Fission Gas Bubble Behavior in Uranium Carbide Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/p8418r053

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  • The need for cheap reliable energy, while simultaneously avoiding uranium supply constraints makes uranium carbide (UC) fueled Gas Fast Reactors offer an attractive nuclear reactor design. In order to qualify the fuel, an enhanced understanding of the behavior of uranium carbide during operation is paramount. Due to a reduced re-solution rate, uranium carbide suffers from a buildup of very large fission gas bubbles. While these bubbles serve to reduce total fission gas release through the trapping of diffusing gas atoms, they lead to high swelling and ultimately dominate the microstructure of the fuel. The bubble size distribution is determined by the competing absorption rate and the rate of knock-out, or re-solution. As a result of the enhanced thermal dissipative properties of uranium carbide fuel, the atom-by-atom knockout process was shown to be an accurate representation of re-solution in uranium carbide. Furthermore, the Binary Collision Approximation was shown to appropriately model the re-solution event, bypassing computationally expensive Molecular Dynamics simulations. The code 3DOT was developed as an off-shoot of the code 3DTrim, both of which utilize the TRIM algorithm to calculate the kinematics of ions traveling through a material. Benefiting from modern methods and enhanced computational power, the model created in 3DOT results in a more fundamental understanding of the re-solution process in uranium carbide. A re-solution parameter that was an order of magnitude lower than previously determined was cal- culated in 3DOT. A decrease in the re-solution parameter as a function of radius occurred for low bubble radii, with a nearly constant re-solution parameter for bubble radii above 50 nm. Through comparative studies on the re-solution parameter for various values of implantation energy and atomic density in the bubble, we found that while the re-solution parameter did change slightly, the overall shape did not. A new application, BUCK, was built using the MOOSE framework to simulate the fission gas bubble concentration distribution. In order to build a bare-bones foundation, the simplistic yet historically prevalent physics that can be used to model fission gas bubble nucleation, growth, and knock-out were implemented as stepping stones until more advanced models for each physical process can be created. As the first step towards models that are based on first-principles, the new re-solution parameter was included and tested within BUCK. BUCK was tested using different parameters and behaved normally. However, from studies using representative simulation parameters, it is clear that the currently implemented theory does not adequately identify the growth mechanism that leads to larger bubbles. While this currently limits the applicability of BUCK in a full fuel pin calculation, it provides the baseline structure in which new physics can be implemented, and represents an important step towards understanding the complex behavior of fission gas bubbles.
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