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5 Mission Applications If a nuclear electric propulsion (NEP) or nuclear thermal propulsion (NTP) system is successfully developed for a crewed Mars mission, it will also be able to support the accomplishment of additional space missions. Separately, the Department of Energy (DOE) and the Department of Defense (DoD) are developing small nuclear fission systems for terrestrial applications. These programs are expected to precede the baseline Mars mission and, if planned synergistically, may provide space nuclear propulsion technology advancement. Potential synergies across these missions and development programs are summarized in this chapter. SCIENCE MISSIONS The use of NEP has been considered repeatedly over the decades for robotic exploration missions to Mars, Saturn, Neptune, and Pluto and for a range of sample return missions. NEP systems can potentially provide extraordinary power capability to science instruments in addition to propulsion. Power levels considered have generally been 100 kWe or less.1 The Jupiter Icy Moons Orbiter (JIMO) mission would have visited three Jovian moons with an NEP system designed to produce 200 kWe. Most recently, an NEP system at power levels of 1 to 8 kWe has been examined for outer planet missions.2 NTP systems and megawatt electric (MWe)-class NEP systems have seldom been considered for these science missions, primarily due to the large total cost and mass of the system, the inability to launch these systems on a single launch vehicle, the lack of significant transfer time constraints, and the desire to avoid in-space assembly of science missions. Mission concepts for destinations from 100 to 1,000 astronautical units from Earth have focused on NEP systems or even more advanced propulsion concepts. An NEP system developed for the baseline Mars mission would provide a starting point for developing an NEP system for an interstellar mission. The latter would need to provide a higher specific impulse (Isp) at a lower specific mass (in kilograms per kilowatt-electric [kg/kWe]) than is needed to execute the 1 âPriorities in Space Science Enabled by Nuclear Power and Propulsion,â Committee on Priorities for Space Science Enabled by Nuclear Power and Propulsion, National Academies Press, (2006). 2 Gibson, M. A. et al., âNASAâs Kilopower Reactor Development and the Path to Higher Power Missionsâ NASA/TMâ2017-219467. PREPUBLICATION COPYâSUBJECT TO FURTHER EDITORIAL CORRECTION 74
MISSION APPLICATIONS 75 baseline mission to Mars.3 The lower Isp of NTP systems makes them less suited for missions beyond the solar system.4 POTENTIAL FOR HIGHER PERFORMANCE SPACE NUCLEAR PROPULSION SYSTEMS Beyond 2040, both NTP and NEP offer the potential for higher performance, beyond that required for the baseline mission. For NTP, increasing Isp from 900 s to 1,000 s, for example, would require a propellant temperature of approximately 3100 K at the reactor exit. Fundamentally, this challenge derives from the thermal propellant acceleration process, because Isp scales as the square root of the reactor temperature. Increasing the operating temperature by 400Â°C would significantly increase development risk for materials and fuel forms, ground testing, and spaceflight. In contrast, NEP offers several different approaches to future higher-performance systems. First, a scaled-up power system using existing technology would produce more power without increasing reactor temperature. Second, advanced power conversion subsystems could potentially be developed with a lower specific mass, which would reduce the specific mass of the NEP system as a whole. Third, use of a higher-Isp electric propulsion (EP) system with the same power and heat rejection system could enable high total velocity increment (ïV) missions, albeit with lower acceleration levels (unless power is increased). Finally, developing a reactor capable of operating at 1500 K without a significant increase in support system mass would also reduce the specific mass of the system. Ongoing research and technology development for both NTP and NEP is necessary to allow them to achieve their potential, even if they are not selected as the propulsion system for the first human Mars exploration mission. SURFACE POWER USE OF NEP REACTORS Nuclear fission power has been identified as a technology priority for sustained human presence on both the Moon and Mars.5 The development of the reactor and power conversion subsystems of an NEP system may contribute to the development of surface power systems and vice versa, especially if the megawatt electric capacity of the NEP system greatly exceeds the power requirements for the surface power system. Even so, key differences in the operational environment, such as gravitational effects and the presence of a potentially corrosive atmosphere or dust layer (on Mars) impose different design requirements on the reactor, core cooling, and thermal management system. There would also be different design requirements for the radiation shield. NEP systems use shadow shielding to reduce radiation only in the conical region where equipment and personnel on the spacecraft are located. The shield for a planetary-based reactor would need to reduce radiation in all directions, although it could be buried to allow regolith to provide some of the required shielding. Thermal management systems for surface applications 3 K. T. Nock, âTAU-A Mission to a Thousand Astronomical Units,â AIAA-87-1049, 19th AIAA/DGLR/JSASS Intâl Electric Propulsion Conf, Colorado Springs, May 11-13, 1987. 4 James R. Powell, J. Paniagua, G. Maise, H. Ludewig, Michael Todosow, âHigh performance nuclear thermal propulsion system for near term exploration missions to 100 A.U. and beyond,â Acta Astronautica, Volume 44, Issues 2â4, JanuaryâFebruary 1999, Pages 159-166. 5 Fission surface power systems have not been identified as a priority for NASA science missions. PREPUBLICATION COPYâSUBJECT TO FURTHER EDITORIAL CORRECTION
76 SPACE NUCLEAR PROPULSION FOR HUMAN MARS EXPLORATION would need to account for the effects of gravity on coolant flow and the presence of planetary surface rather than space for one half of the view factor for radiation heat rejection. The Kilopower systemâs output power of 7 to 10 kWe is estimated to be suitable for life support and, with multiple units, in situ resource utilization (ISRU) for initial lunar bases. Some studies of augmented ISRU production estimate power level requirements of 40 to 125 kWe. Potential long-term growth of lunar basing could drive power requirements to the 100s of kWe, at which point a derated NEP reactor and/or power system could prove to be advantageous. Mars ISRU power requirements were also assessed in planning the Kilopower program. An early power level for ISRU is estimated to be 40 kWe, which could be provided by four 10 kWe Kilopower units. A larger base could require power levels on the order of 150 kWe, similar to longer-term lunar requirements. SYNERGIES WITH NATIONAL SECURITY MISSIONS Space nuclear propulsion and power systems have the potential to provide the United States with military advantages. DoD and other federal agencies with an interest in national security have historically been interested in nuclear power and propulsion for space. The utility provided by either NTP or NEP is mission dependent. An NTP system could provide DoD with a rapid response capability in cislunar space to address counter-space and anti-satellite threats on critical timescales. The primary differentiator between these two systems is whether the vehicle needs to move rapidly (which would require an NTP system) or if it can remain quasi-stationary or accelerate slowly (which is compatible with an NEP system). Additionally, an NEP system could potentially provide megawatts of power to a spacecraft dedicated to power beaming, long- distance communications, and long-distance sensing. The Defense Advanced Research Projects Agency (DARPA) presently has an NTP program named Demonstration Rocket for Agile Cislunar Operations (DRACO).6 NASA could benefit from lessons learned by the DRACO flight demonstration (currently planned for late 2025) and could work collaboratively with DARPA to develop technologies and subsystems that contribute to the mission needs of both agencies. Threats to U.S. space assets are increasing. They include anti-satellite weapons and counter- space activities.7,8 Crossing vast distances of space rapidly with a reasonably sized vehicle in response to these threats requires a propulsion system with high Isp and thrust. This could be especially important in a high-tempo military conflict. For high ïV missions, an NTP system that fits within the mass and volume limits of a single launch vehicle would be ideal, whereas an in-space chemical system might be prohibitively large. This is the driving rationale behind the selection of an NTP system for the DRACO program.9 Some of the technologies and methods that are applicable to the development and construction of an NTP system for DRACO could contribute to NASAâs development of an NTP system for the baseline mission to Mars, despite the difference in scale between the two systems. 6 DRACO Program Page, DARPA website, https://www.darpa.mil/program/demonstration-rocket-for-agile- cislunar-operations. 7 U.S.-China Economic and Security Review Commission, 2019 Report to Congress, November 2019. 8 U.S.-China Economic and Security Review Commission, Hearing on China in Space: A Strategic Competition? written testimony of Namrata Goswami, April 25, 2019, 82. 9 Broad Agency Announcement, DARPA, Demonstration Rocket for Agile Cislunar Operations, HR001120S0031, June 29, 2020. PREPUBLICATION COPYâSUBJECT TO FURTHER EDITORIAL CORRECTION
MISSION APPLICATIONS 77 Areas of common interest include (1) fundamental questions about the development and testing of materials (such as reactor fuels and moderators) that can survive NTP conditions and (2) advancing modeling and simulation (M&S) capabilities that are relevant to NTP, such as in the area of dynamic, time-dependent reactor predictions. Furthermore, a NASA NTP system could potentially use a scaled-up version of a DoD reactor, depending on the design. Additionally, NASA could benefit programmatically by working with a DoD program having national security objectives, which establishes a level of prioritization for use of national assets. SYNERGIES WITH TERRESTRIAL NUCLEAR SYSTEMS Dozens of companies are currently pursuing advanced reactor designs for various applications. Several of these efforts focus on development of terrestrial microreactors, which are on the scale of hundreds of kilowatts to a few megawatts of electric power for both commercial and military applications. As such, they are on the same scale as the NEP systems under consideration for the baseline mission. Funded by DOE, DoD, and private industry, developers of terrestrial microreactors are focused on similar concepts of interest to NEP systems, such as factory assembly and fueling, easy transportability, autonomous or semi-autonomous operation, and long-life operation (e.g., on the order of 5 to 10 years) without refueling and minimal maintenance. Although terrestrial systems seek to be transportable by standard means (truck, rail, barge, and aircraft), they likely have less stringent mass and volume constraints relative to systems intended for space. Additionally, these systems would be accessible for maintenance in the event of a sensor malfunction or equipment degradation, despite the overall desire to operate without intervention throughout the planned fuel cycle length. Demonstration of the first terrestrial microreactors is expected in the mid-2020s, offering operational data for fuels and materials that can support code validation that is also applicable to NEP designs, and with some but reduced applicability to NTP designs. These demonstrations will be supported by the Advanced Reactor Demonstration Program of the DOE Office of Nuclear Energy (DOE-NE), the DOE National Reactor Innovation Center, and the DoD Pele Program.10 The acquired operational data can support evaluation of system integrity and reliability, reducing risk to mission success for areas common to terrestrial and space propulsion systems, and providing confidence in the ability to obtain launch approval. There is significant private investment in development of some of these systems, either via private-public partnerships or fully private investment. Microreactor concepts include heat pipe, gas, and liquid metal cooled designs, as have been evaluated for NEP across various historical programs; gas-cooled designs may also provide some similarity to NTP designs, albeit limited due to the significant differences in operational approaches. There are also many similarities in the nuclear fuel forms under consideration for terrestrial and space systems, including high-assay, low-enriched uranium (HALEU; e.g., uranium dioxide and uranium nitride) and TRISO. Hence, the NASA program may be able to leverage the fuel and component fabrication and testing facilities and resultant property 10 More information can be found for these programs at the following sites: ARDP, https://www.energy.gov/ne/nuclear-reactor-technologies/advanced-reactor-demonstration-program; NRIC, https://inl.gov/nric/; DoD Pele, https://www.defense.gov/Newsroom/Releases/Release/Article/2105863/dod-awards- contracts-for-development-of-a-mobile-microreactor/. PREPUBLICATION COPYâSUBJECT TO FURTHER EDITORIAL CORRECTION
78 SPACE NUCLEAR PROPULSION FOR HUMAN MARS EXPLORATION measurements, performance characterization, and test data to accelerate the development roadmap for space missions. Moderator materials are considered for many advanced reactor designs to allow use of HALEU fuels. Development programs for such moderators, including yttrium hydride, are in process for the DOE-NE Microreactor program. These programs could support the needs of either NEP or NTP designs that include a moderator block, but additional testing points at higher temperatures may need to be included to ensure that the data covers the operational envelope for NEP or NTP applications. Approaches for manufacturing and assembly may also be similar across terrestrial and space applications for some of these concepts. The recently established DOE-NE Transformational Challenge Reactor and Advanced Methods for Manufacturing programs seek to advance the state of the art for nuclear component fabrication. These programs will expand and demonstrate the methods by which nuclear equipment, components, and plants are manufactured and assembled. Similar approaches may be of interest to space nuclear systems as a means to reduce cost, increase reliability, and establish a secure supply chain. The fabrication experience, mechanical testing data, and material characterization data (pre- and post-irradiation) will support the case for use of advanced manufacturing in nuclear systems, providing a jump start on the regulatory and launch approval paths for crewed nuclear missions. Operational temperatures are expected to be lower and operating lifetimes longer for terrestrial systems relative to NEP, such that test data on these components will likely need to be extended. Some microreactor and NEP designs rely on advanced Brayton power conversion systems, including supercritical carbon dioxide and helium working fluid designs, for electricity generation, allowing for lessons learned from terrestrial systems development to inform NEP systems for both cargo and crewed missions. FINDING. Synergies with Terrestrial and National Defense Nuclear Systems. Terrestrial microreactors, which operate at a power level comparable to NEP reactors, are on a faster development and demonstration timeline than current plans for space nuclear propulsion systems. Development of microreactors may provide technology advances and lessons learned relevant to the development of NEP systems. Similarly, technology advances within the DARPA DRACO program could potentially contribute to the development of NTP systems for the baseline mission. RECOMMENDATION. Synergies with Terrestrial and National Defense Nuclear Systems. NASA should seek opportunities for collaboration with the Department of Energy and Department of Defense terrestrial microreactor programs and the Defense Advanced Research Projects Agency DRACO (Demonstration Rocket for Agile Cislunar Operations) program to identify synergies with NASA space nuclear propulsion programs. PREPUBLICATION COPYâSUBJECT TO FURTHER EDITORIAL CORRECTION