Workers have successfully floated a balloon in the Fukushima Daiichi unit 1 equipment hatch up to the fifth floor.
They are trying to learn the state of the refuelling floor in order to engineer a way to extract spent fuel from the spent fuel pool, among other work. Unit 1 is covered by steel-framed tent to protect it from weather and to reduce the spread of radioactivity.
The images show masses of rubble from collapsed roof trusses, overhead travelling crane beams and the refuelling crane, which were all damaged by explosions in March 2011.
The survey found that the fuel handling machine did not collapse into the spent fuel pool, and that there is some space between the floor and the collapsed ceiling around the spent fuel pool.
The maximum radiation dose was found to be about 54 mSv/hr, although the highest radiation discovered, three times that, was experienced on the second floor.