The US Defense Advanced Research Projects Agency (DARPA) has awarded start-up Space Nuclear Power Corporation (SpaceNukes) an Other Transaction Agreement (OTA) under the Fast Capacitance (Fat Cat) programme.
The Fat Cat program, managed by Dr Tabitha Dodson within DARPA’s Defense Sciences Office, focuses on advancing autonomous reactor operations for national security missions. The primary goal is to prove that a nuclear fission reactor can safely regulate itself without continuous human oversight or ground-control intervention. This is a vital requirement for remote bases and deep-space platforms plagued by light-time communication delays.
SpaceNukes previously delivered the foundational 2018 KRUSTY (Kilopower Reactor Using Stirling Technology) demonstration alongside Los Alamos National Laboratory (LANL), which was passively controlled. The new Fat Cat agreement takes this a step further by introducing active control rods to handle extreme operational transients and dynamic power management.
The programme is divided into strict design, procurement, and integration gates. Component integration will take place at SpaceNukes’ workshop in Medina, Ohio, led by Chief Engineer Marc Gibson. The project will culminate in a full, physical nuclear ground test of the newly designed hardware and autonomous software logic.
SpaceNukes leverages the exact engineering and management team that executed the 2018 KRUSTY project. This includes CEO Andy Phelps, CTO David Poston, COO Patrick McClure, and Chief Engineer Marc Gibson. SpaceNukes operates out of its headquarters in Los Alamos, New Mexico, using licensed Kilopower technology originally developed by LANL.
The KRUSTY ground test was the first new US space reactor design to be tested in over 40 years. The test, conducted at the Nevada National Security Site (NNSS) between late 2017 and March 2018, concluded with a 28-hour continuous full-power run. The prototype achieved a thermal output of 4-5 kWt at a core operating temperature of 800-880 degrees C.
The primary breakthrough was demonstrating the reactor’s inherent physics-based stability. When researchers simulated cooling losses or Stirling engine failures, the reactor automatically dialled its fission rate up or down. The core temperature varied by less than 4% even when slapped with extreme 175% thermal load variations. The system converted nuclear heat to electricity using two Sunpower Stirling engines, generating roughly 183 We) at an overall thermal-to-electric efficiency of about 25–34%.
The defining feature of a NASA Kilopower reactor is its absolute simplicity. By eliminating complex pumped liquid coolant loops, which are prone to leaking, freezing, or mechanical failure, the entire assembly relies on passive physics.
At the heart is a solid, cast uranium-235 alloyed with molybdenum core (about the size of a paper towel roll). A single boron carbide control rod is inserted to jumpstart the fission chain reaction and then pulled back. Surrounding the solid core are multiple sodium heat pipes. The sodium, sealed inside under a vacuum, vaporises when exposed to the core’s intense heat, rushes to the cool end of the pipe, releases its thermal energy, condenses back into a liquid, and returns back to the core to repeat the cycle.
At the cold end of the heat pipes sit high-efficiency Stirling engines. The transferred heat pressurises a gas (typically helium) inside the engine, driving a reciprocating piston. This kinetic movement is directly coupled to an alternator, generating constant electricity.
In deep space, distance creates an operational vacuum. Radio signals traveling at the speed of light take 3-22 minutes to travel between Earth and Mars (one way), and up to several hours if a spacecraft is navigating near the outer planets like Saturn or Neptune.
If a spacecraft’s power plant suffers a critical anomaly (like a sudden drop in grid voltage or an asymmetrical thermal spike), relying on a ground-control station on Earth to diagnose the issue and beam back a fix would take hours. By the time the instructions arrived, the reactor could suffer permanent thermal damage, or the probe could lose power and die.
Autonomous controllers solve this by cutting humans out of the tactical loop. An onboard autonomous nuclear controller uses integrated software to monitor internal temperatures, neutron flux levels, and mechanical stress at millisecond intervals. Instead of waiting for a command from Earth, the autonomous controller computes complex transient physics locally. It commands active control rods or dynamically trims the Stirling engine loads immediately to stabilise the core during unforeseen anomalies.
If an engine component fails, the controller re-routes power demands instantly, ensuring the reactor keeps the space probe alive while transmitting a diagnostic update back to Earth asynchronously.
“This award recognises the maturity and continuity of the team that designed, built, and tested KRUSTY in 2018,” said SpaceNukes COO Patrick McClure. “We are proud to work alongside … Los Alamos National Laboratory to deliver the next demonstrated step in American nuclear power.”
Andy Phelps, CEO of SpaceNukes, noted: “There is no substitute for demonstrated technology when American national security objectives are at stake,” said. “This award lets us advance the work begun in the KRUSTY programme and bring another tested capability forward for the nation.”
