As much as 50 percent to 90 percent of radioactive cesium on the ground in forested areas as a result of the Fukushima No. 1 nuclear power plant accident is concentrated on fallen leaves and branches, according to a measurement by experts.
The discovery indicates it is possible to reduce large amounts of ground radiation by removing fallen forest materials, and likely will become basic data for decontamination measures.
A research team led by Tsukuba University Prof. Yuichi Onda reported the results of the measurement, which was carried out between June and August, at a review meeting of the Education, Culture, Sports, Science and Technology Ministry on Tuesday.
The research team compared densities of radioactive cesium and its cumulative quantities at a coniferous cedar forest and a forest with ample broad-leaved beech trees, both in Kawamatamachi, Fukushima Prefecture, and within the government-designated evacuation zone.
The cumulative amount of radioactive cesium in living leaves at the cedar forest was found to be higher than in the broad leaf forest. However, in fallen leaves at the broad leaf forest, the cumulative amount of radioactive cesium was three times to six times higher than in fallen material at the cedar forest.
Of the cumulative quantity of radioactive cesium in the cedar forest, about 50 percent to 90 percent was found to be concentrated on fallen branches. In the broad leaf forest, more than 90 percent of radioactive cesium was found to be accumulated on fallen leaves, according to the research.
The result likely indicates that larger amounts of radioactive cesium contaminated ground covered with fallen leaves in the broad leaf forest because there were fewer living leaves in the forest in March, when the crippled nuclear plant began spewing large quantities of radioactive material.
Decontamination of forests has become a problem in evacuation zones in the prefecture. Onda said removing fallen leaves would be an effective way of decontaminating forests. "In addition to this, we want the government to consider trimming branches and cutting down trees as decontamination measures," he said.