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Cs) on the coastal seafloor near the Fukushima Daiichi Nuclear Power Plant inferred from radiocesium distributions in long coresNakanishi, Takahiro; Tsuruta, Tadahiko; Misono, Toshiharu; Shiribiki, Takehiko; Urabe, Yoshimi*; Sanada, Yukihisa
Journal of Coastal Research, 116(SI), p.161 - 165, 2024/01
Continuous data of Cs-137 concentration in surface seabed sediment around the Fukushima Daiichi Nuclear Power Plant (FDNPP) from 2012 to 2022 was compiled and its temporal change was fitted by exponential function. In general, Cs-137 concentrations were gradually declining with time. However, at some monitoring points in shallow region, long half-lives and/or large deviations of Cs-137 concentrations were noticed. To gain insight into cesium dynamics in shallow seafloor, long sediment cores were collected at shallow region near the FDNPP, and the vertical profiles of Cs-137 concentration and particle size distribution were determined. At the beach, Cs-137 concentration and particle size distribution were very homogenized from several tens of cm to more than 1 m deep, probably due to strong vertical mixing by wind waves and tides. Therefore, beach sediments have significant Cs-137 reserves in deeper layers at present. It was suggested that the Cs-137 supply from deep layers as well as the land area might suppress the decline of surface Cs-137 concentration in this area. At sampling points located at the bases of cliffs and depressions at the offshore zone, the vertical distributions of Cs-137 concentration were highly heterogeneous, showing the particle size dependency. Moreover, Cs-137 profiles obtained at the same point for several years were quite different. Therefore, it might cause the large fluctuation of Cs-137 concentration at surface sediment over time.
Sanada, Yukihisa; Misono, Toshiharu; Shiribiki, Takehiko*
Kaiyo Riko Gakkai-Shi, 27(2), p.37 - 44, 2023/12
This paper summarizes the general situation of marine monitoring conducted after the Fukushima Daiichi Nuclear Power Plant accident, the experience of development and operation of USVs, and the possibility of applying unmanned vessels as a tool for nuclear disaster prevention in the future. 0.01 Bq/L or less for seawater and 10 Bq/L or less for seabed soil. Operational tests of three USVs have been continuously conducted for use in such environmental radiation monitoring. Development of these UAVs is underway with a view to utilizing them for seawater sampling, direct measurement of the seafloor soil surface layer, and seafloor soil sampling, depending on their performance. It is necessary to promote the development of USVs for future nuclear power plant accidents.
Misono, Toshiharu; Nakanishi, Takahiro; Sanada, Yukihisa; Shiribiki, Takehiko; Urabe, Yoshimi*; Tsuruta, Tadahiko
JAEA-Research 2022-010, 134 Pages, 2023/02
JAEA-Research-2022-010.pdf:8.45MB
An accident occurred at the TEPCO's Fukushima Daiichi Nuclear Power Station (1F) in 2011 and a large amount of radioactive materials were deposited around the 1F. Japan Atomic Energy Agency has continued to conduct research on the dynamics of radioactive materials after the accident. This report summarizes the results of the survey conducted in FY 2021 on the status of marine monitoring survey on radioactive substances. Furthermore, a seabed topography and sediments distribution survey was conducted in the coastal area off the Mano River from the Ohta River to understand the topography and sediment distribution. Furthermore, in order to evaluate the inflow of radioactive Cs from the river, the horizontal distribution of the radioactive Cs concentration on the surface sediment in front of the rivers was measured. As basic information on the effects of radioactive materials on marine products, the distribution status of fish was investigated. In addition, a demonstration test of water sampling and sediment sampling was conducted using an unmanned observation vessel. From these results, we estimated the distribution and its dynamics of radioactive Cs in the sediments in the front area on the 1F.
Misono, Toshiharu; Nakanishi, Takahiro; Shiribiki, Takehiko; Nagao, Seiya*; Ochiai, Shinya*; Sanada, Yukihisa; Tsuruta, Tadahiko
Gekkan Kaiyo, 54(11), p.558 - 561, 2022/11
Sediment traps were set up around estuaries and in coastal areas to collect sinking particles at the time of river discharge. Around the estuary, changes in radiocesium concentration and mass flux of sinking particles linked to the discharge of radiocesium from the river were observed.
Misono, Toshiharu; Nakanishi, Takahiro; Tsuruta, Tadahiko; Shiribiki, Takehiko; Sanada, Yukihisa
Marine Pollution Bulletin, 178, p.113597_1 - 113597_9, 2022/05
Times Cited Count：3 Percentile：59.35(Environmental Sciences)It is essential to evaluate secondary migration caused by riverine input and resuspension from seabed sediments to estimate the future distribution of radioactive cesium (Cs) in the coastal area off Fukushima Prefecture. In particular, the inflow from rivers cannot be ignored because most of the Cs inflow from rivers is deposited on the coast without elute into seawater. Two mooring systems were installed near the Ukedo River's mouth (Fukushima Prefecture) from February 2017 to February 2018. The first contained a sediment trap system, collecting sinking particles during the period. The second comprised a turbidity sensor and a current sensor. The contribution of resuspension and inflow from the river to the mass flux was quantitatively evaluated using multiple regression equations. The results showed that resuspension caused 79%-83% of secondary Cs migration in nearshore areas, whereas the influence of riverine Cs input on the sediment was only 7% per year.
Misono, Toshiharu; Nakanishi, Takahiro; Sanada, Yukihisa; Shiribiki, Takehiko; Urabe, Yoshimi*; Tsuruta, Tadahiko
JAEA-Research 2021-004, 214 Pages, 2021/11
JAEA-Research-2021-004.pdf:12.76MB
After the accident of TEPCO's Fukushima Daiichi Nuclear Power Station (1F), the project of marine monitoring survey on radioactive substances have been conducted by the contract research from the Nuclear Regulatory Agency in FY2020. Results obtain in the project are presented in this report. Based on the monitoring results of radioactive substances of seawater and sediments, we suggested the evaluation method for optimizing the survey points and frequency, and examined the proper monitoring method on marine monitoring. In addition, core samples were collected at 70 points at the coast of Fukushima Prefecture to reveal the accumulation of radioactive Cs in the sediments. Furthermore, in order to evaluate the inflow of radioactive Cs from the river, sediment traps were installed at the coastal area to collect sinking sediment, and the horizontal distribution of the radioactive Cs concentration on the surface sediment in front of the rivers was measured. We carried out the revaluation of the towed radiation monitoring data conducted from 2013 to 2018, taking into account the natural radionuclides, and improved the radioactive Cs distribution map in the coastal sediments. In addition, a seabed topography and sediments distribution survey was conducted in the silt band area off the 1F to understand the topography and sediment distribution. From these results, we estimated the distribution and its dynamics of radioactive Cs in the sediments in the front area on the 1F.
Sanada, Yukihisa; Urabe, Yoshimi*; Misono, Toshiharu; Shiribiki, Takehiko; Nakanishi, Takahiro; Watanabe, Yusuke; Tsuruta, Tadahiko
Scientific Reports (Internet), 11(1), p.23175_1 - 23175_13, 2021/11
Times Cited Count：2 Percentile：13.39(Multidisciplinary Sciences)After the accident at the Fukushima Daiichi Nuclear Power Station (FDNPP) on March 15, 2011, a large amount of volatile radionuclides were released into the atmosphere and hydrosphere. Monitoring of radioactive cesium in sediments is important for assessing the behavior and effects of radioactive cesium in the environment. In this study, the distribution of radioactive cesium in the superficial deposits around FDNPP was visualized as a radioactive cesium map using regular survey data from a towed gamma-ray detection system.
Misono, Toshiharu; Tsuruta, Tadahiko; Shiribiki, Takehiko; Nakanishi, Takahiro; Nagao, Seiya*; Ochiai, Shinya*; Sanada, Yukihisa
Journal of Coastal Research, 114(SI), p.315 - 319, 2021/10
Times Cited Count：2 Percentile：17.67(Environmental Sciences)For understanding future radioactive Cs dynamics and its distribution in the coastal area of Fukushima Prefecture, it is important to evaluate the supply of radioactive Cs from river discharge. Especially particulate radioactive Cs supplied in floods might have a significant impact on the coastal area. However, there are few studies on the dynamics of particulate radioactive Cs at the coastal area linked with rivers at the flood event. As a result of measuring the Cs concentration of the sinking particles collected by the sediment trap, at the observation point installed in front of the river, the Cs concentration collected immediately after the typhoon Bualoi (October 25th) was about higher than before the typhoon. However, the Cs concentration of the sinking particles collected two days after the typhoon was almost the same as before the typhoon. The impact of rivers on coastal areas during torrential rain stuck is extremely limited.
Tsuruta, Tadahiko; Shiribiki, Takehiko; Misono, Toshiharu; Nakanishi, Takahiro; Sanada, Yukihisa; Urabe, Yoshimi*
Journal of Coastal Research, 114(SI), p.320 - 324, 2021/10
Times Cited Count：0 Percentile：0.01(Environmental Sciences)no abstracts in English
Misono, Toshiharu; Tsuruta, Tadahiko; Nakanishi, Takahiro; Sanada, Yukihisa; Shiribiki, Takehiko; Miyamoto, Kenji*; Urabe, Yoshimi*
JAEA-Research 2020-008, 166 Pages, 2020/10
JAEA-Research-2020-008.pdf:13.11MBJAEA-Research-2020-008(errata).pdf:0.92MB
After the accident at TEPCO Fukushima Daiichi Nuclear Power Station (1F), marine monitoring survey on radioactive substances have been conducted with financially supported by the Nuclear Regulatory Agency from FY2019. Results obtain in the project in FY2019 are presented in this report. Based on scientific grounds, the concept necessary for "progress of sea area monitoring" was arranged for the future medium- to long-term investigation of radiocesium concentrations. As basic information of survey frequency revise, a seabed topography and sediment distribution survey was conducted, and an attempt was made to understand the relationship between the seabed topography and the grain size distribution of bottom sediment. A columnar core sample was collected in the coastal area and analyzed for radioactive cesium concentration. In order to understand the dynamics of radioactive cesium contained in suspended matter flowing in from a river, suspended solids was collected using a sediment trap and the concentration of radioactive cesium was measured. We re-analyzed the towed monitoring data that had been implemented since 2013, and tried to improve the accuracy of the radioactive cesium distribution estimation map in the coastal area.
Misono, Toshiharu; Nakanishi, Takahiro; Tsuruta, Tadahiko; Shiribiki, Takehiko; Nagao, Seiya*; Ochiai, Shinya*; Sanada, Yukihisa
no journal, ,
The tsunami caused an accident at the Fukushima Daiichi Nuclear Power Station, causing a large amount of radioactive cesium (RCs) to release and deposit in the surrounding environment. The load of RCs on the coastal area is considered to be resuspension from seafloor sediments and inflow to land through rivers. Quantification of land load is important for understanding the dynamics of RCs. In this study, the RCs concentration and mass flux of sinking particles were measured. We also measured the carbon/nitrogen ratio and stable carbon isotope ratio of sinking particles and attempted to evaluate the effects of land load.
Misono, Toshiharu; Tsuruta, Tadahiko; Nakanishi, Takahiro; Shiribiki, Takehiko; Sanada, Yukihisa
no journal, ,
Radioactive material was released from the Fukushima Daiichi Nuclear Power Station (1F) and deposited in the surrounding area. Radioactive cesium is thought to affect coastal areas via rivers. This tendency is remarkable when the river water level increase due to a typhoon. Therefore, it is important to evaluate the influence of rivers on the dynamics of radioactive cesium in coastal areas. In this study, we used sediment traps to collect sinking particles in coastal areas and investigated the dynamics of radioactive cesium.
Misono, Toshiharu; Funaki, Hironori; Nakanishi, Takahiro; Tsuruta, Tadahiko; Shiribiki, Takehiko; Sanada, Yukihisa
no journal, ,
Typhoon Hagibis (on 11-12 October 2019) and the subsequent typhoon Bualoi (on 24-25 October 2019) caused considerable flood damage in Fukushima region. The Ukedo River has a catchment with a high Cs inventory due to the FDNPP accident. Thus, there was concern about re-contamination of the seabed sediment in the coastal area due to the deposition of particulate Cs discharged from river. The results showed that the Cs concentration in the seabed sediment two years after the large-scale discharge was higher than that before the river flood. We plan to clarify the details of the mechanism through continuous monitoring.
Chaboche, P.-A.*; Wakiyama, Yoshifumi*; Takata, Hyoe*; Wada, Toshihiro*; Evrard, O.*; Misono, Toshiharu; Shiribiki, Takehiko; Funaki, Hironori
no journal, ,
Chaboche, P.-A.*; Wakiyama, Yoshifumi*; Takata, Hyoe*; Wada, Toshihiro*; Evrard, O.*; Misono, Toshiharu; Shiribiki, Takehiko; Funaki, Hironori
no journal, ,
Misono, Toshiharu; Funaki, Hironori; Shiribiki, Takehiko; Urabe, Yoshimi*; Sanada, Yukihisa
no journal, ,
no abstracts in English
Misono, Toshiharu; Nakanishi, Takahiro; Shiribiki, Takehiko; Funaki, Hironori
no journal, ,
no abstracts in English
Cs in a river-sea system boundary areaTakata, Hyoe*; Wada, Toshihiro*; Wakiyama, Yoshifumi*; Hirao, Shigekazu*; Sato, Shun*; Aono, Tatsuo*; Nakanishi, Takahiro; Misono, Toshiharu; Shiribiki, Takehiko
no journal, ,
Chaboche, P.-A.*; Wakiyama, Yoshifumi*; Takata, Hyoe*; Wada, Toshihiro*; Misono, Toshiharu; Shiribiki, Takehiko; Funaki, Hironori
no journal, ,
The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) incident, triggered by the Great East Japan Earthquake and subsequent tsunami in March 2011, resulted in the release of significant quantities of radionuclides into the terrestrial and marine environments of Fukushima Prefecture. While the activity of radiocesium (i.e., 
Cs and Cs) in these environments has decreased since the accident, continuous inputs through rivers draining and eroding the main terrestrial radioactive plume have been measured, maintaining elevated levels of Cs in riverine and coastal sediment deposited off the Prefecture's coast. Consequently, identifying the sources of sediment is required to elucidate the links between terrestrial and marine radiocesium dynamics and to anticipate the fate of persistent radionuclides in the environment.
