microreactor
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Apollo Atomics
Apollo Atomics is a Cambridge, Massachusetts nuclear startup building the most power-dense pressurized water reactor. By redesigning the steam generator - the bulkiest part of a conventional plant - into a proprietary Compact Steam Generator, the company shrinks a proven reactor design by roughly an order of magnitude, turning a decade-long construction project into a factory-built machine that fits on a truck and deploys in under 24 months. Founded by MIT nuclear PhD Assil Halimi and operator Drew Walker, Apollo is in Y Combinator's Spring 2026 batch and is testing its design in collaboration with MIT.
Antares
Antares is a nuclear fission energy company building factory-produced, transportable microreactors that deliver strategic power for mission-critical systems across land, sea, and space. Founded in 2023 and headquartered in Torrance, California, its R1 reactor uses TRISO fuel, sodium heat pipes, and a nitrogen Brayton cycle to produce 100kWe-1MWe for six-plus years. In June 2026 its Mark-0 reactor became the first privately developed advanced reactor to achieve criticality under the U.S. Department of Energy's Reactor Pilot Program.