A survey of the flooded Fukushima Daiichi unit 1 torus room, which is adjacent to the reactor suppression chamber, recorded radiation levels in water in excess of 10,000 mSv/hr.
TEPCO was unable to record water radiation levels exactly; engineers suspect that the probe malfunctioned since it registered constant values of between 100 million-1 billion mSv/hr in the water. Radiation levels increased steadily as the probe descended, from 19.5 mSv/hr about 6m above the water to about 1300 mSv/hr about 4m above the water to a peak of 10300 mSv/hr just before it was submerged. The probe's first reading at the water surface was 8190 mSv/hr.
According to a report by Japanese broadcaster NHK, the radiation figures are 10 times higher than levels in units 2 and 3, which also reported TEPCO official Junichi Matsumoto speculating that radiation levels were higher in unit 1 because more fuel rods melted down. NHK also said that the 10300 mSv/hr dose would be fatal to humans in less than an hour.
The survey discovered that the water was flooded to about Ohama Port 4.0m, or about 5.2m deep. Water surface temperature was 37°C, compared with an air temperature of 22°C in the reactor building.
The survey also determined that the accumulated water was transparent at least 60cm below the surface, and that there is floating sediment on the bottom of the torus room.
