Antares Mark – Antares’ Mark 0 small modular nuclear reactor has reached criticality in a first test at a U.S. Department of Energy lab, validating reactor modeling and producing safety data for licensing. Engineers say the design relies on TRISO fuel pellets wrapped in mult
A small nuclear reactor has reached criticality for the first time in its initial test—an early milestone that signals the Antares team has managed to get a self-sustaining chain reaction going, while still keeping the most sensitive pieces of the design separated from power generation.
The test centers on what the company calls a Mark 0 reactor. It is not yet connected to the part of the system meant to produce electricity. Instead. the goal is narrower and more procedural: validate the company’s modeling of physical conditions inside the reactor and generate safety data that can be used during licensing applications. A full run that includes electrical generation is expected next year, after this modeling and safety work is completed.
At the heart of the Antares approach is the way its fuel is handled. The reactor is designed to keep TRISO pellets contained. In that framework. the company argues there should be no risk of meltdown—or even the release of the most dangerous isotopes—so long as the pellets remain contained. There are still safety concerns, though. Neutrons can escape, and those escaping neutrons could potentially convert surrounding material into unstable isotopes. The Antares design addresses this by enclosing the TRISO with a graphite sheath. intended to slow most of those neutrons down.
Heat management is handled without opening the loop to the outside environment. The design uses sodium to take heat from the reactor and move it into a heat exchanger. From there, the system transfers that heat to pressurized nitrogen, which drives a turbine in a closed Brayton cycle setup.
The work was done at a Department of Energy lab, but Antares isn’t building only for demonstration. The company is working with the Department of Defense’s Project Pele program to develop a mobile nuclear reactor. Support has also come from NASA. underscoring how the effort fits into a broader push for compact. deployable nuclear power rather than fixed. grid-only systems.
For now. the Mark 0 test is about trust—trust in the physics inside the vessel. and trust in the safety data that will be needed for licensing. Next year, engineers plan to attempt the wider goal: running the entire system, including the electrical generation portion. Until then. the message from this first criticality run is both simple and consequential: the reactor can reach operating conditions. and the team now has a path to prove what happens afterward.
small modular reactor SMR criticality test Antares TRISO fuel graphite sheath sodium heat transfer closed Brayton cycle pressurized nitrogen mobile nuclear reactor Project Pele NASA Department of Energy lab nuclear safety data licensing