No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

Computed tomography (CT) is an invaluable diagnostic tool in current medical practice. Unfortunately, the radiation dose imparted during a CT scan can be significant. This thesis seeks to develop, verify, and validate appropriate computational methods for computing this dose accurately and efficiently. The components of the model are the nuclear data, transport methods, and computer codes. Monte Carlo transport methods are employed primarily for their ability to accurately capture most of the relevant physical phenomena. Deterministic transport methods are subsequently verified and validated. The work is divided into three stages: experimental, verification, and validation. The experimental stage involves gathering high-fidelity data to aid in the validation procedures. Multiple radiation detection devices are employed to give greater certainty to the results. In addition, an important task is gathering data using a geometrically simplified phantom which is easier to model than the detailed Rando phantom. Towards this end, a CTDI FDA phantom is imaged. Exposure and dose measurements were taken in air and in the phantom center and periphery. The second stage, verification, involves the testing of the deterministic model for correctness of the methodology and the physics data, i.e. cross section library. Primarily, there are a few key assumptions which must be tested. The first is the importance of the secondary electron transport. Using Monte Carlo methods, it is found that the transport is unimportant for the accurate computation of the dose deposition distribution given the relatively low energy photons produced by x-rays tubes employed in CT scan machines. This makes the deterministic transport calculations much simpler. Next, the discretization of space, energy, and angle in the deterministic model is examined to ensure sufficient refinement capable of delivering accurate results. The Monte Carlo method is an excellent complement to deterministic methods, serving as reference as though it were an actual experiment, thus allowing the testing of these issues in a straightforward and highly controlled manner. In each discretization, the deterministic model proved capable, although some flux spectrum results differed by fifteen percent or more, mostly a result of the multigroup cross section set. Finally, after ensuring that the deterministic model was functioning as expected, a comparison was made of the simulations to the experimentally measured data. This was the most difficult of the tasks, in great part because of the lack of precise knowledge of detailed information concerning some of the parameters comprising the experimental setup. However, much effort was placed into conforming the simulations to the experiment as closely as possible. The ratio of exposures in the CTDI FDA phantom periphery-to-center is computed to within experimental uncertainty of about ten percent, while the absolute computed exposures have greater errors. The absolute exposures differed from the measured values by less than 35 percent.

Several methods of reprocessing are currently available to separate recyclable materials from spent nuclear fuel. Electrochemical processing, also known as pyroprocessing, represents a non-aqueous method of reprocessing that uses high temperature molten-salt based electrochemical technology. This method provides several advantages over conventional aqueous processing with respect to proliferation resistance. With electrochemical processing there is no pure plutonium separation and the presence of large decay heat and high radiation barriers dissuades diversion attempts. As the current electrochemical processing relies on a batch operation, the total throughput of the system is inherently limited and nuclear materials accounting is difficult due to the nonhomogeneous nature of the process. This results in much larger uncertainties in the total amount of material processed compared to the aqueous UREX+ or PUREX processes. Continuous electrochemical processing was considered as a way to address these concerns. The objective of this research was to investigate the feasibility of a continuous electrochemical processing operation to achieve the desired separation performance by using computer based simulation. The conceptual design of the continuous electrochemical processing includes two separate stages in a molten salt medium. First, a pure uranium deposit is collected at a solid cathode during the uranium extraction stage. When the amount of plutonium in electrorefiner becomes comparable or higher than the amount of uranium in the electrorefiner, a liquid cathode is employed to extract both uranium and plutonium in the second stage. In this approach, molten salt, as the material carrier, flows through the electrorefiner while chopped spent fuel is continuously fed into the system. Simulations of electrochemical reactions at the electrode surfaces were based on the kinetic modeling capability of a time-dependent code, REFIN. Based on a screening study performed for the most significant process parameters over a broad range of values, a functional combination of initial uranium and plutonium concentrations at the anode and in the molten salt was determined for continuous operation. This dictated the use of a higher concentration of uranium than plutonium at the anode and a lower concentration of uranium than plutonium in the molten salt. Furthermore, using design of experiment technique for computers, a refinement of initial concentrations was performed to maximize the total throughput and minimize the operational time. The flow velocity profiles and chemical concentration distributions of elements in molten salt have been determined through three dimensional Computational Fluid Dynamics simulations using ANSYS CFX. This approach resulted in the need to evaluate the diffusion layer thickness at the cathode â molten salt interface, an important parameter for the electrochemical process. Computer based simulations of the continuous electrochemical processing concept presented in this study have provided an indication that electrochemical processing could be a viable technology for closing the nuclear fuel cycle.

Diffraction Enhanced Imaging (DEI) with x-ray radiation provided by a synchrotron source has been shown to provide good image contrast at lower radiation dose for materials with small x-ray attenuation coefficient As a result, DEI has received significant interest for digital mammography and other medical imaging applications. However, deployment of a synchrotron source at a medical facility is not currently feasible due to its size and costs. Consequently, a compact x-ray source capable of delivering x-ray intensities and beam collimation similar to a synchrotron accelerator is desirable. A wide beam area x-ray source has been suggested as a possible alternative to a synchrotron source, with the x-rays generated by electron bombardment of a suitable target material. Previous research work demonstrated a prototype scale cylindrical shaped oxygen free copper target with a layer of molybdenum to be feasible from an engineering perspective. An industrial size DEI facility requires a scale-up of the proof-of-principle design. The x-ray flux necessary for high image quality implies significant heat loading on the x-ray source. Safe operation of a full scale DEI facility is reliant upon a thermal management solution capable of rejecting this heat. An active target cooling system has been proposed and its performance has been evaluated through CFD simulation. The design ensures the maximum target temperature is maintained at reasonable levels and coolant boiling is not reached under the most demanding operating conditions.

F-18 is a short-lived radioisotope commonly used in Positron Emission Tomography (PET). This radionuclide is typically produced through the O-18(p,n)F-18 reaction by proton bombardment of O-18 enriched water. Thermosyphon batch targets have been proposed as a means to increase F-18 production due to their enhanced heat rejection capabilities. These boiling targets have been operated with up to 3.2 kW of beam power with manageable O-18 enriched water volumes. The primary purpose of this work has been to develop a fundamental approach to target design from a modeling perspective, and to implement this approach to design new thermosyphon targets with enhanced production capabilities. Computational methods have been developed to predict target thermal performance and have been validated with experimental test data from the Duke University Medical Cyclotron and the Wisconsin Medical Cyclotron. These methods have been used to design a new production target for the Duke cyclotron with enhanced F-18 production capabilities. Low volume test targets have been successfully operated at the Wisconsin cyclotron with beam powers in excess of the desired 1.6 kW.