A US regulator is set to approve Tennessee Valley Authority’s proposal to build a small nuclear reactor in an important sign of progress for the fission industry. The Nuclear Regulatory Commission is expected to issue a construction permit Tuesday for TVA to build a 300-megawatt system at its Clinch River site in Tennessee, west of Knoxville, according to a media alert from the company.

TVA is considering as many as four of the BWRX-300 reactors from GE Vernova Hitachi Nuclear Energy, though this permit is only for the initial system, confirming a report last week by Bloomberg News. GE Vernova Hitachi is one of dozens of companies developing new, smaller reactors that are expected to be produced in factories and assembled on-site, an approach aimed at reducing costs and construction time.

However, the idea remains unproven, with only two under construction outside of China or Russia. This is the second US construction permit for a small commercial modular reactor, after Terra Power LLC received approval in March for a project in Wyoming. In Canada, Ontario Power Generation began construction last year on the world’s first BWRX-300 system.