<p>A series of analytic and numerical models have been developed for the prediction and interpretation of the energy spectrum of the neutron emission from thermonuclear deuteriumtritium (DT) plasmas. The main component of the neutron emission, due to reactions between thermal fuel ions, has been modeled as well as minority components due to the presence of fast (supra-thermal) ions. In particular, the so-called alpha-particle knock-on neutron (AKN) emission has been analyzed and found to carry information on the con- finement of fast alpha particles inside the plasma. The alpha particles carry one fifth of the fusion power generated in the plasma and provide the plasma self-heating. This thesis is devoted to this central endeavor of fusion research and the possibilities for its study in today’s largest magnetic confinement devices, the tokamaks.</p><p>The developed models have been used for the interpretation of experimental data taken during the first deuterium-tritium experiment (DTE1) at the Joint European Torus (JET) in 1997. The data were taken with a neutron spectrometer of the magnetic proton recoil (MPR) type developed at the Department of Neutron Research (INF) of Uppsala University. The MPR was used to measure the neutron emission from DT plasmas representing record high fusion power levels of up to 16 MW and correspondingly high quality in the neutron emission observations. These studies in DT plasmas were complemented with theoretical and empirical studies of the 14-MeV triton burn-up neutron (TBN) emission from deuterium plasmas.</p><p>The predicted neutron energy spectra were found to be able to describe observations leading to positive identification of previously unobserved spectral features such as the very weak AKN and TBN signature in the neutron emission. In this summary, the developed models are presented as well as the experimental findings. Finally, a discussion is included of the possible application of the presented models and experimental techniques to next-step fusion experiments such as the proposed ITER tokamak.</p>

<p>Fe-Cr alloys form the basis of many industrially important steels. Due to their excellent resistance to radiation induced swelling, ferritic steels are expected to be used for critical structural components in advanced nuclear systems, such as fast breeder reactors, accelerator driven systems and fusion reactors. In this thesis project, theoretical modelling of bulk properties of Fe-Cr alloys has been performed for a wide range of phenomena. Electronic structure calculations, based on density functional theory, have been used to determine equilibrium properties for different magnetic states of the alloy. Ferromagnetic alloys of low Cr concentration (<10% Cr) are anomalously stable, which is related to the variation in sign of the mixing enthalpy which was predicted for the first time in this work. This finding is in agreement with experimental evidence of long range ordering in Fe-Cr alloys with low Cr concentration, as well as the observed phase separation for compositions with higher Cr content. </p><p>The character of the interaction of point defects with solute Cr atoms in an iron matrix was investigated <i>ab initio</i>. It was found that due to magnetic interactions, interstitial defects are bound by Cr atoms in bulk iron. Vacancies, on the other hand, interact only weakly with Cr. These results may offer qualitative explanations to the observed concentration dependence of radiation induced swelling in Fe-Cr model alloys.</p><p>The <i>ab initio</i> predictions inspired an effort to develop an interatomic alloy potential capable of reproducing both the thermodynamic bulk behaviour of the alloy, such as the mixing enthalpy, and the point defect interactions, in order to perform large scale atomistic and stochastic simulations on scales out of reach for density functional theory. A two-band extension of the embedded atom method of interatomic potentials was developed in order to model ferromagnetic Fe-Cr alloys of arbitrary composition. Kinetic Monte-Carlo simulations of thermal aging, using this two-band potential, reproduce the experimentally measured formation and evolution of solute precipitation as a function of concentration for temperatures relevant to structural materials in nuclear reactors.</p>

A series of nuclear fission experiments have been performed in the intermediate energy range. The dynamics of the fission process have been studied by means of detection of neutrons emitted from compound nuclei formed by ion beams of protons, α-particles, and 12C-ions, all at a beam energy of 100 MeV. The neutron emission data have been interpreted using a statistical model analysis, and the fission time scale has been found to vary between 6.4 and 170ⅹ10-21 s. Neutron-induced fission cross section measurements have been performed on targets of 208Pb, 209Bi, and 238U. The results have been compared with other measurements as well as with model calculations and evaluations. The comparisons agreed reasonably well with previous measurements, but disagreed substantially both with calculations and evaluations. The neutron flux in the neutron-induced fission was determined relative to the np-scattering cross section which was measured simultaneously. The np-scattering cross section has been measured with an unprecedented precision. Using a normalization procedure which takes advantage of the total np cross section, which has been determined with great precision, differential np cross sections have been presented in the angular range 74° to 180° at 96 MeV and 72° to 180° at 162 MeV. They were found to disagree with as much as 10—15 % compared with older measurements in the world data base, and with Partial Wave Analysis solutions. The data are, however, in agreement with another recent, high-quality measurement.

A series of analytic and numerical models have been developed for the prediction and interpretation of the energy spectrum of the neutron emission from thermonuclear deuteriumtritium (DT) plasmas. The main component of the neutron emission, due to reactions between thermal fuel ions, has been modeled as well as minority components due to the presence of fast (supra-thermal) ions. In particular, the so-called alpha-particle knock-on neutron (AKN) emission has been analyzed and found to carry information on the con- finement of fast alpha particles inside the plasma. The alpha particles carry one fifth of the fusion power generated in the plasma and provide the plasma self-heating. This thesis is devoted to this central endeavor of fusion research and the possibilities for its study in today’s largest magnetic confinement devices, the tokamaks. The developed models have been used for the interpretation of experimental data taken during the first deuterium-tritium experiment (DTE1) at the Joint European Torus (JET) in 1997. The data were taken with a neutron spectrometer of the magnetic proton recoil (MPR) type developed at the Department of Neutron Research (INF) of Uppsala University. The MPR was used to measure the neutron emission from DT plasmas representing record high fusion power levels of up to 16 MW and correspondingly high quality in the neutron emission observations. These studies in DT plasmas were complemented with theoretical and empirical studies of the 14-MeV triton burn-up neutron (TBN) emission from deuterium plasmas. The predicted neutron energy spectra were found to be able to describe observations leading to positive identification of previously unobserved spectral features such as the very weak AKN and TBN signature in the neutron emission. In this summary, the developed models are presented as well as the experimental findings. Finally, a discussion is included of the possible application of the presented models and experimental techniques to next-step fusion experiments such as the proposed ITER tokamak.

Fe-Cr alloys form the basis of many industrially important steels. Due to their excellent resistance to radiation induced swelling, ferritic steels are expected to be used for critical structural components in advanced nuclear systems, such as fast breeder reactors, accelerator driven systems and fusion reactors. In this thesis project, theoretical modelling of bulk properties of Fe-Cr alloys has been performed for a wide range of phenomena. Electronic structure calculations, based on density functional theory, have been used to determine equilibrium properties for different magnetic states of the alloy. Ferromagnetic alloys of low Cr concentration (&lt;10% Cr) are anomalously stable, which is related to the variation in sign of the mixing enthalpy which was predicted for the first time in this work. This finding is in agreement with experimental evidence of long range ordering in Fe-Cr alloys with low Cr concentration, as well as the observed phase separation for compositions with higher Cr content. The character of the interaction of point defects with solute Cr atoms in an iron matrix was investigated ab initio. It was found that due to magnetic interactions, interstitial defects are bound by Cr atoms in bulk iron. Vacancies, on the other hand, interact only weakly with Cr. These results may offer qualitative explanations to the observed concentration dependence of radiation induced swelling in Fe-Cr model alloys. The ab initio predictions inspired an effort to develop an interatomic alloy potential capable of reproducing both the thermodynamic bulk behaviour of the alloy, such as the mixing enthalpy, and the point defect interactions, in order to perform large scale atomistic and stochastic simulations on scales out of reach for density functional theory. A two-band extension of the embedded atom method of interatomic potentials was developed in order to model ferromagnetic Fe-Cr alloys of arbitrary composition. Kinetic Monte-Carlo simulations of thermal aging, using this two-band potential, reproduce the experimentally measured formation and evolution of solute precipitation as a function of concentration for temperatures relevant to structural materials in nuclear reactors.

The accelerator-driven system (ADS) is recognized as a promising system for the purpose of nuclear waste transmutation and minimization of spent fuel radiotoxicity. The primary cause for this derives from its accelerator-driven, sub-critical operating state, which introduces beneficial safety-related features allowing for application of cores employing fuel systems containing pure transuranics or minor actinides, thereby offering increased incineration rate of waste products and minimal deployment of advanced (and expensive) partitioning and transmutation technologies. The main theme of the thesis is safety and kinetics performance of accelerator-driven nuclear reactors. The studies are confined to the examination of ADS design proposals employing fast neutron spectrum, uranium-free lattice fuels, and liquid-metal cooling, with emphasis on lead-bismuth coolant. The thesis consists of computational studies under normal operation and hypothetical accidents, and of evaluation and identification of safety design features. By itself, subcritical operation provides a distinct safety advantage over critical reactor operation, distinguished by high operational stability and additional margins for positive reactivity insertion. For a uranium-free minor actinide based fuel important safety parameters deteriorate. Specific analyses suggest that operation of such cores in a critical state would be very difficult. The studies of unprotected transients indicate that lead-bismuth cooled accelerator-driven reactors can be effective in addressing the low effective delayed neutron fraction and the high coolant void reactivity that comes with the minor actinide fuel, but some supportive prompt negative feedback mechanism might be considered necessary to compensate for a weak Doppler effect in case of a prompt critical transient. Although lead-bismuth features a high boiling point, the work underlines the importance of maintaining a low coolant void reactivity value. The transient design studies identified a molybdenum-based Ceramic-Metal (CerMet) fuel with favourable inherent safety features. A higher lattice pitch is suggested to avoid mechanical failure during unprotected loss-of-flow. Detailed coupled neutron kinetics and thermal hydraulic analyses demonstrated that the point kinetics approximation is capable of providing highly accurate transient calculations of subcritical systems. The results suggest better precision at lower keff levels, which is an effect of the reduced sensitivity to system reactivity perturbations in a subcritical state resulting in small spatial distortions. In the course of a beam reliability study, the accelerator was identified as responsible for frequent beam interruptions. It is clear that extensive improvement in the mean-time between beam failures is required. / QC 20101011

Thesis (Ph. D.)--Florida State University, 2002. / Advisor: Dr. Horst Wahl, Florida State University, College of Arts and Sciences, Dept. of Physics. Title and description from dissertation home page (Oct. 6, 2003). Includes bibliographical references.

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