Engineering design and analysis for new moderated space propulsion reactors utilizing advanced HEU and LEU fuels

Abstract

Space exploration is a realistic and profitable goal for not only understanding the universe and protecting our planet from hazardous asteroids, but also pushing technology advancement and ensuing valuable spinoffs, insuring long-term humanity survival, etc. Nuclear-based systems for both power and propulsion in space are meritorious measures to overcome the harsh environment of space and thus to efficiently perform deep-space missions. In particular, a Nuclear Thermal Rocket (NTR) is a leading and promising option for near-term human exploration into deep-space such as to Mars and beyond because of its high thrust, improved specific impulse (Isp: propellant efficiency), proven technology, bimodal capability, and resultantly enhanced mission safety and reliability (by reducing a mission trip time), compared with the modern chemical rockets. The NTR technology has been investigated and tested by the United States and the Russia/former U.S.S.R since 1950s. The traditional NTRs typically pursued large-size and high power reactor designs utilizing Highly Enriched Uranium (HEU) fuels, and fast or epithermal neutron energy spectrums to be utilized as the first stage engines with high thrust levels for intercontinental ballistic missiles and space launch vehicles. However, the early large-NTR designs are not suitable for the recent design requirements to perform in-space orbital transfer and upper-stage missions. Rather, as a recent trend, small-NTR designs with lower thrusts, when used in a clustered engine arrangement, can enhance not only mission versatility and flexibility but also mission safety (reducing the risk against engine-out). In addition, the HEU-fueled designs inevitably provoke nuclear proliferation obstacles for research-and-development activities and uses by civilians and non-nuclear weapon states, and for potential commercialization, even though an HEU-fuel is the best in terms of rocket performance. Therefore, the purpose of this research is to conceptually design new moderated space propulsion reactors to enhance rocket performance and nuclear non-proliferation.

To begin with, this dissertation offers a design methodology for advanced space propulsion reactors, which require the characteristics of small-size, low-thrust, improved Isp and/or nuclear non-proliferation capability. The design methodology includes the design requirements, the key methods, the design option assessment, the engineering constraints and the design process as the general to specific guidance for a conceptual design. In particular, the key methods suggest the important design considerations to achieve the reactor characteristics.

Then, based on the design methodology, innovative and advanced NTR engine concepts of the Korea Advanced NUclear Thermal Engine Rockets (KANUTER) are proposed to reduce engine size and thrust level, but to improve Isp and/or to implement a Low Enriched Uranium (LEU) fuel in a compact reactor. In particular, this study focuses on the design and analysis of new moderated space propulsion reactors, which are installed in the KANUTERs. To achieve the goals, the new NTR reactor concepts consider the key methods of advanced fuels with a high uranium density, and heat and corrosion resistances

thermal neutron energy spectrum

and compact and integrated fuel element core designs with protective cooling capability. The new reactors are classified into two versions depending on their characteristics: the ultra-small and high Isp space propulsion reactor utilizing an HEU-fuel, and the non-proliferative and comparable Isp space propulsion reactor utilizing an LEU-fuel.

In order to simulate and to analyze the new space propulsion reactor designs, integrated with an expander cycle engine system, a new computational thermofluid dynamics design analysis model was developed. The purpose-built NTR engine system code provides propellant thermodynamic state in the entire engine system, thermal state of the reactor components (by multi-channel analysis) and ensuing rocket performance. The result of each reactor component state calculated by the in-house system code was validated by comparison with the corresponding 3-D Computational Fluid Dynamics (CFD) analysis result.

To demonstrate the feasibility of the newly proposed reactor concepts, the design analysis regarding neutronics and thermohydraulics was performed by using the general-purpose Monte Carlo codes for neutronics, the purpose-built NTR engine system code for thermohydraulics and their coupling model. The major parameters for the analysis includes material options for fuel and moderator, fuel element pitch to diameter ratio, reactor power and fuel assembly geometry. The design analysis suggests the potential design space in terms of neutronics and thermohydraulics, and provides the feasible design points by the integration process between neutronics and thermohydraulics. In addition, the material candidates for fuel and moderator were assessed to determine baseline options in the particular reactor operating conditions. The assessment results indicate that both the ternary carbide and CERMET fuels can be meritorious baselines according to certain purposes, and the zirconium hydride moderator is preferable to be a baseline because of its better thermo-mechanical properties for the advanced space propulsion reactors. The cooling capability of the new integrated fuel element core design was also proved to protect the fuel and moderator components. Besides the inherent passive safety feature of the LEU version reactor due to temperature effects on reactivity was discovered. Eventually, the analysis results demonstrate the feasibility of the proposed reactor designs in steady-state operating conditions to achieve the leading design requirements, and indicate that the innovative designs have great potentials to enhance rocket performance and/or non-proliferation capability, compared with the existing HEU-NTR designs. At each feasible design point, the rocket performance of KANUTERs are estimated to be a 19.5 kN thrust, a 4.2 thrust to engine weight ratio $(T/W+{eng})$ and a 945 s $I_{sp}$ for the HEU version

and a 51.1 kN thrust, a 4.2 $T/W_{eng}$ and 897 s Isp for the LEU version. The performance levels are better or comparable, compared with that of the state-of-the-art SNRE (with a 71.7 kN thrust, a 3.1 $T/W_{eng}$ and 860 s $I_{sp}$) during the Rover/NERVA Program.

Finally, this dissertation presents important recommendations to complete the conceptual design and to carry forward a R&D program including testing and designing in detail for the KANUTERs in the future.