Tritium inventory and its temporal variation in Fukushima front sea area. Comparison between coastal and offshore tritium inventories and 1F treated water and operational target values for discharge per year

We estimate the inventory of tritium in two sea areas corresponding to coastal and offshore ones around Fukushima Daiichi Nuclear Power Plant (1F) on the basis of the measured seawater tritium concentrations monitored constantly from 2013 to Jan. 2021 by using the Voronoi tessellation scheme. The obtained results show that the offshore area inventory and its temporal variation amount correspond to approximately 1/5 and 1/50 of that of the treated water accumulated inside 1F, respectively. These results suggest that the amount of tritium included in seawater as the background is non-negligible in evaluating the environmental impact of releasing the accumulated treated water into the sea area. We also estimate the offshore area inventory before the 1F accident and find that it had exceeded the 1F stored inventory over 30 years from 1960s to 1980s, being approximately 4 times larger in the peak decade 1960s. This finding means that we had already experienced more contaminated situations over 30 years in the past compared with the conservative case emerged by just releasing whole the present 1F inventory. Here, it should also be emphasized that the past contamination situation was shared by the entire world. We further extend the estimation area to include the offshore area from Miyagi to Chiba Prefectures and find that the average area inventory is now comparable to half the present 1F inventory. Finally, we estimate the internal dose per year by ingesting fishes caught inside the area when the 1F inventory is added only inside the area and kept there for one year. The result indicates that it approximately corresponds to 1.0 x 10^-6 of the dose from natural radiation sources. From these estimation results, we find that all the tritium inventories stored inside 1F never contribute to significant dose increment even when it is instantly released into the area. (author)