Increasing Cermet Fuel Thermal Margin With Thoria For Nuclear Thermal Propulsion PDF Download

Nuclear Thermal Propulsion (NTP) is currently a topic of research for NASA. NASA has the goal of sending humans to Mars and Nuclear Thermal Propulsion (NTP) is an appealing technology to aid in this endeavor. In the simplest terms, an NTP system is a nuclear reactor that utilizes hydrogen that is expanded through a convergent-divergent nozzle and ejected for propulsion. NTP can shorten the travel time to Mars and reduce the mass that must be lifted into low earth orbit. Furthermore, strategies to use low enriched uranium (LEU) in NTP systems have been identified which could make NTP significantly more affordable to develop than for past NTP development efforts. Beyond a human Mars mission, NTP has the potential to assist in other human and robotic missions beyond low earth orbit. The overarching goal of this work is to better understand NTP technology so that it may one day help with a human Mars mission. This work focuses on a subset of NTP systems that use a combination of LEU and tungsten cermet fuel and addresses the issues related to 135Xe in these systems. LEU cermet NTP systems have a unique operational regime where 135Xe has a profound impact on performance and controllability. LEU cermet NTP have extremely high power densities, operate with a thermal neutron spectrum, and the reference human mission to Mars requires restarting the reactor 4 to 8 hour after full power operation. In this work, two LEU cermet NTP point designs are presented and used as reference systems for the study of 135Xe in LEU cermet NTP systems. These point designs were produced with a thorough search of the rocket performance design space. Using infinite lattice burnup calculations, it was found that MCNP 6.1.1 Beta and Serpent 2 produced very similar results and that burnup cells across the fuel element were not needed to capture spatial self shielding effects. The infinite lattice results were used to inform the approach undertaken for the full core burnup calculations. Full core burnup calculations indicate that the reactivity loss during operation of a LEU cermet NTP system has a maximum value of 210 pcm during a 25 minute burn and the maximum reactivity loss after operation peaks at approximately 3500 pcm. The possible effect of 135mXe on xenon worth in LEU cermet NTP systems was found by using the model based TENDL-2014 nuclear data library and Serpent 2. Model based TENDL cross sections were used because no experimentally determined cross sections are available for 135mXe. A relationship to estimate the performance (in terms of Isp loss) as a function of control drum angle is derived and presented. Mitigation strategies are identified that show promise for counteracting the effects of 135Xe and maximizing the performance of LEU cermet NTP systems. It is recommended for future work that an integrated reactor systems code be developed to examine 135Xe in LEU cermet NTP more thoroughly. In addition, this work has identified a need for basic nuclear data experiments to measure the cross section of 135mXe instead of relying on models.