Polonium-210 in mussels and its implications for environmental alpha-autoradiography

Alpha-autoradiographic and radiochemical studies of the distributions of transuranic nuclides in the tissues and organs of mussels collected from the vicinity of the British Nuclear Fuels plc reprocessing plant at Sellafield, Cumbria, England, appeared to require assessment also of baseline alpha-activities of natural ²¹⁰ Po levels. Results for the latter were found to be in excess of the anthropogenic activities of Pu and Am isotopes. To ensure that the levels of ²¹⁰ Po in Cumbrian mussels were not artificially enhanced by local discharges and in the absence of published data, mussels from remote British and French coastal sites were also analyzed. General similarities in ²¹⁰Po concentrations (111 to 459 Bq kg⁻¹ dry) found in mussels soft parts suggest that the ²¹⁰Po levels in the Ravenglass mussels (279 Bq kg⁻¹) are natural and largely unsupported by ²¹⁰Pb; however these levels are as much as four times greater than the present day ^239+240Pu concentrations in the same samples. As the transuranic nuclide content of Cumbrian mussels produces a major component of the local critical group radiation exposure from the Sellafield discharges, this finding places in some perspective the significance of the baseline natural radionuclide concentrations in generating total exposure of the public. More specifically, these findings severely limit the usefulness of alpha-autoradiographic studies for transuranic nuclides performed on such samples. Because of the recently lower concentrations of alpha-emitting transuranic nuclides (mainly ²³⁸Pu, ^239+240Pu and ²⁴¹Am) in the Ravenglass environment, natural ²¹⁰Po is now a major contributor to alpha-track distributions in autoradiographic studies.