Accumulation of 63Ni, 106Ru and 125Sb in phytoplankton

Sammanfattning: Background: Nuclear Power Plants (NPPs) release small amounts of radionuclides which contribute with a low radiation dose to the environment as well as humans and animals. In this context, marine ecosystems are critical, since most NPPs in Sweden are located by the sea. The Concentration Factor (CF) is a model parame ter that is used to study the transport and uptake of these radionuclides. However, the published CF values for many elements range across several orders of magnitude. Aims: The aim of this project was to experimentally determine phytoplankton CF for some of the radionuclides released from various nuclear facilities in Sweden, in order to improve their uncertainties and to enable a more realistic dose assessment. The elements analyzed in this study were nickel, ruthenium and antimony. Materials and Methods: Seawater samples from two separate stations (Anholt E and Karlsödjupet) near NPP in Sweden, were collected by SMHI. Phytoplankton were then cultured in these seawater samples with addition of relevant radionuclides and nutrients. The cultures were filtered when the phytoplankton concentration reached around 1·106 cells/mL. A HPGe-detector was used to measure the activity of the filtrates and filters from 106Ru and 125Sb. Likewise, a liquid scintillation counter (LSC) was used to measure the activity of the samples containing 63Ni. Results and Discussion: The mean calculated phytoplankton dry weight were 33 ± 8 and 26 ± 4 pg for Anholt E and Karlsödjupet, respectively. The phytoplankton growth rates and dry weights in the different seawater samples were not significantly different. The mean calculated phytoplankton CF for the respective elements were as follows: 4000 and 3800 L/kg for Ni; 15000 and 20000 L/kg for Ru and 250 and 700 L/kg for Sb. The obtained values are in good agreement with published data. Conclusion: The phytoplankton CF for the elements studied were as follows: 4000 and 3800 L/kg for Ni; 15000 and 20000 L/kg for Ru and 250 and 700 L/kg for Sb. The CF obtained in this study, had a smaller variation compared to the literature data, thus the uncertainties in the CF have been improved considerably. These CF will therefore provide a more realistic dose assessment. Keywords: concentration factor (CF), phytoplankton, phaeodactylum tricornutum, uptake, radioactive releases, nickel, ruthenium, antimony, gamma spectroscopy, liquid scintillation counting.