Abstract
We estimate the monthly discharged inventory of 90Sr from the port of Fukushima Daiichi Nuclear Power Plant (1F) from Jun. 2013 to Mar. 2022 by the Voronoi tessellation method inside the port, following the monitoring of 90Sr seawater radioactivity concentration inside the port that started in Jun. 2013. From the estimation results, we find that closing the Seaside Impermeable Wall on Oct. 2015 was the most effective method in reducing the discharged inventory in the period. In addition, after its closure, we confirm that a major source of the discharged inventory is narrowed down to the drainage channels flowing inside the 1F site. As for the nonmonitoring period from Apr. 2011 to May 2013, using the technique proposed by the authors, i.e., the activity ratio of 90Sr to 137Cs of stagnant water measured at the beginning of the accident and the ratio of the discharged inventory for the period until the Seaside Impermeable Wall closure, in which both radionuclides 90Sr and 137Cs were measured, we estimate the monthly discharged inventory of 90Sr and report it from the initial month including the direct discharge accident (Apr. 2011) to the latest month (Mar. 2022) for 11 years in addition to that of 89Sr only for the initial three months. Moreover, we compare the temporal variation of 90Sr with those of 137Cs and tritium. The comparisons reveal that the discharged inventory of 90Sr is the most effectively reduced by closing the Seaside Impermeable Wall and its temporal variation is the most sensitively dependent on precipitation and seasonal changes compared with 137Cs and tritium. Since the riverine input of 90Sr into the sea is negligible compared with those of 137Cs and tritium, we can evaluate the impact of the discharged inventory of only 90Sr from the 1F port in the coastal and offshore area centered on 1F. The results reveal the rough monthly discharged inventory required to observe the visible enhancement of the sea radioactivity concentration from the background level in each area. Such an outcome is significant for considering the environmental impact on the planned future release of the treated water accumulated in the 1F site.
