On the use of dated sediments to investigate historical nuclear discharges

Studsvik, a Swedish nuclear research facility, has been releasing aquatic radioactive discharges in the Baltic Sea, through the bay Tvären, since 1959. The permissible discharge levels are regulated by the Swedish Radiation Safety Authority (SSM) but only information about ⁶⁰Co, ¹³⁷Cs, ¹⁵²Eu, total alpha and beta activities were reported up to 2002. Since then, the reports cover most a more comprehensive set of radionuclides. In this context, the seabed can be utilized as a chronological archive to investigate historical Studsvik releases. To this end, 23 sediment cores covering the whole area of the bay were studied and 5 of them were dated using ²¹⁰Pb-dating methods. Since the discharges from Studsvik contain both plutonium and caesium, neither can be used to validate the ²¹⁰Pb-dating method. Instead, stable lead with maximum deposition, known to be dated to 1970, was used. Cobalt-60, ¹³⁷Cs, and ¹⁵²Eu depth distributions were studied from the dated sediment cores and compared with reported levels of aquatic discharges. The expected sediment ¹³⁷Cs-peak from the fallout from the Chornobyl accident was however smeared out, indicating an ongoing inflow of ¹³⁷Cs with the Baltic seawater. Our findings show that reported releases of aquatic discharges of ⁶⁰Co and ¹⁵²Eu are consistent with measured sediment activity distribution. The sediments from the deepest parts of Tvären, with intact chronology and with a high time resolution, are ideal for investigating historical nuclear discharges and can be a tool to investigate unreported radionuclide releases. Dated sediment can in this way be a tool for nuclear safeguards to evaluate past and present activities in nuclear facilities.