Workers are currently inside the reactor of the crisis-hit Fukushima Daiichi nuclear power plant, Kyodo News reports.
They are there to measure the levels of radioactivity inside.
Tokyo Electric Power Co (TEPCO) said the doors of the No. 1 reactor building connecting it to the adjacent turbine building at the nuclear power plant were opened last night.
This paves the way for the utility to proceed with efforts to stabilise the damaged reactor.
The move came after the government's Nuclear and Industrial Safety Agency gave the go-ahead, saying it has assessed that opening the double-entry doors at the troubled facility would not have adverse impact on the environment.
Residents living within 20km of the plant have been evacuated and there are no plans as yet to allow them to return to their abandoned homes.