(A) physics and conceptual study on LEU-loaded nuclear thermal propulsion reactors

Abstract

The present thesis provides details and insights for the resolution of the key neutronic difficulties currently facing the development of Low Enriched Uranium Nuclear Thermal Propulsion (LEU-NTP). These critical issues are the reduction in rocket performance during full power operation and the water submersion criticality accident that would occur in the case of a water landing during a mission abort. This thesis presents a set of solutions to these difficulties that require minimal changes to the current system architecture and enable their implementation into existing LEU-NTP designs. The proposed solutions include the introduction of a rapid depletion neutron absorber (BORGalloy) in the tie-tube elements, use of variable hydrogen density in the tie-tube loop (HYPOSPRA), implementation of enhanced worth control drums, and spectral shift absorbers in the core. Additional design modifications were made with existing parameters including the addition of a core exit axial reflector, smaller coolant channels in the fuel elements, and Inconel structural elements. With these solutions developed and characterized using a previously designed baseline core (SULEU), they are then combined into a new conceptual core design: SULEU-TNG. This new design demonstrates the successful integration of these solutions into a single core that meets the necessary neutronic operating requirements of a NTP core (excess reactivity for life-time operation and subcriticality during a submersion accident). With the successful resolution of these difficulties, it is concluded that with minimal additional development, LEU-NTP systems can replace historical High Enriched Uranium (HEU)-NTP systems without any loss in performance or safety.