# Reduction of Outdoor and Indoor Ambient Dose Equivalent after Decontamination in the Fukushima Evacuation Zones

• Accepted : 2016.11.28
• Published : 2017.03.31

#### Abstract

Background: One of the most urgent issues following the accident at the Fukushima Daiichi nuclear power plant (FDNPP) was the remediation of the land, in particular, for residential area contaminated by the radioactive materials discharged. In this study, the effect of decontamination on reduction of ambient dose equivalent outdoors and indoors was evaluated. The latter is essential for residents as most individuals spend a large portion of their time indoors. Materials and Methods: From December 2012 to November 2014, thirty-seven Japanese single-family detached wooden houses were investigated before and after decontamination in evacuation zones. Outdoor and indoor dose measurements (n = 84 and 114, respectively) were collected based on in situ measurements using the NaI (Tl) scintillation surveymeter. Results and Discussion: The outdoor ambient dose equivalents [$H^*(10)_{out}$] ranged from 0.61 to $3.71{\mu}Sv\;h^{-1}$ and from 0.23 to $1.32{\mu}Sv\;h^{-1}$ before and after decontamination, respectively. The indoor ambient dose equivalents [$H^*(10)_{in}$] ranged from 0.29 to $2.53{\mu}Sv\;h^{-1}$ and from 0.16 to $1.22{\mu}Sv\;h^{-1}$ before and after decontamination, respectively. The values of reduction efficiency (RE), defined as the ratio by which the radiation dose has been reduced via decontamination, were evaluated as $0.47{\pm}0.13$, $0.51{\pm}0.13$, and $0.58{\pm}0.08$ ($average{\pm}{\sigma}$) when $H^*(10)_{out}$ < $1.0{\mu}Sv\;h^{-1}$, $1.0{\mu}Sv\;h^{-1}$ < $H^*(10)_{out}$ < $2.0{\mu}Sv\;h^{-1}$, and $2.0{\mu}Sv\;h^{-1}$ < $H^*(10)_{out}$, respectively, indicating the values of RE increased as $H^*(10)_{out}$ increased. It was found that the values of RE were $0.53{\pm}0.12$ outdoors and $0.41{\pm}0.09$ indoors, respectively, indicating RE was larger outdoors than indoors. Conclusion: Indoor dose is essential as most individuals spend a large portion of their time indoors. The difference between outdoors and indoors should be considered carefully in order to estimate residents' exposure dose before their returning home.

#### References

1. Hosoda M, Tokonami S, Sorimachi A, Monzen S, Osanai M, Yamada M, Kashiwakura I, Akiba S. The time variation of dose rate artificially increased by the Fukushima nuclear crisis. Sci. Rep. 2011;1:87. https://doi.org/10.1038/srep00087
2. Akahane K, Yonai S, Fukuda S, Miyahara N, Yasuda H, Iwaoka K, Matsumoto M, Fukumura A, Akashi M. The Fukushima Nuclear Power Plant accident and exposures in the environment. Environmentalist 2012;32:136-143. https://doi.org/10.1007/s10669-011-9381-2
3. Minato S. Diagonal elements fitting technique to improve response matrixes for environmental gamma ray spectrum unfolding. Radioisotopes 2001;50:463-471. https://doi.org/10.3769/radioisotopes.50.10_463
4. Minato S. Distribution of terrestrial ${\gamma}$ ray dose rates in Japan. J. Geography 2006;115:87-95. https://doi.org/10.5026/jgeography.115.87
5. Iyogi T, Ueda S, Hisamatsu S, KondoK, Haruta H, Katagiri H, Kurabayashi M, Nakamura Y, Tsuji N. Environmental gammaray dose rate in Aomori prefecture, Japan. Health Phys. 2002;82: 521-526. https://doi.org/10.1097/00004032-200204000-00013
6. Yoshida-Ohuchi H, Hosoda M, Kanagami T, Uegaki M, Tashima H. Reduction factors for wooden houses due to external ${\gamma}$-radiation based on in situ measurements after the Fukushima nuclear accident. Sci. Rep. 2014;4:7541.
7. Yoshida-Ohuchi H, Saito J, Kanagami T, Kobayashi I, Hirasawa N. Survey report on personal dose equivalent and indoor and outdoor staying time for children in the southern Miyagi Prefecture after the Fukushima nuclear accident. Radioisotopes 2015;64: 319-333. https://doi.org/10.3769/radioisotopes.64.319

#### Cited by

1. Comparison of external doses between radio-contaminated areas and areas with high natural terrestrial background using the individual dosimeter ‘D-shuttle’ 75 months after the Fukushima Daiichi nuclear power plant accident vol.38, pp.1, 2018, https://doi.org/10.1088/1361-6498/aaa147