Wednesday, March 29, 2017

Oceanic dispersion of Fukushima-derived radioactive cesium: a review

This paper focuses on the radioactive Cs in seawater and summarizes estimates of the total amount of released radioactive Cs from the FNPP site, spatio–temporal changes in the concentrations of 134Cs and 137Cs not only off the coast of Fukushima and adjacent prefectures, but also in the North Pacific, and adjacent seas such as Japan Sea, East China Sea, based on measurement results and simulation models published during 4 years since the FNPP accident.

14 decembre 2016

Oceanic dispersion of Fukushima-derived radioactive cesium: a review

Abstract

This review summarizes the more than 70 papers published during the 4 years since the Fukushima Dai-ichi nuclear power plant accident that occurred on 11 March 2011, and details the radioactive cesium dispersion pattern in the North Pacific and adjacent seas. The total amount of Fukushima-derived radioactive cesium released into the North Pacific via atmospheric deposition and direct release, spatial and temporal changes in the Pacific coast around the accident site, and the concentration levels of radioactive cesium around the Japanese Islands, not only the Pacific coast but also in adjacent seas, such as Japan Sea, East China Sea are summarized. Based on observational data mostly obtained during 2 years since the accident, and simulation results, oceanic dispersion of radioactive cesium in the entire area of the North Pacific is described. The Fukushima-derived radioactive cesium dispersed eastward as surface water and extended to the eastern side of the North Pacific in 2014, and was also observed via a southward intrusion to subsurface waters as Subtropical Mode Water and Central Mode Water. The radioactive cesium movement related to mode water is important in terms of the circulation of cesium into the ocean interior. Some new technologies and techniques concerning emergency monitoring of radioactivity in the ocean environment are also reported, the effectiveness of which has been demonstrated by use in relation to the Fukushima accident.

Introduction

On 11 March 2011, the Great East Japan Earthquake (Mw 9.0) occurred at the plate boundary off the coast of Tohoku, northeastern Japan. A huge tsunami was generated and caused 15 729 fatalities and 4539 missing in the Hokkaido, Tohoku and Kanto regions (The National Police Agency, as of 24 August 2011). Preliminary surveys reported tsunami waves with run-up heights exceeding 30 m (Mori et al., 2011). The tsunami also hit the Fukushima Dai-ichi Nuclear Power Plant (FNPP) sites located at 37˚25'N, 141˚02'E, and a loss of electric power at FNPP resulted in overheated reactors and hydrogen explosions. Radioactive materials were then released into the ocean through atmospheric fallout (such as aerosols and precipitation) and as direct releases (controlled releases related to safety issues at FNPP) as well as uncontrolled leaking of the heavily contaminated coolant water (Buesseler et al., 2011; Chino et al., 2011; Takemura et al., 2011). This accidental release of anthropogenic radionuclides (mostly iodine-131, cesium-134 and -137; 131I, 134Cs and 137Cs) resulted in severe elevations of these radionuclides in fisheries products in the coastal areas of Fukushima and adjacent prefectures (Buesseler, 2012; Yoshida and Kanda, 2012; Wada et al., 2013; Nakata and Sugisaki, 2015). Owing to its relatively long half-life (2.07 years for 134Cs and 30.07 years for 137Cs), the evaluation of these radioactive Cs isotopes in the marine environment is important for addressing risks to both marine ecosystems and public health through consumption of fisheries products. Generally, cesium is a conservative element and mostly occurs in the dissolved phase in the marine environment. The concentration of radioactive cesium in marine organisms is strongly affected by its concentration in the surrounding seawater. Actually, temporal changes in radioactive Cs concentrations of many pelagic fish species in the near coastal area off Fukushima and adjacent prefectures were associated with those in seawater after the FNPP accident (e.g., Wada et al., 2013; Takagi et al., 2015; Morita et al., unpublished data). Kaeriyama et al. (2015) and Morita et al. unpublished data revealed the time-lagged temporal changes in radioactive Cs in organisms (zooplankton and Pacific saury) and seawater under non-steady-state conditions after the FNPP accident, and showed that the concentration ratios in these organisms had been elevated when compared with those before the FNPP accident. With regard to zooplankton, Baumann et al. (2015) discussed the possible uptake of Fukushima-derived radioactive Cs from phytoplankton dominated suspended particles. As a consequence, radioactive Cs would be transferred to the higher trophic level not only via surrounding seawater but also by prey-predator interactions in the pelagic ecosystem. Shigenobu et al. (2014) reported the radioactive Cs concentrations of fat greenling (Hexagrammos otakii) caught off the coast of Fukushima Prefecture, and reported two outlier specimens caught in August 2012 and May 2013 which had ambiguously high 137Cs concentrations of more than 1000 Bq/kg-wet. Probability analysis indicated that the two outlier fat greenlings had migrated from the port of FNPP. In the port of FNPP, extremely high 137Cs concentrations were reported from Japanese rockfish (Sebastes cheni), brown hakeling (Physiculus maximowiczi) and fat greenling (H. otakii) caught during January and February 2013 (Fujimoto et al., 2015). The maximum concentration of 137Cs (129 kBq/kg-wet) was detected from fat greenlings. Wada et al. (2013) with the corrigendum (Wada et al., 2014) summarized the monitoring results of radioactive Cs concentrations in fisheries products from Fukushima Prefecture and revealed time-series trends. Clear trends include a slower decrease of radioactive Cs in demersal fish compared to pelagic fish as well as spatial heterogeneity; specimens sampled in the area south of FNPP tended to have higher concentrations of radioactive Cs than those caught in the area north of FNPP. Sohtome et al. (2014) reported the time-course trends in concentration of radioactive Cs in invertebrates in the coastal benthic food web near the FNPP. The difference in decreasing trends observed within the organisms and the concentrations of radioactive Cs in some of the sea urchins (Echinocardium cordatum and Glyptocidaris crenularis) were clearly affected by the contaminated sediments taken into their digestive tract.

This paper focuses on the radioactive Cs in seawater and summarizes estimates of the total amount of released radioactive Cs from the FNPP site, spatio–temporal changes in the concentrations of 134Cs and 137Cs not only off the coast of Fukushima and adjacent prefectures, but also in the North Pacific, and adjacent seas such as Japan Sea, East China Sea, based on measurement results and simulation models published during 4 years since the FNPP accident.

Total Amount of FNPP–Released Radioactive Cesium

Information on the total amount of the FNPP-released radioactive Cs into the North Pacific is critical information to enable effective monitoring and resource management. However, despite its importance, estimation of atmospheric deposition is complex due to lack of the observational data in the oceanic environment. The activity ratios of 134Cs/137Cs, decay corrected to March–April 2011, were reported to be almost 1.0 for the entire North Pacific (e.g., Buesseler et al., 2011, 2012; Kaeriyama et al., 2014). This ratio means an equivalent amount of 134Cs and 137Cs was released into the ocean. Under the limitation of data concerning not only the amount of radioactive Cs in aerosols but also on precipitation in the North Pacific, estimation of atmospheric deposition remains a source of considerable uncertainty (5–15 PBq of 134Cs and 137Cs; 1 PBq = 1015 Bq, Table 1). In contrast, the direct release of radioactive Cs (134Cs and 137Cs) into the ocean as uncontrolled leaking of the heavily contaminated coolant water is well estimated as approximating the value of 3.5 PBq, with the exception of Bailly du Bois et al. (2012) and Charette et al. (2013) (Table 1). Dietze and Kriest (2012) discussed the possible overestimates by Bailly du Bois et al. (2012) as a result of methodological issues. Charette et al. (2013) estimated the direct release inventory from the observational data of radioactive Cs with radium isotopes in May–June 2011, and no atmospheric deposition was assumed. Their estimates of direct releases may be included in the atmospheric deposition. Tsumune et al. (2012) clearly showed that direct releases started on 26 March 2011 using 131I/137Cs activity ratios, which varied much more before 26 March 2011 when the atmospheric deposition was the major source. The most recent estimations have revealed that 3–4 PBq of 134Cs and 137Cs were directly released into the ocean and 12–15 PBq of 134Cs and 137Cs were deposited on the surface seawater in the North Pacific (Aoyama et al., 2015a).

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Table 1. Estimated total inventory of 137Cs (PBq) in the North Pacific in 2011

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Figure 1.

Schematic view of current system: (a) in the North Pacific and (b) around the Japanese Islands. Solid lines indicate surface current and dashed lines indicate the movement of mode waters. FNPP: Fukushima Dai-ichi Nuclear Power Plant; STMW: Subtropical Mode Water; CMW: Central Mode Water. Based on Kumamoto et al. (2014); Oka et al. (2011, 2015); Talley (1993) and Yasuda (2003) [Colour figure can be viewed at wileyonlinelibrary.com].

Read more : http://onlinelibrary.wiley.com/doi/10.1111/fog.12177/full



Original Page: http://nuclear-news.net/2017/03/28/oceanic-dispersion-of-fukushima-derived-radioactive-cesium-a-review



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