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

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.