Dr. Tsubokura's Radiation Lecture Vol.110

Author: Masaharu Tsubokura PhD.

Editors: Akihiko Ozaki M.D.,PhD., Yuki Senoo

219. Bonding characteristics of cesium to soil


Some portion of the radioactive materials scattered from the nuclear accident fell on the surrounding mountains and forests. Over time, radioactive cesium particles that had bonded to trees in the mountains migrated by rain into the soil and fallen leaves on the ground. Now, more than eight years after the Fukushima Daiichi Nuclear accident, most of the cesium has migrated into the soil.

Cesium has a property by which it strongly bonds to soil particles. The bond between radioactive cesium and soil is so strong that cesium particles barely dissolve in water once bonded with soil.

The migration of radioactive cesium flowing from upstream to downstream in the forest only occurs during heavy rainfall. Moreover, previous studies have shown that only a limited amount of radioactive cesium migrates from the soil in forests to rivers, with 1/5,000 to 1/300 of its amount annually found in rain-basin soil.

Even when radioactive cesium flows out of the mountains along with the soil, a purification system removes it from the water along with the soil particles with which it is bonding. Then, radioactivity concentrations in the tap water of Fukushima Prefecture are measured to confirm the water’s safety.

222. Almost no internal exposure detected in people living in Fukushima Prefecture


More than eight years have passed since the Fukushima Daiichi nuclear power plant accident. Radiation-related environmental impacts have significantly weakened thanks to multiple factors. These contributing factors include the reduced amounts of radionuclide and radioactivity over time, migration of radioactive contaminants by rain, and implementation of various measures to protect against radiation such as consistent long-term monitoring and decontamination.

The level of internal radiation exposure among residents in Fukushima Prefecture was one of the values that saw a reduction. Due to tight controls on food distribution, almost no internal radiation exposure exceeding the limits was detected among residents in Fukushima by 2012, one year after the nuclear accident. Consequently, internal radiation exposure was detected only when a person consistently consumed food products under restricted distribution, though it was rare. In Minamisoma immediately after the accident, the highest level of annual internal radiation exposure detected in tens of thousands of investigations was slightly over one millisievert.

The internal radiation exposure detected in Minamisoma residents does not reach detectable levels in most cases. In addition, detection of internal radiation exposure became rarer even as people continued to consume food products that were subject to restricted distribution due to contamination levels. This contributed to lowering the internal radiation exposure of the residents as well. The maximum level of internal exposure detected over the past two years has been about one-fifth lower than was detected in 2012.

  Besides the fact that an internal exposure level exceeding the limit has not been detected in most residents, detected levels are much lower than one millisievert per year.


The Japanese version of the manuscript was originally published in Fukushima Minyu, a local newspaper in Fukushima prefecture, Japan, 24th and 31th March 2019 was reproduced for MRIC Global under the author's permission.

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