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Radiation Doses Due to Contamination

4.2.2.1 Sievert

Besides becquerels and grams, there is one other unit of measurement—sievert— that we must understand in order to comprehend effects of radiation on human bodies resulting from radioactive contamination such as the data presented in Fig. 4.3. When nuclei decay, they emit energized particle(s), such as electrons, neutrons, protons, photons, and helium nuclei. These particles lose their energy while in motion whenever they interact with and transfer kinetic energy to other matter that exists along their trajectory, such as air, concrete, paper, water, and human tissue. When an energized particle hits a human body, it transfers its energy to human tissue, and in some cases causes irrecoverable damage (see Chap. 13). The severity of damage is dependent on the energy and type of particle, and on part of the body hit by the particle. While the first two factors are physical, the third is biological. Sievert (Sv) is a unit of measurement for a radiation dose that takes into account these three factors. Sievert expresses the combined effects (i.e., severity) of emitted energetic particles on a human body.

4.2.2.2 Pathways that Cause Radiation Dose

To estimate how much radiation dose (Sv) would be caused by the observed contamination of Cs-134 and Cs-137 in the environment, various pathways need to be taken into account. A report [3] published by the International Atomic Energy Agency (IAEA) shows a generic model for radiation dose evaluation.

Fig. 4.4 Generic models for assessing the impact of discharges of radioactive substances to the environment [3]

Figure 4.4 depicts multiple pathways that affect radiation dose to a resident in a contaminated area. The box labeled as “Total dose” at the right of Fig. 4.4 indicates that the total dose results from various causes, such as inhalation of radionuclides fl in the atmosphere, external radiation due to immersion in the radionuclide plume in the atmosphere, external radiation exposure to radionuclides deposited on the ground surface, ingestion of foodstuffs contaminated by radionuclides, etc.

Among those, the first two pathways, i.e., inhalation and plume immersion, occurred within a few weeks after the initial accident. Due to failure in conducting systematic measurement at the early stage of the accident, however, only an indirect way is now possible for dose evaluation for these pathways.[1] The ingestion pathway through contaminated foodstuffs can be avoided by applying stringent inspection for foodstuffs before they enter the commercial market. Thus, in this analysis, we focus on the external radiation due to exposure to radionuclides deposited on the ground surface.

4.2.2.3 Hourly Dose

For radiation due to exposure to radionuclides deposited on the ground surface, the relation between the surface concentration and the hourly radiation dose to a resident is given in the IAEA report by the conversion factor 2.1 × 10−3 (µSv/h)/(kBq/ m2) for Cs-137, and the factor 5.6 × 10−3 (µSv/h)/(kBq/m2) for Cs-134. A study in Fukushima [4] indicates that the radioactivity of Cs-137 and Cs-134 observed in the environment was approximately the same soon after the accident. Therefore, for example, at a location with contamination of 1,000 kBq/m2, 500 kBq/m2 is due to Cs-137 and 500 kBq/m2 is due to Cs-134. Using these values, we can calculate the total hourly radiation dose to a resident located at a point with 1,000 kBq/m2 of contamination in the following way: 2.1 × 10−3 (µSv/h)/(kBq/m2) × 500 (kBq/ m2) + 5.6 × 10−3 (µSv/h)/(kBq/m2) × 500 (kBq/m2) = 3.8 µSv/h. This means that if you stay at a location contaminated by these two cesium isotopes with a total concentration of 1,000 kBq/m2, then you will get 3.8 µSv of radiation dose every hour. It should be noted that 2.8 µSv/h is contributed by Cs-134 because of the greater conversion factor. With the shorter half-life for Cs-134, this contribution decreases faster than that by Cs-137.

4.2.2.4 Annual Dose

The guidelines of the decontamination measures announced by the government are expressed in terms of the annual dose, as shown in the next section. To obtain the conversion relation between the annual dose and the hourly dose, we need to make assumptions about people's daily life and living conditions. Suppose that (1) a person stays outside of buildings for 8 h and inside for 16 h a day, and (2) while inside, because of shielding effects by the building's walls, the radiation dose is reduced to 40 % of that observed outside. In such a scenario, 3.8 µSv/h for example can be converted as follows: [3.8 (µSv/h) × 8 (hours-outside/day) + 3.8 × 0.4 (µSv/h) × 16 (hours-inside/day)] × 365 (days/year) = 20,000 µSv/year or 20 mSv/year.[2] In this manner, the surface radioactivity concentration of Cs-134 and Cs-137 can be related to an annual dose of radiation.

Regulatory Guidelines

The Japanese government enacted a law on special measures on August 30, 2011 [5]. It stated that (1) the annual dose is to be made less than 20 mSv/year within 2 years, and (2) 1 mSv/year or lower at any location in the long term.

Returning again to Fig. 4.3, the surface concentrations of cesium in the yellow and red regions exceed the 1,000 kBq/m2 level, in which case, as the calculation above illustrates, annual doses exceed the 20 mSv/year level. This fact indicates that efforts to reduce the surface concentration of cesium should be focused in these regions to achieve the first guideline. To achieve the second guideline requires decontamination of a much broader area. With the proportionality between the surface concentration and the annual dose, the target area of decontamination would be all places with a surface contamination greater than 50 kBq/ m2, in other words the areas corresponding to the first through the seventh bars in the legend for Fig. 4.3.

  • [1] In such indirect estimates, first, evolution of radioactivity plumes with time is simulated by utilizing detailed information on the source term, i.e., how much radioactivity was released from the Fukushima Daiichi site, and on the meteorological data. Then, information on traces of people's movements during the first few weeks needs to be collected. Finally, the radiation dose can be estimated for individual evacuees.
  • [2] 1 mSv (milli sievert) is equal to 1,000 µSv.
 
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