Image Courtesy: US Department of Energy
A next-generation small modular nuclear reactor has reached criticality for the first time in the United States, marking a significant milestone for an industry seeking to revive nuclear power with smaller, safer, and more flexible reactor designs.
The reactor, developed by US startup Antares and tested at Idaho National Laboratory, successfully achieved a self-sustaining nuclear chain reaction this week. Reaching criticality means the reactor can maintain fission without external intervention, although it is not yet producing electricity.
The achievement comes just over a year after the Trump administration issued an executive order aimed at accelerating nuclear energy development in the United States. The order called for multiple advanced reactor designs to reach criticality on an accelerated timeline as policymakers seek reliable low-carbon energy sources capable of supporting growing electricity demand.
Antares’ reactor uses TRISO fuel, a technology widely viewed as one of the most promising developments in advanced nuclear energy. Unlike conventional reactor fuel, TRISO consists of tiny uranium fuel particles encased in multiple protective layers of carbon and ceramic materials. These layers are designed to contain radioactive byproducts even under extreme temperatures, significantly reducing the risk of a meltdown.
The reactor also incorporates a graphite structure to absorb and slow neutrons while using liquid sodium to transfer heat away from the core. That heat is ultimately intended to power a turbine through a closed Brayton cycle system using pressurized nitrogen gas, an approach designed to improve efficiency while simplifying reactor operations.
For now, the company is operating what it calls its Mark 0 reactor, a non-power-generating test platform intended to validate engineering models and gather safety data for future licensing efforts. Full demonstrations that include electricity generation are expected to begin next year.
The project has attracted support from multiple US government agencies. In addition to working with the Department of Energy, Antares is involved in the Department of Defense’s Project Pele initiative, which aims to develop transportable nuclear reactors for military applications. The company has also received backing from NASA, highlighting growing government interest in compact nuclear systems for both terrestrial and potential future space-based uses.
The milestone underscores renewed momentum in the advanced nuclear sector, where dozens of startups are racing to commercialize alternative reactor designs. While many concepts remain years away from deployment, successful criticality tests are viewed as a key step toward proving that next-generation reactors can move beyond simulations and into real-world operation.