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Transfer of elements related to the nuclear fuel cycle – Evaluation of linearity in boreal ecosystems

Public examination of a doctoral dissertation in the field of Radioecology
Doctoral candidate: MSc Tiina Tuovinen 
Date and venue: 16.12.2016, 14.00, MS301, Medistudia, Kuopio campus 
Language of the dissertation: English
Language of the public examination: Finnish

ABSTRACT

In radioecological models, the transfer of radionuclides into organisms is commonly described by the concentration ratio (CR), which assumes that the uptake into an organism is linear with respect to the concentration of the element in the medium. This study focused on evaluating this assumption of linearity by examining samples collected from the natural world and experimental meso- and microcosms. 

The transfer of essential (Mo, Ni, Zn) and non-essential elements (U, Pb) from soil into three understory species and two tree species was studied at two forest sites. The radionuclides of these elements are potentially important in the risk assessment of radioactive waste disposal. An evaluation of the CRs, as a function of the soil element concentration, revealed that CR was not constant but decreased with increasing soil concentrations, and thus transfer was described better with a non-linear equation than with the linear model.

The transfer of 137Cs from water to fish was studied in two lakes. The water-to-fish CR decreased with increasing 137Cs concentration in water, and the data fitted well with a non-linear equation, similar to that used for describing transfer of elements from soil to plants. There were other findings of potential importance for radioecological modeling; the 137Cs concentration was threefold higher in piscivores than in non-piscivores, but no differences were detected between three non-piscivorous species with different feeding habits.

The transfer of U, Co, Mo, Ni, Pb, Th and Zn into three plant species was studied also in experimental mesocosms. Non-linear transfer was observed in all of the studied elements and plant species, confirming the findings based on samples collected from a natural forest. The mesocosm experiments, and additional microcosm experiments, were also designed to study the transfer of soil elements into snails and earthworms. The data indicated that also transfer from soil or food to animals is non-linear for many of the elements studied. The uptake of U was nearly linear, indicating that different modeling approaches may be needed for individual elements. However, the results concerning the uptake of Pb did not support the previously proposed simplified hypothesis that transfer would be non-linear only for essential elements.

Finally, the impact of non-linear transfer on radioecological modelling was studied by comparing the linear model, a non-linear model derived from the observations of this study, and a novel model based on the observation that non-linear transfer leads to a practically constant total element concentration in plant tissues. Three models were used to predict transfer of 234U, 59Ni and 210Pb into spruce needles. The predictions of the non-linear model and the novel model were essentially the same, but the linear model underestimated the uptake of radionuclides when the total element concentration in soil was low as typically is the case in a boreal forest. Linear modeling could advantageously be replaced by the new modeling approach since this more realistically reflects the actual processes involved in the uptake of these elements into plants. The proposed new modeling approach could potentially reduce the uncertainty in model predictions. It is also simple to perform and the data needed to estimate the model parameters are readily available in the published literature.

The doctoral dissertation of Master of Science Tiina Tuovinen, entitled Transfer of elements related to the nuclear fuel cycle – Evaluation of linearity in boreal ecosystems will be examined at the Faculty of Science and Forestry. The opponent in the public examination will be Professor Jukka Lehto, University of Helsinki and the custos will be Professor Jukka Juutilainen, University of Eastern Finland.