Computational Modeling of Thermionic Energy Conversion for Nuclear Electric Propulsion

With interplanetary human spaceflight missions on the horizon, technological advancements for efficiently transporting astronauts beyond our Earth-Moon system are a necessity. One propulsion design that could enable human missions to Mars is nuclear electric propulsion. This propulsion system includes a fission reactor, radiation shield, power conversion system, thermal management system, and high-powered electric propulsion thrusters for directing the trajectory of the spacecraft bus and payloads. A primary challenge with this design is simultaneously providing sufficient power to support the high-powered propulsion system while also achieving sufficient heat rejection to manage the thermal load produced by the reactor power subsystem. Utilizing dynamic power conversion such as with Brayton, Rankine, and Stirling cycle engines introduces a working fluid and additional mechanical components to the design which increases both the complexity of the subsystem and the number of potential failure points. This proposal intends to explore and develop thermionic power conversion technologies to eliminate the necessity of moving parts in the power conversion process by using thermionics to convert heat directly to energy. Thermionics are uniquely suited to nuclear electric propulsion due to their ability not only to serve as the energy conversion system but also provide a mode of heat rejection, reducing the radiator surface area required to support the high thermal loads from the nuclear power system. The intent is to create a model that combines the physical mechanisms that accelerate thermionic emission, such as ion-enhanced emission and quantum tunneling effects, with thermal transport and heat dissipation modes. The method for accomplishing this thermionic energy conversion technology development is separated into the following milestones. First, a comprehensive literature review will be completed. This literature review will be followed by an investigation of optimal thermionic emitter candidates for use in high-power systems. Then, a theoretical model will be developed to characterize the emitter current density, specific to the material properties and including enhanced emission effects. A study of the heat transport and conversion process for the emitter will follow the development of the theoretical model and will occur simultaneously to the investigation into thermionic energy conversion efficiency improvement methods. Finally, the development of the computational model is integral to the success of this project and will include the ion-enhanced emission and quantum tunneling effects of the thermionic emitter. To summarize, my research goals include the successful modeling of advanced emission methods including ion-enhanced emission and quantum tunneling techniques. I will investigate thermionic emitters most effective for high temperature, high emission current applications in nuclear electric space propulsion systems and develop both theoretical and computational models. Then, I will perform an optimization of the thermionic energy conversion system to balance the energy conversion efficiency with the heat rejection needed to offset the high thermal loads produced by the nuclear power source.

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