Determination of 240Pu/239Pu atom ratio in coastal surface seawaters from the western North Pacific Ocean and Japan Sea

Surface seawater samples were collected from a site in the vicinity of the nuclear fuel reprocessing facility at Rokkasho, Japan and sites along the Japan Sea coast. 239+240Pu activities and 240Pu/239Pu atom ratios were determined by α-spectrometry and isotope-dilution sector-field ICP-MS. The 240Pu/239Pu atom ratio with the mean value of 0.227±0.006 was significantly higher than the mean global fallout ratio of 0.18. The contribution of the Pacific Proving Grounds close-in fallout was estimated to be 33% of the 239+240Pu.