<p>The search for an enhancement of nuclear energy suggests that breeding of fissile fuel from fertile materials as well as the rejuvenation of spent nuclear fuel is of major importance. With the development of linear accelerator breeders it is possible, in principle, to enrich depleted nuclear fuel and to reenrich spent thermal reactor fuel for continued burnup without reprocessing. The management of separated fissile isotope is thus largely avoided.</p> <p>The symbiotic nuclear reactor system analyzed here consists of a linear accelerator breeder and a conventional nuclear converter reactor capable of burning natural uranium fuel. With repeated breed-burn cycles the system nuclear fuel can be brought to fuel burnups of up to and exceeding 27 times the conventional system once-through fuel burnup.</p> <p>The analysis undertaken here shows considerable potential for uranium based fuel cycles relative to that for thorium fuel cycles for the case of low fuel burnup rejuvenation without reprocessing. Consequently uranium-plutonium fuel cycles are analyzed for schemes with periodic chemical fuel reprocessing as well as without reprocessing.</p> <p>This analysis involves the development of a detailed symbiotic system dynamic mass-energy conservation formulation, yielding results ranging from detailed system isotopic concentration histories to general system fuel mass flow and energy flow. A comparison is made between the major parameters of the symbiotic reactor system and its equivalent conventional system.</p> <p>Optimum symbiotic system characteristics are generally obtained for net accumulated fuel burnups below 9 times the conventional once-through fuel burnup. For the presently estimated fuel to capital-operational cost fraction a marginally favorable symbiotic system is only obtained with an energy self-sufficient breeder subsystem. For increased fuel cost fractions, however, the symbiotic system shows, highly improved system characteristics even for net power consuming breeder subsystems.</p> / Doctor of Philosophy (PhD)

<p>The neutron image unsharpness which generally limits radiographic resolution to a qualitative assessement has been studied in detail and found to exhibit unique features at object boundaries and material discontinuities. These radiographic features provide intrinsic criteria for accurate dimensional measurements and the determination of material boundaries.</p> <p>As a basis for our investigation, the analysis of the underlying radiation-conversion and transport phenomena of the neutron imaging process was undertaken. The resultant functional image characterizations were found to be dependent on the radiographed object geometry, object dimensions, and material composition. Two avenues for extracting quantitative details from radiographs have thereupon been identified. Firstly, the coordinates bounding an object or regions of material discontinuities were found to be obtainable by an adaptation of the experimental optical density data to the functional representation. Secondly, the optical density variations due to image unsharpness was found to possess functional inflections at object boundaries and material discontinuities.</p> <p>The applications of both methods to the determination of limits of internal material discontinuities and dimensions of objects have been investigated in some detail. Experimental tests of the inflection approach show excellent agreement with theory. A mathematical-theoretical characterization of the neutron imaging phenomenon has thus been established and developed into a practical tool for extracting dimensional information from a neutron radiographic image.</p> / Doctor of Philosophy (PhD)

<p>Heavy water moderated thermal nuclear fission reactors have a greater inherent neutron economy than light water or graphite moderated reactor designs. Consequently, such units, operating on a variety of fuel cycles, may play an ever-increasing role in meeting future global energy demands. This thesis explores, analytically, the operational advantages and challenges associated with the use of a low enriched uranium (LEU) fuel cycle in advanced reactors based on the CANDU heavy water moderated, pressure tube design concept. The flexibility afforded through the use of LEU fuel is applied to enhance the operational and safety characteristics of reactors utilizing this fuel cycle. An investigation of factors influencing coolant void reactivity is conducted. Design modifications are introduced to reduce the coolant void reactivity, while maintaining the continued capability of high power operation. An enrichment and element radius graded fuel bundle design is developed with a central graphite core, an inner ring of 14 fuel elements, and an outer ring of 21 fuel elements. Fuel and lattice design perturbations are investigated to examine the effect of lattice pitch variations, capability of radioisotope production, the use of burnable poisons, and light water coolant. Xenon override requirements with LEU fuel are addressed. The efficacy of using modified two group (M-2) neutron diffusion theory for LEU fuel management studies is investigated. A modelling strategy is developed for the simulation of reactivity devices and fuel lattice properties using the M-2 methodology with a fixed energy cut-off. Detailed fuel management studies are conducted to examine the operational intricacies of LEU fuelling. Improved checkerboard type fuelling strategies are developed. Finally, the CANDU - Spectral Shift Advanced Thermal Reactor (CANDU-SSATR) is introduced and characterized. This multi-spectrum high burnup advanced reactor design utilizing simplified fuel management strategies holds great promise for the future.</p> / Doctor of Philosophy (PhD)

<p>The ability to predict the flow patterns and flow pattern transitions in a two-phase flow process is useful for an accurate prediction of the pressure drop, heat and mass transfer rates, and also for the choice of appropriate two-phase flow design parameters for the system. During a loss of coolant accident (LOCA) in a nuclear reactor, two-phase flow may exist in the primary heat transport loop, and a knowledge of the flow patterns that are occurring at the various flow conditions is needed to accurately model the accident scenario.</p> <p>Horizontal gas-liquid two-phase flow patterns and flow pattern transitions have been investigated both theoretically and experimentally for a pipe of 5.08 cm i.d., annulus geometries of outer tube diameter 5.08 cm i.d. and inner-to-outer diameter ratios from 0.375 to 0.625, and for a 37-rod nuclear fuel type bundle flow system having an outer tube diameter of 10.16 cm i.d. and rods of diameter, 1.27 cm. The 28-rod bundle flow geometry was also studied theoretically. The flow conditions were at inlet pressures of about 1 to 2 bar and at near room temperature.</p> <p>In this study, the time averaged void fraction and pressure drop measurements were also successfully obtained. The instantaneous and time averaged void fraction measurements were achieved by the ring type capacitance transducers based on the differences in the dielectric constants of the liquid and gas phases. The various flow patterns occurring in the pipe, annulus and rod bundle flow systems were successfully characterized by direct visual observation through the transparent test sections and also from the signal waveforms of the instantaneous fluctuations in the void fraction, the pressure drop measurements and the ultrasonic transmission waveforms. Flow pattern transitions were determined from both the results of the measured void fraction and direct visual observation.</p> <p>The experimental results show that the flow pattern structures occurring in horizontal annulus and rod bundle geometries are similar to those observed for the pipe flow case, except the annulus flow system where we characterized two additional flow patterns, namely, "Annulus-Slug" and "Annulus-Plug". These flow patterns are similar to the Slug and Plug flow structures observed for the pipe geometry, but are restricted to the lower annulus channel gap below the annulus rod. These occur at the flow conditions that would otherwise lead to Stratified flow patterns for pipe flow cases.</p> <p>The results show that the flow pattern transitions for the annulus and rod bundle flow geometries are significantly different from those of the normal pipe flow. The flow pattern transitions for the annulus flow geometries were observed to be significantly influenced by different inner-to-outer diameter ratios, except the Stratified Smooth to Stratified Wavy transition. The Stratified to Intermittent and the Intermittent to Dispersed Bubble transitions occur at lower superficial liquid velocities, while the Intermittent to Annular transition occurs at higher superficial gas velocities for larger inner-to-outer diameter ratios. In the rod bundle geometries, the flow pattern transitions were observed to vary slightly with the particular angle of orientation of the bundle within the enclosing tubeshell. The various influences on the flow pattern transitions observed in the present study are mainly due to the differences in geometries and force distributions.</p> <p>From direct visual observation results, we also observed that interfacial waves in the rod bundle flow geometry were generated and dissipated at the rod bundle end plates. No significant effect of the rod bundle end plates on the other flow patterns was observed, except a slight effect on the regularity of these intermittent flow patterns, usually becoming more apparent at higher flow rates.</p> / Doctor of Philosophy (PhD)

<p>In a pressurizer water nuclear power plant, the pressure of the primary heat transport system is maintained mainly through the operation of a pressurizer. Existing pressurizer models are not satisfactory in predicting pressurizer behaviour in response to operating or accidental transients. This is mainly due to lack of understanding on non-thermodynamic equilibrium and other local two-phase phenomenon in the pressurizer. The primary objectives of this work are to prepare necessary tools for a systematic study of the pressurizer and to investigate pressurizer phenomena under quasi-steady-states and to determine the effects of major pressurizer control parameters to the behaviour of the pressurizer.</p> <p>To achieve the first objective a rate form of equation of state is analytically derived. It is used as an analytical expression of pressurizer pressure response as well as to support and to guide the rest of the work. lDRIFF, a two-phase simulation code consisting of a lumped homogeneous model and a differentially formulated drift-flux model, is developed to accommodate any physical assumptions in pressurizer modelling. A laboratory scale pressurizer system, complete with a glass pressurizer tank, a simulated primary heat transport system and substantial monitoring facilities is also developed. Together, they provide analytical, numerical and empirical tools needed in a systematic study of pressurizer phenomena.</p> <p>To achieve the second objective, various quasi-steady-state conditions in the pressurizer are simulated experimentally and four distinct flow regimes are identified in the pressurizer. With the help of numerical extrapolation by using the IDRIFF code, the data are analyzed to produce, among other things, a correlation of average void fraction, a pressurizer flow-regime map with the transitions of flow-patterns semi-analytically modelled, and a general understanding of pressurizer behaviour under quasi-steady-states.</p> <p>In addition, the effects of changes in pressurizer heater, steam-bleed flow and pressurizer surge-line flow to the behaviour of the pressurizer are investigated. Perturbation transients, where the dynamic effect of the parameters can be isolated from other transient factors, are empirically and numerically simulated. The resulting overall pressurizer behaviour and observed local phenomena are analyzed in terms of their relation to quasi-steady-state behaviour, global and local pressure response, local phase distribution and pressurizer stability. Several flow and heat transfer mechanisms, such as the temperature of the steam being controlled by the liquid saturation pressure, are also summarized.</p> <p>It is believed that the preparation of research tools and the accumulation of information learnt during the course of the current endeavour has formed an essential basis for a further systematic study of nuclear power plant pressurizer and has brought the study one step closer to the goal of achieving a complete understanding of pressurizer phenomena.</p> / Doctor of Philosophy (PhD)

In this dissertation, techniques that have potential applications to the digital operation of nuclear reactors are developed. A digital controller is developed to plan feasible control actions and digital transient identifiers are developed to detect unexpected events. Given an operational objective, the model based controller identifies a sequence of control actions that will fulfill the operational objective. The hazard anticipator checks the feasibility of the control. Only by means of the hazard anticipator, can the digital control system safely perform control actions without violating technical specifications and the limits of the physical system. In the controller, a precursor population meter implemented as a finite filter of the power history provides effective and accurate estimation of the delayed neutron precursor population. In this dissertation, techniques of transient identification are developed to detect unexpected events. Neural networks are used to identify different types of transients based on the distinct features extracted from the transients by matching pursuit decomposition or by shiftable wavelet transformation. The techniques have been shown to work well in extensive tests.

<p> Conventional ceramic oxide nuclear fuels are limited in their thermal and life-cycle properties. The desire to operate at higher burnups as is required by current utility economics has proven a formidable challenge for oxide fuel designs. Metallic formulations have superior thermal performance but are plagued by volumetric swelling due to fission gas buildup. In this study, we consider a number of specific microstructure configurations that have been experimentally shown to exhibit considerable resistance to porosity loss. Specifically, a void sizing that is bimodally distributed was shown to resist early pore loss and could provide collection sites for fission gas buildup. We employ the phase field model of Cahn and Hilliard, solved via the finite element method using the open source Multi-User Object Oriented Simulation Environment (MOOSE) developed by INL.</p>

<p> <b>Purpose:</b> The purpose of this project was two-fold. One, to test the accuracy and usefulness of a clinically unused dose calculation algorithm for the Leksell Gamma Knife Perfexion radiosurgery unit that accounts for heterogeneities in the patient volume. This process included designing, fabricating, and testing a novel phantom from idea stage through production and use. Two, to facilitate and provide dosimetry for irradiating a large number of mice and rats to develop a murine model of radiation induced necrosis in the brain.</p><p> <b>Methods/Materials:</b> To test the dose calculation algorithm, we used a commercially available anthropomorphic head phantom and EBT2 radiochromic film to evaluate predicted vs measured dose delivery for the clinically accepted algorithm, which assumes a homogeneous treatment volume, and the convolution algorithm, which takes into account heterogeneities within the treatment volume. In addition, we designed and fabricated a novel phantom that could accommodate various heterogeneities along with EBT2 film and an ion chamber. We again evaluated predicted vs measured dose with varying material configurations for both algorithms. To assist the murine necrosis model, we developed a novel mouse positioning and irradiation system utilizing the Gamma Knife Perfexion that was designed to be accurate, repeatable and efficient. We designed an animal immobilizing planform that could be incorporated into the clinical protocol for acquiring patient image data, image registration, and treatment planning.</p><p> <b>Results:</b> We demonstrated that the convolution algorithm is accurate to within a clinically acceptable three percent in cases of extreme heterogeneities, and it is clinically significantly more accurate than the standard homogeneous algorithm when large heterogeneities are present in the treatment volume. In addition, we were able to facilitate the development of a robust murine radiation necrosis model by irradiating more than 1,000 mice to a spatial accuracy of within 0.5 millimeters in all directions and to within five percent accuracy of prescription dose.</p><p> <b>Conclusion:</b> During the course of this work we successfully completed two large undertakings that are representative of tasks often asked of a clinical medical physicist. First, to evaluate a treatment delivery option in the radiation oncology clinic and make evidence based recommendations for clinical protocols. And second, to provide a reliable and scientifically sound service to collaborators and outside research groups when physics expertise is required.</p><p>

<p> Radiation therapy accounts for more than half of cancer treatments in the US, and in order to provide the most effective treatment to patients, new developments are implemented each year. Before a novel radiation therapy device or technique can be used to treat patients in the clinic, it must first undergo testing. One of the most effective testing methods is preclinical small animal testing, because the testing environment provides a large sample population on which treatment variations can be tested for efficacy and possible side effects. To improve the effectiveness of preclinical testing, the devices and methods used on small animals should closely resemble those used in the clinic. These include irradiators, fractionation schedules, repeatability methods, and results characterization. Results characterization will provide a translational pathway between the preclinical and clinical environments of a small animal irradiation and human treatments to account for variation in treatment beams and subject size between preclinical and clinical irradiations. </p><p> The Biomedical Physics Laboratory at Washington University in St. Louis, with whom I was working, developed a preclinical small animal image guided microirradiator, the microIGRT, and we characterized the device using clinical methods, such as those used for machine acceptance and quality assurance. In order to provide treatment verification and subject positioning repeatability, we designed, developed, and characterized a micro electronic portal imaging device (&mu;EPID), similar to the portal devices used on clinical linear accelerators. Using the microIGRT and the &mu;EPID, we developed treatments for small animal brain, lung, liver, and spinal tumors using clinical treatment planning methodologies translated to preclinical small animal models. We characterized the treatment results with several metrics and compared these to clinical treatments. The metrics were compared, side by side, and conclusions were drawn for the efficacy of the small animal treatment to establish the first steps for a pathway to translate preclinical results to clinical trials. Considering the difficulties of dosimetry for small fields commonly used in small animal irradiations, we also designed and developed a fiber scintillating microdosimeter. This dosimeter allowed for more accurate orthovoltage beam characterization, thus improving treatment planning and translational treatments.</p><p>

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1998. / Includes bibliographical references (p. 137-138). / by Imad Maurice Jureidini. / Ph.D.