Evaluation of dose conversion coefficients for environmental radionuclides

Study

Dose assessment study on the basis of the 2007 recommendations of ICRP

Evaluation of dose conversion coefficients for environmental radionuclides

Due to the accident at the TEPCO Fukushima Daiichi Nuclear Power Plant, a large amount of radionuclides had been discharged to the environment. Then, air, water, and soil are contaminated with radionuclides in the environment. In order to make a remediation strategy for the contaminated environment, it is important to assess the radiation dose from external exposure to radionuclides distributed in the environment.

To meet the demand, we have evaluated the dose conversion coefficients for radionuclides (1252 nuclides) in environment with latest knowledge of radiation protection, and the data were prepared for typical ages from newborn to adult.

In this work, the evaluation of the dose conversion coefficients has been performed as follows:

Radiation fields are calculated on the basis of particle transport simulation in a huge geometry of the soil, atmosphere and water considering interactions of photons and electrons with the materials by using radiation sources that emit mono-energetic photons and electrons.
The voxel phantoms that precisely represent a human body including internal organs and tissues [1](Fig. 1) from newborn to adult are located in the radiation field calculated in the first step. The effective dose is derived from the calculation results of organ absorbed doses according to the ICRP 2007 Recommendations[2].
Dose conversion coefficients are derived for each radionuclide from the effective dose for mono-energetic sources using decay data library employed in the ICRP Publication 107[3].
Data are prepared for dose assessment at typical age from newborn to adult.

As an example of the calculation results, Fig. 2 shows absorbed doses calculated using the ICRP male and female reference phantoms exposed to radiations from radioactive cesium in soil. The conversion coefficients to the effective dose computed from those absorbed doses are summarized in Table 1.

We will release a software to assess the external radiation doses for radionuclides distributed in the environment based on the dose conversion coefficients evaluated in this study.

Fig. 1 The ICRP/ICRU adult reference computational phantoms [1] used in effective-dose calculations (left: male, right: female).

Fig. 2 Organ doses of adult male and female for ¹³⁴Cs and ¹³⁷Cs distributed at a depth of 0.5 g/cm² in soil [4].

Table 1 Coefficients to convert from the activity concentration of ¹³⁴Cs and ¹³⁷Cs in soil to effective dose rate of adults (mSv/h per kBq/m²) [4].

References
[1] ICRP Publication 110 (2009).
[2] ICRP Publication 103., Ann. ICRP 37 (2-4) (2007).
[3] Ann. ICRP 38 (3) (2008).
[4] D. Satoh et al., J. Nucl. Sci. Technol., 1021286 (2015).

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