Harmonisation of standards for permissible radionuclide activity concentrations in foodstuffs in the long term after the Chernobyl accident.

The article critically examines the practice of post-Chernobyl standardisation of radionuclide concentrations (mainly 137Cs and 90Sr) in food products (FPs) in the USSR and the successor countries of Belarus, Russia and Ukraine. Recommendations are given on potential harmonisation of these standards of radionuclide concentrations in FPs among the three countries, taking into account substantial international experience. We propose to reduce the number of product groups for standardisation purposes from the current amount of several dozens to three to five groups to optimise radiation control and increase the transparency of the process. We recommend five product groups for the standardisation of 137Cs and three groups for 90Sr in food in radiocontaminated areas. The values of standards for individual product groups are recommended to be set proportionally to the measured specific activity in each of these groups, which will reduce unreasonable food rejection. The standards might be set for the entire country, and could be also used to control imports from other countries as well as exports to other countries. The developed recommendations were transferred in 2015-2016 to the regulatory authorities of the three countries.

[Contamination of agricultural production with 90Sr in Ukraine at the late phase of the Chernobyl accident].

The specific activity of 90Sr in milk and vegetables for the last 15 years does not exceed the permissible level in Ukraine outside the Chernobyl exclusion zone. Exceeding the acceptable level of 90Sr is registered only in food grain. Specific activity of 90Sr in grain, which is produced in the high contaminated areas of the Kiev region, nowadays may reach 60-70 Bq/kg, which exceeds the permissible level of 20 Bq/kg for bread-grain. The part of 90Sr activity in the biologically available form has reached its maximum values for the post-accidental period due to the fuel particle dissolution. Contamination of grain with this radionuclide has slowly decreased in recent years. Values of concentration ratios and aggregated transfer factors of 90Sr from soil to rye, oat and winter wheat grain are inversely proportional to the exchangeable calcium content in soil. The transfer factors and dependences are in good accordance with those that have been obtained in our previous works and with generalized data of the IAEA for sandy soils. Application in Ukraine of such countermeasures as liming, fertilizing and manuring makes it possible nowadays to produce grain that meets the requirements of hygienic regulations on the 90Sr content in bread-grain.

[Prognosis of dynamics and risk of exceeding permissible levels of 137Cs and 90Sr contents in fish in the Kiev Reservoir at the late phase of the Chernobyl accident].

On the basis of the radionuclide specific activity measurements made on 832 samples of fish in 2009-2011 and taking into account literature data, the parameters of the stochastic model have been derived to describe the 137Cs and 90Sr contents in typical commercial fish species in the Kiev Reservoir at the late phase of the Chernobyl accident, including: statistical variability, seasonal changes and monotonous long-term trends. At any fixed moment of the year the standard deviations of logarithms of the 137Cs and 90Sr specific activities in carnivorous and benthophage fish species do not reliably differ, making up at average 0.4. The maximum vari- ation of the 137Cs specific activity (a four-fold decrease from April to November) was observed in pike. The obtained values of the ecological half-life periods for 137Cs and 90Sr (1.3-14 years) in fish of the Kiev reservoir in 2002-2012 were significantly lower than both the radioactive decay periods and the estimates of the IAEA Chernobyl Forum. Based on the obtained model parameters, the dynamics of the 137Cs and 90Sr specific ac- tivities in main commercial fish of the Kiev reservoir has been described and the risk of exceeding the permis- sible levels of these radionuclides in fish at the late phase of the Chernobyl accident has been estimated. Now the risk of catching fish with the specific activities of 137Cs and 90Sr above the permissible levels (150 Bq/kg and 35 Bq/kg, respectively) does not exceed 10% (except perch in the spring spawning period that is banned for fishing in Ukraine). Corresponding risks for roach, white bream and rudd are less than 0.1%.

Prussian Blue to reduce radiocaesium accumulation in fish in lakes affected by the Chornobyl accident.

Fish in lakes situated within the Chornobyl exclusion zone have been highly contaminated since the accident and have not been utilized as food for humans. In the present study, field experiments with less-contaminated silver Prussian carp (Carassius gibelio (Bloch, 1782)) caged in contaminated lake within the Chornobyl exclusion zone was performed from June to October 2021 to investigate the effectiveness of clean feed containing potassium ferric hexacyanoferrate (KFCF) KFe[Fe(CN)6], a kind of Prussian Blue as a countermeasure to reduce 137Cs accumulation in fish. The addition of clean feed containing 0.1% or 1% KFCF resulted respectively in 2.4 ± 0.4 or 4.2 ± 0.7 times lower activity concentration of 137Cs in muscle tissue of the carp compared to control fish with clean feed without KFCF and in 7-16 or 12-27 times lower activities compared to fish without additional clean feeding. After 18 weeks exposure, 137Cs levels in all the caged fish except for the control group were below the European permissible level (600 Bq kg-1) for consumption. In contrast, KFCF did not affect the intake of 90Sr in fish, although additional feed increased the growth rate and thereby accumulation in bone tissues. Hence, the use of clean feed containing KFCF is judged to be an effective and inexpensive countermeasure to reduce the 137Cs contamination of edible fish muscles.

Variability of activity concentrations and radial distributions of 137Cs and 90Sr in trunk wood of Scots pine and Silver birch.

This study analyzes the variability of 137Cs and 90Sr concentrations in wood and their radial distributions in the trunks of Scots pine and Silver birch trees in the small uniformly contaminated forest stands in the Chornobyl Exclusion Zone. Concentrations of both radionuclides follow a lognormal distribution with a large scatter of values measured in the trees within the stands (GSD ranges from 1.6 to 2.0). No correlation was found between the concentrations of the two radionuclides measured in individual trees, or between their concentrations and tree diameter. The average 137Cs and 90Sr Tag were 8.4 × 10-4 m2 kg-1 and 8.8 × 10-3 m2 kg-1 for pine, respectively, and 9.3 × 10-4 m2 kg-1 and 1.1 × 10-2 m2 kg-1 for birch, indicating a much higher availability of 90Sr for uptake by the studied species. For 137Cs, the Tag values are within the range recommended by the IAEA Handbook (IAEA, 2010), while the values for 90Sr exceed the recommended range for birch and are close to its upper value for pine. The highest concentrations of 137Cs in pine at the height of 1.3 m were measured in the youngest sapwood rings; they were lower in the rest of the sapwood and decreased further in the heartwood, but remained relatively high even in annual rings that were the heartwood at the time of deposition, suggesting sapwood-to-heartwood translocation of the radionuclide by diffusion and/or ray transport. In contrast, 90Sr concentrations increased through the sapwood from the trunk periphery in pine trees up to 80 years old and remained stable through the sapwood in older trees (except for higher concentrations in the young annual rings), but dropped to zero in physiologically inactive heartwood tissues. In most birch trees, regardless of age, 137Cs concentrations demonstrated an increasing trend from the trunk periphery towards the pith, while concentrations of 90Sr were relatively stable in the whole trunk except in the oldest annual rings, where they increased sharply, likely indicating active transport of the radionuclide to senescing tissues.

ReSCA: decision support tool for remediation planning after the Chernobyl accident.

Radioactive contamination of the environment following the Chernobyl accident still provide a substantial impact on the population of affected territories in Belarus, Russia, and Ukraine. Reduction of population exposure can be achieved by performing remediation activities in these areas. Resulting from the IAEA Technical Co-operation Projects with these countries, the program ReSCA (Remediation Strategies after the Chernobyl Accident) has been developed to provide assistance to decision makers and to facilitate a selection of an optimized remediation strategy in rural settlements. The paper provides in-depth description of the program, its algorithm, and structure.

[Dependences of 137Cs and 90Sr concentration ratios in fish on the potassium and calcium concentrations in the freshwater reservoirs].

Activities of 137Cs and 90Sr, concentrations of the potassium and calcium ions in water and accumulation of the radionuclides in the organisms of various freshwater fish have been measured in the stagnant and semistagnant water reservoirs of Ukraine contaminated as a result of the Chernobyl accident. On the basis of the numerous experimental data for different regions the statistical parameters were derived describing the dependencies of the 137Cs and 90Sr concentration ratios in the muscle tissue of various fish species on the potassium and calcium concentrations in water, respectively.

Optimising sampling strategies for emergency response: Vegetation sampling.

The lessons learned from the plant sampling campaigns implemented in the most Chernobyl affected countries are described. The variability of 137Cs activity concentrations in plants taken from a variety of sampling sites, as well as the uncertainties around the aggregated transfer factors (Tag) from soil to plants were estimated. The sampling sites covered both agricultural and natural lands in different landscapes: floodplain, plains, and watershed meadows. To determine parameters of the lognormal distribution of the 137Cs activity concentration in plants and the values of corresponding aggregated transfer factor (Tag) values, from 25 to 49 plant and soil samples were collected at each sampling site with the grid increment that varied from 1 to 10 m. The gradients of deposition i.e. monotonic changes (trends) of the contamination density conditioned by the global (in respect to study area) gradient of fallout were not observed in any of the study sites. Therefore, the variability of radionuclide contamination density (and activity concentrations in the soil) within the study sites were determined by only random factors such as microheterogeneity of radioactive deposition in a sampling point. The mean standard deviation of the logarithms of 137Cs activity concentrations in plants sampled in all such sites and the corresponding transfer factors were similar for all sites studied and were not dependent on the mean soil contamination density at the site, the type of radioactive fallout and the vegetation type. The values of the average standard deviation of the 137Cs activity concentration logarithms in plants and the corresponding transfer factors for the vegetation sampling area ≥1 m2 and the relative activity measurement uncertainties ≤10% were estimated as 0.4 ± 0.1 and 0.5 ± 0.1, respectively. A new simple method for optimization of the number of linked (conjugated) plant and soil samples as well as estimates of the activity concentration and transfer factor uncertainties when measuring composite samples were proposed. Based on the results of these studies, the recommendations were made to the sampling of plants for radionuclides.

Current radiological situation in areas of Ukraine contaminated by the Chornobyl accident: Part 2. Strontium-90 transfer to culinary grains and forest woods from soils of Ivankiv district.

Some of the highest 90Sr activity concentrations recorded beyond the Chornobyl Exclusion Zone occur in the Ivankiv district of Ukraine, located approximately 50 km south of the power plant, an area which nonetheless remains important for agricultural production. Although characterized by soils with low exchangeable calcium values, which can enhance the bioavailability of certain radionuclides, information on the transfer of 90Sr to food crops and trees in the region has remained limited to date. Analysis of 116 grain samples (wheat, rye, oat, barley or Triticale) collected from fields in 13 settlements in the region between 2011 and 2019 revealed 90Sr and 137Cs activity concentrations above Ukrainian limits in almost half of those samples, with annual averages exceeding this limit in four of those nine years (most recently in 2018) and with no clear evidence for a declining trend over time. Analysis of paired sandy soil samples from the same fields yielded concentration ratios for transfer of 90Sr from soils to grains that were on average 3 times higher than that specified by the IAEA. In addition, three quarters of wood samples collected from the trunks of trees (primarily pines) from 12 locations in the same district between 2015 and 2019 contained 90Sr activity concentrations in excess of the Ukrainian limits for firewood (60 Bq/kg), with levels more than four times that limit at one location and again no evidence for decline over time. A single sample of ash collected from a domestic wood-burning oven in the district contained 90Sr at a level 25 times higher than in the most contaminated wood sample collected in this study. Overall these results reveal additional facets of the ongoing legacy of Chornobyl contamination within the Ivankiv district, and the diversity of pathways by which local residents may be exposed to radionuclides. They also highlight the dangers associated with the current lack of routine and comprehensive environmental and food monitoring programs within the region, especially at a time in which the use of locally-sourced wood for biomass energy generation is set to expand markedly.

Monitoring in animal breeding in response to nuclear or radiological emergencies: Chernobyl experience.

The main techniques of animal product sampling used in different time periods after the Chernobyl accident are summarised and lessons learned from this analysis are presented. It was shown that simple instruments for measurement γ-radiation in the environment can also be effectively implemented for measurement of γ-emitters in animal products even though these were not originally developed to measure radioactivity in food. The lessons learned related to the major tasks of the monitoring such "what to sample", "where to sample" and "when to sample". The role and example of application of supplementary data on radionuclide of concern properties for sampling planning are also discussed. Based on the statistical analysis of the data obtained in the affected settlement it was shown that radionuclide concentration in the animal products can be fitted by the log-normal distributions whilst the dispersion of the logarithms of the activity 137Cs concentrations in milk is not dependent on the local settlement specific factors. Based on these findings the novel approach for justification of the number of samples that should to be taken to obtain the GM estimate with predefined precision for given variability of the data is suggested.