## **Kilopower and the Kilowatt Reactor Using Stirling TechnologY (KRUSTY)** ![Artist's concept of a Kilopower system on Mars](https://www.scientificamerican.com/sciam/cache/file/FC8FE515-CFEB-49F1-A568AE1723865331_source.png) Kilopower is an ongoing project headed by the NASA Glen Research Center, in partnership with Los Alamos National Laboratory, the Department of Energy, to develop and demonstrate a suite of compact, modular, scalable nuclear fission reactors for spacecraft propulsion and to provide surface power for human missions to Mars or the Moon. The rapidly maturing work simplifies reactor controls and imbeds safety features to ensure reliable, long-lived, low-maintenance operation. The latest planetary surface reactor architecture, based on well-established physics, within the Kilopower project is referred to by NASA as KRUSTY (Kilowatt Reactor Using Stirling TechnologY). The Kilopower project cleared the feasibility stage in 2012, and work since then has been on moving towards a full-scale demonstration. The current (as of July, 2017) focus is on ground testing a fully functional highly enriched uranium Sterling core 1 kW system, with the ultimate goal of developing a systems capable of delivering 1 - 10 kW of electric power with a specific power of 2.5 - 6.5 W/kg [4]. **BACKGROUND** A 2006 NASA Glenn Research Center study compared three designs of nuclear reactor/power conversion units suitable for lunar (and Martian) surface power generation. Each unit is of the 50 kWe-class, and scales from 25~200 kWe. The 3 design concepts studied were gas-cooled Brayton, liquid- metal Stirling, and liquid-metal thermoelectric. [1] Ultimately, the GRC study resulted in NASA selecting two Stirling designs [2] for the KRUSTY project. Under NASA’s Space Technology Mission Directorate, KRUSTY researchers aim to demonstrate a full-scale 1kWe fission reactor unit by the end of 2017, bringing the technology to TRL5 [3][4]. ![Kilopower and TDU](https://i.ytimg.com/vi/PcmZ554_-zE/hqdefault.jpg) >"The ultimate goal of the KRUSTY experiment is to show that a nuclear system can be designed, built, nuclear tested, and produce electricity via a power conversion system in a cost effective manner.[3]” Kilopower research began with studies that compared small reactors across a range of power levels with the aim of meeting both science and exploration needs. These studies found that the 40 kW _Affordable Fission Surface Power system_, which was studied in the 1970s, should be reduced to a 1-10 kW power level system[1, 4-6]. Work since 2006 has resulted in the design, construction, and testing of a 10 kW demonstration unit[4-12]. ![evolving testing](https://k61.kn3.net/taringa/B/D/5/D/8/A/Info_Set/691.png) >“The reduction in power level from 40 kW class to the 1-10 kW power range allowed for several design simplifications including the use of monolithic cores and eliminating the need for pumped liquid metal loops. Another benefit of the lower power systems is that existing government facilities can be used to perform full system nuclear ground tests, which would have required building new facilities at great cost for higher power systems.[4]” Kilopower flight (propulsion) hardware underwent preliminary nuclear ground testing (_Demonstration Using Flattop Fissions_) in 2012, proving that the design can produce 1 kWe at a “reasonable cost” [4, 13-14]. Subsequently, KRUSTY underwent higher fidelity DUFF testing: >“The KRUSTY test is based on the 1 kWe version Kilopower design which consists of a 4 kWt highly enriched Uranium-Molybdenum reactor operating at 800 °C coupled to sodium heat pipes. The heat pipes deliver heat to the hot ends of eight 125 W Stirling convertors producing a net electrical output of 1 kW. Waste heat is rejected using titanium-water heat pipes coupled to carbon composite radiator panels. The KRUSTY test, based on this design, uses a prototypic highly enriched uranium-molybdenum core coupled to prototypic sodium heat pipes. The heat pipes transfer heat to two 60 W Advanced Stirling Convertors and six thermal simulators, which simulate the thermal draw of full scale power conversion units. Thermal simulators and Stirling engines are gas cooled.[4]”. ![2016 NASA overview of Kilopower](https://s5.postimg.org/qfc7ro36f/2017-05-14_22-34-17.png) _Latest configuration of 1 kWe KRUSTY nuclear demonstration_ Some key reactor technology challenges that have been addressed during the Kilopower development 1 kWe configuration are: * conceptual design * materials selection * UMo fuel casting * final machining and geometric tolerances * core structural integrity * phase stability and creep at operating temperature * heat pipe-to-core materials compatibility * diffusion * interface coatings * heat transfer from core to heat pipes and heat pipes to Stirling * verification of predictable reactivity feedback * model validation for core temperatures, power, and reactivity * component fabrication * component testing * inherent safety features * system level testing (with both stainless steel surrogate core, and (in 2017) with a depleted uranium core to simulate highly enriched uranium) [3, 4, 15] **Future Development -- _Megapower_** >"Lessons learned from the kiloPower development program are being leveraged to develop a Mega Watt class of reactors termed MegaPower reactors. These concepts all contain intrinsic safety features similar to those in kiloPower, including reactor self-regulation, low reactor core power density and the use of heat pipes for reactor core heat removal. The use of these higher power reactors is for terrestrial applications, such as power in remote locations, or to power larger human planetary colonies. The MegaPower reactor concept produces approximately two megawatts of electric power. The reactor would be attached to an open air Brayton cycle power conversion system. A Brayton power cycle uses air as the working fluid and as the means of ultimate heat removal." specific power of 50 W/kg. "MegaPower design and development process will rely on advanced manufacturing technology to fabricate the reactor core, reactor fuels and other structural elements. Research has also devised methods for fabricating and characterizing high temperature moderators that could enhance fuel utilization and thus reduce fuel enrichment levels." –_Dasari V. Rao, director of the Office of Civilian Nuclear Programs, Patrick McClure, System Design and Analysis, of Los Alamos National Laboratory_ [16] ![MegaPower](https://www.nextbigfuture.com/wp-content/uploads/2017/07/43da83f1f54f2f77653c06ce4e5e18de-1024x820.png) _A scaled up 2 megawatt system would be expected to weigh about 35 - 40 metric tons (specific power of 50 W/kg)._[17] **Shifting to a Lunar Perspective** It should be noted that depending on the date of publication, NASA literature states Kilopower/KRUSTY as applicable to one of either the surface of Mars or the surface of the Moon, or both the Moon and Mars. However, it is mentioned that the research extends to broader applications as it is intended to facilitate the “long sought return of a real U.S. space nuclear program[2]”. The most recent NASA literature adds lunar power to the list key applications (“This technology . . . could be used to provide surface power for manned missions to the Moon or Mars.[4]”). Taking Mars exploration as a technological analog to establishing a lunar colony, it can be inferred that NASA’s research into KRUSTY would be highly applicable to the establishment of a lunar colony. “NASA and its commercial partners are focused on putting humans on Mars within the following two decades as the next great step in human exploration. The Mars Design Reference Architectures (DRA) have base-lined fission power as the primary power system for surface operations and have recently established the 10kWe Kilopower reactor as the leading technology. [2]”. For Mars exploration, NASA states that both its phase one (precursor to crew arrival) and phase two (human habitat) require an autonomous power system for In-Situ Resource Utilization (ISRU) for life support systems, propellant production, and for the construction of related architecture. (for oxygen to be used in fuel for an ascent vehicle). For a crew of 4-6, NASA estimates 40kWe is needed to support early Mars missions[1][2][16]. The most recent NASA studies have focused on the use of KiloPower for potential Mars human exploration. NASA has examined the need for power on Mars and determined that approximately 40 kilowatts would be initially needed. Five 10-kilowatt KiloPower reactors (four main reactors plus one spare) could solve this power requirement. **Design** _Inherent Safety_ Kilopower uses steady-state reactor technology. Anomalies in a reactor's operation arise when the core deviates from its steady-state value. When the number of neutrons in the core remains unchanged from generation to generation, it is said to be in "steady state". Sudden deviations in the steady state cause the core temperature to rise, and the reactor's thermal limits are rapidly exceeded. Instead of relying on complex control systems to regulate the steady state, as has been traditionally done, Kilopower uses reflectivity feedback to self-regulate. Automatically, the reactor power level decreases as core temperature rises, and power increases as core temperature drops. > "Our objective is to design-in self-regulation as the front-line feature in order to minimize technical and programmatic risk and to demonstrate via testing that self-regulation is both reliable and repeatable." - Dasari V. Rao, Patrick McClure, Los Alamos National Laboratory (Feb., 2017) [16] In the design of Kilopower, engineers and physicists have run detailed simulations and studies "that explicitly examined (a) how choices related to fabrication, alloying and bonding techniques would affect the internal crystalline structure of each nuclear component and in turn (b) how that morphology affects that components thermal, mechanical and nuclear performance at conditions of interest." These studies and simulations have found that even when the Kilopower core sustains a sudden and "rather large power mismatch" resulting from a 25% loss of Stirling engines, the "reactor recovers from this perturbation and regains steady state, assuring us that there is no need for advanced autonomous control system." Hardware testing in a simulated space environment confirmed that the system self-regulates without compromising electric power conversion. ![Full-scale system being readied for engineering demonstration. In this case, core is electrically heated to demonstrate overall system – including Stirling engines – performance. Radial reflectors and Stirling engines are missing from the picture. Heat pipes, core, steel rings used to attach the heat pipes and instrumentation wires are visible. This system was subjected to numerous thermal cycles over prolonged periods of time to examine its thermos-mechanical performance. Source: Los Alamos National Laboratory](https://abm-website-assets.s3.amazonaws.com/rdmag.com/s3fs-public/styles/content_body_image/public/embedded_image/2017/02/nuclear%20react.jpg?itok=RfSIsloY) _The core, heat pipes, wires, and rings (used to attach the heat pipes) being preparing for a full-scale system demonstration. Not shown, are the Sterling engines and radial reflectors. (source: Los Alamos National Laboratory)_ NASA’s Kilopower system is a modular, compact, low cost, scalable fission reactor and electrical power conversion unit. It uses Stirling converters, passive sodium heat pipes, and is fueled by Uranium-235. It is intended to “bridge the gap between Radioisotope Power Systems (RPS) and 40 kWe class fission power technology.[1]” ![basics](https://s5.postimg.org/gd212odnb/2017-05-14_22-40-31.png) Kilopower uses only available fuel (U-235) and available component technologies, and is slated to be tested at existing facilities. ![range of Stirling systems](https://k60.kn3.net/taringa/D/5/B/D/C/B/Info_Set/F74.png) ![close-up](https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcT6ZHdOPPPeR2I1sC-piDYZMZ7Lp6OwX6NWMh5Ilf46tfCSk4F0) _KRUSTY_ **Power Conversion** ![Diagram of power conversion unit](https://abm-website-assets.s3.amazonaws.com/rdmag.com/s3fs-public/styles/content_body_image/public/embedded_image/2017/02/figure%201.jpg?itok=zmVQH1Z7) _Solid metallic U-Mo reactor core, with eight heat pipes and Stirling engines that convert core thermal power into electricity. (Source: Los Alamos National Laboratory)_ **Power output** 0.5~10kWe. **Fuel** Highly enriched U-235 eases the burden on plutonium-238 production.[1-15]. **Cooling** Passive sodium pipes transfer heat waste. **Scalability, Modulation, and Size** The compact modular design of the KRUSTY system serves as an alternative to putting a large single power plant on a planetary surface. The Evolvable Mars Campaign has found that: ![concept designs for varying sizes](https://hsto.org/getpro/geektimes/post_images/e61/5c1/ea9/e615c1ea933f59dd5adce6f6fe1e124e.png). Smaller unit size and mass permits easier packaging in surface landers * Multiple units provide a greater level of redundancy and fault tolerance * Units can be deployed as needed in timeline for flexibility in buildup approach. * Human missions can benefit from first user’s establishment of nuclear infrastructure (material handling, testing, safeguards) and launch approval process [1]. The small size of the Kilopower unit and low power level benefit safety, transport, cost, testing, and demonstration. The small size also reduces complexity. * 800 W Stirling - Approx. 2.5 m long 400 kg or 2 W/kg * 1 kW Thermoelectric - Approx. 4 m long 600 kg or 1.7 W/kg * 3 kW Stirling - Approx. 5 m long 750 kg or 4 W/kg * 10 kW Stirling - Approx. 4 m tall 1800 kg or 5 W/kg **Current Status** The surrogate stainless steel core of the prototype Kilopower reactor was swapped out for a depleted uranium (DU) core in July, 2017, and subsequent testing ensued. The testing process included a KRUSTY “dress rehearsal” of the fuelling and assembly process. The resulting paper concludes that the 1 kW subsystem level testing was “a major milestone for the Kilopower project” and that “there were no issues encountered during DU testing that caused unexpected operational issues which would need to be addressed prior to HEU [highly enriched uranium] testing.” It says that the successful testing has allowed for the fabrication of molds and the development of procedures for the HEU to core sections to already be completed, and that Kilopower technology is now ready for “final testing with the HEU core” and that the DU testing “has reduced the risk of any unexpected issues in fueling, assembly, or test operations”. The conclusion suggested that with increased funding the thermal efficiency could be improved by replacing the ASC heat acceptor plate, which caused “a 200 °C temperature drop between the core and the Stirling convertor hot end” during the DU testing, with customized convertors and interfaces. Even without the suggested change, the Kilopower system “as it stands is capable of delivering 120 W electric from two ASC convertors with a maximum thermal power draw of roughly 3000 W, which is sufficient to verify neutronics models at the nominal Kilopower operating condition.” Immediate future plans for the Kilopower system are to run tests related to improving thermal efficiency, and then later to install the HEU core into KRUSTY and run nuclear ground testing. Based on 2006 qualification specifications [1], the latest (2017) DU testing puts the Kilopower/KRUSTY system at a technology readiness level 5 (TLR-5). To reach TLR-6 (2018~), NASA will conduct irradiation studies, prototype reactor control, prototype radiation shielding, conduct end-to-end system testing, conduct environmental testing, and work on nuclear launch safety. [1]. The summary of 2016 report, Nuclear Systems Kilopower Overview (_NASA Space Technology Mission Directorate Game Changing Development Program_) states: >“We have the team; We have available, leveraged infrastructure and experience; We have mission pull from HEOMD [_Human Exploration, Operations, and Mission Directorate_] and willingness to use the technology from SMD [Science Mission Directorate]; We have an NASA-wide directive on a role to fulfill; We are making progress today with successful technology development accomplishments; We’re seizing the opportunity to demonstrate system-level technology readiness of space fission power.” HEOMD has expressed interest in using Kilopower in a 2024-2026 ISRU Mars surface demonstration[3]. **Lifespan and Maintenance** Lifespan of over 10 years **Safety** The total amount uranium fuel needed for a human Mars mission would be less mass than the mass of a toy marble; a single medical x-ray gives off more radiation than one year of standing next to a nuclear propulsion engine before launch; [3]. **Ownership** Kilopower is being developed in conjunction with US _Department of Energy_ (DOE) and the _National Nuclear Security Administration_ (NNSA). NNSA will “own, keep, and dispose of Kilopower demonstration reactor core”. Aerojet/Rocketdyne has funded and conducted independent research on the reactor core materials research and testing, and expressed interest in continuing and broadening the partnership with the project. The two Kilopower Stirling technology contracts have been awarded to Infinia ($3.7M) and Sunpower ($3.5M)[3]. **SUGGESTED LINKS** *NASA "Game changing development" Video on its Nuclear Systems program: https://youtu.be/PcmZ554_-zE *NASA "Space Fission Power and Propulsion" Overview of history, development, and physics of nuclear power for space: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160004957.pdf MORE TO COME **References** >[1]https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160012354.pdf [2] https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20170002010.pdf [3] https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150021391.pdf [4] _Electrically Heated Testing of the Kilowatt Reactor Using Stirling TechnologY_ (KRUSTY) (NASA Glenn) published July 30, 2017 Retrieved 2017-08-30T06:09:09+00:00Z from https://ntrs.nasa.gov/search.jsp?R=20170007986 / https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20170007986.pdf [5] Mason, Lee, Marc A. Gibson, and David I Poston. "_Kilowatt-Class Fission Power Systems for Science and Human Precursor Missions_." Nuclear and Emerging Technologies for Space. Albuquerque: NASA TM-2011-216541, 2011. [6]Mason, Lee, Carmichael, C.. "_A Small Fission Power System With Stirling Power Conversion for NASA Science Missions_." Nuclear and Emerging Technologies for Space. Albuquerque: NASA TM-2013-216541, 2013. [7] Fission Surface Power Team, “_Fission Surface Power System Initial Concept Definition_,” NASA/TM—2010- 216772, 2010. [8] Mason, L. et.al “_Design and Test Plans for a Non-Nuclear Fission Power System Technology Demonstration Unit_” NASA/TM-2011-217100 [9] Geng, S., Briggs, M., Hervol, D., “_Performance of a Kilowatt-Class Stirling Power Conversion System in a Thermodynamically Coupled Configuration_,” Nuclear and Emerging Technologies for Space, Paper 3269, 2011. [10] Briggs, M., Geng, S., Pearson, J., Godfroy, T., “_Summary of Test Results from a 1 kWe-Class Free-Piston Stirling Power Convertor Integrated with a Pumped NaK Loop_,” International Energy Conversion Engineering Conference, Paper 7173, 2010. [11] Briggs, M., “_Dynamic Behavior of Kilowatt Class Stirling Convertors with Coupled Expansion Spaces_,” Nuclear and Emerging Technologies for Space, Paper 3032, 2012. [12] Briggs, M., et.al. “_Cold-End Subsystem Testing for the Fission Power System Technology Demonstration Unit_” NASA/TM-2013-216545 [13] Gibson, Marc A., Max Briggs et al. "_Heat Powered Stirling Conversion for the Demonstration Using Flattop Fission (DUFF) Test_." Nuclear and Emerging Technologies for Space. Albuquerque: NETS paper 6812, 2013. [14] Poston, David I., and Patrick R. McClure. "_The DUFF Experiment-What was Learned_." Nuclear and Emerging Technologies for Space. Albuquerque: NETS paper 6967, 2013. (Retrieved Aug. 29, 2017). [15] Gibson, Marc A., Poston, David, McClure, Pat "_NASA’s Kilopower Reactor Development and the Path to Higher Power Missions_" IEEE Aerospace Conference. Big Sky, MT, 2017. (Retrieved Aug. 29, 2017). [16] Dasari V. Rao, Patrick McClure, Los Alamos National Laboratory "_Nuclear Reactors to Power Space Exploration_" https://www.rdmag.com/article/2017/02/nuclear-reactors-power-space-exploration Feb. 14, 2017. (Retrieved Aug. 31, 2017) [17] Brian Wang, "_Los Alamos self regulating reactors from tens of kilowatts for NASA to several megawatts_" https://www.nextbigfuture.com/2017/07/los-alamos-self-regulating-reactors-from-tens-of-kilowatts-for-nasa-to-several-megawatts.html July 12, 2017 (Retrieved Aug. 31, 2017) other http://www.mdcampbell.com/Mason2006SmallNPS.pdf