We present an application of multi-isotopic fingerprints (i.e., ²³⁶U/²³⁸U, ²³³U/²³⁶U, ²³⁶U/¹²⁹I and ¹²⁹I/¹²⁷I) for the discovery of previously unrecognized sources of anthropogenic radioactivity. Our data indicate a source of reactor ²³⁶U in the Baltic Sea in addition to inputs from the two European reprocessing plants and global fallout. This additional reactor ²³⁶U may come from unreported discharges from Swedish nuclear research facilities as supported by high ²³⁶U levels in sediment nearby Studsvik, or from accidental leakages of spent nuclear fuel disposed on the Baltic seafloor, either reported or unreported. Such leakages would indicate problems with the radiological safety of seafloor disposal, and may be accompanied by releases of other radionuclides. The results demonstrate the high sensitivity of multi-isotopic tracer systems, especially the ²³³U/²³⁶U signature, to distinguish environmental emissions of unrevealed radioactive releases for nuclear safeguards, emergency preparedness and environmental tracer studies.

A simple method for direct measurement of uranium (238U) in seawater using triple quadrupole inductively coupled plasma mass spectrometry (ICP-MS) was established. The method provides a good analytical performance with respect to detection limit, accuracy, precision and sample throughput. During the method development and application, several interesting facts were observed: 1) Comparison results for reference material using different quantitation approaches indicate that isotope dilution (using 233U) is the most reliable to achieve accurate 238U measurement. The results obtained for direct 238U measurement in 50-fold diluted seawater samples (n = 112) also underline the difference between isotope dilution and internal (or external) standardization. 2) Appropriate dilution of seawater is important to minimize the matrix effect on the ICP-MS measurement and 20-50 dilution is recommended for natural seawater samples. 3) The sensitivity of ICP-MS was observed to increase in the beginning of sample measurement, and then decrease with the continuous injection of samples, which is believed as a consequence of matrix effect from the seawater to the ionization efficiency in the plasma. 4) When measuring samples taken from large volume of seawater stored in immovable containers for relatively long period (i.e., several months), the uranium concentration and salinity data showed slightly increasing trends with the increase of water depth in the container. Therefore, cautions need to be paid in sampling representativeness when performing 238U measurement for such long-term stored large volume samples.