Four privately developed reactor designs sustained a fission chain reaction between 4 June and 4 July. The Department of Energy had set a presidential target of bringing at least three advanced test reactors to criticality by Independence Day; Antares Nuclear, Valar Atomics, Deployable Energy and Aalo Atomics put four across the line. [E1]
Antares ran Mark-0 at Idaho National Laboratory on 4 June. [E2] Valar followed with Ward 250 at Utah’s San Rafael Energy Lab on 18 June, the first reactor in the program authorized outside a national laboratory. [E3] Deployable Energy reached the same state with Unity at INL on 30 June. [E4] Aalo-X completed the sequence there on 4 July. [E5]
Criticality is a physics threshold, not a power contract. Each experiment was conducted at zero power: the chain reaction became self-sustaining, but the apparatus produced no useful heat or electricity. DOE’s environmental review for Unity states the distinction without promotional insulation. The test would generate neither electric power nor useful thermal energy. [E4]
The month still changed the hardware ledger. Four teams moved from drawings, fuel models and non-nuclear assemblies to fueled cores under public authorization. Antares tested a kilowatt-class lineage; Deployable describes a one-megawatt electrical product; Aalo’s commercial architecture aims at ten megawatts per reactor. Those future ratings do not belong to the June and July experiments. [E2][E4][E5]
DOE authorized the demonstrations. Commercial reactors will still meet the Nuclear Regulatory Commission’s licensing system, while factories that handle enriched fuel require their own permissions. A Federal Register notice published Monday opened a hearing window on Radiant Nuclear’s application to possess coated uranium particles and load fuel assemblies at an Oak Ridge facility. Petitions are due on 25 September. [E6] The document concerns a fuel plant, not an operating Kaleidos reactor.
Portability also arrives in pieces. Valar demonstrated that its experimental unit could travel by military cargo aircraft, and Deployable brought Unity to Idaho by truck. [E3][E4] Transport proves that the package can move. It does not prove that a customer can unload it, license a site, connect a turbine, secure fuel and run through an endurance cycle.
The counter-case to the scepticism is the calendar itself. Four separate teams reached the same physical threshold inside one month, and none can dismiss a fueled chain reaction as a slide deck. The boring null is equally physical: no unit sent electricity to a base, mine, data centre or grid. DOE compressed the path to criticality. The power line still begins after the experiment.