This study utilizes multi-isotopic fingerprints (²³⁶U/²³⁸U, ²³³U/²³⁶U, ²³⁶U/¹²⁹I, and ¹²⁹I/¹²⁷I) to identify an unknown source of anthropogenic radioactivity in the Baltic Sea. The data reveal an additional source of reactor ²³⁶U beyond known inputs from European reprocessing plants and global fallout. This extra ²³⁶U is potentially linked to unreported discharges from Swedish nuclear research facilities (like Studsvik) or accidental leakages from spent nuclear fuel disposed on the seafloor. The high sensitivity of the multi-isotopic tracer system, especially the ²³³U/²³⁶U signature, highlights its effectiveness in distinguishing unrevealed radioactive releases, aiding nuclear safeguards, emergency preparedness, and environmental tracer studies.

Jixin Qiao, Haitao Zhang, Peter Steier, Karin Hain, Xiaolin Hou, Vesa-Pekka Vartti, Gideon M. Henderson, Mats Eriksson, Ala Aldahan, Göran Possnert, Robin Golser