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Thesis (Ph. D.)--Syracuse University, 2003. / "Publication number AAT 3113223."

Magister Scientiae - MSc / The inclusive double di®erential cross sections of 3He, 4He, and 6He par- ticles produced from the interaction of 12C with 93Nb at 400 MeV incident energy were extracted from the prescaled singles spectra which were mea- sured in conjunction with the correlation between 8Be fragments and ® par- ticles. A further aim of this project was to test the detector e±ciencies and energy calibrations by comparing the alpha and 8Be prescaled singles data with existing inclusive cross sections of the same reaction. The absolute cross sections of 3He and 6He fragments have been extracted based on the absolute normalization of the alpha particle spectra. A study of the inclusive spectra of 3He and 6He suggest similar features to those seen in the alpha particle spectra. These features are described by the projectile break-up and nucleon coalescence mechanisms. A theoretical model which takes both these mech- anisms into account was applied to describe 3He and 6He fragments. The model is able to reproduce reasonably well the inclusive energy spectra of the 3He and 6He fragments. These results are found to be consistent with previous studies of the emission of intermediate mass fragments. / South Africa

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<p>Nearly mono-energetic, high intensity (~10<super>7</super> gamma/s), and approximately 100% linearly polarized gamma-ray beams at energies between 5.15 and 7.60 MeV were used to induce photofission of <super>232</super>Th, <super>233,235,238</super>U, <super>237</super>Np, and <super>239,240</super>Pu. Prompt fission neutron yields parallel and perpendicular to the plane of beam polarization were measured using arrays of 12-18 liquid scintillator detectors. Prompt neutron polarization asymmetries close to zero were found for the even-odd actinides (<super>233,235</super>U, <super>237</super>Np, and <super>239</super>Pu), while significant asymmetries were found for the even-even actinides (<super>232</super>Th, <super>238</super>U, and <super>240</super>Pu). Predictions based on previously measured fission fragment angular distributions combined with a model of prompt neutron emission agree well with our experimental results. Finally, we describe a new method of measuring the enrichment of special nuclear material based on our results.</p> / Dissertation

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<p>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.</p><p>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.</p>