Workers at Japan's Fukushima Daiichi nuclear plant have entered one of its reactor buildings for the first time since it was damaged by a powerful earthquake in March.
Officials say the workers are installing ventilation systems in the No 1 reactor on Thursday to filter out radioactive material from the air.
The quake disabled reactor cooling systems, causing fuel rods to overheat.
Radiation levels inside reactor buildings must be lowered before new cooling systems can be installed, The BBC reports.
The No 1 reactor was one of four damaged by explosions in the days immediately after the 9.0-magnitude quake and tsunami on 11 March.
The Tokyo Electric Power Company (Tepco) says 12 engineers would work inside the reactor building in shifts of 10 minutes.
Footage filmed by cameras mounted on robots sent into the reactor had already established that there were no leaks of potentially radioactive water inside the building.
Tepco says it hopes to begin operating the ventilator later on Thursday. It then plans to connect up a new cooling system outside the reactor to bring temperatures down.
The company faces similar problems at three other reactors at the six-reactor plant.
Tepco is also dealing with highly radioactive waste-water leaking from the No 2 reactor which it is moving to secure storage on site.