Radiocaesium in mosses from the Kopački rit Nature Park in Croatia: searching for undeclared releases from nuclear facilities in war-torn Ukraine

In our previous study (1) we sampled moss at forty sites across Kopački rit and its vicinity and obtained an in-depth view of the distribution or radiocaesium. For the work presented here, we reduced the number of sites to ten to get an overview of the more recent situation as quickly as possible. The sampling sites were either at Kopački rit or close to its borders (Figure 1). Since site accessibility changes with floods, road conditions, and so on, we could not take samples at the same sites as before.

Figure 1

The experimental procedures were essentially the same as described in our previous study (1). Briefly, in April and May 2023, we again sampled Anomodon viticulosus, which is the most abundant moss at Kopački rit. As before, the moss was thoroughly cleaned, dried to constant mass (hence our results refer to dry mass), and packed into 1 L Marinelli beakers. Activity concentrations (A) of the samples were determined by means of high-resolution gamma-ray spectrometry, using two high-purity coaxial detectors – an Ortec GMX and an Ortec GEM (Advanced Measurement Technology, Oak Ridge, TN, USA) with respective relative efficiencies of 74.3 % and 57 %, energy resolutions of 2.23 and 1.70 keV, and ⁶⁰Co 1.33 MeV both. ¹³⁷Cs and ¹³⁴Cs activity concentrations were studied at energies of 661.7 and 604.7 keV, respectively. Measurements were carried out 1–2 months after the sampling, and since the half-lives of both radionuclides are much longer, no correction for radioactive decay was required. However, the branching ratio of ¹³⁴Cs for gamma emission at the specified energy was corrected for true coincidence summing (2).

The distances between Kopački rit and the Ukrainian nuclear power plants (NPPs) are about 780 km for the Khmelnytsky NPP, 820 km for the Rivne NPP, 970 km for the South Ukraine NPP, and 1220 km for the Zaporizhzhia NPP. These are, in fact, small distances for the spread of airborne radionuclides via air masses, having in mind the spread of radioactive matter from the Chernobyl NPP (about 1040 km from Kopački rit) during the 1986 accident (3) or the fact that even the radioactive matter originating from the disaster in Fukushima was detected in Croatia (4). The Chernobyl radioactive spread was not the same in all directions (3), so eastern Croatia received less fallout than many other equally distant areas from the source of radioactive plume. This is evident from generally lower activity concentrations of ¹³⁷Cs in soil in this part of Croatia than elsewhere in the country (5). Our earlier finding (1) that the A of ¹³⁷Cs in moss samples from the Kopački rit was unusually low (0.7–13.1 Bq/kg) in 2018 is consistent with that, since the maximum value measured (and extrapolated to 2018) was 65 Bq/kg in Serbia (6), 400 Bq/kg in Greece (7), 300 Bq/kg in Armenia (8), 10,680 Bq/kg in Austria (9), and 360 Bq/kg in Turkey (10). In all these locations, the primary source of measured ¹³⁷Cs was the Chernobyl accident, and the distances between all these countries and Chernobyl are not significantly different.

There are several methods for the detection of airborne radionuclides, each having advantages and disadvantages. The most common is to measure the activity of a filter through which ground-level air has been pumped for some time (5). While this method is the most direct, its disadvantage is that pumping must be carried out while the radioactive matter concentration in the air is high enough, which may not last long (4). Another method is to measure radioactivity concentrations in surface soil, where airborne radionuclides eventually end up. Radionuclides remain in soil for a long time, but their concentrations stabilise only after a long deposition and diffusion. The third approach, which we took in this study, is to use bioindicator organisms which take up airborne radionuclides quickly and retain them for long (11,12,13). This method is particularly suitable for those radionuclides which have similar chemical properties as some biogenic elements, which is the case with caesium. Having the electronic structure similar to those of potassium and sodium, caesium has a potential for mimicking either of these two elements in a tissue.

The results of our measurements are shown in Table 1. We detected ¹³⁷Cs in all samples, the activity concentrations ranging from 1.2 to 5.3 Bq/kg. By comparing these values with the 0.7–13.1 Bq/kg range from our previous study (1), we conclude that that there has been no significant uptake of ¹³⁷Cs by mosses in Kopački rit between August 2018 and May 2023. This implies no contribution of additional ¹³⁷Cs to Kopački rit from any source, including the nuclear facilities in Ukraine. Regarding ¹³⁴Cs, Table 1 shows that activity concentration for each site (sample) is lower than the decision threshold. In other words, no ¹³⁴Cs was detected in Kopački rit in April and May 2023. These clearly suggests that the war in Ukraine has not caused a significant release of radioactive matter from any facility where the fission of ²³⁵U has been taking place.

Therefore, Kopački Rit remains the low radiocaesium background area, suitable for future airborne radionuclide pollutant monitoring.