Oh my gosh you are NOT going to believe me — Antares' test reactor at Idaho National Laboratory reached criticality last week (Thursday, June 5th specifically), making it the FIRST new design to cross that monumental threshold since Trump signed an executive order last summer accelerating US nuclear development! Now, here's why this actually matters: only one of all these startup designs has been fully licensed so far — and nobody plans on building any instances of THAT design yet — so crossing criticality is a much more significant milestone than just getting another nod from regulators. Criticality means the nuclear reactions inside are now self-sustaining, which is beautiful but still NOT generating power like actual electricity (that comes later).
Let me tell you about what's cooking in that reactor because this fuel system called TRISO is genuinely fascinating — those tiny pellets have a uranium oxide core surrounded by several layers of carbon that moderate both the neutrons and lighter nuclei released during fission, all encased in an insanely strong ceramic shell designed to withstand peak temperatures. The big safety argument here: as long as your reactor keeps the TRISO contained there's essentially zero risk of meltdown or even release of dangerous isotopes, though some escaping neutrons can still convert surrounding material into unstable forms (mitigated by Antares' graphite sheath wrapping everything up). Instead of boiling water they use SODIUM to transfer heat from core out through a heat exchanger pressurized nitrogen drives their turbines in something called the closed Brayton cycle — and honestly this whole setup is beautiful engineering.
Right now it's what Antares calls the "Mark 0" reactor being tested at the Department of Energy lab but not yet connected to electrical generation; its job right now is validating physical condition modeling plus generating safety data for licensing applications, with a full system run (including actual power) expected NEXT YEAR! Bonus: they're working alongside DOD's Project Pele program for developing mobile reactors AND have NASA support too — so this little startup is juggling multiple major agencies while most of their competitors are still figuring out how to boil water. John Timmer wrote it up beautifully at Ars Technica and honestly if you love reading about clean energy tech moving forward, this whole article will get you pretty hyped!
Source: https://arstechnica.com/science/2026/06/first-us-test-of-modular-reactor-reaches-criticality/
Let me tell you about what's cooking in that reactor because this fuel system called TRISO is genuinely fascinating — those tiny pellets have a uranium oxide core surrounded by several layers of carbon that moderate both the neutrons and lighter nuclei released during fission, all encased in an insanely strong ceramic shell designed to withstand peak temperatures. The big safety argument here: as long as your reactor keeps the TRISO contained there's essentially zero risk of meltdown or even release of dangerous isotopes, though some escaping neutrons can still convert surrounding material into unstable forms (mitigated by Antares' graphite sheath wrapping everything up). Instead of boiling water they use SODIUM to transfer heat from core out through a heat exchanger pressurized nitrogen drives their turbines in something called the closed Brayton cycle — and honestly this whole setup is beautiful engineering.
Right now it's what Antares calls the "Mark 0" reactor being tested at the Department of Energy lab but not yet connected to electrical generation; its job right now is validating physical condition modeling plus generating safety data for licensing applications, with a full system run (including actual power) expected NEXT YEAR! Bonus: they're working alongside DOD's Project Pele program for developing mobile reactors AND have NASA support too — so this little startup is juggling multiple major agencies while most of their competitors are still figuring out how to boil water. John Timmer wrote it up beautifully at Ars Technica and honestly if you love reading about clean energy tech moving forward, this whole article will get you pretty hyped!
Source: https://arstechnica.com/science/2026/06/first-us-test-of-modular-reactor-reaches-criticality/