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Sr (
Sr) from port of Fukushima Daiichi Nuclear Power Plant; Analysis of the temporal variation from the accident to March 2022 and evaluation of its impact on Fukushima coast and offshore areasMachida, Masahiko; Iwata, Ayako; Yamada, Susumu; Otosaka, Shigeyoshi*; Kobayashi, Takuya; Funasaka, Hideyuki*; Morita, Takami*
Nihon Genshiryoku Gakkai Wabun Rombunshi (Internet), 22(4), p.119 - 139, 2023/11
We estimate monthly discharged inventory of Sr from port of Fukushima Daiichi Nuclear Power Plant (1F) from Jun. 2013 to Mar. 2022 by using the Voronoi tessellation method inside the port, following the monitoring of Sr sea water radioactivity concentration inside the port. The results suggest that the closure of sea side impermeable wall is the most effective for the reduction of discharged one. In addition, the results roughly reveal the monthly discharged inventory required to observe visible enhancement of the sea radioactivity concentration from the background level in each area. Such outcome is significant for considering environmental impacts on the planned future releasing of the treated water accumulated in 1F site.
Machida, Masahiko; Iwata, Ayako; Yamada, Susumu; Otosaka, Shigeyoshi*; Kobayashi, Takuya; Funasaka, Hideyuki*; Morita, Takami*
Journal of Nuclear Science and Technology, 60(3), p.258 - 276, 2023/03
Times Cited Count：1 Percentile：31.61(Nuclear Science & Technology)We estimate monthly discharge inventory of tritium from the port of Fukushima Dai-ichi Nuclear Power Plant (1F) from Jun. 2013 to Mar. 2020 using the Voronoi tessellation scheme, following the tritium monitoring results inside the port started in Jun. 2013. As for the missing period from the first month of 1F accident, Apr. 2011 to May 2013, we calculate the tritium discharge inventory by utilizing the concentration ratio of tritium to 
Cs in stagnant contaminant water during the initial direct run-off period until Jun. 2011 and the discharge inventory correlation between tritium and Cs for the next-unknown continuous-discharge period from Jul. 2011 to May 2013. From all the estimated results over 9 years, we find that the monthly discharge inventory sharply dropped immediately after closing the seaside impermeable wall in Oct. 2015 and consequently coincided well with the sum of input inventories of drainage water and subdrain etc. purified water into the port. By comparing the above estimated results with those in the normal operation period before the accident, we point out that the discharge inventory from 1F port after the accident is not so large. Even the estimation value for the accident year 2011 is found to be comparable to the maximum of operating pressurized water reactors releasing relatively large inventories in the order. At the national level, the total domestic release inventory in Japan significantly decreased after the accident owing to the operational shutdown of most plants. Furthermore, the total Japanese discharge inventory including 1F are found to be minor compared with those of nuclear reprocessing plants and heavy-water reactors on a worldwide level. From the above results, 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 as well as those of other nuclear facilities.
Machida, Masahiko; Iwata, Ayako; Yamada, Susumu; Otosaka, Shigeyoshi*; Kobayashi, Takuya; Funasaka, Hideyuki*; Morita, Takami*
Nihon Genshiryoku Gakkai Wabun Rombunshi (Internet), 22(1), p.12 - 24, 2023/01
We estimate inventory of tritium in two sea areas corresponding to coastal and offshore ones around Fukushima Daiichi Nuclear Power Plant (1F) based on the measurement results of sea-water tritium concentration monitored constantly from 2013 to Jan. 2021 by using Voronoi tessellation scheme. The obtained results show that the offshore area inventory and its temporal variation amount correspond to approximately 1/5 and 1/40 of that of the treated-water accumulated inside 1F, respectively. These results just suggest that the presence of tritium already included in sea-water as the background is non negligible in evaluating the environmental impact by releasing the accumulated treated-water into the sea region. We also estimate the offshore area inventory before 1F accident and find that it had exceeded over 1F stored inventory over about 30 years from 1960s to 1980s with approximately 4 times larger in the peak decade, 1960s. This fact means that we had already experienced more contaminated situation over 30 years in the past compared to the conservative case appeared by just releasing whole the present 1F inventory. Here, it should be also emphasized that the past contaminated situation was shared by the entire world. We further extend the estimation region into a wider region including an offshore area from Miyagi to Chiba prefectures and find that the area average inventory is now comparable to a half of the present 1F one. Finally, we estimate internal dose per year via ingesting fishes caught inside the area when 1F inventory is just added inside the area and kept for a year. The result indicates that it approximately corresponds to 1.0
of the dose from natural radiation sources. From these estimation results, it is found that all the tritium inventory stored inside 1F never contribute to significant dose increment even when it is instantly released into the area.
Machida, Masahiko; Iwata, Ayako; Yamada, Susumu; Otosaka, Shigeyoshi*; Kobayashi, Takuya; Funasaka, Hideyuki*; Morita, Takami*
Nihon Genshiryoku Gakkai Wabun Rombunshi, 21(1), p.33 - 49, 2022/03
We estimate monthly discharge inventory of tritium from the port of Fukushima Daiichi Nuclear Power Plant (1F) from Jun 2013 to Mar 2020 by using the Voronoi tessellation scheme, following that the tritium monitoring inside the port has started since Jun 2013. As for the missing period from the initial month, Apr 2011 to May 2013, we calculate it by utilizing the concentration ratio of tritium to that of Cs in stagnant contaminant water during the initial direct discharged period to Jun 2011 and the discharge inventory correlation between tritium and Cs for the next-unknown continuously-discharged period up to May 2013. From the all- estimated results over 9 years, we find that the monthly discharge inventory sharply dropped just after closing the sea-side impermeable sea-wall in Oct. 2015 and subsequently coincided well with the sum of those of drainage and subdrain. By comparing the estimated results with those in the normal operation period before the accident, we point out that the discharge inventory from 1F port is not so large compared to those during the normal operation. Even the estimated one in year 2011 is found to be comparable to the maximum of operating pressurized water reactors discharging relatively large inventory in the order. In the nation level, the whole Japan domestic discharge inventory significantly decreased after the accident due to operation shutdown of most plants. Furthermore, 1F and even Japanese total discharge inventory are found to be entirely minor when comparing those of nuclear reprocessing plants and heavy-water reactors in world-wide level. From the above, we suggest that various scenarios can be openly discussed on the management in tritium stored inside 1F with help of the present estimated data and its comparison with the past discharge inventory.
H discharge from rivers; Comparison of discharge from the Fukushima Dai-ichi and inventory in seawater in the Fukushima coastal regionSakuma, Kazuyuki; Machida, Masahiko; Kurikami, Hiroshi; Iwata, Ayako; Yamada, Susumu; Iijima, Kazuki
Science of the Total Environment, 806(Part 3), p.151344_1 - 151344_8, 2022/02
Times Cited Count：5 Percentile：35.21(Environmental Sciences)Machida, Masahiko; Yamada, Susumu; Iwata, Ayako; Otosaka, Shigeyoshi; Kobayashi, Takuya; Watanabe, Masahisa; Funasaka, Hideyuki; Morita, Takami*
Journal of Nuclear Science and Technology, 57(8), p.939 - 950, 2020/08
Times Cited Count：9 Percentile：71.58(Nuclear Science & Technology)After direct discharges of highly contaminated water 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 by his estimation method. However, the estimation period was limited to up to September 2012. Therefore, this paper estimates the discharge inventory 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 Cs up to June 2018 by two methods, i.e., Kanda 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 1 F 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 magnitude. Using these results, we evaluate the impact of the discharge inventory from the 1F port into the coastal area and radiation dose upon fish ingestion.
Machida, Masahiko; Yamada, Susumu; Iwata, Ayako; Otosaka, Shigeyoshi; Kobayashi, Takuya; Watanabe, Masahisa; Funasaka, Hideyuki; Morita, Takami*
Nihon Genshiryoku Gakkai Wabun Rombunshi, 18(4), p.226 - 236, 2019/12
After direct discharges of highly-contaminated water from Unit 2 and 3 in Fukushima Daiichi Nuclear Power Plant (1F) in April to May 2011, Kanda suggested that relatively small run-off of radionuclides from 1F port into Fukushima coastal region has subsequently continued by using his estimation scheme. However, the estimation period was limited until September 2012, and there has been no report on the issue since the work. Therefore, this paper focuses on discharge inventory from 1F port until June 2018. In the missing period, the central government and Tokyo Electric Power Company Holdings have done continuous efforts to stop the discharge, and consequently sea water concentration inside 1F port has diminished gradually. We show monthly discharge inventory of Cs-137 until June 2018 by two schemes, i.e., Kanda's scheme partially improved by authors and more sophisticated one using Voronoi tessellation reflecting the increment of the number of monitoring points inside 1F port. The results show that the former always presents overestimated results compared to the latter but the ratio of former to latter is less than one order. Based on these results, we evaluate impact of discharge inventory from 1F port into the coastal area and radiation does via fish digestion.
Nunoshiba, Tatsuo*; Yamauchi, Ayako*; Iwata, Rika*; Sato, Katsuya; Ono, Yutaka; Narumi, Issey*
JAEA-Review 2014-050, JAEA Takasaki Annual Report 2013, P. 124, 2015/03
Machida, Masahiko; Iwata, Ayako; Yamada, Susumu
no journal, ,
no abstracts in English
Yamada, Susumu; Machida, Masahiko; Iwata, Ayako; Otosaka, Shigeyoshi*; Kobayashi, Takuya; Watanabe, Masahisa; Funasaka, Hideyuki*; Morita, Takami*
no journal, ,
no abstracts in English
Machida, Masahiko; Iwata, Ayako; Yamada, Susumu
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no abstracts in English
Machida, Masahiko; Iwata, Ayako; Yamada, Susumu
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no abstracts in English
Cs discharge from rivers to the ocean near the Fukushima Dai-ichi Nuclear Power Plant using a simple modelSakuma, Kazuyuki; Aoyama, Michio*; Nakanishi, Takahiro; Kurikami, Hiroshi; Machida, Masahiko; Yamada, Susumu; Iwata, Ayako
no journal, ,
MERCURY is one of the models developed to simply predict Cs discharge from rivers to the ocean for understanding of Cs migration from seawater and sediment to the ecosystem and for estimating Cs discharge under heavy rainfall immediately. It is composed of a tank model, relationships between water discharge and suspended solids load, and two-component exponential models for river water Cs concentration. Using the MERCURY, Cs discharge to the ocean from five rivers near the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) in 2018 and 2019 was estimated to be 0.23 and 0.81 TBq. Cs discharge in 2019 was larger than in 2018 due to the two huge typhoons in October 2019, Hagibis and Bualoi. Although the model has some limitations such as underestimating the Cs discharge during a heavy rainfall due to lack of modeling dependence of Cs concentration on sediment size, it can quickly evaluate the effect of Cs discharge from rivers to the coastal area near FDNPP.
Machida, Masahiko; Iwata, Ayako; Yamada, Susumu
no journal, ,
no abstracts in English
Machida, Masahiko; Iwata, Ayako; Yamada, Susumu
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no abstracts in English
Miyamura, Hiroko; Sato, Tomoki*; Numata, Yoshiaki*; Tobita, Yasuhiro*; Iwata, Ayako*; Machida, Masahiko
no journal, ,
We propose a prototype of a decommissioning support system using an XR device, considering the system is effective for realizing the safety for the decommissioning workers the efficiency of the work. In this paper, we report the outline of the new function as we have added it with the use of Avatar in cyberspace for evaluating the exposure dose of the decommissioning workers. This function allows to create the work plan which is taken into account the worker's exposure dose rate.
