Pu Isotopic Composition Indicates Its Sources and Environmental Changes in the Vicinity of Daya Bay Nuclear Power Plant

The plutonium (Pu) isotopic compositions of surface sediments and two sediment cores from Daya Bay were measured by ICP–MS for the sake of investigating the sources of Pu and their historical evolution. The surface 240Pu/239Pu atomic ratios in Daya Bay are in the range of 0.247–0.287 (average = 0.264 ± 0.011, n = 14), which are higher than the characteristic value of global fallout (∼0.178) and indicate the input from non-global fallout. Correspondingly, the 239+240Pu activities were in the range of 0.029–0.386 Bq kg–1 (average = 0.197 ± 0.091 Bq kg–1, n = 14), and their spatial distribution shows relatively high Pu activity at the mouth and western part of Daya Bay, which is mainly attributed to finer particle sizes. The high 240Pu/239Pu atomic ratios were confirmed to be from the contribution to Pacific Proving Grounds (65% ± 7%) transported via the Kuroshio current. There was no additional Pu input released from the normal operation of Daya Bay nuclear power plants. Additionally, the Pu chronology of the sediment cores showed that changes in the sedimentary environment impact the Pu isotopic content. The Advection-Diffusion Equation model was used to further evaluate and predict the vertical migration of Pu isotopes in Daya Bay, suggesting that the diffusion of Pu over time may impact its usefulness as a time marker. Finally, we provide a baseline value of Pu isotopes in Daya Bay for environmental risk assessments in the future.