Deposition and distribution of Chernobyl fallout fission products and actinides in a Russian soil profile

In this article the distribution of fission products and actinides in a soil profile from Novo Bobovicky in Russia, which was contaminated due to the Chernobyl nuclear power plant accident, is described. The ground deposition of long-lived fission products determined by γ-spectrometry was (recalculated to 26 April 1986) 1600 kBq 137Cs/m2, 900 kBq 134Cs/m2 and 60 kBq 125Sb/m2. Of these radionuclides 137Cs shows the dominating activity at the present time. After 6.5 years 90% of the Cs and Sb activity was contained in the upper 4 cm. A 239,240Pu ground deposition of 77.4±8.0 Bq/m2 was determined by α-spectrometry. The 238Pu/239,240Pu activity ratio of 0.30±0.03 and 241Pu/239,240Pu activity ratio of 115±14 (in 1986) measured in the soil profile, indicates that the analysed Pu originates mainly from the Chernobyl accident. The average 234U/238U activity ratio of 1.06±0.29 indicates that the uranium in this soil is dominated by naturally occurring uranium. The α- and β-autoradiography revealed that the activity is mainly present in particulate form. It could further be observed that the spots containing α- or β-activity originated from different particles. A comparison of α-autoradiography with the bulk Pu and Am activity showed that 92% of the α-activity was present as clearly detectable α-spots. The β-active particles, located by β-autoradiography were correlated with γ-spectrometric measurements and contained only 137Cs. These hot spots ranged from 0.02 to 0.15 Bq. It could be concluded that the vertical transport of 137Cs and fuel fragments occurs mainly by movement of particles through the soil. It could also be concluded that the fuel fragments found, in this soil were depleted in respect to Cs, Sb and Eu. Comparison of the analysed 238Pu/239,240Pu, 241Pu/239,240Pu and 241Am/239,240Pu ratios with the ratios calculated with ORIGEN-S code gave an estimate of the average burn-up of the fuel particles to be in the range of 11–12 GWd/tU. The results presented in this article are valid for this single soil profile and should not be generalised unless validated in a more rigorous study of a larger number of soil profiles.