The Nuclear Regulation Authority conducted a one-day investigation Friday at the Oi nuclear power plant in Fukui Prefecture to check whether a disputed fault running underneath it should be viewed as active.
Depending on the outcome of the investigation, Japan's only operating nuclear plant could be told to shut down, just months after two of its reactors were allowed to restart.
It is the first time the NRA, established in September, has conducted an on-site inspection at a nuclear plant.