The smallest nuclear reactor core ever tested was about the size of a paper towel roll. NASA’s KRUSTY prototype, built for space missions, had a uranium core just 25 centimeters tall (about 10 inches) and 11 centimeters wide (roughly 4.3 inches), weighing 32 kilograms. That tiny cylinder sustained a controlled fission chain reaction and generated usable electricity. But how small a reactor can get depends entirely on what you mean by “reactor” and what you need it to do.
The Physics That Set the Lower Limit
Every fission reactor needs enough fuel to sustain a chain reaction, a point called critical mass. For a bare sphere of uranium-235 at normal density, that minimum is about 47 kilograms (104 pounds). Plutonium-239 reaches criticality at roughly 10 kilograms (22 pounds). These are the absolute baselines with nothing helping the reaction along, and they produce a sphere you could hold in your hands.
Reflectors change the math dramatically. Surrounding the fuel with a material that bounces escaping neutrons back into the core lets you shrink the required fuel mass. Beryllium oxide is one of the most effective reflectors: it can reduce the minimum critical mass of water-moderated uranium-235 from about 1,420 grams all the way down to around 220 grams. That’s less than half a pound of fissile material. The core itself would be tiny, though you’d still need shielding, cooling, and power conversion equipment around it.
The Smallest Reactor That Actually Worked
NASA’s Kilopower project tested the KRUSTY reactor (Kilowatt Reactor Using Stirling Technology) in 2018 at the Nevada National Security Site. The core was a cylinder of highly enriched uranium alloyed with molybdenum, with a hollow center about 4 centimeters wide running through it. The entire core weighed 32.2 kilograms, and it was designed to produce 1 to 10 kilowatts of electricity, enough to power a few homes or a future Moon base habitat.
What made KRUSTY so compact was its simplicity. Instead of water pumps and complex piping, it used heat pipes to passively move thermal energy from the core to Stirling engines that converted heat into electricity. No moving coolant loops, no pressurized water systems. The reactor had very few moving parts and could operate autonomously for years.
How Heat Pipes Shrink a Reactor
Traditional nuclear power plants are enormous partly because of their cooling infrastructure. Pressurized water loops, steam generators, massive pumps, and backup cooling systems all add bulk. Heat pipe cooled reactors eliminate the liquid coolant loop entirely. Each heat pipe is a sealed tube containing a small amount of fluid that evaporates at the hot end, travels to the cool end, condenses, and wicks back. No pumps needed.
This passive cooling approach allows for high power density in a compact structure. It also makes modular assembly practical, since the reactor, power conversion unit, and heat rejection system can be integrated into a single transportable package. Nearly every microreactor design in development today relies on some version of heat pipe cooling.
Microreactors: Small Enough for a Truck
The U.S. Department of Energy defines microreactors as units producing between 1 and 20 megawatts. The key feature isn’t just low power output, it’s that they can fit on a standard flatbed truck and be transported to remote locations. Several designs are working toward commercial deployment.
Westinghouse’s eVinci microreactor is one of the furthest along. It’s designed as a transportable nuclear battery: fuel is loaded at the factory, the sealed unit ships to the site, it runs for years, and then the entire reactor is transported offsite for storage and decommissioning. No spent fuel is ever handled at the deployment location. The design uses heat pipes and operates at low pressure, keeping the overall footprint small enough to serve remote mines, military bases, or communities far from the electrical grid.
Other designs in the pipeline include Oklo’s Aurora and Ultra Safe Nuclear’s Micro Modular Reactor, which range from less than 1 megawatt up to 25 megawatts of thermal output. For comparison, a single large conventional reactor produces around 1,000 megawatts of electrical power, so these microreactors are roughly 100 to 1,000 times smaller in output.
Submarine Reactors: Compact but Powerful
Some of the most proven compact reactors are the ones you never see. U.S. Navy submarines have run on nuclear power since the 1950s, and the reactors have gotten progressively smaller and more energy-dense over the decades. The S9G reactor powering Virginia-class attack submarines fits inside a hull just 10 meters (33 feet) in diameter. Exact dimensions of the reactor compartment are classified, but the entire propulsion system, including shielding, produces enough energy to drive a 7,800-ton submarine at speeds over 25 knots for the life of the vessel without refueling.
Naval reactors use highly enriched uranium fuel, which allows for a much smaller core than commercial reactors running on low-enrichment fuel. The tradeoff is that this fuel requires the kind of security infrastructure only a military can provide.
Nuclear Batteries: Tiny but Not Reactors
If your search was inspired by headlines about coin-sized “nuclear batteries,” those are a different technology entirely. Betavoltaic batteries, like those developed by the Chinese company Betavolt, use thin films of radioactive material to generate tiny amounts of electricity from beta particle decay. They’re the size of a piece of candy and could theoretically last 50 years. But they don’t sustain a fission chain reaction. They’re converting the natural radioactive decay of an isotope into current, more like a very long-lasting chemical battery than a reactor.
NASA’s Mars rovers use a similar concept on a larger scale: radioisotope power systems that convert heat from plutonium decay into electricity through thermoelectric generators. These are nuclear power sources, but they aren’t reactors. The distinction matters because a true fission reactor can be controlled, throttled, and scaled in ways that radioactive decay devices cannot.
What Determines Practical Size
The fuel core is never the size bottleneck. As KRUSTY demonstrated, you can build a functioning reactor core smaller than a wastebasket. What adds bulk is everything around it: radiation shielding (often the heaviest component), heat exchangers, power conversion equipment, and control systems. A core producing 5 megawatts of heat might weigh a few hundred kilograms, but the complete system with shielding and power conversion could weigh tens of tons.
The practical lower bound also depends on purpose. A reactor heating a space habitat needs less infrastructure than one feeding electricity into a grid. A reactor on a submarine needs robust shock resistance. A reactor in a remote Arctic village needs years of unattended operation. Each application drives different size constraints, but the engineering trend is clearly toward smaller, simpler, and more transportable units that blur the line between a power plant and an appliance.