Nuclear technology is key to enabling the establishment of scientific bases on the Moon or Mars, or for exploring deep space. Its use can take several forms (RTG, NTP among others), but this thesis focuses on Nuclear Electric Propulsion (NEP): heat produced by a nuclear reactor is converted into electricity to power an ionic propulsion engine. Various concepts have been studied in the past (PROMETHEUS, MEGAHIT and DEMOCRITOS, typically for Jupiter satellite exploration missions), while currently design studies are underway at CEA for a 100 kWe nuclear-electric NEP system.
The system of interest combines several specific design choices: uranium nitride fuel, direct gas cooling (helium-xenon mixture) and energy conversion system based on a Brayton cycle, as well as waste heat evacuation through thermal radiation. These choices address requirements to minimize mass and volume, and to ensure performance and reliability for the duration of the scientific mission. Analysis of the dynamic behavior of the nuclear-electric system is therefore crucial for project success. However, the issue of transient modeling of a complete spatial nuclear-electric system is very poorly addressed in the state of the art, especially for NEP.
The thesis objectives are therefore to research and develop physical models adapted to a NEP system, to propose an approach for their validation, and finally to implement them to analyze the dynamic behavior of the reactor and contribute to improving its design. Several mission phases will be studied: reactor startup in space, power variation transients for the ionic propulsion engine, reactor response in case of failure, and its potential shutdown with the problem of safe residual power evacuation.
The thesis will be conducted at IRESNE Institute (CEA Cadarache), in a stimulating scientific environment, and integrated into a team designing innovative nuclear reactors. CNES will also be involved in monitoring the work, particularly to define the ionic propulsion engine characteristics and exploration missions of interest for the nuclear-electric system. The thesis topic, combining modeling, fluid mechanics, thermodynamics, neutronics, and space mechanics, will lend itself to scientific communication and allow the development of key skills for an academic or industrial career.