Radiocesium Contamination Research Articles

Characteristics of radiocesium contamination of dry riverbeds due to the Fukushima Daiichi Nuclear Power Plant accident assessed by airborne radiation monitoring

The Great East Japan Earthquake that occurred on 11 March 2011 generated a series of massive tsunami waves that caused severe damage to the Fukushima Daiichi Nuclear Power Plant, following which large amounts of radioactive materials were discharged from the power plant into the environment. Given the recently developed technologies for unmanned helicopters and their potential application in airborne radiation monitoring, we developed a radiation monitoring system for aerial use. We then used unmanned helicopters to measure the radiation level in areas with soil contaminated by radiocesium emitted from the nuclear power plant to evaluate the ambient dose rate distribution around the site. We found that in dry riverbeds downstream of the highly contaminated watershed, especially in the upstream parts, that the ambient dose rate was higher than that in protected inland areas. In addition, we carried out soil sampling in dry riverbeds. The soil sample data agreed with the results obtained from the unmanned helicopter. A possible scenario is that river sediment and adjacent soil containing radioactive materials were carried downstream due to an event such as a rise in water level caused by, e.g., a typhoon, and that these remained in the riverbed after the water drained away. The vertical profile of radiocesium in the soil and the measurements of river sediment at river mouth areas corroborated this scenario.

Identification and temporal decrease of 137Cs and 134Cs in groundwater in Minami-Soma City following the accident at the Fukushima Dai-ichi nuclear power plant.

The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident on March 11, 2011, caused severe radioactive contamination in Fukushima Prefecture. In order to clarify the safety of drinking water, we have conducted radiocesium monitoring of public tap water and groundwater in Minami-Soma City, which is 10-40km north of the nuclear power plant. The source of tap water for Minami-Soma City is groundwater, which is treated by rapid filtration before distribution in two of the three treatment plants. The tap water was collected from six stations during 2012-2016 and groundwater was collected from 11 stations with wells between 5 and 100m deep during 2014-2016. Radiocesium contamination of groundwater has been considered unlikely in Japan because of the small vertical migration velocity of radiocesium in Japanese soil. However, radiocesium was detected in public tap water after 2012, and the maximum 137Cs concentration of 292 mBq L-1 was observed in 2013. In all the well water, radiocesium was detected between 2014 and 2015, at concentrations similar to those observed in tap water in the same period. In tap water and groundwater, radiocesium was decreased to below the detection limit in 2016 except for four stations. Radiocesium concentration in shallow water reached a maximum between 2013 and 2015, 2-4 years after the FDNPP accident, and then decreased. The results are interpreted that dissolved 137Cs migrated in the soil and reached aquifers of various depth.

Development of beta ray scanner for imaging foliar uptake of radiocesium

Large quantities of radionuclides were released from the Fukushima Daiichi Nuclear Power Plant (FDNPP) into the atmosphere, which then contaminated the soil and vegetation surrounding the FDNPP. The research on radiocesium contamination of agricultural products and wild plants is important. Therefore, we developed a simple beta ray scanner to obtain radiographs of the transport and foliar uptake of radiocesium. This simple beta ray scanner comprised a beta ray detector, a motorized sample stage, a shielding box, and a personal computer. Beta rays released from radiocesium were detected effectively using a plastic scintillator plate coupled to multipixel photon counter devices. The spatial resolving power was approximately 6 mm $$\times $$ 4 mm (FWHM). In a preliminary experiment, a drop of radiocesium solution was placed on the upper surface of a radish leaf. Time-lapse images of radiocesium in the leaf were obtained by the beta ray scanner. Images of isotope transport in the leaf were successfully obtained with the beta ray scanner. The beta ray scanner will be used in our future research on the mechanism of the foliar uptake of radiocesium.

Assessing Fukushima-Derived Radiocesium in Migratory Pacific Predators.

The 2011 release of Fukushima-derived radionuclides into the Pacific Ocean made migratory sharks, teleosts, and marine mammals a source of speculation and anxiety regarding radiocesium (134+137Cs) contamination, despite a lack of actual radiocesium measurements for these taxa. We measured radiocesium in a diverse suite of large predators from the North Pacific Ocean and report no detectable (i.e., ≥ 0.1 Bq kg-1 dry wt) Fukushima-derived 134Cs in all samples, except in one olive ridley sea turtle (Lepidochelys olivacea) with trace levels (0.1 Bq kg-1). Levels of 137Cs varied within and across taxa, but were generally consistent with pre-Fukushima levels and were lower than naturally occurring 40K by one to one to two orders of magnitude. Predator size had a weaker effect on 137Cs and 40K levels than tissue lipid content. Predator stable isotope values (δ13C and δ15N) were used to infer recent migration patterns, and showed that predators in the central, eastern, and western Pacific should not be assumed to accumulate detectable levels of radiocesium a priori. Nondetection of 134Cs and low levels of 137Cs in diverse marine megafauna far from Fukushima confirms negligible increases in radiocesium, with levels comparable to those prior to the release from Fukushima. Reported levels can inform recently developed models of cesium transport and bioaccumulation in marine species.

Radiocesium Contamination of Cover Crops and Soybeans in Ibaraki after the Fukushima Daiichi Nuclear Power Plant Accident

Radiocesium Contamination of Cover Crops and Soybeans in Ibaraki after the Fukushima Daiichi Nuclear Power Plant Accident

Low-cesium rice: mutation in OsSOS2 reduces radiocesium in rice grains

In Japan, radiocesium contamination in foods has become of great concern and it is a primary issue to reduce grain radiocesium concentration in rice (Oryza sativa L.). Here, we report a low-cesium rice mutant 1 (lcs1) with the radiocesium concentration in grain about half that in the wild-type cultivar. Genetic analyses revealed that a mutation in OsSOS2, which encodes a serine/threonine-protein kinase required for the salt overly sensitive (SOS) pathway in plants, is responsible for the decreased cesium (Cs) concentrations in lcs1. Physiological analyses showed that Cs+ uptake by lcs1 roots was significantly decreased under low-potassium (K+) conditions in the presence of sodium (Na+) (low K+/Na+). The transcript levels of several K+ and Na+ transporter genes, such as OsHAK1, OsHAK5, OsAKT1, and OsHKT2;1 were significantly down-regulated in lcs1 grown at low K+/Na+. The decreased Cs+ uptake in lcs1 might be closely related to the lower expression of these genes due to the K+/Na+ imbalance in the lcs1 roots caused by the OsSOS2 mutation. Since the lcs1 plant had no significant negative effects on agronomic traits when grown in radiocesium-contaminated paddy fields, this mutant could be used directly in agriculture for reducing radiocesium in rice grains.

Temporal changes in radiocesium contamination derived from the Fukushima Dai-ichi Nuclear Power Plant accident in oceanic zooplankton in the western North Pacific

We investigated temporal changes of the contamination of oceanic zooplankton with radiocesium (134Cs and 137Cs) derived from the Fukushima Dai-ichi Nuclear Power Plant accident one month to three years after the accident at subarctic and subtropical stations (1900 and 900–1000 km from the plant, respectively) in the western North Pacific. The maximum activity concentrations of 137Cs in zooplankton were two orders of magnitude higher than the pre-accident level. In the first four months after the accident, the activity concentrations of radiocesium in subtropical zooplankton decreased rapidly, but no similar change was observed at the subarctic station. The radiocesium derived from atmospheric deposition rapidly decreased as a result of seawater mixing. Thus, most of the subtropical zooplankton (with short lifespans) that had taken up radiocesium just after the accident were probably replaced by newly hatched zooplankton within four months of the accident, whereas subarctic zooplankton (with long lifespans) that were highly contaminated with radiocesium were still alive four months after the accident. By the end of the study, 137Cs activity concentrations in subtropical zooplankton were still high, whereas the activity concentrations in subarctic zooplankton had decreased to nearly the pre-accident level. The former concentrations were probably influenced by a secondary supply of radiocesium via advection of subtropical mode water that was highly contaminated with Fukushima-derived radiocesium. Unexpectedly, at the subarctic station, the radiocesium activity concentrations in surface zooplankton were lower than those in subsurface zooplankton, whereas the opposite relationship was observed in surface and subsurface seawater. Because carnivores predominated in the subsurface zooplankton community, we hypothesize that the higher radiocesium activity concentrations in subsurface zooplankton were influenced by bioaccumulation. We conclude that radiocesium activity concentrations in zooplankton are influenced not only by the supply of radiocesium to the environment but also by the characteristics of the zooplankton community.

Whole-Body Counter(WBC) and food radiocesium contamination surveys in Namie, Fukushima Prefecture.

PurposeThis study examined the internal Cs exposure of residents and the Cs present in food products produced in Namie. Whole-body counter (WBC) was used for the measurement of internal exposure per each whole body of examinees.MethodsThe food products which appeared to be used for consumption, were brought by residents and commercially available food items were excluded. Most of them were wild plants or food items produced by residents. Four years of data from April 2012 to March 2013 (fiscal 2012) and April 2015 to March 2016 (Fiscal 2015) were analyzed and studied.ResultsThe average radioactivity measured by WBC was approximately 5 Bq for Cs-134, and 20 Bq for Cs-137 and the average committed effective dose was approximately 1 μSv. The average for the residents with detectable radioactivity was 25 μSv, and the human health effects are considered to be extremely low risk. However, the radioactivity of the affected individuals showed a higher value than the theoretical attenuation rate. The majority (83.2%) of individuals exhibiting radioactivity were over 50 years old. The number of food products brought in for detection decreased as the study period progressed, but the number of food products with radioactivity had increased. While the items with a higher detection rate of radioactivity included fruits such as citron and persimmon, shiitake mushrooms exhibited the highest radioactivity. Moreover, the radioactivity of seven items in these 10 items decreased from fiscal 2012 to fiscal 2015. Mushrooms had high radioactivity and were produced over a wide area.ConclusionWe suggest that the elderly try to enjoy life and eat wild plants in moderation while inspecting food products. Therefore, we will continue to work in raising awareness of radiation and its potential presence in food products and thus the continuing necessity of monitoring radioactivity in food in the future.

Radiocesium contamination of aquatic organisms in the estuary of the Abukuma River flowing through Fukushima

AbstractIn the summer of 2013, we measured the radiocesium (134Cs and 137Cs) concentrations in aquatic organisms inhabiting the estuary of the Abukuma River, which flows through the Fukushima and Miyagi Prefectures. Radiocesium concentrations in four muscle‐tissue samples of blackhead seabream (Acanthopagrus schlegeli) [1240, 914, 202 and 106 Bq kg−1 wet weight (w.w.)] and two whole‐body samples deriving from multiple individuals of flathead grey mullet (Mugil cephalus) (129 and 110 Bq kg−1 w.w.) exceeded the Japanese standard limit for radiocesium levels in foods (100 Bq kg−1 w.w.). However, radiocesium concentrations in the other fish samples were lower than the Japanese standard limit. In addition, radiocesium concentrations in crustaceans, major prey items of omnivorous and carnivorous fish species in the Abukuma River estuary, were generally low (0.444–15.1 Bq kg−1 w.w.). Stable isotope analysis indicated that the feeding habit of the three heavily contaminated blackhead seabream, which the radiocesium concentrations were about several times or more higher than the Japanese standard limit, were similar to those of other individuals collected from the same sampling site. Therefore, the difference in the radiocesium level between the three heavily contaminated individuals and the less‐contaminated individuals of blackhead seabream was not because of a recent feeding habit in the Abukuma River estuary. Environmental conditions in the Abukuma River estuary in the summer of 2013 would not have had the potential to increase radiocesium concentrations in the muscle tissues of fish inhabiting the estuary to levels greater than 100 Bq kg−1 w.w.

Whole-Body Counter Evaluation of Internal Radioactive Cesium in Dogs and Cats Exposed to the Fukushima Nuclear Disaster.

As a result of the 2011 nuclear incident that occurred at the Fukushima Nuclear Power Plant, a large number of abandoned dogs and cats were left within the disaster zone. A small number of these animals were rescued and cared for at shelters. Prior to the dispersal of these animals to their owners or fosterers, we evaluated the degree of internal radiocesium contamination using a specially designed whole-body counter. We conducted 863 non-invasive measurements of gamma rays due to internal radioactive cesium for 68 dogs and 120 cats at one shelter. After plotting graphs of 137Cs density we generated exponential functions of decay from seven dogs and six cats. From the regression formulae, we were able to determine the biological half-lives as 38.2 days for dogs and 30.8 days for cats. We found that in dogs there was a correlation between the biological half-life of radioactive cesium and age. Using our data, we estimated whole-body densities for each cat and dog at the time when they were rescued. We found that there were deviations in the data distributions among the different species, likely due to the timing of rescue, or living habits prior to rescue. A significant correlation was found when extracted feline reproductive organs were analyzed; the coefficients for the estimation of whole-body densities were approximately 7-fold higher than those based on the extracted feline reproductive organs. This may be due to the fact that majority of the radioactive cesium accumulates within muscular tissue with less distribution in other organs. It is possible to plan the appropriate management period in an animal shelter based on the use of the biological half-life of radioactive cesium calculated in this study. We believe that the correlations we uncovered in this work would be of great use for the management of companion animals in the event of a future nuclear accident.

ワカサギ中の放射性セシウム濃度の体重依存性と経年変化

The accident of the Fukushima Dai-ichi Nuclear Power Plant in March, 2011, released large amounts of radiocesium into the atmosphere, and contaminated the environment of Gunma Prefecture in eastern Japan. In particular, 640 Bq kg–1-wet of radiocesium concentration was found in wakasagi (Hypomesus nipponensis) in Lake Onuma on Mt. Akagi in August, 2011. Thus, to elucidate the body-size effect in weight-dependent and dynamics of radiocesium in the ecosystem of Lake Onuma, we determined the age of wakasagi, the body-weight class of the radiocesium concentration in wakasagi and the effective ecological half-life (Teff) of radiocesium in wakasagi 0+ from 2012 to 2016. The body-size effect was found for the 137Cs concentration of wakasagi fished from 2012 to 2015, i.e., the concentration of 137Cs increased with the increase in its body weight. On the other hand, no body size effect was found in 2016. This result may be caused by the following two factors: the 137Cs concentration of the lake water reached a steady state after May 2014; wakasagi is a small plankton-feeding fish, while it is known that larger piscivorous fishes show the strong body size effect. Teff of the 137Cs concentration in wakasagi 0+ consists of two components, fast- and slow-term ones, and the decay rate of the 137Cs concentration in wakasagi 0+ was greatly reduced. As stated above, the radiocesium contamination in Lake Onuma has still been lasting; we are thus continuing our monitoring studies.

Ⅰ-2. 内水面魚類におけるモニタリング調査の概要

Ⅰ-2. 内水面魚類におけるモニタリング調査の概要

Processes of Radiocesium Contamination in Marine Fish

Processes of Radiocesium Contamination in Marine Fish

Transfer of 40K and 137Cs from diet into meat of ruminants: analogy or not?

The study of transfer coefficients and concentration ratios of 40K and 137Cs was performed in order to investigate the bioavailability and potential analogy relationships of observed radionuclides in the chain of animal production of ruminants. Sampling of material was performed at small individual farms situated inside the three regional livestock areas in Bosnia and Herzegovina. Transfer coefficients and concentration ratios of 40K and 137Cs were calculated for beef and sheep meat. Obtained results were in accordance with literature data with average transfer coefficient values for 40K and 137Cs in beef of 0.017 and 0.058 d kg-1, respectively; whilst the same values for the sheep meat were higher (0.104 and 0.341). Average values for concentration ratios of 40K and 137Cs were equilibrated between the species (beef: 0.125 and 0.418; sheep meat: 0.136 and 0.443). Obtained results indicated on the hidden analogy between observed radionuclides, covered by the homeostatic control of potassium in ruminants. The study confirmed the use of potasssium compounds for reduction of radiocesium contamination in ruminants.

Radiocesium contamination of three dwarf bamboo species in the forest floor of deciduous broad-leaved forest

Radiocesium contamination of three dwarf bamboo species in the forest floor of deciduous broad-leaved forest

Investigation of radiocesium contamination in wild boars (Sus scrofa) in the Yamizo mountainous region, including Tochigi, Ibaraki and Fukushima prefectures

Investigation of radiocesium contamination in wild boars (<i>Sus scrofa</i>) in the Yamizo mountainous region, including Tochigi, Ibaraki and Fukushima prefectures

Forest type effects on the retention of radiocesium in organic layers of forest ecosystems affected by the Fukushima nuclear accident.

The Fukushima Daiichi nuclear power plant disaster caused serious radiocesium (137Cs) contamination of forest ecosystems over a wide area. Forest-floor organic layers play a key role in controlling the overall bioavailability of 137Cs in forest ecosystems; however, there is still an insufficient understanding of how forest types influence the retention capability of 137Cs in organic layers in Japanese forest ecosystems. Here we conducted plot-scale investigations on the retention of 137Cs in organic layers at two contrasting forest sites in Fukushima. In a deciduous broad-leaved forest, approximately 80% of the deposited 137Cs migrated to mineral soil located below the organic layers within two years after the accident, with an ecological half-life of approximately one year. Conversely, in an evergreen coniferous forest, more than half of the deposited 137Cs remained in the organic layers, with an ecological half-life of 2.1 years. The observed retention behavior can be well explained by the tree phenology and accumulation of 137Cs associated with litter materials with different degrees of degradation in the organic layers. Spatial and temporal patterns of gamma-ray dose rates depended on the retention capability. Our results demonstrate that enhanced radiation risks last longer in evergreen coniferous forests than in deciduous broad-leaved forests.

Quantifying the dilution of the radiocesium contamination in Fukushima coastal river sediment (2011–2015)

Fallout from the Fukushima Dai-ichi nuclear power plant accident resulted in a 3000-km2 radioactive contamination plume. Here, we model the progressive dilution of the radiocesium contamination in 327 sediment samples from two neighboring catchments with different timing of soil decontamination. Overall, we demonstrate that there has been a ~90% decrease of the contribution of upstream contaminated soils to sediment transiting the coastal plains between 2012 (median – M – contribution of 73%, mean absolute deviation – MAD – of 27%) and 2015 (M 9%, MAD 6%). The occurrence of typhoons and the progress of decontamination in different tributaries of the Niida River resulted in temporary increases in local contamination. However, the much lower contribution of upstream contaminated soils to coastal plain sediment in November 2015 demonstrates that the source of the easily erodible, contaminated material has potentially been removed by decontamination, diluted by subsoils, or eroded and transported to the Pacific Ocean.

Effects of the nuclear disaster on marine products in Fukushima: An update after five years

Original data (134Cs and 137Cs, and sampling location) of marine products in Fukushima Prefecture monitored during 2011–2015 (n = 32,492) were analyzed to present an updated detailed description of radiocesium contamination after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident and to examine taxon/habitat-specific decreasing trends in different areas. Furthermore, marine species data presented by the Tokyo Electric Power Company (TEPCO) during 2012–2015 (n = 5458) were analyzed to evaluate the decreasing trends of 137Cs inside and outside (within a 20 km radius) of the FDNPP port. Monitoring results by Fukushima Prefecture show that percentages of samples higher than the Japanese regulatory limit of 100 Bq kg−1-wet (>RL%) were higher, whereas those below the detection limit (<DL%) (mean 8.3 and 7.4 Bq kg−1-wet for 134Cs and 137Cs, respectively) were lower in demersal fishes than in pelagic fish or other taxa. However, >RL% and <DL% of demersal fish respectively decreased dramatically and increased gradually to 0.06% and 86.3% in 2015, although slightly elevated radiocesium concentrations were still observed in shallow areas south of the FDNPP. The drastic decrease in radioactivity was supported by the spatiotemporal distribution of radiocesium concentrations in demersal fish, in which higher concentrations that were frequently observed in 2011 and 2012 were rarely detected in 2015, even within the 20 km radius area (maximum 220 Bq kg−1-wet in Japanese rockfish Sebastes cheni). Statistical analyses of TEPCO data revealed that 137Cs concentrations both inside and outside of the FDNPP port decreased exponentially with time: The respective geometric mean days of ecological half-lives were 218 d and 386 d. These results show clearly that the contamination level of marine products in Fukushima Prefecture, even within the 20 km radius area, has decreased drastically during the five years after the FDNPP accident, although 137Cs concentrations higher than 10 kBq kg−1-wet were still detected in some specimens of sedentary rockfishes (S. cheni, Sebastes oblongus, and Sebastes pachycephalus) in the FDNPP port. Fishing operations started on a trial basis in June 2012 have gradually expanded the target areas and species. Careful monitoring should be continued to accelerate the restoration of coastal fisheries in Fukushima Prefecture.

Radiocesium contamination in living and dead foliar parts of Japanese cedar during 2011–2015

Radiocesium (137Cs) activity concentrations, mainly derived from the Fukushima accident of March 2011, were measured in green foliar parts without separation by age (bulk green foliar parts; GL) and litterfall (LF) of Japanese cedar (Cryptomeria japonica) from 2011 to 2015. In all samples, 137Cs concentrations decreased exponentially over time, but were always higher in LF (7.36–0.58 Bq g-DW−1) than in GL (2.10–0.06 Bq g-DW−1). The difference in the decreasing rate between GL and LF would reflect a difference in the dominant factor of the decrease between living and dead tissues (i.e., internal translocation and weathering, respectively). Over this same timeframe, potassium (K) concentrations in both GL and LF experienced repetitive periodical changes within a certain range (0.38–3.0 mg g-DW−1 for LF and 2.08–4.77 mg g-DW−1 for GL, respectively). Thus, there was no specific correlation between 137Cs and K concentrations in LF and GL. However, analyses of the age classified green foliar parts (GL-S) and dead foliar parts still retained on trees (DL) could indicate another view. The annual changes in residual rates of both 137Cs and K concentrations in GL-S demonstrated very similar two-phase reductions (i.e., a faster reduction in each expansion year than in the following years) and an obvious linear correlation between each other. Radiocesium concentration in DL were always higher than in any part of GL-S sampled at the same timing, but K concentrations showed the reverse relation. It is probable that 137Cs is basically translocated from older parts to the developing parts (as long as the former are alive) via a seasonal nutritional flow of K; however, a part of 137Cs translocation would cease considerably earlier than the cessation of K translocation.