Relationship between stable cesium concentration and body size of Japanese flounder Paralichthys olivaceus and the effect of a size-dependent shift in diet.

To understand the relationship between the radioactive cesium (Cs) concentration in muscle of Japanese flounder Paralichthys olivaceus and the species' biological characteristics (size, sex, and age) under conditions of ecological equilibrium (i.e., distributed among ecosystem components over sufficient time, and with nearly constant ratios of Cs concentration in organisms to the concentration in water) as existed before the accident at the Fukushima Dai-ichi Nuclear Power Station (FDNPS), Japan, in 2011, we examined stable Cs, as it is thought to exist in equilibrium in the environment and behave similarly to radioactive Cs in aquatic animals. The concentration of stable Cs in 241 P. olivaceus (range 216-782 mm total length [TL]) collected in Sendai Bay, approximately 90 km north of the FDNPS, in June-July 2015 was expressed as an exponential function with size as an independent variable; the results show the concentration of stable Cs doubled with an increase in TL of 442 mm. Next, to evaluate the cause of the size-dependent change in stable Cs concentration, we examined 909 individuals (200-770 mm TL) collected in September 2013-July 2015 to determine their feeding habit based on size. Analysis of the frequency of occurrence of prey organisms in stomach contents showed that sand lance Ammodytes japonicus (55-180 mm standard length [SL]) was the most consistently consumed across size classes. Analysis on a wet-mass basis showed that A. japonicus and anchovy Engraulis japonicus (65-130 mm SL) were the main food of P. olivaceus sized 200-599 mm TL, whereas chub mackerel Scomber japonicus (120-230 mm SL) and two species of flatfishes (180-205 mm SL) were abundant in the diet of P. olivaceus sized ≥600 mm TL. All these prey items were presumed to have similar concentrations of stable Cs. Based on the above, the effect of diet on the relationship between stable Cs in muscle and fish size was considered negligible. That the diet of P. olivaceus largely did not change with size was also confirmed by C and N stable isotope ratios in P. olivaceus and their prey species. Therefore, the Cs-size relationship is probably determined by changes in the balance between the rate of Cs intake from food and seawater and the excretion rate during growth, both of which change as functions of body mass. Values of stable Cs concentrations among environmental components and animals appear to be a valid indicator for understanding the radioactive Cs distribution in the marine environment and aquatic animals under the equilibrium state, as existed before the 2011 nuclear accident.

Immobilization of 137Cs as a crystalline pollucite surrounded by amorphous aluminosilicate.

To date, radiocesium (137Cs) has been considered stable in the form of pollucite mineralized through high-temperature heat treatment. This study presented a possibility through experimental results that the entire medium exists as amorphous aluminosilicate at a relatively low temperature, but cesium is partially and preferentially converted from a composite adsorbent into pollucite. Cesium lowers the eutectic point within the system and initiates the nucleation of pollucite prior to other elements. We confirmed that the partial mineral phase of cesium showed the same chemical stability as when the entire medium was converted to pollucite. X-ray absorption spectroscopy provided direct evidence for this phenomenon; also, the stability results of radioactive cesium shown through a series of sintering experiments supported the conclusion. This method can be applied as a method to immobilize radioactive cesium under relatively mild temperature conditions of atmospheric pressure, while eliminating the problem of diffusion due to its volatilization.

Effect of radioactive cesium-rich microparticles on radioactive cesium concentration and distribution coefficient in rivers flowing through the watersheds with different contaminated condition in Fukushima.

Radioactive cesium-rich microparticles (CsMPs) derived from the Fukushima Daiichi Nnuclear Power Plant accident were detected from soils and river water around Fukushima Prefecture, Japan. Because CsMPs are insoluble and rich in radioactive cesium (RCs), they may cause the overestimation of solid-water distribution coefficient (Kd) for RCs in the water. Previous studies showed the proportion of RCs derived from CsMPs on RCs concentration in soils collected from areas with different contaminated levels. Because the proportion of RCs concentration derived CsMPs to the RCs concentration of soils in the less contaminated areas is higher than that in the highly contaminated areas, the effect of CsMPs on particulate RCs concentration in river water may be larger in the less contaminated areas. However, the difference in the effects of CsMPs on the particulate RCs concentration and Kd in river water flowing through watersheds with different contaminated levels has not been clarified. In this study, we investigated the effect of CsMPs on the particulate RCs concentration and Kd in two rivers, Takase River and Kami-Oguni River, flowing through the watersheds with different RCs contaminated levels in Fukushima Prefecture. CsMPs might enter rivers due to soil erosion because they were detected only in some samples collected from both rivers during flood events. CsMPs accounted for more than half of particulate RCs concentration in some water samples collected in the flood condition. In particular, the proportion of CsMPs in particulate RCs for the Kami-Oguni River was greater than that for the Takase River. However, when evaluating for the entire water sampling in the flood condition, a proportion of RCs concentration derived from CsMPs in the average RCs concentrations per unit mass of SS in both river waters collected in the flood condition was not large. CsMPs might temporarily increase the particulate RCs concentration and Kd in the flood event, but CsMPs did not significantly affect them when evaluated throughout the event.

Vertical migration of cesium in weathered granite soil under flowing water condition depending on Cs concentration and states of dissolved organic matter.

After the accident at the Fukushima Daiichi nuclear power plant in Japan, the migration of radioactive cesium (Cs) in soils has become a crucial issue since this can negatively affect human health and the surrounding environment. Dissolved organic matter (DOM) may have different influences on Cs migration in soils depending on Cs adsorption sites with different selectivity. It is unclear how DOM affects the rapid migration of Cs in soils under flowing water conditions during rainfall events. This study evaluated the effects of DOM on Cs migration in weathered granite soil depending on Cs adsorption sites by conducting laboratory experiments under different DOM conditions and Cs concentrations in the liquid phase. Cs concentration can affect the fraction of Cs adsorbed onto differently selective sites, and DOM can have different influences on Cs migration in the soil accordingly. Under condition of high-Cs concentration, the DOM adsorbed on the soil reduced Cs migration due to increasing Cs electrostatic adsorption to less selective sites in the soil. Meanwhile, under low-Cs concentration, the DOM adsorbed on the soil enhanced Cs migration because the DOM on the soil decreased the Cs adsorption to highly selective sites. Furthermore, DOM in the liquid phase detached the Cs adsorbed on the less selective sites and enhanced Cs migration in the soil, regardless of the Cs concentration.

A Highly Efficient Biomass Compound Aerosol Suppressant in Purifying Radioactive Cesium Droplet Aerosols.

Nuclear accidents and decommissioning in the nuclear industry would release a large number of radioactive aerosols which endangers the natural environment and the health of workers. Therefore, there is an urgent need for environment-friendly aerosol suppressants to control and handle environmental pollution problems caused by radioactive aerosols. In this paper, sodium alginate (SA), a type of polyphenol material (TP), and alkyl glycosides (APGs) were selected as the components of the compound aerosol suppressant and the optimal proportion was generated via the method of D-optimal mixture design. Furthermore, the cesium aerosol sedimentation effect of the optimized compound aerosol suppressants was evaluated via sedimentation efficiency, the change in particle concentration cumulative concentration fraction of the cesium aerosol sedimentation process. The results showed that the aerosol sedimentation efficiency was 99.82% which was much higher than nature settlement, 18.6% and water spraying sedimentation, 43.3%. Moreover, after spraying the compound suppressant, it displayed a good effect on settling the cesium aerosol particles with a diameter of less than 1 µm, as the concentration of particles was reduced from 55.49% to 44.53%. Finally, the sedimentation mechanism of the compound aerosol suppressant and cesium aerosol particles, such as the coagulation effect, was analyzed using the particle size distribution.

Effective removal of radioactive cesium from contaminated water by synthesized composite adsorbent and its thermal treatment for enhanced storage stability.

A composite adsorbent for the removal of radioactive cesium (137Cs) was synthesized by immobilizing potassium cobalt ferrocyanide in the micro pores of the zeolite chabazite. The synthetically optimized composite adsorbent demonstrates a rapid cesium adsorption rate under both salt-free and high-salt conditions with a high distribution coefficient of cesium (≥105 mL/g). Although both components have the same ion-exchange reaction between potassium and cesium, the reaction by ferrocyanide component was predominant, which derived hundred times higher distribution coefficient of the composite adsorbent than that of pure chabazite. A thermal stabilization process was studied for improving the storage and/or disposal stability of the spent adsorbent. The formation of a eutectic system within the spent adsorbent reduced the stabilization temperature to 1000 °C from 1200 °C. Accordingly, the leaching of cesium was remarkably reduced by the remineralization to the stable pollucite. The stable impregnation of ferrocyanide component in the chabazite pores derived the reduction of cesium volatility enabling the high temperature stabilization method. Our experimental results provide evidence that the composite adsorbent has clear advantages on the cesium removal from contaminated water and its stabilization via thermal-treatment.

Electroactive magnetic microparticles for the selective elimination of cesium ions in the wastewater.

To improve operability as well as the removal efficiency for cesium ions in the wastewater treatment, a novel electrochemically switched ion exchange (ESIX) technique by using electroactive Prussian-blue(PB)-based magnetic microparticles (PB@Fe3O4 microparticle) with different uniform particle sizes in the range of 300-900 nm as the adsorption materials was developed. The obtained PB@Fe3O4 microparticle were characterized by Scanning electron microscopy (SEM), Transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and Thermogravimetric analysis (TGA). It is found that the PB can be well coated on the surface of Fe3O4 microsphere, which can be easily adsorbed on the magnetic electrode substrate for the electrochemical adsorption of Cs+ ions. Electrochemical adsorption of 97% Cs+ on PB/Fe3O4 was achieved in less than 10 min, and the maximum adsorption capacity was 16.13 mg/g, and the distribution coefficient (KD) of Cs+ ions reached as high as 3938. In addition, the electrochemical adsorption behavior of PB@Fe3O4 microparticle fitted well with the Freundlich adsorption isotherm and the Pseudo-second-order kinetic models. It is expected that such an ESIX technique using PB@Fe3O4 microparticle can be applied for the separation and recovery of dilute Cs+ ions from cesium-contaminated solution in a practical process.

Selective Binding and Precipitation of Cesium Ions from Aqueous Solutions: A Size-Driven Supramolecular Reaction.

The nuclear disasters of Chernobyl and Fukushima presented an urgent need for finding solutions to treatment of radioactive wastes. Among the by-products of nuclear fission is radioactive 137 Cs, which evokes an environmental hazard due to its long half-life (>30 years) and high solubility in water. In this work, a water-soluble organic ligand, readily obtained from alloxan and 1,3,5-benzenetriol, has been found to selectively bind and precipitate Cs+ ions from aqueous solutions. The special rigid structure of the ligand, which consists of a "tripodal" carbonyl base above and below an aromatic plane, contributes to the size-driven selectivity towards the large Cs+ ions and the formation of a giant, insoluble supramolecular complex. In addition to the low costs of the ligand, high yields and effectiveness in precipitating Cs+ ions, the Cs-complex revealed a high endurance to continuous doses of γ-radiation, increasing its potential to act as a precipitating agent for 137 Cs.

Evaluation of physicochemical properties of radioactive cesium in municipal solid waste incineration fly ash by particle size classification and leaching tests.

After the Fukushima Daiichi-Nuclear Power Plant accident, environmental recovery was a major issue because a considerable amount of municipal solid waste incineration (MSWI) fly ash was highly contaminated with radioactive cesium. To the best of our knowledge, only a few studies have evaluated the detailed physicochemical properties of radioactive cesium in MSWI fly ash to propose an effective method for the solidification and reuse of MSWI fly ash. In this study, MSWI fly ash was sampled in Fukushima Prefecture. The physicochemical properties of radioactive cesium in MSWI fly ash were evaluated by particle size classification (less than 25, 25-45, 45-100, 100-300, 300-500, and greater than 500 µm) and the Japanese leaching test No. 13 called "JLT-13". These results obtained from the classification of fly ash indicated that the activity concentration of radioactive cesium and the content of the coexisting matter (i.e., chloride and potassium) temporarily change in response to the particle size of fly ash. X-ray diffraction results indicated that water-soluble radioactive cesium exists as CsCl because of the cooling process and that insoluble cesium is bound to the inner sphere of amorphous matter. These results indicated that the distribution of radioactive cesium depends on the characteristics of MSWI fly ash.

[Concentration of Radioactive Cesium in Domestic Foods Collected from the Japanese Market in Fiscal Years 2014-2016].

Between fiscal years 2014 and 2016, we surveyed the concentration of radioactive cesium in commercial foods produced in areas where there is a risk of radiation contamination due to the Fukushima Daiichi nuclear disaster. The number of samples with a concentration of radioactive cesium that exceeded the regulatory limit （100 Bq/kg for general foods） was 9 out of 1,516 （0.6％） in fiscal 2014, 12 out of 900 （1.3％） in fiscal 2015, and 10 out of 654 （1.5％） in fiscal 2016. Even though some samples were expected to be contaminated with radioactive cesium, because wild mushrooms and edible wild plants were intentionally included in this survey, the percentage of samples that exceeded the regulatory limit was only around 1％. The surveillance results confirmed that the pre-shipment food monitoring conducted by local governments was properly and efficiently performed, although continuous monitoring of the concentration of radioactive cesium in cultivated and wild mushrooms, edible wild plants, and wild animal meats is still required.

Effects of aging of radioactive fallout on timely decontamination of concrete using low and mild pressure washing.

Timely decontamination will reduce the consequences of a radiological contamination event. For this purpose, pressure washing can be rapidly deployed, but its effectiveness will change if the interactions between the surface and radionuclides changes as the contamination "ages" under the influence of time and precipitation. While effects of this aging have been reported for dissolved cesium, they have not been studied for radionuclides present as particulate, e.g., fallout. This work studied the effects of aging on decontamination with low (<280 kPa/40 psi) and mild (14,000 kPa/2000 psi) pressure washing, on concrete contaminated with surrogate fallout consisting of soluble Cs-137, 0.5 µm silica particles, and 2 µm silica particles. The samples were aged up to 59 days (time between contamination and decontamination) with and without simulated precipitation. The percent removal following decontamination of the soluble cesium decreased over the first ten days of aging until the removals were less than 10 % for both low and mild pressure washing. The particle decontamination was independent of aging time but decontaminating via mild pressure washing (>80 % particle removal) significantly outperformed decontaminating by low pressure washing by flowing solution across (parallel to) the contaminated surface (<25 % particle removal). The observed changes in decontamination efficacy are explained via measurements of the penetration depth of contaminants. For soluble cesium, the results compared favorably with prior studies and theoretical treatment of cesium penetration, and they yielded additional insight into the effect of washing pressures on decontamination. There are no comparable studies for particulate contamination, so this study resulted in several novel observations which are operationally important for timely decontamination of surfaces following a radiological incident. It also suggests an evidence-based pressure washing procedure for timely decontamination of soluble and insoluble radionuclides which can be used throughout the emergency phase and into the early recovery phase.

Magnetic hierarchical titanium ferrocyanide for the highly efficient and selective removal of radioactive cesium from water.

Selective adsorption is the most suitable technique for eliminating trace amounts of 137Cs from various volumes of 137Cs-contaminated water, including seawater. Although various metal ferrocyanide (MFC)-functionalized magnetic adsorbents have been developed for the selective removal of 137Cs and magnetic recovery of adsorbents, their adsorption capacity for Cs remains low. Here, magnetic hierarchical titanium ferrocyanide (mh-TiFC) was synthesized for the first time for enhanced Cs adsorption. Hierarchical TiFC, comprising 2-dimensional TiFC flakes, was synthesized on SiO2-coated magnetic Fe3O4 particles using a sacrificial TiO2 shell as a source of Ti4+ via a reaction with ferrocyanide under acidic conditions. The resultant mh-TiFC exhibited the highest maximum adsorption capacity (434.8 mg g-1) and enhanced Cs selectivity with an excellent Kd value (6,850,000 mL g-1) compared to those of previously reported magnetic Cs adsorbents. This enhancement was attributed to the hierarchical structure, which reduced intracrystalline diffusion and increased the surface area available for direct Cs adsorption. Additionally, mh-TiFC (0.1 g L-1) demonstrated an excellent removal efficiency of 137Cs exceeding 99.85% for groundwater and seawater containing approximately 22 ppt 137Cs. Therefore, mh-TiFC offers promising applications for the treatment of 137Cs-contaminated water.

Effect of incorporated transition metals on the adsorption mechanisms of radioactive cesium in Prussian blue analogs.

Extensive efforts were made to remove radioactive cesium (137Cs) from the environment, with Prussian blue analogs (PBAs) emerging as highly selective and efficient materials for 137Cs removal. However, limited studies systematically compared Cs+ adsorption across different transition metals in PBA. This study investigates the influence of the choice of transition metal ion (Co, Cu, Fe, Mn, Ni, Zn) on Cs+ adsorption mechanisms and efficiency. PBAs were synthesized and characterized based on their specific surface area, ion exchange capacity, lattice parameter, and defect sites (as indicated by water molecule content). Cs+ adsorption mechanisms varied significantly with transition metals. In CoFe and FeFe PBAs, ion exchange with K+ dominated, while CuFe and MnFe PBAs, with more defect sites primarily used ion exchange between H+ and Cs+. NiFe and ZnFe exhibited enhanced Cs+ adsorption under light irradiation, likely due to their light-absorbing properties facilitating a reduction reaction. The Langmuir adsorption isotherm was applied to model the adsorption behavior, confirming that each performance of PBA depends on the transition metal used. These findings suggest that PBAs with various transition metals can efficiently remove 137Cs under diverse environmental conditions by using distinct adsorption mechanisms.

Occurrence of radioactive cesium-rich micro-particles (CsMPs) in a school building located 2.8 km south-west of the Fukushima Daiichi Nuclear Power Plant.

Radioactive Cs-rich microparticles (CsMPs) released from the Fukushima Daiichi Nuclear Power Plant (FDNPP) are a potential health risk through inhalation. Little has been documented on the occurrence of CsMPs, particularly their occurrence inside buildings. In this study, we quantitatively analyze the distribution and number of CsMPs in indoor dust samples collected from an elementary school located 2.8 km to the southwest of FDNPP. The school had remained deserted until 2016. Then, using a modified version of the autoradiography-based "quantifying CsMPs (mQCP) method," we collected samples and determined the number of CsMPs and Cs radioactive fraction (RF) values of the microparticles (defined as total Cs activity from CsMPs/bulk Cs activity of the entire sample). The numbers of CsMPs ranged from 653 to 2570 particles/(g dust) and 296-1273 particles/(g dust) on the first and second floors of the school, respectively. The corresponding RFs ranged between 6.85 - 38.9% and 4.48-6.61%, respectively. The number of CsMPs and RF values in additional outdoor samples collected near the school building were 23-63 particles/(g dust or soil) and 1.14-1.61%, respectively. The CsMPs were most abundant on the school's first floor near to the entrance, and the relative abundance was higher near the stairs on the second floor, indicating a likely CsMP dispersion path through the building. Additional wetting of the indoor samples combined with autoradiography revealed that indoor dusts had a distinct absence of intrinsic, soluble Cs species, such as CsOH. These combined observations indicate that a significant amount of poorly soluble CsMPs were likely contained in initial radioactive airmass plumes from the FDNPP and that the microparticles penetrated buildings. CsMPs could still be abundant at the location, with locally high Cs activity in indoor environments near to openings.

Tannic acid-assisted in-situ interfacial formation of Prussian blue-assembled adsorptive membranes for radioactive cesium removal.

There is a growing interest in advanced materials that can effectively treat wastewater contaminated with radioactive cesium (137Cs), which is an extremely hazardous material. Here, we report a new class of Cs-adsorptive membranes compactly assembled with Cs-adsorptive Prussian blue (PB) particles. The PB particle assembly was formed via an in-situ interfacial reaction between two PB precursors in the presence of tannic acid (TA) as a binder on a porous support. While the interfacial reaction enabled the formation of a defect-less PB network, TA enhanced the PB-PB and PB-support compatibilities, consequently producing a uniform, densely packed PB assembly near the support surface. The fabricated TA-assisted PB membrane (PB/TA-M) synergistically rejected Cs via a combination of adsorption and membrane filtration, although adsorption predominantly determined Cs rejection initially. Hence, the PB/TA-M membrane showed considerably higher Cs removal performance than commercial nanofiltration (NF) and reverse osmosis (RO) polyamide (PA) membranes for a sufficiently long operation time. Furthermore, the PB/TA-M membrane displayed excellent radioactive 137Cs removal performance, significantly exceeding those of commercial NF and RO PA membranes due to its higher radiation stability, indicating its viability for application in treating actual radioactive wastewater.

Kinetic properties of 137Cs uptake by the cesium-accumulating eustigmatophycean microalga.

Cesium-137 (137Cs) is one of the radioactive substances that was released into the environment as a result of the Fukushima nuclear disaster. Radiocesium exposure is of great concern due to its potential environmental implications. However, research on 137Cs removal using algae is still limited. This is the first report to describe the kinetic properties of 137Cs uptake by Vacuoliviride crystalliferum in the presence and absence of potassium. In this work, we studied the kinetic properties of 137Cs uptake using a freshwater microalga, V. crystalliferum (NIES 2860). We also analyzed the effects of temperature, light, and potassium (K) on the 137Cs uptake. Results showed that V. crystalliferum can remove up to 90% of 157 nM 137Cs within an hour. At 20 °C, the removal increased by up to 96%, compared to less than 10% at 5 °C. However, the removal was inhibited by nearly 90% in the dark compared to the removal in the light, implying that V. crystalliferum cells require energy to accumulate 137Cs. In the inhibition assay, K concentrations ranged from 0 to 500 µM and the inhibitory constant (Ki) for K was determined to be 16.7 µM. While in the uptake assay without potassium (- K), the Michaelis constant (Km) for Cs was 45 nM and increased to 283 nM by the addition of 20 µM potassium (+ K), indicating that V. crystalliferum had a high affinity for 137Cs. In addition, the maximum uptake velocity (Vmax) also increased from 6.75 to 21.10 nmol (mg Chl h)-1, implying the existence of Cs active transport system. In conclusion, V. crystalliferum is capable of removing radioactive 137Cs from the environment and the removal was favorable at both normal temperature and in the light.

Origin and pathway of a subsurface maximum of 137Cs detected in the winter of 2012 after the Fukushima accident.

This study investigates the dispersion behavior of 137Cs and evaluates its origin (atmospheric deposition or direct ocean release) from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident using a Lagrangian particle tracking model. The ocean circulation fields based on the Modular Ocean Model Version 5 (MOM5) were adopted for the simulation. The MOM5 results represented the formation and migration of subtropical mode water (STMW) comparable with observations and reanalysis data. Particularly, anticyclonic eddies south of the Kuroshio extension promoted surface mixing over 300 m in the cooling season. The particle tracking simulation reproduced well the maximum subsurface activity between 142 and 146°E, where STMW is deep owing to anticyclonic eddies, compared to the activity found via measurements conducted around 149°E in the winter of 2012. It also demonstrated that the 137Cs of the tropical and subtropical regions (10-35°N, 142-146°E) in the winter of 2012 almost entirely originated from atmospheric deposition.

Surface decontamination of radioactive cesium by a reversibly cross-linkable hydrogel using poly(vinyl alcohol) and phenylboronic acid-grafted poly(methyl vinyl ether-alt-mono-sodium maleate).

Wide-area surface decontamination is essential during the sudden release of radioisotopes to the public, such as nuclear accidents or terrorist attacks. A self-generated hydrogel comprising a reversible complex between poly(vinyl alcohol) (PVA) and phenylboronic acid-grafted poly(methyl vinyl ether-alt-mono-sodium maleate) (PBA-g-PMVE-SM) was developed as a new surface decontamination coating agent to remove radioactive cesium from surfaces. The simultaneous application of PVA and PBA-g-PMVE-SM aqueous polymer solutions containing sulfur-zeolite to contaminated surfaces resulted in the spontaneous formation of a PBA-diol ester bond-based hydrogel. The sulfur-zeolite suspended in the hydrogel selectively removed 137Cs from the contaminated surface and was easily separated from the dissociable used hydrogel. This removal was performed by simple water rinsing without costly incineration to remove the organic materials for final disposal/storage of the radioactive waste, making it suitable for practical wide-area surface decontamination. In radioactive tests, the hydrogel containing sulfur-chabazite (S-CHA) showed substantial 137Cs removal efficiencies of 96.996% for painted cement and 63.404% for cement, which are 2.33 times better than the values for the commercial surface decontamination coating agent DeconGel. Due to its excellent zeolite ion-exchange ability, our hydrogel system has great potential for removing various hazardous contaminants, including radionuclides, from the surface.

[Study on the Concentration of Radioactive Cesium in the Environment after the Fukushima Daiichi Nuclear Power Plant Accident].

The Tohoku-Pacific Ocean Earthquake that occurred on March 11, 2011 and the resulting tsunami caused the loss of many people and extensive damage in a wide area. Among the anthropogenic radionuclides dispersed from the Fukushima Daiichi Nuclear Power Plant, 134Cs and 137Cs have very long half-lives of approximately 2 years and 30 years, respectively, and there are concerns about their uptake into soil and living things. This paper describes a study conducted by the authors' group on radiocesium activity concentrations in the environment.

Cesium-137 and 137Cs/133Cs atom ratios in marine zooplankton off the east coast of Japan during 2012-2020 following the Fukushima Dai-ichi nuclear power plant accident.

We measured the concentrations of cesium isotopes (133Cs, 134Cs, and 137Cs) in zooplankton samples collected in waters off the east coast of Japan from May 2015 to June 2020. By combining these data with those obtained previously from May 2012 to February 2015, we evaluated the long-term impacts of the Fukushima Dai-ichi Nuclear Power Plant accident on marine zooplankton. Relatively high 137Cs concentrations in zooplankton, exceeding 10 Bq/kg-dry weight, were sporadically observed until June 2016, regardless of year or station. After May-June 2017, 137Cs concentrations decreased to below 1 Bq/kg-dry at most stations, and by May 2020, concentrations were below 0.5 Bq/kg-dry except those off Fukushima Prefecture. Since the accident, the 137Cs/133Cs atom ratios of zooplankton samples were higher than those of ambient seawater until 2019, but in May-June 2020 the ratios matched those of seawater except off Fukushima Prefecture. Highly radioactive particles were not detected in zooplankton samples by autoradiography using imaging plates after May-June 2017, although they were before. Therefore, the persistence of elevated 137Cs/133Cs ratios in zooplankton relative to seawater for nine years after the accident was probably due to the incorporation of highly radioactive particles (cesium-bearing particles or clay-mineral aggregates with highly adsorbed radiocesium) onto/into zooplankton for several years after the accident. However, since at least May-June 2017, these elevated ratios have likely been caused by small highly radioactive particles (or larger particles disaggregated into small pieces) entering the ocean from land via rivers or directly discharged from the Fukushima Nuclear Power Plant. Microplastics enriched with radiocesium with higher 137Cs/133Cs ratios than seawater may have also contributed 137Cs to the zooplankton.