A series of accidents at the Fukushima Dai-ichi Nuclear Power Plant (1F NPP) following the Great East Japan Earthquake and tsunami of 11 March 2011 resulted in the release of radioactive materials to the ocean. We used the Regional Ocean Model System (ROMS) to simulate the 137Cs activity in the oceanic area off Fukushima, with the sources of radioactivity being direct release, atmospheric deposition, river discharge, and inflow across the domain boundary. The direct release rate of 137Cs after the accident until the end of 2016 was estimated by comparing simulated results with measured 137Cs activities adjacent to the 1F NPP. River discharge rates of 137Cs were estimated by multiplying simulated river flow rates by the dissolved 137Cs activities, which were estimated by an empirical function. Inflow of 137Cs across the domain boundary was set according to the results of a North Pacific Ocean model. Because the spatiotemporal variability of 137Cs activity was large, the simulated results were compared with the annual averaged observed 137Cs activity distribution. Normalized annual averaged 137Cs activity distributions in the regional ocean were similar for each year from 2013 to 2016. This result suggests that the annual averaged distribution is predictable. Simulated 137Cs activity attributable to direct release was in good agreement with measurement data from the coastal zone adjacent to the 1F NPP. Comparison of the simulated results with measured activity in the offshore area indicated that the simulation slightly underestimated the activity attributable to inflow across the domain boundary. This result suggests that recirculation of subducted 137Cs to the surface layer was underestimated by the North Pacific model. During the study period, the effect of river discharge on oceanic 137Cs activity was small compared to the effect of directly released 137Cs.

中文翻译：

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直接释放和河流排放对福岛第一核电站事故造成的海洋137C的影响。

2011年3月11日东日本大地震和海啸后，福岛第一核电站（1F NPP）发生了一系列事故，导致向海洋释放了放射性物质。我们使用区域海洋模型系统（ROMS）来模拟福岛附近海洋区域中137Cs的活动，放射性的来源是直接释放，大气沉积，河流排放和跨域边界的流入。通过将模拟结果与与1F NPP相邻的137Cs活动进行比较，估算出事故发生后直至2016年底137Cs的直接释放率。通过将模拟河流流速乘以溶解的137Cs活动，可以估算出137Cs的河流排放速率，并通过经验函数对其进行估算。根据北太平洋模型的结果，确定了137Cs跨域边界的流入。由于137Cs活性的时空变化很大，因此将模拟结果与137Cs活性的年平均分布进行了比较。从2013年到2016年，区域海洋中标准化的137Cs的年均平均活动分布相似。这一结果表明年平均分布是可预测的。直接释放引起的137Cs模拟活动与1F NPP附近沿海地区的测量数据高度吻合。将模拟结果与海上地区测得的活动进行比较表明，该模拟略微低估了跨域边界流入的活动。该结果表明，北太平洋模型低估了被俯冲的137Cs再循环到表层的情况。在研究期间，与直接释放137Cs的影响相比，河流排放对海洋137Cs的影响很小。