High Fidelity Design Tools and a New Hydrogen Containment Process for Nuclear Thermal Engine Ground Testing

The significant capability for long distance space travel offered by nuclear thermal propulsion (NTP) systems has the potential to revolutionize NASA's space exploration efforts. As with any new propulsion system, ground testing is an important part of development. However, the 2014 Draft NASA Technology Roadmap, TA2: In-Space Propulsion Technologies, notes that ground testing of NTP systems in a way that demonstrates full system qualifications and meets current federal environmental regulations is a significant challenge that must still be resolved. A critical issue in NTP ground testing is the large amount of hot radioactive hydrogen exhausted during the tests. A safe, robust and affordable hydrogen containment process is necessary to enable the testing required for NTP development. At this point, such a process does not exist. A system-level concept has been developed for converting the exhaust hydrogen to steam by combusting it with oxygen, condensing the superheated steam to form water, and then completely containing the resulting water and remaining oxygen. But system level designs do not adequately account for the finite rate chemistry and complex multi-phase mixing and thermodynamics in the hydrogen treatment process. The omission of these first order physics create significant doubt that the hydrogen will be removed to satisfactory levels. The quantity of water spray required to quench the hot hydrogen exhaust is also quite uncertain, though it will surely be quite large. These are safety and cost issues, respectively, that would result in failure to meet the design objectives of a safe, robust and affordable hydrogen containment system. This effort proposes to design and conduct a high-fidelity, simulation-based demonstration of a containment process that meets these design objectives with innovative simplicity. Preliminary efforts indicate a safe and cost effective solution is possible. To meet the design safety objectives, the new system must be confidently shown to minimize, if not eliminate, the existence of residual hydrogen in the system. The cost objective is addressed by use of a non-contact heat exchanger that minimizes the water spray required, thus greatly reducing the cost associated with collection and storage of this contaminated water. A unique, state of the art design process using high-fidelity tools that account for the first order physics not included in the system-level design will be used to produce a detailed design and simulation of the system to demonstrate that it meets both design goals. Since this level of fidelity has not previously been applied to the design of such a system, the risk is somewhat high, but the potential payoff is significant in terms of both safety and cost. Nuclear Thermal Propulsion (NTP) is an enabling technology for delivering large payloads to Mars with reasonable transit time because of its high thrust and high Isp. NTP is baselined for Mars missions in the 2030 time frame. NTP features a solid-core nuclear reactor consisting of hundreds of heat generating fuel elements. Each fuel element contains tens of tubular flow channels through which the hydrogen working fluid acquires energy and expands in a high expansion nozzle to generate thrust. The 2014 Draft NASA Technology Roadmap, TA2: In-Space Propulsion Technologies, notes that ground testing of NTP systems in a way that demonstrates full system qualifications and meets current federal environmental regulations is a significant challenge that must still be resolved. A critical issue in the ground testing is dealing with the large amount of hot radioactive hydrogen exhausted during the tests. This hot hydrogen exhaust cannot be vented into the atmosphere without potential for explosion and/or possible contamination to surrounding areas. It cannot be stored either, due to safety concerns. A safe, robust and affordable hydrogen containment process is necessary to enable the testing required for NTP development. The objective of this effort is to design and conduct a simulation-based demonstration of such a system. This demonstration will be the first of its kind to include the first order physics at a meaningful fidelity level. To date, hydrogen containment processes have been designed based on results from system-level models. One such hydrogen containment process currently under consideration utilizes hot hydrogen exhaust being shocked down through a diffuser and then largely combusted to steam in an oxygen rich burner. Even with a higher than stoichiometric oxygen-to-hydrogen ratio, there are still appreciable amounts of hydrogen and radicals such as hydrogen, oxygen, and hydroxyl atoms remaining in the exhaust. So, the remaining hydrogen and associated radicals must also be converted into steam. This is accomplished by reducing the system temperature via a direct water spray cooler. The large volume of now-contaminated water from the test article exhaust and the spray must be collected and stored. There are significant levels of uncertainty associated with this simplified system-level approach to the complex set of coupled physics found in the NTP hydrogen containment system design and operation. The chemical equilibrium assumption used in the system-level design model is based solely on thermodynamics. Thus, it necessarily assumes infinite residence time, infinite reaction rate, and infinite mixing rate between the hot gas and the cooling water spray. In actuality, none of those assumptions is valid here. Due to the finite dimension of the direct water spray cooler, the residence time of the hot exhaust flowing through the cooler is finite rather than infinite. The chemical reaction rate is always finite, not infinite. The multiphase mixing rate between the gaseous exhaust and the liquid water spray is also unlikely infinite. Further, the above discussion highlights the real possibility that much more water could be required to achieve the design goal than predicted by the system-level calculations. This would require costly storage of very large amounts of contaminated water. More »