Beyond the Solid Core Nuclear Thermal Rocket: A Computational Investigation into Criticality and Neutronics Performance of Advanced Liquid and Gas Core Reactor Approaches for Next Generation Performance

Abstract

Space exploration enables humans to not only explore unknown worlds but also to satisfy the curiosity about the contents and origins of the Universe. The last and only extraterrestrial body to be explored by humans is the Moon. The next natural step is human exploration of Mars and beyond. The long trip times associated with space exploration beyond low Earth orbit pose many risks to humans such as psychological effects associated with confinement and isolation, cancer and disease associated with increased radiation dose, and deleterious effects of extended exposure to microgravity which affects a range of physiological systems such as ocular and cardiac. One approach to overcoming these challenges is limiting exposure time via high-speed transit. This requires advanced propulsion beyond conventional systems that feature high thrust and high fuel efficiency. A propulsion system that can enable such quick trip times to destinations throughout the solar system is the gas core nuclear rocket concept. The gas core nuclear rocket offers substantial advantages over chemical or electric or even solid core nuclear propulsion systems. Operating at high temperatures, the gas core nuclear rocket achieves high specific impulse and high thrust, essentially eclipsing conventional solid-core nuclear thermal rockets. Indeed the core itself is in the gaseous state and thus conventionally can operate at temperatures so high that the core itself is in the plasma state – thereby enabling the prospect of heating propellant to many times higher than the melting point of the nuclear fuel. Challenges to the realization of this technology include 1) stably confining the fissioning gas core, 2) preventing plasma erosion due to mixing, 3) optimizing the heat transfer from uranium plasma to the hydrogen propellant, 4) protecting the nozzle from the high-temperature exhaust, and 5) obtaining a self-sustaining critical nuclear rocket engine. In this research, the criticality of an open cycle gas core nuclear rocket engine and the heat transfer from uranium to hydrogen was studied. Material properties at high temperature and high pressure were calculated or obtained from the literature. Criticality analysis was carried out in the MCNP transport code for spherical non-homogenous gas core nuclear rocket geometry. These criticality analyses proved that a self-sustaining engine can be achieved by optimizing the geometry and the heat transfer in this engine. The heat transfer analyzes were carried out from the basic heat transfer equations. The hydrogen propellant temperatures varied from 3000 K to 40,000 K depending on the uranium core temperature which varied from 10,000 K to 55,000 K. For these temperatures, a critical engine could be obtained by adjusting the amount of uranium in the system. In this modeling activity, confinement of the uranium core is assumed to be achieved hydrodynamically to reduce the loss of uranium from the system. An optimized nozzle was designed for the gas core nuclear rocket commensurate with the high chamber pressure required for this engine to operate. Using this nozzle design and energy transferred to the hydrogen propellant, the gas core nuclear rocket’s performance was calculated. The calculated specific impulse ranged from 1000 s to 6200 s and the thrust ranged from 50,000 N to 300,000 N with a mass of approximately 100,000 kg. This analysis confirms that high-performance is achievable from a gas core nuclear rocket. Indeed, criticality is possible provided technical challenges of confinement and nozzle survivability are addressed.