Uncertainties in assessing radiation ecological risk for biocenosis of the regional radioactive waste storage

Modern methodology of regulatory actions for radiation effects on biota or ecosystems is based on the postulate of the threshold impact of radiation on natural populations. This postulate has been used in this paper to assess the risk and to characterize uncertainties in risk assessment for biota found in the ecosystem adjacent to the regional radioactive waste storage. The aim of this paper is to characterize uncertainties in assessing radiation ecological risk in the “space” of each module of the risk assessment technology. The test object is the ecosystem of a regional radioactive waste storage: soil, vegetation - great nettle ( Urtica dioica ), mollusks - bush snail ( Bradybaena fruticum ). The external 90Sr beta-radiation intensity from the own shell and from natural radionuclides in soil was calculated from Marinelli or Levinger formulae. The technology of ecological risk assessment is presented as five modules: 1) selection of the ecosystem-receptor of radiation effects; 2) determination of reference species of living organisms and their survival indices; 3) the critical load as an absorbed dose rate is calculated from the dependence " the absorbed 90Sr radiation dose rate - the coefficient of radioactive strontium accumulation in mollusk shells”; the critical dose rate is estimated as a median value of the obtained power functional dependence of accumulation coefficient on the exposure rate; 4) risk is assessed from a part of the ecosystem territory with increased mollusk loading; 5) uncertainties appeared at each stage of risk assessment are characterized. The values of critical loads on mollusk populations taken from other literature sources are used in uncertainty characterization. The R-Development software package has been used at all stages of risk assessment. Uncertainty of the first module of risk assessment results from the “assignment” of ecosystem boundaries for which the ecological risk is estimated. Uncertainties of the second module result from the choice of the reference species - mollusks (bush snail). These mollusks turned out to be most radiosensitive of the tested living organisms, however, the procedure of determining the critical load has some uncertainties. The risk value 90.1% is calculated from the obtained critical load 18.2 mGy/year. According to literature data, the critical load for all mollusks is 87.6 mGy/year and presupposes the risk load below the permissible value 5%. Hence, the greatest uncertainty appears at the stage of selecting reference species. It can be reduced by correct determination of the dose dependence for the most radiosensitive biological index. In this case the choice of one reference species of organisms forecasts the risk of existence for this population only but not the entire system.