Small modular nuclear reactor achieves first criticality in test

Just over a year ago, the Trump Administration issued an executive order aimed at accelerating nuclear power development in the United States. Since then, a vibrant startup ecosystem has emerged around innovative reactor designs, typically smaller than traditional models. However, only one design has received full licensing, and there are no concrete plans to build any operational versions of it.

The executive order tasked the Department of Energy with achieving criticality for three different reactor designs within roughly 13 months. On Thursday, a startup named Antares announced that a test reactor installed at the Idaho National Laboratory had reached criticality, marking the first new design to cross this milestone. Criticality indicates that nuclear reactions within the hardware have become self-sustaining; it does not yet mean the reactor is generating power.

Antares is among several companies basing its design on a novel fuel system called TRISO. This system shifts much of the complexity and safety burden from the reactor itself into the fuel design. The fuel consists of tiny pellets with a uranium oxide core, surrounded by multiple layers of carbon that moderate the energy of neutrons and lighter nuclei released during fission. All of this is encased in a hard ceramic shell engineered to withstand the highest temperatures produced by the uranium inside.