The Energy Department says a small nuclear reactor under development at a national lab has reached a crucial milestone that could allow it to produce electricity in the next few years. The microreactor being developed by Antares Nuclear Inc. at the Idaho National Lab reached "criticality" on Thursday, Energy Secretary Chris Wright said. The milestone occurs when a nuclear reactor achieves a self-sustaining chain reaction capable of producing a steady release of energy. Antares is the first private company to bring an advanced reactor to criticality under a pilot program begun last year by the Trump administration.