| Summary: | 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 137Cs and 134Cs 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 137Cs and 134Cs 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 134Cs 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 137Cs radionuclide originated from the Fukushima NPP was assumed to be the same as that of the 134Cs 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 134Cs inventory was found at a depth of 5–10 cm. In the period September 2011– September 2012, the inventory of 134Cs declined by ~26% at four plots selected for long-term observations. The decline in the 134Cs inventory closely corresponded to the reduction (29%) of 134Cs activity due to radioactive decay. Pre-accidental inventory of 137Cs 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 137Cs 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 137Cs 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.
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