Search

The March 2011 earthquake and tsunami resulted in significant damage to the Fukushima Daiichi Nuclear Power Plant (FDNPP) and the subsequent release of radionuclides into the ocean. Here, we investigated the spatial distribution of strontium-90 ( 90 Sr) and cesium-134/cesium-137 ( 134, 137 Cs) in surface seawater of the coastal region near the FDNPP. In the coastal region, 90 Sr activity was high, from 0.89 to 29.13 mBq L -1 , with detectable FDNPP site-derived 134 Cs. This indicated that release of 90 Sr from the power plant was ongoing even in May 2013, as was that of 134 Cs and 137 Cs. 90 Sr activities measured at open ocean sites corresponded to background derived from atmospheric nuclear weapons testing fallout. The FDNPP site-derived 90 Sr/ 137 Cs activity ratios in seawater were much higher than those in the direct discharge event in March 2011, in river input, and in seabed sediment; those ratios showed large variability, ranging from 0.16 to 0.64 despite a short sampling period. This FDNPP site-derived 90 Sr/ 137 Cs activity ratio suggests that these radionuclides were mainly derived from stagnant water in the reactor and turbine buildings of the FDNPP, while a different source with a low 90 Sr/ 137 Cs ratio could contribute to and produce the temporal variability of the 90 Sr/ 137 Cs ratio in coastal water. We estimated the release rate of 90 Sr from the power plant as 9.6 ± 6.1 GBq day -1 in May 2013 on the basis of the relationship between 90 Sr and 137 Cs activity ( 90 Sr/ 137 Cs = 0.66 ± 0.05) and 137 Cs release rate.