Dr. Tsubokura's Radiation Lecture Vol.60
Author: Masaharu Tsubokura
Editors: Akihiko Ozaki M.D., Yuki Senoo
119. Cesium is rarely found in water
Cesium is usually tightly bound to soil particles and is hardly detected in water. When radioactive cesium is detected in water, it is generally due to mud floating at the surface that had accumulated at the bottom; almost no contamination has been found in water from ponds or dams.
On the other hand, soil-bound radioactive cesium is sometimes present at the bottom of a pond or dam. However, most of this cesium is not from surroundings such as forests; the majority has been present in lakes since the early phase of the post-disaster period.
When the water level of the pond is 1 meter, radiation will be shielded, and the air dose on the ground is reduced to 1/100. Furthermore, when the water level is more than 2 meters, the air dose is reduced to less than 1/100,000.
Full bodies of water can prevent the air dose rates from increasing due to the radioactive substances accumulated at the bottom. On the other hand, if there is a risk of radiation contamination in agricultural products or an increased air dose rate in the environment due to the withdrawal of water from the lake, such as during a drought, decontamination of the field will be undertaken.
120. Tritium is also present in nature
A radioactive substance called tritium is often referenced in media outlets regarding the radiation contamination in water following the accident at the nuclear power plant. Tritium is an isotope of hydrogen, and it is one of the radioactive substances released into the environment after the Fukushima Daiichi nuclear disaster, similar to cesium and strontium.
Tritium originally existed in nature, and about 7 quadrillion becquerels (70,000 trillion becquerels) of tritium are produced annually by the chemical reaction between nitrogen and oxygen in the atmosphere and radiation emitted from outer space. Therefore, it can be said that tritium is not only an artificial radioactive substance but also a naturally occurring radioactive substance.
Tritium reacts with oxygen and becomes tritium water, which is present everywhere in the world, including water vapor in the atmosphere, seawater, rivers, rain, groundwater, and drinking water.
As mentioned, the extent of the exposure dose determines the impact of radiation on health. Approximately a few dozens of becquerels of tritium are present in our bodies, but the impact of internal exposure on our health is much lower compared to other radioactive substances such as potassium: the level of internal exposure from tritium accounts for only about 1 / 100,000 millisievert per year.
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The Japanese version of the manuscript was originally published in Fukushima Minyu, a local newspaper in Fukushima prefecture, Japan, on 23rd and 30th April 2017 was reproduced for MRIC Global under the author's permission.