Investigation of a lower thrust nuclear thermal propulsion design utilizing low-enriched uranium fuel

Due to concerns over proliferation and international security, several high-profile initiatives within the past decade including the Global Threat Reduction Initiative and the Reduced Enrichment for Research and Test Reactors Program have sought to convert any existing nuclear reactors utilizing high-enriched uranium (HEU) to low-enriched uranium (Leu). Following in this trend, nuclear thermal propulsion for crewed interplanetary travel purposes has also been evaluated for its conversion to low-enriched uranium fuel. Previous studies successfully found that, at the full-scale level of 16-kblf and greater, a nuclear thermal rocket fueled with Leu is possible without necessitating a significant mass increase. Up to this date, however, Leu has not been evaluated at smaller thrusts, which may be a desirable size for purposes such as small-scale qualification testing. Preliminary efforts during the Nuclear Cryogenic Propulsion Stage Program found that a 7.5-kblf engine is the smallest size an HEU core could reasonably reach before more than halving the thrust to weight ratio. As Leu-Ntp is inherently limited by its lower enrichment, and therefore lower U-235 density, the belief holds that Leu-Ntp can only sustain comparable system masses at the higher thrust levels, where thermal hydraulic requirements limit the minimum size of an HEU core. Nevertheless, the point of diminishing returns for Leu-Ntp relative to minimum thrust has yet to be concretely defined. The following paper begins progress towards this effort by attempting to convert the previously mentioned 7.5-kblf HEU core to low-enriched uranium. (Author)