U.S. reactor startup Antares has reached a key milestone in developing a next-generation small reactor after its Mark 0 test unit achieved criticality, meaning a self-sustaining fission reaction. Antares said the reactor at Idaho National Laboratory is a test device not connected to power generation equipment, focusing instead on design validation and safety data. The development aligns with U.S. policy goals to advance new reactor designs.

The U.S. government is pursuing a plan to use some plutonium that has been in long-term storage and designated for disposal as fuel for next-generation reactors. The Energy Department selected five nuclear power startups to negotiate with the government on plutonium use. The move is tied to decades-long challenges in managing plutonium produced for nuclear weapons. Security, potential conflicts of interest and fuel supply chains remain key variables.