Radiation levels of up to 1,800 millisieverts per hour have been detected at four locations at the crippled Fukushima nuclear plant, as the operator checks storage tanks following a leak of 300 tons of radioactive water.
Tokyo Electric Power Co. said Aug. 31 it found between 70 and 1,800 millisieverts per hour around five tanks in three areas.
An exposure of 1,800 millisieverts per hour for about four hours is considered fatal to a human. But a worker can be shielded from the radiation with proper protection because it is mostly beta rays, which have weak penetrating power.
High radiation levels may be discovered at more locations as the utility continues to examine other storage tanks.
TEPCO detected between 220 and 1,800 millisieverts per hour around joints at the bottom of two tanks in an area called H3 on Aug. 31. Radiation levels between 70 and 100 millisieverts per hour had been found at the same locations on Aug. 22.
The utility also detected 230 millisieverts per hour at a section connecting two tanks in an H5 area and 70 millisieverts per hour around the bottom of a tank in an H4 area.
In the H5 area, a drop of water fell when a worker on patrol pressed insulation outside the connecting section of the tanks. A radiation level of 230 millisieverts per hour was measured on a section of the floor.
TEPCO officials said they believe radioactive water has not spread outside barriers surrounding tanks. They said water levels had not dropped in any of the five tanks, and drain valves on the barriers had been closed.
The five tanks are a so-called flange type that utilizes steel sheets connected by bolts. They are of the same design as the one from which TEPCO said on Aug. 19 an estimated 300 tons of radioactive water leaked.
The Nuclear Regulation Authority assessed the severity of the leakage at Level 3 (serious incident) on the eight-level International Nuclear and Radiological Event Scale (INES).
Flange-type tanks are said to be durable for about five years, and they account for 350 of the approximately 1,000 storage tanks at the Fukushima No. 1 plant.
TEPCO has announced plans to switch to tanks whose sheets are welded together, which are said to be more durable. But at least one month is required to complete a welded tank, while a flange-type tank can be installed in a week or so.
The company will have to continue to use flange-type tanks for the time being, which leaves the possibility that more radioactive water may escape them.
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