Special thanks for their very important work to Kurumi Sugita of the Fukushima 311 Voices Blog and to Mr Yoichi Ozawa of the citizen’s measurement group named the “Fukuichi Area Environmental Radiation Monitoring Project“.
We have published several articles in this blog saying that to protect the population the Japanese goverment should take into account the soil contamination as well as the radiation dose in the air. The policy to open the evacuation zones and encourage the population to return to live there (with the end of financial compensation and relocation aid) is based only on the airborn radiation dose measurements (the evacuation order is lifted when the radiation dose is under 20mSv/year). We have been saying that this is very dangerous, even criminal, for the air radiation dose rate (indicating the amount of radioactive dose received by a person within a certin period time) is useful with a well-identfied fixed source of radiation, but is not adequate to reveal the overall environmental contamination after a nuclear accident. It doesn’t account for the internal radiation exposure induced health hazards (note 1).
Now we would like to point out another problem related to hotspots: it is nearly impossible to find hotspots by the usual measuring practice of the airborn radiation dose rate (in sieverts per unit of time). To illustrate this difficulty, we are translating here a Facebook post of Mr Yoichi Ozawa of the citizen’s measurement group named the “Fukuichi Area Environmental Radiation Monitoring Project“.
Here are the radiation dose rates, captured vertically above a highly radioactive substance (“black substance” or “black dust”) of 4,120,000 Bq/kg, measuring 79 μ Sv/h.
Below are measurements at different distances from the ground.
Conclusion: it is impossible to discover micro-hotspots right under your feet when you walk around measuring radiation doses at 1m of distance from the ground.
Measurements carried out by Mr Yoichi Ozawa.
For 0cm from the ground, Aloka TCS172B, which cannot measure over 30µSv/h, was replaced by Polimaster PM1703M and Radex RD1706. The value is the average of the measurements of these two devices.
Here is the video of the measurement.
As we can see from the graph above, the value in terms of Sieverts decreases drastically with the distance from the ground. At 1m, which is the usual reference height to measure the radiation dose rate, the value becomes very small even with the soil of over 4 million Bq/kg, which is absolutely enormous (note 2).
Some readers might be familiar with the image of a Japanese citizen measuring radioactivity with a device at about 1m from the ground. This practice, almost unknown before the Fukushima Daiichi nuclear accident, has become widespread among citizens, although it has become rather a rare practice nowadays as many people have more or less become accustomed to live with radiation. Besides the fact that it is hard to live a life worrying about radiation around the clock and some people prefer to stop thinking about it, this “normalisation” of radiation is strongly enforced by a governmental security campaign. One of the methods employed is to focus on the external irradiation risk, neglecting the internal irradiation risk, by spreading the knowledge and data only in terms of the radiation dose in the air (measured by Sieverts), at the expense of other measurements such as the radiocontamination density in soil (surface contamination density in terms of Becquerel/unit of surface).
One of the now well-known problems of radiocontamination of the environment is that the contamination is not homogeneous, but dispersed with what is called a hot spot. This is a serious problem for the population, as the absorption of radioative particles contained in these hot spots can cause internal irradiation related health damage. And as we see above, it is extremely difficult to detect these hotspots, from 1m and above, even with the extremely highly contaminated substance such as “black dust”.
It is widespread belief among the public that if the value of the airborn radiation dose at 1m from the ground is under 0.23µSv/h (note 3), it is safe. This value, diffused by the authorities as well as by media, is indeed applied as the lower limit to carry out decontamination work. Yet, as we have seen, even with the extremely highly contaminated substance such as “black dust”, at 1m, the radiation dose is only 0.25µSv/h, that is to say, only slightly over the limit of the 0.23µSv/h, which is believed to be the “safety level”.
It is unfortunate to say that for most of the residents taking the measurements of the airborn radiation dose by themselves, the values they observe have become rather an “encouraging” factor to continue living there or to return to live, than an alarming factor, as these values do not reveal but rather conceal the presence of hotspots which can cause internal radiation exposure induced health damage.
It is difficult to find hotspots anyway. So when the soil contamination is high (see the concentration maps in this blog, for Namie, Minamisoma), it is better to keep the zone closed, continuing to aid the evacuated people.
Note 1: In opposition to the external radiation exposure which occurs when the human body is exposed to an external source, the internal radiation exposure is an exposure from inside the body due to the incorporation of radioactive particles through ingestion, inhalation or adhesion to skin.
Note 2: This extremely high level of contamination is understandable, for what is measured here is the infamous “black substance” or “black dust”, a kind of Cyanobacteria, about which we invite you to listen to podocast of Marco Kaltofen with English transcription.
Note 3: In fact, the 0.23µSv/h value is problematic in itself. This is based on the 1mSv/year value following the ICRP (International Commission on Radiological Protection) recommendations on the public health. However, the 0.23µSv/h value is not the result of a simple division of 1mSv by 365 days x 24 hours. The calculation of 0.23µSv/h presupposes that people stay inside for 16 hours/day and that the radiation is reduced by 60% because of the building structure. Then, the background of 0.04µSv/h is added. (1000µSv÷365÷(8 + 〈16×0.4〉) + 0.04 But in the real life in rural areas such as Fukushima, people spend more time outdoors. Besides, some recent research has shown that in some cases the radiation dose can be higher indoors than outdoors because of the infiltration of hot particles. Thus, the reality is much more complex to apply uniformly the value of 0.23µSv/h as a safety threshhold. Lastly, many people in Fukushima were victimes of the initial exposure right after the accident. For such population, any exposure, whatever the quantity is, is to be avoided.