Detection of an unknown emission source in the Baltic Sea using the new oceanographic tracer U-233/U-236

By analysing the two long-lived anthropogenic Uranium (U) isotopes U-233 and U-236 in different compartments of the environment affected by releases of nuclear power production or by global fallout from nuclear weapons tests, we showed that the corresponding isotopic ratios U-233/U-236 differ by one order of magnitude. Based on these experimental results which were obtained with the ultra-sensitive detection method Accelerator Mass Spectrometry, we suggested a representative ratio for nuclear weapons fallout of U-233/U-236 = (1.40 ± 0.15) ·10^-2 and (0.12 ± 0.01) ·10^-2 for releases from nuclear power production. Consequently, the U-233/U-236 ratio not only has the potential to become a novel sensitive fingerprint for releases from nuclear industry, but could also serve as a powerful oceanographic tracer due to the conservative behaviour of U in ocean water which does not suffer from chemical fractionation.

As a first application of this paired tracer, we studied the distribution of U-233 and U-236 concentrations in addition to I-129 in the Baltic Sea which is known to have received inputs of radionuclides from various contamination sources including the two European reprocessing plants, global fallout from weapons testings and fallout from the Chernobyl accident. Our data indicate an additional unidentified source of reactor U-236 in the Baltic Sea demonstrating the high sensitivity of the U-233/U-236 ratio to distinguish different emission sources in water mixing processes.