Small modular nuclear reactor reaches criticality in first test

The reactor, from a startup called Antares, isn’t ready to generate power yet.

Just over a year ago, the Trump Administration issued an executive order meant to accelerate the development of nuclear power in the US. While an entire startup ecosystem has developed around the use of different—and typically smaller—reactor designs, only one of them has been fully licensed so far, and there are no plans to actually build any instances of that design.

The executive order directed the Department of Energy to have three different reactor designs reach criticality in a bit over a year. On Thursday, a startup called Antares announced that a test reactor it had placed at the Idaho National Laboratory had reached criticality, making it the first new design to cross this threshold. Criticality means that the nuclear reactions inside the hardware had become self sustaining; it does not mean the reactor had started to generate power.

Antares is one of a number of companies that is basing its design on a new fuel system called TRISO that takes some of the complexity and safety out of the reactor design and places them in the fuel design. The fuel design is based on tiny pellets with a uranium oxide core. The pellets are surrounded by several layers of carbon that can moderate the energy of both the neutrons and lighter nuclei that are released by fission reactions. All of that is encased in a hard ceramic shell that’s designed to withstand the highest temperatures that can be produced by the encased uranium.

As long as your reactor can keep the TRISO pellets contained, then there should be no risk of meltdown or even the release of the most dangerous isotopes produced from the reactions. There are still some safety concerns, as neutrons will still escape and can potentially convert some of the surrounding material into unstable isotopes. But the Antares design surrounds the TRISO with a graphite sheath, which should slow most of these neutrons down.

To mitigate non-radioactive risks, the Antares design uses sodium to take heat from the reactor to a heat exchanger. The heat is transferred to pressurized nitrogen, which then drives a turbine in a closed Brayton cycle setup.

At the moment, Antares is just testing what it calls a Mark 0 reactor, which is not connected to the power-generation portion. Instead, it’s being used to validate the company’s modeling of the physical conditions in its reactors and generate safety data that can be used during licensing applications. Attempts to run the entire system, including electrical generation, are expected to happen next year.

While the work was done at a Department of Energy Lab, the company is working with the Department of Defense’s Project Pele program for developing a mobile nuclear reactor. The company has also received support from NASA.

Source

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Posted Saturday 6 June 2026 at 8:01 am AEST (my time).

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