Presentation Internal dose measurements of 131I for three persons staying at Tokai-mura during the arrival of radioactive plume released from the Fukushima Daiichi nuclear power station

Kurihara, Osamu  ,  Nakagawa, T.  ,  Takada, C.  ,  Tani, Kotaro  ,  Kim, E.  ,  T.Momose

The dose reconstruction of members of the public is an on-going important task in the accident at the Fukushima Daiichi nuclear power station in 2011. Limited data on human measurements in this accident make it difficult to estimate internal doses to the public in affected areas, especially due to the intake of short-lived radionuclides, one of which was 131I, the largest contributor to the internal thyroid dose. This paper demonstrates results of internal dose measurements for three employees of Japan Atomic Energy Agency (JAEA) who stayed at Tokai-mura in Ibaraki prefecture (located around 100 km south from the station) during the arrival of radioactive plume in this area. The three persons were measured with two types of whole-body counters and a NaI(Tl) probe to determine the contents of the radionuclides possibly incorporated via inhalation. As a result, thyroid equivalent doses to the three persons from 131I were estimated to be 1.0 – 1.5 mSv on the assumption on a single intake event on 15 March 2011. The effective intake ratios of 131I to 137Cs were calculated as 1.5 – 2.5 assuming the deposition of these radionuclides in the human respiratory tract for elemental iodine and particulate caesium aerosols with 1 m in Activity Median Aerodynamic Diameter (AMAD). A remarkable finding is that the effective intake ratio for the three persons was much lower than the accumulated air concentration ratio at the site of JAEA by the end of March 2011. The intake amounts of 131I and 137Cs via inhalation by adults based on the observed air concentrations were calculated as 18 kBq and 2.3 kBq, respectively; whereas those estimated from the direct measurements were 3 kBq and 1.5 kBq on average. The large difference between these two estimations for 131I is partly caused by the uptake factor of iodine to the thyroid in Japanese which is smaller than the corresponding parameter in the iodine biokinetic model for internal dose estimations; however, it is still difficult to interpret the results obtained from the three persons.
International Conference on Individual Monitoring of Ionising Radiation 2015(iM2015)

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