After the Fukushima Dai-ichi Nuclear accident, many efforts were put into the determination of the presence of 137Cs, 134Cs, 131I 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 129I, 236U and Pu isotopes (239Pu and 240Pu) in seawater sampled during four different cruises performed 2, 3 and 4 years after the accident, and compare the results to 137Cs collected at the same stations and depths.
Our results show that concentrations of 129I are systematically above the nuclear weapon test levels at stations located close to the FDNPP, with a maximum value of 790 x107 at·kg-1, that exceeds all previously reported 129I concentrations in the Pacific Ocean.
Yet, the total amount of 129I released after the accident in the time 2011-2015 was calculated from the 129I/137Cs ratio of the ongoing 137Cs 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 236U and Pu-isotopes has been found in this study, although further monitoring is encouraged to elucidate the origin of the highest 240Pu/239Pu atom ratio of 0.293±0.028 we found close to FDNPP.