Abstract
Previous studies conducted by the Federal Service for Surveillance on Consumer Rights Protection and Human Well-Being (Rospotrebnadzor) in Sakhalin Region, Russia in 2011–2012 have shown that soils on the southern Kuril Islands were contaminated with 137Cs and 134Cs due to the accident at the Fukushima-1 NPP. In this study, we evaluated the air kerma rate from 137Cs and 134Cs using earlier published data on the vertical distribution of Fukushima-derived radiocaesium in the soil at grasslands on the islands of Kunashir, Shikotan and Iturup. The air kerma rate due to gamma-rays from pre-Fukushima 137Cs was estimated as well. In the period from May 2011 to September 2012, the air kerma rate due to the fresh fallout of radioactive caesium varied from 0.31 to 0.84 nGy h−1. On average, the contribution of the radiation from Fukushima-derived 134Cs and 137Cs to the total air kerma rate from radioactive cesium was 35% in the fall of 2011 and 25% in the fall of 2012. The mean normalized kerma rate from Fukushima-derived 137Cs was estimated at a level of 1.63 nGy h−1 per kBq m−2 in mid-May 2011, 1.53 nGy h−1 per kBq m−2 in autumn 2011 and 1.45 nGy h−1 per kBq m−2 in autumn 2012. The mean normalized air kerma rate from pre-Fukushima 137Cs at reference undisturbed grasslands was estimated as 0.77 nGy h−1 per kBq m−2. The results of our calculations of the air kerma rate from Fukushima-derived 137Cs were in good agreement with the forecast of the 2013 UNSCEAR model, which was used to estimate the external doses of the Japanese population after the Fukushima accident. For pre-Fukushima 137Cs, the UNSCEAR model underestimated the calculated normalized air kerma rates in Sakhalin Region by approximately 40%. Our calculations showed that, overall, the aboveground biomass of herbaceous plants had practically no effect on the air kerma rate from radioactive cesium contained in the soil. The fallout of 134Cs and 137Cs after the accident at the Fukushima-1 NPP did not lead to any significant increase in the gamma dose rate in the air at grasslands on Kuril Islands. In 2019–2020, the contribution of radiocaesium to the total gamma dose rate in air will not exceed 5%.
Highlights
Estimation of gamma-ray dose rate in air at an open undisturbed field is an essential component of a radiological survey after a nuclear accident resulting in a large-scale environmental contamination by gamma-ray emitting radionuclide(s) [1]
The value of gamma dose rate in air at such undisturbed lands is often used as a reference parameter in dosimetric models for assessment of the external exposure of the public [2,3,4,5]
During the major nuclear reactor accidents at the Chernobyl and Fukushima-1 NPPs, the release of 137Cs was accompanied by the release of 134Cs (T1/2 = 2.06 y; the range of the gamma-ray energy, E, = 242–1365 keV), which contributed significantly to the gamma dose in air and the external gamma radiation dose of the population living in contaminated areas [8, 9]
Summary
Estimation of gamma-ray dose rate in air at an open undisturbed field (grassland, meadow) is an essential component of a radiological survey after a nuclear accident resulting in a large-scale environmental contamination by gamma-ray emitting radionuclide(s) [1]. An infinite plane source located below a soil slab of the mass thickness of 0.5 g cm−2 [3, 11, 16] was selected as the initial distribution of the radionuclides in the ground in the case of wet fallout. For this source, the calculated normalized kerma rate in air is 1.72 and 4.68 nGy h−1 per kBq m−2 for 137Cs+137mBa and 134Cs, respectively (see Table 1 in [18]). The subsequent time-dependent decrease in the dose rate in air due to the vertical migration of the radionuclides in the soil column (excluding radioactive decay) can be presented by the attenuation function for air kerma rate, r(t), [3, 16]: Введение
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