A radiation hot spot on a sidewalk in Tokyo's Setagaya Ward was probably not related to the accident at the Fukushima No. 1 nuclear power plant, Setagaya Mayor Nobuto Hosaka said Oct. 13.
A radiation hot spot on a sidewalk in Tokyo's Setagaya Ward was probably not related to the accident at the Fukushima No. 1 nuclear power plant, Setagaya Mayor Nobuto Hosaka said Oct. 13.
Hosaka, citing the results of the ward's investigation, said bottles found in a cardboard box under the floor of a neighboring house could be the source of the hot spot. High radiation levels were measured from those bottles.
He also said a company that remeasured radiation levels on the sidewalk found 3.35 microsieverts per hour at a point 1 meter above the ground.
Hosaka announced on Oct. 12 that the area in the 5-chome neighborhood of the ward's Tsurumaki district had radiation levels of 2.707 microsieverts per hour, higher than the 2.115 microsieverts measured at a monitoring post on Oct. 12 in Iitate, Fukushima Prefecture, which is within the 20-km exclusion zone around the Fukushima plant.
An individual exposed to such radiation levels for eight hours a day and living in a wooden house could be exposed to about 14.2 millisieverts over the course of a year, below the 20-millisievert standard set by the central government for evacuation.
The sidewalk is used by students attending Matsugaoka Elementary School, and school officials were guiding pupils away from the hot spot on Oct. 13.
A 34-year-old mother taking her third-grade son to school said: "My child used this sidewalk for the past six months. I am worried about whether there really was no effect on his health."
A 48-year-old mother said: "I want the ward to check the routes to school as well as parks."
Ward officials maintain that just walking through the hot spot will not seriously affect human health, but they have given instructions to limit access. They will start emergency measurements of radiation levels at 258 parks in the ward that have sandboxes.
The ward learned of the problem earlier this month from a resident who had made private measurements and found abnormally high radiation levels. Officials then conducted their own survey along a sidewalk by the fence of the home where the radiation was initially detected.
Measurements were taken at nine spots, each about 2.5 meters apart. At each spot, measurements were taken at three heights above ground: 1 meter, 50 centimeters and 5 cm. Five readings were made at the three heights at each location. An average measurement was calculated for each height at the nine locations.
The 2.707-microsievert average reading was found at one spot where the measurement was made at a height of 1 meter. Radiation levels were lower closer to the ground at the same location. At another location, the measurement at the lowest height found radiation levels of only 0.088 microsievert.
A company specializing in radiation measurements was commissioned to recheck the levels near the sidewalk and also found radiation exceeding 2.7 microsieverts in one location.
The sidewalk where the high radiation level was detected is about 20 to 30 cm below the road.
One ward official said, "We believe the radiation level became high as a result of the accumulation of rain water."
However, officials admit they cannot explain some aspects of the case, including the fact that the highest radiation levels were found at the highest height measured and that much lower levels were measured in locations nearby. Radiation levels are normally highest the closer to the ground a measurement is taken.