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Depth profile and inventory of fallout 237 Np in sediment cores from the Mikata Five Lakes and Nyu Bay located in the coastal area of the Sea of Japan have been studied together with those of Pu isotopes ( 238 Pu, 239 -24°P u and 241 Pu), 241 Am and 137 Cs to understand the sedimentation behaviors of these elements under different aquatic environments. Mikata Five Lakes has different water salinity levels in a narrow area: fresh water (Lake Mikata), brackish water (Lakes Kugushi, Suigetsu and Suga) and sea water (Lake Hiruga).The inventory of 239,240 Pu estimated from measured depth profiles ranged from 109 to 347 MBq/km 2 . The depth profiles of 237 Np and 241 Am in sediment cores were similar to that of 239,240p u Gn the basis of a comparison of the 237 Np/ 239 -240 Pu inventory ratio obtained in sediment cores with that in surface soils, Np is considered to be more soluble than Pu in an aquatic environment. The 241 Am depth profile observed can well be explained by the ingrowth from parent 241 Pu by assuming the USA nuclear tests during the late 1950's as a substantial source of this nuclide and the immobilization of Pu and Am in the sediment. The 239,240 Pu/ 137 Cs inventory increased from 0.027 to 0.38 with the increase of salinity of the water.

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Low-level 23, Np analysis/Alpha-ray spectrometry/Environmental soil and sediment Abstract An accurate and reliable analytical method has been developed for the measurement of low-level 23, Np in environmental soil and sediment by alpha-ray spectrometry, with emphasis on the purification from other interfering alpha-ray emitting nuclides, especially uranium isotopes. The method consists of leaching of neptunium by aqua regia, separation and purification by TOA (tri-n-octylamine) extraction, LaF, coprecipitation and two steps of anion exchange. 239 Np was used as a chemical yield tracer. The chemical yield of neptunium ranged from 50% to 84%. The minimum detectable activity of " 7 Np was about 0.1 mBq/sample at the counting time of 10 4 minutes.