The caesium-134 and caesium-137 radionuclides released into the atmosphere as a result of the Fukushima accident were dispersed over the entire Northern Hemisphere. To assess the risks associated with the exposure due to Fukushima fallout, a comprehensive radiological survey was performed in the Russian Far East. One of the objectives of the project was to determine the densities of ground contamination by 137 Cs and 134 Cs on Sakhalin and Kuril Islands that constitute the Sakhalin oblast, an administrative region of Russia. In 2011, soil samples were collected at grasslands on Sakhalin, Kunashir and Shikotan Islands and results of the 2011 survey were published earlier. In the present study, activities of 137 Cs and 134 Cs were measured in soil samples obtained on Kunashir, Iturup, Urup and Paramushir Islands in 2012. From the studies carried out in 2011-2012, it was estimated that the Fukushima-derived 134 Cs inventory at 37 undisturbed grassland sites in the Sakhalin oblast varied from 8 Bq m −2 to 345 Bq m −2 (as of 15 March 2011). For this date, the inventory of the 137 Cs radionuclide originated from the Fukushima NPP was assumed to be the same as that of the 134 Cs radionuclide. The southern Kuril Islands were the most contaminated due to Fukushima fallout. In 2011 and 2012, Fukushima-derived radiocaesium was detected only in the top 5 cm layer of soil at all sites, excluding one, where ~20% of the 134 Cs inventory was found at a depth of 5-10 cm. In the period September 2011-September 2012, the inventory of 134 Cs declined by ~26% at four plots selected for long-term observations. The decline in the 134 Cs inventory closely corresponded to the reduction (29%) of 134 Cs activity due to radioactive decay. Pre-accidental inventory of 137 Cs in the top 20 cm layer of soil ranged from 53 Bq m −2 to 3630 Bq m −2. The mean reference inventory of pre-accidental 137 Cs for 13 representative sites was amounted as 2600 Bq m −2. Hence, the Fukushima accident added relatively small quantities of radioactivity to the reference preaccidental inventory of 137 Cs in grassland soils in the Sakhalin region: about 3% (~80 Bq m −2) on the average and 15% (~350 Bq m −2) at the maximum. Such small additional radioactive contamination is absolutely safe from a radiological point of view.

Samples of vegetation (vascular plants)

1Российская академия наук, Москва, Россия 2 Федеральная служба по надзору в сфере защиты прав потребителей и благополучия человека, Москва, Россия 3 Санкт-Петербургский научно-исследовательский институт радиационной гигиены имени профессора П.В. Рамзаева, Санкт-Петербург, Россия Романович Иван КонстантиновичСанкт-Петербургский научно-исследовательский институт радиационной гигиены имени профессора П.В. Рамзаева.

Федеральная служба по надзору в сфере защиты прав потребителей и благополучия человека, Москва, Россия 3 Санкт-Петербургский научно-исследовательский институт радиационной гигиены имени профессора П.В. Рамзаева, Федеральная служба по надзору в сфере защиты прав потребителей и благополучия человека, Санкт-Петербург, Россия Ключевые слова: радиационно-гигиенический паспорт организации, радиационно-гигиенический паспорт территории, источники ионизирующего излучения, радиационный объект, дозы облучения, радиоактивное загрязнение, радиационное обследование, радиационный контроль, удельная активность.

Vertical distribution of natural and man-made radionuclides in the soil profile is a decisive parameter when calculating the dose rate of gamma radiation in the air above the ground and the effective dose of external human exposure. The main purpose of this work was to determine vertical distribution of 137Cs in soddy-podzolic sandy and sandy-loam soils in forests and grasslands in the south-western districts of the Bryansk region in the remote period after the Chernobyl accident. In 2015–2016, soil cores were sampled in 7 virgin meadows and 13 forested areas to a depth of 20 cm. The cores were cut into horizontal layers 2 cm thick. The 137Cs activity in the samples was determined using a semiconductor gamma spectrometer. The activity concentration in samples of dry soil (n = 200) ranged from 6.35 Bq/kg to 83300 Bq/kg with an average of 4550 Bq/kg. In the meadows in three cases, the maximum activity concentration was determined in the uppermost layer. With increasing depth, the activity concentration decreased and reached a minimum in the deepest layers. A difference between the surface layer and the deepest layer was two to three orders of magnitude. The three other meadows showed a relatively uniform distribution of 137Cs in the upper 4–6 cm, followed by a decrease in activity concentration with an increase in depth. In one meadow area, a peak of the 137Cs activity was found at a depth of 4–6 cm. In the forest, the most typical (in 10 cases) was the presence of a pronounced maximum activity concentration of 137Cs in the uppermost layer. The experimentally obtained values of the 137Cs inventory in the upper 20 cm of soil at the surveyed sites ranged from 42 to 1940 kBq/m2. The values of 137Cs inventory positively and statistically significantly correlated with officially established levels of 137Cs surface ground contamination for the territory of nearby settlements. Vertical migration of 137Cs in the soil in the surveyed areas was mainly limited to the upper 10 centimeters. The layer on average contained 94% of the total 137Cs inventory. Less than 1% of the total 137Cs inventory was found in the deepest soil layer sampled (18–20 cm). The obtained 137Сs activity distributions were used to calculate kerma rate in the air at a height of 1 m above the ground at the surveyed sites. The air kerma rate ranged from 52 to 2240 nGy/h (on average, 807 nGy/h). The caesium-137 deposit in the upper 6 cm of soil determined about 95% and 90% of the air kerma rate in the forests and in meadows, respectively. Radioactive caesium, which migrated into the soil to a depth of more than 10–12 cm, gave a negligible contribution (less than 1%) to the gamma-radiation dose rate in the air. In practical terms, it indicates that the depth of soil sampling equal to 20 cm is currently quite sufficient to estimate the dose rate of gamma radiation in the air in virgin grasslands and in forests.