Time Dependent Full Core Coupled Multiphysics Analysis of Nuclear Thermal Propulsion Reactors
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Krecicki, Matthew A.
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Abstract
A novel full-core multiphysics analysis framework for Nuclear Thermal Propulsion (NTP) Reactors is developed in this dissertation. To achieve the high specific impulses and thrust levels required for crewed space exploration missions NTP systems operate at very high temperatures, rely on complex counter-flow needed to drive the turbopump, and exhibit dynamic behavior for short pulse-like operation. Therefore, the design and analysis of a NTP reactor-core requires multiphysics computational tools that can capture the heat transfer and flow complexities and dynamic haviour during the engine operation. Existing higher-order codes, such as ANSYS, can analyze complex flow paths in a NTP reactor, but incur prohibitively large computational costs and are not applicable for full-core multiphysics analysis. The development and verification of the reduced-order, ntpThermo, code is a novel contribution as it is capable of accurately modeling the complex flow paths and heat transfer within an NTP reactor. In addition, ntpThermo can perform coupled thermal-hydraulic thermo-mechanical analysis to capture the impact of thermal expansion with an acceptable computational cost. The ntpThermo code is coupled to the Monte Carlo Neutron transport Serpent code via the novel Basilisk multiphysics framework. The Basilisk framework enables full-core time-dependent multiphysics analysis by leveraging the pre-existing depletion solvers implemented into the Serpent code. The framework also enables the user to perform a critical drum search during each depletion step to account for the impact of control drum rotation during operation. Previous NTP-related research that focused on full core design has applied decoupled analysis approaches where the impact of thermal-hydraulic and thermo-mechanical feedback on the neutronic solution is neglected. In an effort to provide useful insights for current programs a reactor design which adheres to the current industry ground rules was developed. The subsequent analysis demonstrates that such decoupled approaches can introduce significant errors in the spatial power distributions and thus predicted thermal and mechanical safety margins. More specifically, for heavily moderated High Assay-Low Enriched Uranium fueled designs the fuel and moderator temperature spatial distributions have a significant impact on the neutron economy and spatial power distributions. Additionally, the impact of thermo-mechanical feedback has a significant impact on the mass-flow distribution within the core, and thus the solid material temperatures. Due to the elevated exit gas temperatures required to satisfy rocket engine performance requirements orificing is typically applied to the fuel elements in the core to reduce peak fuel temperatures. When a consistent multiphysics design approach is applied to design the orificing pattern a constant peak fuel temperature can be maintained through a 60-minute full-power burn due to the balance of various multiphysics feedback mechanisms. This dissertation demonstrates the importance of multiphysics tools to design a NTP reactor that can maintain adequate thermal and mechanical safety margins while also satisfying engine performance requirements.
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2023-04-25
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Dissertation