129I level in seawater near a nuclear power plant determined by accelerator mass spectrometer

129I concentration in the seawater samples near a nuclear power plant was determined in the Xiʹan Accelerator Mass Spectrometer (AMS) Center. Isotope dilution method was used via addition of excessive amount of stable iodine (127I) in the sample before separation, and iodine in the seawater was separated by solvent extraction, and the back extracted iodine in iodide form was precipitated as AgI, which was used as AMS target for 129I measurement. 125I tracer was added to monitor the recovery of iodine in the whole separation process. 129I/127I ratios in the prepared target were determined by AMS. The concentration of 127I in seawater samples was determined by inductively coupled plasma mass spectrometry. The results show that the 129I/127I atomic ratios in the seawater range from 8.29×10−11 to 9.45×10−10, approximately one order of magnitude higher than that in seaweed collected in the pre-nuclear era, but fall in the environmental level of global fallout.