Transuranic nuclides Pu, Am and Cm isotopes, and 90 Sr in seafloor sediments off the Fukushima Daiichi Nuclear Power Plant during the period from 2012 to 2019.

The ²³⁸ Pu, ^239+240 Pu, ²⁴¹ Am, ²⁴² Cm, ^243+244 Cm and ⁹⁰ Sr concentrations in seafloor surface sediments collected at three sampling stations off the Fukushima Daiichi Nuclear Power Plant (FDNPP) site during the period from 2012 to 2019 were determined to elucidate the impacts of the FDNPP accident onto their concentrations in coastal sediments and to discuss the sources of the measured radionuclides. The ^239+240 Pu, ²⁴¹ Pu and ²⁴¹ Am concentrations and ²⁴⁰ Pu/ ²³⁹ Pu atom ratios in a sediment core were also determined to allow comparison of their inventories between this study and previously reported values and to identify the Pu sources. The ²³⁸ Pu, ^239+240 Pu, ²⁴¹ Am and ⁹⁰ Sr concentrations showed no remarkable temporal variations; no significant increases in concentrations after the FDNPP accident were observed; these concentrations were comfortably within the previously reported concentration range; and no detectable ²⁴² Cm and ^243+244 Cm amounts were observed in surface sediments. The observed ²³⁸ Pu/ ^239+240 Pu activity ratios were approximately two orders of magnitudes lower than those for the damaged FDNPP reactor core inventories and the observed values in terrestrial samples after the accident. The ^239+240 Pu, ²⁴¹ Pu and ²⁴¹ Am inventories in the sediment core were 389 ± 5, 503 ± 33 and 214 ± 3 Bq m ^-2 , respectively. The ^239+240 Pu inventory was about an order of magnitude greater than the expected cumulative deposition density of global fallout from atmospheric nuclear weapons testing due to an enhanced scavenging effect. The ²⁴⁰ Pu/ ²³⁹ Pu atom ratios in the sediment core ranged from 0.239 to 0.246 with a mean value of 0.242 ± 0.002; these ratios were clearly greater than the mean global fallout ratio of 0.18. The results for ²³⁸ Pu/ ^239+240 Pu activity ratios and ²⁴⁰ Pu/ ²³⁹ Pu atom ratios reflected a mixture of global fallout and Pacific Proving Grounds (PPG) close-in fallout Pu rather than Fukushima accident-derived Pu. The sediment column inventory for ^239+240 Pu originating from the PPG close-in fallout was calculated as 166 Bq m ^-2 , which corresponded to 43% of the total inventory. A significant amount of the PPG-derived Pu has been transported by ocean currents and then preferentially scavenged in the coastal waters of Japan.
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