Potential Releases of ~(129)l, ~(236)U, and Pu Isotopes from the Fukushima Dai-ichi Nuclear Power Plants to the Ocean from 2013 to 2015

摘要

After the Fukushima Dai-ichi nuclear accident, many efforts were put into the determination of the presence of ~(137)Cs, ~(134)Cs, ~(131)I, and other gamma-emitting radionuclides in the ocean, but minor work was done regarding the monitoring of less volatile radionuclides, pure beta-ray emitters or simply radionuclides with very long half-lives. In this study we document the temporal evolution of ~(129)I, ~(236)U( and Pu isotopes (~(239)Pu and ~(240)Pu) in seawater sampled during four different cruises performed 2, 3, and 4 years after the accident, and we compare the results to ~(137)Cs collected at the same stations and depths. Our results show that concentrations of ~(129)I are systematically above the nuclear weapon test levels at stations located close to the FDNPP, with a maximum value of 790 × 10~7 at·kg~(-1), that exceeds all previously reported ~(129)I concentrations in the Pacific Ocean. Yet, the total amount of ~(129)I released after the accident in the time 2011-2015 was calculated from the ~(129)I/~(137)Cs ratio of the ongoing ~(137)Cs releases and estimated to be about 100 g (which adds to the 1 kg released during the accident in 2011). No clear evidence of Fukushima-derived ~(236)U and Pu isotopes has been found in this study, although further monitoring is encouraged to elucidate the origin of the highest ~(240)Pu/~(239)Pu atom ratio of 0.293 ± 0.028 we found close to FDNPP.