著者

町田 昌彦 岩田 亜矢子 山田 進 乙坂 重嘉 小林 卓也 船坂 英之 森田 貴己

出版者

一般社団法人 日本原子力学会

雑誌

日本原子力学会和文論文誌 (ISSN:13472879)

巻号頁・発行日

vol.21, no.1, pp.33-49, 2022 (Released:2022-02-15)

参考文献数

53

被引用文献数

4

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We estimate the monthly discharge inventory of tritium from the port of Fukushima Daiichi Nuclear Power Plant (1F) from Jun. 2013 to Mar. 2020 using the Voronoi tessellation scheme, following the tritium monitoring inside the port that started in Jun. 2013. As for the missing period from the initial month, Apr. 2011 to May 2013, we calculate the tritium discharge by utilizing the ratio of tritium concentration to 137Cs concentration in stagnant contaminant water during the initial direct run-off period to Jun. 2011 and the discharge inventory correlation between tritium and 137Cs for the next-unknown continuous-discharge period up to May 2013. From all the estimated results over 9 years, we found that the monthly discharge inventory sharply dropped immediately after closing the seaside impermeable wall in Oct. 2015 and subsequently coincided well with the sum of those of drainage and subdrain etc. By comparing the estimated results with those in the normal operation period before the accident, we point out that the discharge inventory from the 1F port after the accident is not very large. Even the estimation for the year 2011 is found to be comparable to the maximum of operating pressurized water reactors releasing relatively large inventories in the number of digits. In the national level, the total domestic release inventory in Japan significantly decreased after the accident owing to the operational shutdown of most plants. Furthermore, 1F and even the total Japanese discharge inventory are found to be minor compared with those of nuclear reprocessing plants and heavy-water reactors on a worldwide level. From the above, we suggest that various scenarios can be openly discussed regarding the management of tritium stored inside 1F with the help of the present estimated data and its comparison with the past discharge inventory.

著者

町田 昌彦 岩田 亜矢子 山田 進 乙坂 重嘉 小林 卓也 船坂 英之 森田 貴己

出版者

一般社団法人 日本原子力学会

雑誌

日本原子力学会和文論文誌 (ISSN:13472879)

巻号頁・発行日

pp.J20.036, (Released:2022-01-26)

参考文献数

53

被引用文献数

4

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We estimate the monthly discharge inventory of tritium from the port of Fukushima Daiichi Nuclear Power Plant (1F) from Jun. 2013 to Mar. 2020 using the Voronoi tessellation scheme, following the tritium monitoring inside the port that started in Jun. 2013. As for the missing period from the initial month, Apr. 2011 to May 2013, we calculate the tritium discharge by utilizing the ratio of tritium concentration to 137Cs concentration in stagnant contaminant water during the initial direct run-off period to Jun. 2011 and the discharge inventory correlation between tritium and 137Cs for the next-unknown continuous-discharge period up to May 2013. From all the estimated results over 9 years, we found that the monthly discharge inventory sharply dropped immediately after closing the seaside impermeable wall in Oct. 2015 and subsequently coincided well with the sum of those of drainage and subdrain etc. By comparing the estimated results with those in the normal operation period before the accident, we point out that the discharge inventory from the 1F port after the accident is not very large. Even the estimation for the year 2011 is found to be comparable to the maximum of operating pressurized water reactors releasing relatively large inventories in the number of digits. In the national level, the total domestic release inventory in Japan significantly decreased after the accident owing to the operational shutdown of most plants. Furthermore, 1F and even the total Japanese discharge inventory are found to be minor compared with those of nuclear reprocessing plants and heavy-water reactors on a worldwide level. From the above, we suggest that various scenarios can be openly discussed regarding the management of tritium stored inside 1F with the help of the present estimated data and its comparison with the past discharge inventory.

著者

町田 昌彦 岩田 亜矢子 山田 進 乙坂 重嘉 小林 卓也 船坂 英之 森田 貴己

出版者

一般社団法人 日本原子力学会

雑誌

日本原子力学会和文論文誌 (ISSN:13472879)

巻号頁・発行日

vol.22, no.1, pp.12-24, 2023 (Released:2023-01-18)

参考文献数

54

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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 × 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.

著者

町田 昌彦 山田 進 岩田 亜矢子 乙坂 重嘉 小林 卓也 渡辺 将久 船坂 英之 森田 貴己

出版者

一般社団法人 日本原子力学会

雑誌

日本原子力学会和文論文誌 (ISSN:13472879)

巻号頁・発行日

vol.18, no.4, pp.226-236, 2019 (Released:2019-11-20)

参考文献数

34

被引用文献数

9

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After direct discharges of highly contaminated water from Units 2 and 3 of the Fukushima Daiichi Nuclear Power Plant (1F) from April to May 2011, Kanda suggested that relatively small amounts of run-off of radionuclides from the 1F port into the Fukushima coastal region subsequently continued, on the basis of his estimation method. However, the estimation period was limited to up to September 2012, and there has been no report on the issue since that work. Therefore, this paper focuses on the discharge inventory from the 1F port up to June 2018. In the missing period, the Japanese government and Tokyo Electric Power Company Holdings have continued efforts to stop the discharge, and consequently, the radionuclide concentration in seawater inside the 1F port has gradually diminished. We show the monthly discharge inventory of 137Cs up to June 2018 by two methods, i.e., Kanda’s method partially improved by the authors and a more sophisticated method using Voronoi tessellation reflecting the increase in the number of monitoring points inside the 1F port. The results show that the former always yields overestimated results compared with the latter, but the ratio of the former to the latter is less than one order of magnitnde. Using these results, we evaluate the impact of the discharge inventory from the 1F port into the coastal area and the radiation dose upon fish digestion.