Just over a year ago, the Trump Administration issued an executive order meant to accelerate nuclear power development in the US, because nothing says 'safe energy future' like a last-minute government mandate. While a startup ecosystem has blossomed around smaller reactor designs, only one has been fully licensed, and nobody actually plans to build it - because that would be too straightforward.

The executive order directed the Department of Energy to have three different reactor designs reach criticality in a bit over a year. On Thursday, startup Antares announced that its test reactor at the Idaho National Laboratory had achieved criticality, making it the first new design to cross this threshold. Criticality means the nuclear reactions became self-sustaining - not that the reactor started generating power, which would be useful, but baby steps.

Antares is among companies basing its design on a new fuel system called TRISO, which shifts complexity and safety from the reactor to the fuel itself. The fuel consists of tiny pellets with a uranium oxide core, surrounded by carbon layers that moderate neutrons and lighter nuclei, all encased in a hard ceramic shell designed to withstand the highest temperatures the uranium can produce. Basically, it's a nuclear burrito that refuses to explode.

As long as the TRISO pellets stay contained, there's no meltdown risk or release of dangerous isotopes. However, neutrons will still escape and might convert surrounding material into unstable isotopes - because physics always leaves a loophole. The Antares design wraps the TRISO in a graphite sheath to slow most neutrons down.

To mitigate non-radioactive risks, Antares uses sodium to transfer heat from the reactor to a heat exchanger, which then heats pressurized nitrogen to drive a turbine in a closed Brayton cycle. It's like a fancy steam engine, but with more molten metal.

Currently, Antares is testing a Mark 0 reactor not connected to power generation. Instead, it validates the company's physical modeling and generates safety data for licensing applications. The full system, including electrical generation, is expected next year - assuming the universe cooperates.

While the work occurred at a Department of Energy lab, Antares collaborates with the Department of Defense's Project Pele program for a mobile nuclear reactor and has received NASA support. Because if you're going to build a portable nuclear reactor, you might as well aim for the stars.