High Radiation Levels Found Beyond 30-km Radius

According to the study, the accumulated amount of radiation in the soil at Iitate, Fukushima Prefecture--which is located outside of the 30-km radius--calculated over a three-month period would exceed the annual accumulated amount of 20 millisieverts that the central government is considering as a guideline for evacuating residents.

The government has asked residents living within a 20-km radius of the Fukushima No. 1 plant to evacuate and those living between a 20- to 30-km radius to remain indoors as much as possible.

Government studies have also found evidence of radiation contamination beyond the 30-km radius. The results show that radiation has not been spreading from the nuclear power plant in a concentric manner.

The predicted changes in the level of radiation at the ground surface were calculated after analyzing the amounts of eight kinds of radioactive materials found in the soil and taking into consideration the half-lives of each material.

The study results are considered more accurate than the study conducted by the science ministry, which only released information concerning two types of radioactive material.

Tetsuji Imanaka, an associate professor of nuclear engineering at the Kyoto University Research Reactor Institute, Satoru Endo, an associate professor of radiation physics at Hiroshima University, and other experts visited Iitate in late March.

They collected soil samples from five locations in the village at depths of five centimeters. All the locations were outside the 30-km radius and were by roadways in various hamlets.

The study found cesium-137 at levels between about 590,000 and 2.19 million becquerels per cubic meter.

After the Chernobyl nuclear accident in the former Soviet Union in 1986, residents who lived in areas where cesium-137 levels exceeded 555,000 becquerels were forced to move elsewhere.

According to a report by the International Atomic Energy Agency, the total land area from where residents were forced to move was about 10,000 square kilometers, about the land area of Gifu Prefecture. A total of about 270,000 residents were subject to the forced move.

The amounts of cesium-137 found in Iitate were at most four times the figure from Chernobyl.

If more radioactive materials are emitted from the crippled Fukushima plant, the level of cesium-137 could rise even further.

Calculations were also made of changes in the radiation level in the air one meter above contaminated ground over a three-month period from March 15, when an explosion occurred at the Fukushima plant.

The study found that even after three months there would be between 7 and 21 microsieverts per hour being emitted from the contaminated soil into the air.

If an individual remained outdoors for the entire three-month period, the person would be exposed to between 30 and 95 millisieverts over the period.

Assuming the cesium remains in the soil, the accumulated level of radiation after one year could be between about 70 and 220 millisieverts.

The central government is considering using an accumulated radiation exposure figure of 20 millisieverts over the course of a year as one indicator of whether an evacuation instruction should be issued.

The cesium-137 could move or be washed away by rain and wind, so there is the possibility that the actual accumulated radiation exposure figure could fall below the study's estimates.

Studies by the science ministry have found that contamination of the soil by radiation does not spread out in concentric circles, but is more irregular due to wind direction and other factors.

Soil-sample studies by the science ministry have found levels of cesium-137 per kilogram of soil in other areas beyond the 30-km radius, such as Okuma and Namie, that were higher than those in the study conducted by the experts at Kyoto and Hiroshima universities.

http://www.asahi.com/english/TKY201104080169.html