High levels of radiation were detected on the grounds of the Fukushima No. 1 nuclear power plant one month after hydrogen explosions spewed radioactive materials, according to Tokyo Electric Power Co.
High levels of radiation were detected on the grounds of the Fukushima No. 1 nuclear power plant one month after hydrogen explosions spewed radioactive materials, according to Tokyo Electric Power Co.
The findings were shown in a map depicting radiation levels that TEPCO released for the first time on April 24.
Radiation levels in the air around the No. 1 and No. 3 reactor buildings were especially high, mainly because the hydrogen explosions damaged the buildings and spread radioactive materials.
The air in an area to the northwest of the No. 3 reactor building had radiation levels of up to 70 millisieverts per hour. That building was damaged by a hydrogen explosion on March 14, three days after the Great East Japan Earthquake and tsunami crippled the plant.
TEPCO first compiled the radiation level map on March 22 and has periodically updated it. The map is used to monitor radiation exposure of workers at the Fukushima plant and prepare new work plans for the plant grounds.
Workers check radiation levels in the air every seven to 10 days or before any work procedure starts.
If unusually high radiation levels are detected, further testing of rubble in the area is conducted to determine the cause of the high levels.
Radiation levels as high as 130 millisieverts per hour were confirmed around the No. 1 and No. 3 reactor buildings in late March.
Because radiation decreases with the passage of time, subsequent testing found lower levels.
But if the level of 70 millisieverts per hour continues to the northwest of the No. 3 reactor building, a worker who remains in that area for four hours will have been exposed to more than the upper limit of 250 millisieverts established for individuals engaged in work at the Fukushima plant.
Workers exposed to that total level of radiation will not be allowed to work in the area.
On March 20, concrete rubble found west of the No. 3 reactor building had radiation levels of 900 millisieverts per hour. Even after that rubble was removed, radiation levels in the air measured between 10 and 30 millisieverts per hour.
Another pile of rubble emitting radiation levels of 300 millisieverts per hour was found near the No. 3 reactor building.
Almost all of the contaminated rubble was concrete from the No. 1 reactor building that was damaged in a hydrogen explosion on March 12 as well as from the No. 3 reactor building, hit by an explosion there on March 14.
According to calculations by the Nuclear Safety Commission of Japan, the equivalent of 190,000 terabecquerels of radioactive iodine had been spewed from the reactor buildings by March 15. A terabecquerel is equivalent to 1 trillion becquerels.
That high level meant the Fukushima plant accident had already reached the worst level 7 on the International Nuclear and Radiological Event Scale, matching the assessment given to the 1986 Chernobyl nuclear disaster.
A pipe installed to move radiation-contaminated water from the trench of the No. 2 reactor to a central waste processing facility was found to have radiation levels of 160 millisieverts per hour on its surface.
Areas within the plant ground at a distance from the reactor buildings were also found to have radiation levels that exceeded 1 millisievert per hour.
High radiation levels in the air were found even after rubble in the area was removed.
The rubble, removed by remote-controlled heavy equipment, has been placed in 50 containers and moved to a temporary storage area within the plant grounds. A considerable amount of rubble remains, however.
While TEPCO officials continue to remove the rubble to allow for easier work within the plant, one official said the radiation would not have a major effect on work because the contamination has already been figured into the road map for work procedures.
(This article was written by Keisuke Katori and Hidenori Tsuboya.)