Japan’s nuclear regulators say high-level radiation was detected last month in the No.2 reactor building of the crippled Fukushima Daiichi nuclear plant.
The Nuclear Regulation Authority last October resumed its probe into what caused the accident at the plant following the March 2011 earthquake and tsunami.
A meltdown took place at the reactor after the 2011 accident.
A robot on the floor directly above the reactor detected 683 millisieverts of radiation per hour.
The plant’s operator, Tokyo Electric Power Company, had also detected high levels of radiation there after the accident.
The site remains inaccessible to humans nine years later.
Commissioners and experts were also shown video of the No.4 reactor, which avoided a meltdown but experienced a hydrogen explosion. The video shows a steel frame believed to have been exposed by the blast.
The regulation authority plans to compile the data into a report this year, not only to determine the cause of the accident but also for work to decommission the reactors.