The notion that NASA is currently developing a nuclear power system might not seem that Earth shattering. The agency has flown a number of missions powered by radioisotope thermoelectric generators (RTGs) since 1965, such as on board the two Viking Mars landers, the Curiosity rover, the Apollo expeditions to the Moon, the two Voyager spacecraft, the New Horizons probe to Pluto, and the just-concluded Cassini mission to Saturn. But this time, with the Kilopower Project, NASA aims to shake things up on the surface of other planets.
RTGs produce thermoelectricity via the Seebeck effect—using the natural decay of a radioisotope heat source (typically Plutonium-238) to heat wires made from different types of metal, each shifting electron energy at different times, creating a current. This time, NASA is aiming for a fission nuclear power system that would enable long-duration stays on planetary surfaces.
Beyond solar technology, there is currently no off-the-shelf solution for powering long-term human missions to Mars. The Kilopower project is a near-term technology effort to develop preliminary concepts and technologies for an affordable fission power system that could provide safe, efficient, and plentiful energy for future robotic and human space exploration missions to the Moon, Mars, and other destinations.
While NASA uses solar power extensively to power spacecraft, satellites, and rovers, fission reactors can provide energy even in dark environments where solar cells cannot collect enough light. On Mars, the sun’s power varies widely throughout the seasons, and periodic dust storms can last for months. On the Moon, lunar nights can last for 14 days.
As planned, the Kilowatt reactor technology is intended to provide up to 10 kW of electrical power continuously for at least 10 years—approximately 10 times as much power than the multi-mission RTG used on Curiosity. Four Kilopower units producing a continuous 40 kW would provide enough power to establish an outpost. However, the technology is scalable down to 1 kW of power—which could provide modular energy sources for easier transportation and deployment during human exploration missions.
“We want a power source that can handle extreme environments,” said Lee Mason, NASA’s principal technologist for power and energy storage. “Kilopower opens up the full surface of Mars, including the northern latitudes where water may reside. On the Moon, Kilopower could be deployed to help search for resources in permanently shadowed craters.”
The reactor design includes a novel integration of readily available Uranium-235 (U-235) fuel that could enable further Planetary Science Decadal Surveys without relying on limited Plutonium-238 dioxide fuel. As designed, the reactor, dubbed “KRUSTY” (Kilopower Reactor Using Stirling TechnologY), uses a single rod of boron carbide to initiate the reaction. Passive sodium heat pipes, provided by Pennsylvania-based Advanced Cooling Technologies, transfer reactor heat to high-efficiency Stirling converters, supplied by Ohio-based Sunpower, Inc. An umbrella-like titanium radiator is used to cool the converters.
A beryllium oxide reflector surrounds the 6-in diameter uranium core and creates enough neutron reflection for the reactor to heat and go critical—or where a nuclear chain reaction is self-sustaining, with no increase or decrease in power or temperature. According to NASA, the reactor uses well-established nuclear physics to self-regulate the fission reactions and this feature eliminates the need for a complicated control system.
The Stirling converters use heat to create pressure forces that move a piston, which is coupled to an alternator to produce electricity. The components, even in the test setups, were either designed to be flight-like or flight-ready to ground test at near-flight conditions (i.e., vacuum environment, full thermal power and operating temperature, realistic configuration, and interfaces).
The Kilopower project is part of the NASA Space Technology Mission Directorate (STMD) Game Changing Development (GCD) program, which is managed by the NASA Langley Research Center. While NASA has attempted many space reactor programs since the 1970s, previous programs were limited by funding, schedule, and then-current technologies. The three-year, $20 million Kilopower project, which began in 2015, leverages significant technological breakthroughs from NASA research of the last decade, namely a 2012 proof-of-concept test involving Flattop, a monolithic-core fission reactor that incorporated a heat pipe and Stirling converter at the Nevada National Security Site’s (NNSS’s) National Criticality Experiments Research Center (NCERC).
“The 2012 experiment used an existing nuclear criticality device called Flattop to produce 24 W of electricity. It also confirmed the basics of the nuclear reactor physics and the heat transfer principles necessary to operate this kind of reactor in deep space. KRUSTY will expand on the 2012 experiment by testing a flight-like reactor core at full operating temperature,” said Mark Martinez, President of Mission Support and Test Services, LLC, which manages the NNSS.
The Kilopower project has been managed by the NASA Glenn Research Center (GRC) with partnership from the NASA Marshall Space Flight Center, the Department of Energy (DOE) National Nuclear Security Administration (NNSA), and several DOE laboratories, including Los Alamos National Laboratory, Y-12 National Security Complex, and NNSS.
After conducting numerous system tests with a depleted uranium core, GRC shipped the prototype power system from Cleveland to NNSS in late September 2017. The team at the NNSS National Critical Experiments Research Center began tests on the reactor core in November 2017 and connected the power system to the solid, cast, highly-enriched U-235 core (provided by Y-12) and initiated end-to-end checkouts this past January. According to project officials, the experiments should conclude with a 28-hour, full-power steady-state test (800 °C) in late March.
“The upcoming Nevada testing will answer a lot of technical questions to prove out the feasibility of this technology, with the goal of moving it to a technology readiness level of 5 (TRL 5). It’s a breadboard test in a vacuum environment, operating the equipment at the relevant conditions,” according to lead researcher Marc Gibson.
The next step would be qualifying the Kilopower system in a relevant environment, such as space.
The entire operation is being conducted as if the test was a flight test, with thorough analytical modeling, integrated nuclear test operations, ground safety measures, and interagency support. The only key items missing from the test are the radiator, full suite of Stirling converters, startup-rod, zero gravity environment, launch approval, flight hardware, flight qualification, and spacecraft integration.
NASA has gone through lengths to ensure that all present and future tests comply with the National Environmental Policy Act (NEPA) processes. Beyond needing to complete a final NEPA review before a flight test, several operational safety concerns must be addressed. At launch, the radiological hazard must be limited to less than 5 Ci (the amount of naturally occurring radioactivity of the U-235 core). The reactor, which would not be in operation until reaching the surface of a planet or being placed on a trajectory leaving Earth, will require sufficient radiation shielding to protect crew and sensitive flight equipment. Furthermore, the reactor design includes inherent fault tolerance, so any loss of cooling or failure of one of the heat pipes of Stirling converters would trigger an automatic reduction in fission power, preventing uncontrolled reactions.
Throughout the project, researchers have referred to the technology as an agnostic power system; and have outlined many potential planetary applications such as nuclear electric propulsion for orbiters and landers on Europa, Titan, Enceladus, Neptune, and Pluto; commercial space applications such as space power utility (pay-for-service), resource mining, and settlement; and terrestrial adaptations for powering military forward operating bases, unmanned vehicles, material processing, manufacturing, and electric propulsion.
“The reactor technology we are testing could be applicable to multiple NASA missions, and we ultimately hope that this is the first step for fission reactors to create a new paradigm of truly ambitious and inspiring space exploration,” said David Poston, chief reactor designer at Los Alamos. “Simplicity is essential to any first-of-a-kind engineering project—not necessarily the simplest design, but finding the simplest path through design, development, fabrication, safety and testing.”