It was already known that clouds of radioactive material, known as "radioactive plumes," had spread on March 15 and 16, 2011, but new analysis by the secretariat of the Nuclear Regulation Authority and the Ministry of the Environment shows similar radiation readings for March 20 and 21 as well.
Until now, radiation levels after the disaster have been estimated by comparing observed readings, such as those from aircraft, with computer simulations obtained from the System for Prediction of Environmental Emergency Dose Information (SPEEDI). For the most recent analysis, the Ministry of the Environment used data from constant monitoring devices used to measure vehicle exhaust fumes and other such air pollution. The ministry sought help from institutions including Tokyo Metropolitan University and the Atmosphere and Ocean Research Institute at the University of Tokyo. It collected filter paper that catches airborne particles from around 90 monitoring stations in nine prefectures.
Researchers analyzed radiation levels from March 12 to March 23, 2011.
The analysis showed that at one monitoring station in the city of Fukushima on the night of March 15, the combined activity of cesium-137 and cesium-134 went as high as 45.5 becquerels per cubic meter of air.
Radioactive plumes are believed to have continuously spewed from the plant between March 16 and March 19 as well, but the analysis suggests that due to eastern-blowing winds, they spread out over the Pacific Ocean and did not elevate atmospheric radiation levels over Japan. However, the wind direction later changed, and at 3 p.m. on March 20, the Fukushima city monitoring station registered a reading of 104.1 becquerels per cubic meter of air. Readings around this level continued until the next morning.
It is widely known that a radioactive plume spread around March 15, causing a sharp climb in radiation levels to around 20 microsieverts per hour after rain caused radioactive material to fall on homes and on the ground. Rain did not fall on March 20 and 21, so the already-high radiation levels near homes and on the ground did not climb noticeably. This is thought to be the reason why the second radioactive plume was not noticed until now.
In the Kanto region, two belts of high-concentration radiation were registered -- one on March 15 and one on March 21. In particular, on the morning of March 21 there was a spike in radioactive cesium concentrations in southern Ibaraki Prefecture and northeastern Chiba Prefecture. Afterwards, the plumes appear to have moved southwest to the northeastern coast of Tokyo Bay. Rain is thought to have brought the radioactive material down to the area and created radioactive "hot spots" that were recorded in various areas.
Yuichi Moriguchi, an environmental systems professor at the University of Tokyo who is knowledgeable about environmental pollution from the Fukushima disaster, commented, "This is important data that shows when and where high concentrations of cesium in the atmosphere spread. This information will help in accurately determining residents' radiation exposure at the early stages of the nuclear crisis."