Radiation on Tokyo sidewalk exceeds parts of no-entry zone
Workers commissioned by Setagaya Ward in Tokyo survey a neighborhood where high levels of radiation were confirmed. (Yusaku Kanagawa)Parents accompany their children to school in Setagaya Ward on Oct. 13 after high levels of radiation were confirmed in one neighborhood. (Yusaku Kanagawa)
Radiation exceeding levels in the no-entry zone around the crippled Fukushima No. 1 nuclear power plant has been found on a sidewalk in Tokyo.
According to an Oct. 12 announcement by Setagaya Ward Mayor Nobuto Hosaka, the spot 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 that firm 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.
Ward officials will look into the possibility of radiation coming from a source other than the Fukushima accident and work out ways of decontaminating the area.
Kunikazu Noguchi, an expert on radiation protection at Nihon University, said: "There was probably a condensation of the radiation through an accumulation of rain water. There may be other locations where there are limited areas of high radiation. The local government should be responsible in studying the matter and eliminating the concerns felt by residents."
Estimated 13,000 square km eligible for decontamination2011/10/12
The central government will be responsible for decontaminating about 13,000 square kilometers across eight prefecture, or about 3 percent of Japan's total landmass, under new standards for cleaning up radiation from the Fukushima No. 1 nuclear power plant, according to Asahi Shimbun estimates. The Environment Ministry on Oct. 10 endorsed a basic policy to make the government responsible for decontaminating all areas with radiation levels exceeding 1 millisievert per year. Based on an earlier annual threshold of 5 millisieverts, the ministry initially said about 1,800 square km of land in Fukushima Prefecture would be subject to decontamination. But under the new standard, the size of the area will grow sevenfold. Using airborne radiation measurements by the science ministry, the Environment Ministry drew up maps of regions where additional exposure to radiation due to the nuclear accident lies between 1 and 5 millisieverts annually. The Asahi Shimbun studied the measurements of the affected areas in Fukushima Prefecture as of Aug. 28 and outside the prefecture as of Sept. 18. The calculation found that about 6,200 square km in Fukushima Prefecture had radiation levels between 1 and 5 millisieverts per year, in addition to the estimated 1,800 square km above 5 millisieverts annually. In total, about 8,000 square km, or 60 percent of the landmass of Fukushima Prefecture (13,782 square km), will be eligible for decontamination under the new standard. There were no areas outside Fukushima Prefecture with radiation levels exceeding 5 millisieverts per year. But the following had annual exposure levels between 1 and 5 millisieverts: 2,100 square km in Gunma Prefecture; 1,700 square km in Tochigi Prefecture; 440 square km each in Miyagi and Ibaraki prefectures; 180 square km in Chiba Prefecture; and 20 square km each in Tokyo and Saitama Prefecture. The maps do not reflect "hot spots," or localized spots of high radiation levels. The science ministry is conducting airborne measurements in 22 prefectures. The results for Iwate, Niigata, Nagano and other prefectures have yet to be released, so the total area with radiation levels above 1 millisievert per year could expand. The costs for the cleanup could also grow. In late September, the Environment Ministry said that full decontamination in areas above 5 millisieverts per year and partial decontamination for areas between 1 and 5 millisieverts would involve removing about 29 million cubic meters of surface soil and fallen leaves in forests. It predicted the decontamination measures would cost the central government about 1.2 trillion yen ($15.6 billion). Following protests by local governments, however, the ministry decided that the central government would assume responsibility for decontaminating all areas above 1 millisievert per year. That basic policy plan was accepted by an expert panel on radioactive contamination on Oct. 10. (This article was written by Hiroshi Ishizuka and Harufumi Mori.)