprise. Beyond articulating a possible path forward for the fusion enterprise, the intent of this study is to inform decision makers who will shape energy strategy for the second half of the twenty-first century.

In this project, we have assessed the structural tolerance of advanced refractory alloys to simulated nuclear fusion reactor environments, by using intense proton beams to mimic fusion neutron damage and analysing the proton damaged structures using in-situ/ex-situ transmission electron microscopy and nano-hardness measurements. Refractory metals such as tungsten or tantalum, and their binary alloy combinations, are considered as promising structural materials to withstand the unprecedented high heat loads and fast neutron/helium fluxes expected in future magnetically-confined fusion reactors. Tungsten is currently the frontrunner for the production of plasma-facing components for fusion reactors. The attractiveness of tungsten as structural material lies in its high resistance to plasma-induced sputtering, erosion and radiation-induced void swelling, together with its thermal conductivity and high-temperature strength. Unfortunately, the brittle nature of tungsten hampers the manufacture of reactor components and can also lead to catastrophic failure during reactor operations. We have focused on two potential routes to enhance the ductility of tungsten-containing materials, namely alloying tungsten with controlled amounts of tantalum, and using alternatively tantalum-based alloys containing specific tungsten additions, either as a full-thickness structural facing material or as a coating of first wall reactor components. The aim was to investigate the formation and evolution of radiation-induced damaged structures in these material solutions and the impact of those structures on the hardness of the material. The main results of this work are: (1) the addition of 5wt%Ta to W leads to saturation in the number density and average dimensions of the radiation-induced a/2 dislocation loops formed at 350C, whereas in W the loop length increases progressively and evolves into dislocation strings, and later into hydrogen bubbles and surface blisters, (2) the recovery behaviour of proton irradiated W5wt.%Ta alloy is characterized by dislocation loop growth at 600-900C, whereas voids form at 1000C by either vacancy absorption or loop collapse, (3) the presence of radiation-induced a loops at 590C in Ta hinders the formation and ordering of voids observed with increasing damage levels at 345C, (4) the addition of 5-10wt.%W to Ta delays the evolution of a/2 dislocation loops with increasing damage levels, and therefore the appearance of random voids. These results expand the composition palette available for the safe selection of refractory alloys for plasma facing components with enhanced, or at least predictable, tolerance to the heat-radiation flux combinations expected in future nuclear fusion plants.

Chapter 1 presents the basic principles of Controlled Thermonuclear Fusion, and the approaches to achieve nuclear fusion on Earth. Furthermore, the basic components of the Tokamak, the reactor which will house the fusion reaction, are analyzed. Finally, the chapter ends with a discussion on how the present thesis is related to the Controlled Thermonuclear Fusion. Chapter 2 introduces briefly the basic concepts of the Electromagnetic and Magnetohydrodynamic theories as well as MHD turbulence. Chapter 3 presents a first glance in OpenFOAM CFD library. Chapter 4 introduces the Orszag-Tang vortex flow, which is a benchmark test case for MHD numerical models. Also, the results obtained by the model developed in this thesis are presented and discussed. Chapter 5 describes an analytical solution method for the MHD natural convection in an internally heated horizontal shallow cavity. Also, a finite volume numerical model is presented for solving the aforementioned problem and properly validated. The results of the numerical model are compared with the analytical solutions for a range of Rayleigh and Hartmann numbers. Finally, conclusions based on this work are drawn and recommendations for future work are made.

The promise of a vast source of energy to power the world and protect our planet using fusion technology has been the driving force for scientists and engineers around the globe for more than sixty years. Although the materialization of this ideal still in the distance, multiple scientific and technological advances have been accomplished, which have brought commercial fusion power closer to a reality than it has ever been. As part of the collaborative effort in the pursuit of realizable fusion energy, the International Thermonuclear Experimental Reactor (ITER) is being developed by a coalition of nations of which the United States is a part of. One critical technological challenge for ITER is the development of adequate plasma facing materials (PFMs) that can withstand the strenuous conditions of operation. To date, high heat flux (HHF) testing has been conducted mainly on non-irradiated specimens due to the difficulty of working with radioactive specimens, such as instrument contamination. In this thesis, the new Irradiated Material Target Station (IMTS) facility for fusion materials at Oak Ridge National Laboratory (ORNL), in which the HHFs are provided by water-wall plasma-arc lamps (PALs), is considered for neutron-irradiated specimens, especially tungsten. The facility is being used to test irradiated plasma-facing components materials for magnetic fusion reactors as part of the US-Japan plasma facing components evaluation by tritium plasma, heat and neutron irradiation experiments (PHENIX). In order to conduct HHF testing on the PFMs various sample holders designs were developed to accommodate radioactive specimens during HHF testing. As part of the effort to design sample holders that are compatible with the IMTS facility, numerical simulations were performed for different water-cooled sample holder designs with the commercial computational fluid dynamics (CFD) software package, ANSYS™ FLUENT®. The numerical models are validated against experimental temperature measurements obtained from the IMTS facility. These experimentally validated numerical models are used to assess the thermal performance of two sample holder designs and establish safe limits for HHF testing under various operating conditions. The limiting parameter for the current configuration was determined for each sample holder design. For the Gen 1 sample holder, the maximum temperature reached within the Copper rod limits the allowable incident heat flux to about 6 MW/m². In the case of the Gen 2 sample holder, the maximum temperature reached within the Molybdenum clamping disk limits the allowable incident heat flux to about 5 MW/m². In addition, the numerical model are used to parametrically investigate the effect of the operating pressure, mass flow rate, and incident heat flux on the local heat flux distributions and peak surface temperatures. Finally, a comparative analysis is conducted to evaluate the advantages and disadvantages associated with the main design modifications between the two sample holder models as to evaluate their impact in the overall thermal performance of each sample holder in order to provide conclusive recommendations for future sample holder designs.

The need to make maximum use of renewable resources to the detriment of fossil fuels to achieve environmental goals with an increasing energy demand is driving research into the development of technologies to obtain energy from sources that are not currently being exploited, one of them being fusion energy. The aim of this report is to provide a general overview of fusion and to provide a critical opinion on whether fusion will become a commercial energy source in the future, and if so when. The followed methodology has been a literature review complemented by an interview to B Henric M Bergsåker, teacher and researcher at the KTH on fusion plasma physics and information person for the Swedish fusion research.In the results section the fusion physics and different technological approaches have been presented. Among the studied different projects, the ITER Tokamak magnetic reactor has been selected as the most promising of these projects, as a product of international collaboration, and it has been analyzed in more detail. The obtained results have been that fusion can be an inexhaustible, environmentally friendly and safe energy source. The first-generation fusion commercial reactors are expected to be part of the energy mix before 2100.

Liquid metals (LMs) are compelling candidates for use as plasma facing components (PFCs) in fusion devices to mitigate heat loading, limit damage due to erosion, and possibly breed tritium. When used as electrodes, such as in z-pinch devices, PFCs are subject to large current and magnetic flux densities resulting in large Lorentz forces. Furthermore, if the PFCs are LM, the forces excite wave behavior that has not previously been investigated. The work presented here first characterizes the response of LMs to current pulses which peak between 50 and 200 kA and generate magnetic pressures between 0.5 and 5 MPa. High-speed videography records the liquid metal free surface during and after the current pulse and captures a fast moving, annular jet of LM emerging from the main body. The vertical velocities of the jet range from 0.6 to 5.3 m/s which is consistent with hydrodynamic predictions. Ejection of small droplets is observed from the LM immediately after the current pulse, preceding the LM jet, with velocities ranging from −3.1 to 18.9 m/s in the vertical direction and −14.3 to 6.3 m/s in the radial. A statistical model is developed to predict the likelihood of certain LM PFC material contaminating a core plasma and the severity in such an event. Lastly, effectiveness of bulk wave movement mitigation is investigated with two solid barrier designs, a cylindrical and conical baffle. These designs were fabricated after an iterative design process with assistance from hydrodynamic simulations. A cylindrical baffle design is shown to be preferable for integration into future fusion devices for the reduced likelihood of interference with plasma column formation. / Doctor of Philosophy / Liquid metals are considered for use as a coating on the interior surfaces in nuclear fusion reactors because they can remove heat, reduce damage, and generate additional fuel for the reactor. There has been very little research on what happens to the liquid metal when large amounts of electric current pass through it, as would be necessary in some designs. The work presented here first shows the liquid responds to large amounts of electric current with a fast moving, ring-shaped jet that correlates to the specific amount of current used. A theoretical relationship is used to relate the jet to hydrodynamic scenarios with solid bodies entering liquids. Small droplets are also observed sprayed from the LM earlier in time and the likelihood and severity of liquid metal contaminating the fusion core is analyzed. Finally, solid barriers are used to slow down the jet and minimize the mass it contains. To reduce the likelihood that the jet interferes with the fusion core, certain characteristics of barriers are identified as being preferable for use in plasma devices.

One of the most demanding engineering issues in Inertial Fusion Energy (IFE) reactors is the design of a reaction chamber that can withstand the intense photons, neutrons and charged particles due to the fusion event. Rapid pulsed deposition of energy within thin surface layers of the fusion reactor components such as the first wall may cause severe surface erosion due to ablation. One particularly innovative concept for the protection of IFE reactor cavity first walls from the direct energy deposition associated with soft X-rays and target debris is the thin liquid film protection scheme. In this concept, a thin film of molten liquid lead is fed through a silicon carbide first wall to protect it from the incident irradiations. Numerous studies have been reported in the literature on the thermal response of the liquid film to the intermittent photon and ion irradiations, as well as on the fluid dynamics and stability of liquid films on vertical and upward-facing inclined surfaces. However, no investigation has heretofore been reported on the stability of thin liquid films on downward-facing solid surfaces with liquid injection through (i.e. normal to the surface of) the bounding wall. This flow models the injection of molten liquid lead over the upper end cap of the reactor chamber. The hydrodynamics of this flow can be interpreted as a variation of the Rayleigh-Taylor instability due to the effect of the bounding wall which is continuously fed with the heavier fluid. In order to gain additional insight into the thin liquid film protection scheme, experiments have been conducted to investigate the critical issues associated with this concept. To this end, an experimental test facility has been designed and constructed to simulate the hydrodynamics of thin liquid films injected normal to the surface of and through downward-facing flat walls. In this doctoral thesis, the effect of different design parameters (film thickness, liquid injection velocity, liquid properties and inclination angle) on liquid film stability has been examined. The results address the morphology of the film free surface, the frequency of droplet formation and detachment, the size and penetration depth of the detached droplets, and the interface wave number. These experimental data have been used to validate a novel mechanistic numerical code based on a level contour reconstruction front tracking method over a wide range of parameters. The results of this investigation will allow designers of IFE power plants to identify appropriate windows for successful operation of the thin liquid film protection concept for different coolants.

The chamber walls in inertial fusion reactors must be protected from the photons and ions resulting from the target explosions. One way this can be accomplished is through a sacrificial liquid wall composed of either liquid jets or thin liquid films. The x-rays produced by the exploding targets deposit their energy in a thin liquid layer on the wall surface or in the surface of liquid jets arrayed to protect the wall. The partially vaporized liquid film/jet forms a protective cloud that expands toward the incoming ionic debris which arrives shortly (a few s) thereafter. The charged particles deposit their energy in the vapor shield and the unvaporized liquid, thereby leading to further evaporation. Re-condensation of the vapor cloud and radiative cooling of the expanding plasma allow the energy deposited in the liquid to be recovered prior to the next target explosion (100ms). Chamber clearing prior to the next explosion represents a major challenge for all liquid protection systems, inasmuch as any remaining liquid droplets may interfere with beam propagation and/or target injection. Therefore, the primary objective of this research is to experimentally examine the interaction between liquid droplets and low- temperature, low-pressure plasmas under conditions similar to those expected following inertial fusion target explosions and the subsequent expansion. The data obtained in this research will be useful in validating mechanistic chamber-clearing models to assure successful beam propagation and target injection for the subsequent explosion.