Reassessment of ⁹⁰Sr, ¹³⁷Cs, and ¹³⁴Cs in the Coast off Japan Derived from the Fukushima Dai-ichi Nuclear Accident

The years following the Fukushima Dai-ichi nuclear power plant (FDNPP) accident, the distribution of ⁹⁰Sr in seawater in the coast off Japan has received limited attention. However, ⁹⁰Sr is a major contaminant in waters accumulated within the nuclear facility and in the storage tanks. Seawater samples collected off the FDNPP in September 2013 showed radioactive levels significantly higher than pre-Fukushima levels within 6 km off the FDNPP. These samples, with up to 8.9 ± 0.4 Bq·m^–3 for ⁹⁰Sr, 124 ± 3 Bq·m^–3 for ¹³⁷Cs, and 54 ± 1 Bq·m^–3 for ¹³⁴Cs, appear to be influenced by ongoing releases from the FDNPP, with a characteristic ¹³⁷Cs/⁹⁰Sr activity ratio of 3.5 ± 0.2. Beach surface water and groundwater collected in Sendai Bay had ¹³⁷Cs concentrations of up to 43 ± 1 Bq·m^–3, while ⁹⁰Sr was close to pre-Fukushima levels (1–2 Bq·m^–3). These samples appear to be influenced by freshwater inputs carrying a ¹³⁷Cs/⁹⁰Sr activity ratio closer to that of the FDNPP fallout deposited on land in the spring of 2011. Ongoing inputs of ⁹⁰Sr from FDNPP releases would be on the order of 2.3–8.5 GBq·d^–1 in September 2013, likely exceeding river inputs by 2–3 orders of magnitude. These results strongly suggest that a continuous surveillance of artificial radionuclides in the Pacific Ocean is still required

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 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 we 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 × 10⁷ 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