On the radiocesium distribution on the Romanian territory 30 years after the Chernobyl disasters

The Chernobyl 1986 accident, considered one of the worst of its kind, occupies the highest 7^th position on the seven levels IAEA International Nuclear and Radiological Event Scale. Following the thermal explosions which took parts, an impressive amount of new and spent nuclear fuel, rich in fission and neutron activation products was dispersed into the atmosphere at an altitude up to a few km, but without reaching the stratosphere. In this way, about 20 to 40 % of the total radiocesium inventory estimated to be 280 PBq of ¹³⁷Cs was transported by the atmospheric circulation contaminating significant areas of Ukraine, Belarus, Russia, Scandinavian countries, Central and Eastern Europe. Its presence was signaled also in Japan, Canada, and the United States.

Due to the geographical position of Romania in the vicinity of Ukraine, the total ¹³⁷Cs contamination of Romanian territory was estimated immediately after the Chernobyl accident at 51 ± 2 TBq, an estimation based on more detailed measurements performed during the 1993 y on 62 locations.

Under these circumstances, and for a more accurate estimation of the ¹³⁷Cs contamination, 747 soil samples covering the entire Romanian territory were collected, and the radiocesium inventory was measured by gamma-ray high-resolution spectroscopy performed in the low backgrounder laboratory located in the Slanic-Prahova former salt mine. Soil sampling and radiometric measurements were performed between 2016 and 2018 years, all data being recalculated for May 2016, i.e. 30 years after the Chernobyl accident.

The results showed for the 2016 radiocesium distribution an irregular pattern containing four maxima of which positions were quite different from the 1993 ones. Concerning the ¹³⁷Cs inventory, its total value decreased from 43 ± 2 TBq in May 1993 to 14.1± 0.7 TBq in May 2016, i.e. by a factor of 3 ± 0.3, twice of natural disintegration. This finding could be explained by taking into account that a significant amount of radiocesium was washed out by precipitation and, in a lower measure, was incorporated into plants.

The same data permitted evaluation of the total contribution of ¹³⁷Cs to the population exposure. Accordingly, in 1993 and even more so in 2016, the average supplementary annual effective dose did not exceed 1 mSv, i.e. the maximum annual effective dose considered not harmful for the unexposed population.