Takaki Tsubono

Distribution of oceanic 137Cs from the Fukushima Dai-ichi Nuclear Power Plant simulated numerically by a regional ocean model

Radioactive materials were released to the environment from the Fukushima Dai-ichi Nuclear Power Plant as a result of the reactor accident after the Tohoku earthquake and tsunami of 11 March 2011. The measured (137)Cs concentration in a seawater sample near the Fukushima Dai-ichi Nuclear Power Plant site reached 68 kBq L(-1) (6.8 × 10(4)Bq L(-1)) on 6 April. The two major likely pathways from the accident site to the ocean existed: direct release of high radioactive liquid wastes to the ocean and the deposition of airborne radioactivity to the ocean surface. By analysis of the (131)I/(137)Cs activity ratio, we determined that direct release from the site contributed more to the measured (137)Cs concentration than atmospheric deposition did. We then used a regional ocean model to simulate the (137)Cs concentrations resulting from the direct release to the ocean off Fukushima and found that from March 26 to the end of May the total amount of (137)Cs directly released was 3.5 ± 0.7 PBq ((3.5 ± 0.7) × 10(15)Bq). The simulated temporal change in (137)Cs concentrations near the Fukushima Daini Nuclear Power Plant site agreed well with observations. Our simulation results showed that (1) the released (137)Cs advected southward along the coast during the simulation period; (2) the eastward-flowing Kuroshio and its extension transported (137)C during May 2011; and (3) (137)Cs concentrations decreased to less than 10 BqL(-1) by the end of May 2011 in the whole simulation domain as a result of oceanic advection and diffusion. We compared the total amount and concentration of (137)Cs released from the Fukushima Dai-ichi reactors to the ocean with the (137)Cs released to the ocean by global fallout. Even though the measured (137)Cs concentration from the Fukushima accident was the highest recorded, the total released amount of (137)Cs was not very large. Therefore, the effect of (137)Cs released from the Fukushima Dai-ichi reactors on concentration in the whole North Pacific was smaller than that of past release events such as global fallout, and the amount of (137)Cs expected to reach other oceanic basins is negligible comparing with the past radioactive input.

Simulation of radioactive cesium transfer in the southern Fukushima coastal biota using a dynamic food chain transfer model.

The Fukushima Dai-ichi Nuclear Power Plant (1F NPP) accident occurred on 11 March 2011. The accident introduced (137)Cs into the coastal waters which was subsequently transferred to the local coastal biota thereby elevating the concentration of this radionuclide in coastal organisms. In this study, the radioactive cesium levels in coastal biota from the southern Fukushima area were simulated using a dynamic biological compartment model. The simulation derived the possible maximum radioactive cesium levels in organisms, indicating that the maximum (137)Cs concentrations in invertebrates, benthic fish and predator fish occurred during late April, late May and late July, respectively in the studied area where the source was mainly the direct leakage of (137)Cs effluent from the 1F NPP. The delay of a (137)Cs increase in fish was explained by the gradual food chain transfer of (137)Cs introduced to the ecosystem from the initial contamination of the seawater. The model also provided the degree of radionuclide depuration in organisms, and it demonstrated the latest start of the decontamination phase in benthic fish. The ecological half-lives, derived both from model simulation and observation, were 1-4 months in invertebrates, and 2-9 months in plankton feeding fish and coastal predator fish from the studied area. In contrast, it was not possible to similarly calculate these parameters in benthic fish because of an unidentified additional radionuclide source which was deduced from the biological compartment model. To adequately reconstruct the in-situ depuration of radiocesium in benthic fish in the natural ecosystem, a contamination source associated with the bottom sediments is necessary.

Status of 137Cs contamination in marine biota along the Pacific coast of eastern Japan derived from a dynamic biological model two years simulation following the Fukushima accident

Radiocesium ((134)Cs and (137)Cs) released into the Fukushima coastal environment was transferred to marine biota inhabiting the Pacific Ocean coastal waters of eastern Japan. Though the levels in most of the edible marine species decreased overtime, radiocesium concentrations in some fishes were still remained higher than the Japanese regulatory limit for seafood products. In this study, a dynamic food chain transfer model was applied to reconstruct (137)Cs levels in olive flounder by adopting the radiocesium concentrations in small demersal fish which constitute an important fraction of the diet of the olive flounder particularly inhabiting area near Fukushima. In addition, (137)Cs levels in slime flounder were also simulated using reported radiocesium concentrations in some prey organisms. The simulated results from Onahama on the southern border of the Fukushima coastline, and at Choshi the southernmost point where the contaminated water mass was transported by the Oyashio current, were assessed in order to identify what can be explained from present information, and what remains to be clarified three years after the Fukushima Dai-ichi nuclear power plant (1FNPP) accident. As a result, the observed (137)Cs concentrations in planktivorous fish and their predator fish could be explained by the theoretically-derived simulated levels. On the other hand, the slow (137)Cs depuration in slime flounder can be attributed to uptake from unknown sources for which the uptake fluxes were of a similar magnitude as the excretion fluxes. Since the reported (137)Cs concentrations in benthic invertebrates off Onahama were higher than the simulated values, radiocesium transfer from these benthic detritivorous invertebrates to slime flounder via ingestion was suggested as a cause for the observed slow depuration of (137)Cs in demersal fish off southern Fukushima. Furthermore, the slower depuration in the demersal fish likely required an additional source of (137)Cs, i.e. contaminated detritus or sediment which was entrained with the prey during the active sediment feeding of this fish species.

Distribution of radionuclides in surface seawater obtained by an aerial radiological survey

We investigated the distribution in seawater of anthropogenic radionuclides from the Fukushima Daiichi Nuclear Power Plant (FNPP1) as preliminary attempt using a rapid aerial radiological survey performed by the U.S. Department of Energy National Nuclear Security Administration on 18 April 2011. We found strong correlations between in-situ activities of 131I, 134Cs, and 137Cs measured in surface seawater samples and gamma-ray peak count rates determined by the aerial survey (correlation coefficients were 0.89 for 131I, 0.96 for134Cs, and 0.92 for137Cs). The offshore area of high radionuclide activity extended south and southeast from the FNPP1. The maximum activities of 131I, 134Cs, and 137Cs were 329, 650, and 599 Bq L−1, respectively. The 131I/137Cs ratio in surface water of the high-activity area ranged from 0.6 to 0.7. Considering the radioactive decay of 131I (half-life 8.02 d), we determined that the radionuclides in this area were directly released from FNPP1 to the ocean. We confirm that aerial radiological surveys can be effective for investigating the surface distribution of anthropogenic radionuclides in seawater. Our model reproduced the distribution pattern of radionuclides derived from the FNPP1, although results simulated by a regional ocean model were underestimated.

Radiocaesium derived from the TEPCO Fukushima accident in the North Pacific Ocean: Surface transport processes until 2017

We report temporal variations of 137Cs activity concentrations in surface waters of six regions of the western and central North Pacific Ocean during 2011-2017 using a combination of 1264 previously published data and 42 new data. In the western and central North Pacific Ocean at latitudes of 30-42°N and longitudes of 140°E to 160°W, eastward transport of radiocaesium was clearly apparent. 137Cs activity concentrations in surface water decreased rapidly to ∼2-3 Bq m-3 in 2015/2016, still a bit higher than 137Cs activity concentrations before the FNPP1 accident (1.5-2 Bq m-3). 134Cs/137Cs activity ratios decay-corrected to 11 March 2011 were ∼0.5-0.8. To the south of 30°N and between 130°E and 160°W in the western and central Pacific Ocean, 137Cs activity concentrations were around 1-7 Bq m-3 in 2011/2012 but then stabilized at a few Bq m-3 up to 2017.134Cs activity concentrations were detected at levels of 0.1-0.9 Bq m-3, and 134Cs/137Cs activity ratios decay-corrected to 11 March 2011 were ∼0.3-0.5. Temporal variations of model-simulated 137Cs activity concentrations in surface water in the region of interest showed good agreement with observations, except in the southwestern North Pacific Ocean.

Short-time observation of coastal currents with DBF radar

We have developed a DBF (Digital Beam Forming) radar which can complete one round observation of surface current every 15 minutes by a digital beam forming system. The predominant frequency is 41.9 MHz, range-directional resolution 500 m, maximum reach 25 km, azimuthal resolution 13-17/spl deg/ and velocity resolution 2.13 cm/s. We applied the DBF radar for current observation in the northern area of Ise Bay and compared the data with those collected synchronously by current meters set at 1 m and 2 m depth at two stationary points for eight days. The correlation coefficient between the DBF radar and the current meter at 1 m depth was 0.7-0.8 and the standard error 5-10 cm/s. In view of the fact that the measurement depth for the DBF radar was about 30 cm they seem to be coincided well. Next, we assimilated the continuous observation data of the DBF radar into a quasi three dimensional model of coastal currents by the nudging method in order to infer spatial and temporal changes of the currents. When the inferences of three-dimensional current were compared with the data gained by concurrently implemented ADCP spot observation at 4 measurement points, the current distribution for the surface-to-3 m depth layer was improved well as to speed and direction.

Estimation of the radiation dose equivalent for the hypothetical submergence of a sea-transport package of low-level radioactive waste

ABSTRACT Japanese Electric Power Utilities plans to transport low-level radioactive waste (LLW) in CSD-C (36 canisters) and CSD-B packages (10 canisters) by sea from France to Japan. In this study, we carried out assessments of the dose to the public from a hypothetical release of radioactive materials from submerged LLW packages into the sea. The estimated dose equivalents from the CSD-C and CSD-B packages were 2.8 × 10−8 and 7.9 × 10−9 mSv year−1, respectively, for the near shore case. For the deep sea case, the estimated dose equivalents were 8.6 × 10−8 and 5.8 × 10−8 mSv year−1, respectively. These estimated results were much smaller than those found in a previous study of Type-B packages (spent fuel, high level waste, and mixed oxide fuel) and the ICRP recommendation (1 mSv year−1).

Practicality of VHF-band ocean radar to surface current observation in open sea area

The fundamental characteristics of the high-resolution ocean radar system with the VHF band under severe winter conditions in the open sea area were evaluated through the field observations in Wakasa Bay. As the results, over 60 percentage of the detection ratio of the horizontal currents by the radar was confirmed in the extent of 15km as to the maximum observation range of 25km. The comparison between the current data of the radar and that of the current meter in one-month observation at the close point to Monjyu Power Station showed good correlation with a correlation coefficient over 0.7 and the standard error of 5 to 10cm/s. It was also indicated that the currents from surface to 2.5m below the surface were mainly driven by the wind fluctuation component of the 25.3 hours with the phase of 30 minutes. The ocean surface radar could detect the horizontal current patterns in the irregular current field under the sever weather and oceanic condition, which might largely contribute to comprehend unknown ocean characteristics in an open sea.

Consideration of impact factors on data acquisition for ocean radar with VHF band

An ocean radar with a VHF band is a useful tool for surveying coastal currents in a wide area. We have conducted a field observation of currents in Ariake Bay, the large inner sea located in the western part of Japan. Although the number of valid data acquired (hereafter, data acquisition) varies temporally and spatially, the mechanism of such variation is not yet well understood. Our aim in this study is to investigate the impact factor affecting data acquisition through field observations of current, salinity and wind in Ariake Bay. As a result, the data acquisition and the maximum measurement distance of the radar decreased owing to the decreases in salinity and wind speed. After receiving much rainfall, the data acquisition decreased rapidly in accordance with the decrease in surface salinity influenced by run-off from adjacent rivers and a flood control dike. Also, when wind speed became less than 2 m/s, the intensity of the signals received was not sufficient for the detection of current velocity. In conclusion, salinity and wind speed are important factors for steady-current observation using ocean radars.

Characteristics of Surface Currents around the Mouth of Isahaya Bay using DBF Ocean Radar

This study deals with the field observations on coastal surface currents by means of both Digital Beam Forming marine radar (DBF ocean radar) and Acoustic Doppler Current Profilers (ADCP) in order to realize characteristics of flow around the mouth of Isahaya Bay. In particular, the former has started since 2005. On the other hand, the latter has carried out seven times since 2001. As a result, it is found out that the feature of the M2 consituent flow detected by the DBF ocean radar was widely coincided well with other ADCP field observation data. Not only the river runoff in the head of Ariake Sound but also a flush flow from the sea-dike gate contribute to the surface currents around the mouth of Isahaya Bay significantly.