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Ota, Masakazu; Takahara, Shogo; Yoshimura, Kazuya; Nagakubo, Azusa; Hirouchi, Jun; Hayashi, Naho; Abe, Tomohisa; Funaki, Hironori; Nagai, Haruyasu
Journal of Environmental Radioactivity, 264, p.107198_1 - 107198_15, 2023/08
One of the current major radiation exposure pathways from the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident-fallout is inhalation of the re-suspended Cs occurring in air. While wind-induced soil particle resuspension has been recognized as a primary mechanism of
Cs resuspension, studies following the FDNPP accident suggested that fungal spores can be a significant source of the atmospheric
Cs particularly in the rural areas such as difficult-to-return zone (DRZ). To elucidate the relative importance of the two resuspension phenomena, we propose a model simulating resuspension of
Cs as soil particles and fungal spores, and applied it to DRZ. Our model's calculation showed that soil particle resuspension was responsible for the surface-air
Cs observed during winter-spring, but could not account for the higher
Cs concentrations observed in summer-autumn. The higher concentrations in the summer-autumn were in general reproduced by implementing fungal spore
Cs emission, that replenished low soil particle
Cs resuspension in that period. According to our model's concept,
Cs accumulation in fungal spores and high spore emission rate characterized by the rural environment were likely responsible for the abundance of spore
Cs in the air. It was inferred that the influence of the fungal spores on the atmospheric
Cs would last longer since un-decontaminated forests still exist in DRZ.
Ota, Masakazu; Koarashi, Jun
Isotope News, (784), p.28 - 31, 2022/12
In forests affected by the Fukushima Daiichi Nuclear Power Plant accident, trees became contaminated with Cs. However,
Cs transfer processes determining the tree contamination (particularly for stem wood, a prominent commercial resource) remain insufficiently understood. We propose a model (SOLVEG-R) for simulating dynamic behavior of
Cs in a forest tree-litter-soil system and applied it to contaminated forests of cedar plantation and natural oak stand in Fukushima to elucidate relative impact of distinct
Cs transfer processes determining the tree contamination. The transfer of
Cs to the trees occurred mostly (
99%) through surface uptake of
Cs trapped by needles and bark during the fallout. Root uptake of soil
Cs was several orders of magnitude lower than the surface uptake over a 50-year period following the accident. As a result, internal contamination of the trees proceeded through an enduring recycling (translocation) of
Cs absorbed on the tree surface. A significant surface uptake of
Cs through bark was suggested, contributing to 100% (leafless oak tree) and 30% (foliated cedar tree; the remaining uptake occurred at needles) of the total uptake by the trees. It was suggested that the activity concentration of
Cs in stem wood of the trees at these sites are currently (as of 2021) decreasing by 3% per year, mainly through radioactive decay of
Cs and partly through dilution effect from tree growth.
Ota, Masakazu; Koarashi, Jun
Science of the Total Environment, 816, p.151587_1 - 151587_21, 2022/04
Times Cited Count:2 Percentile:41.02(Environmental Sciences)In forests affected by the Fukushima Daiichi Nuclear Power Plant accident, trees became contaminated with Cs. However,
Cs transfer processes determining tree contamination (particularly for stem wood, which is a prominent commercial resource in Fukushima) remain insufficiently understood. This study proposes a model for simulating the dynamic behavior of
Cs in a forest tree-litter-soil system and applied it to two contaminated forests (cedar plantation and natural oak stand) in Fukushima. The model-calculated results and inter-comparison of the results with measurements elucidated the relative impact of distinct
Cs transfer processes determining tree contamination. The transfer of
Cs to trees occurred mostly (
99%) through surface uptake of
Cs directly trapped by leaves or needles and bark during the fallout. By contrast, root uptake of
Cs from the soil was unsubstantial and several orders of magnitude lower than the surface uptake over a 50-year period following the accident. As a result, the internal contamination of the trees proceeded through an enduring recycling (translocation) of
Cs absorbed on the tree surface at the time of the accident. A significant surface uptake of
Cs at the bark was identified, contributing 100% (leafless oak tree) and 30% (foliated cedar tree; the remaining surface uptake occurred at the needles) of the total
Cs uptake by trees. It was suggested that the trees growing at the study sites are currently (as of 2021) in a decontamination phase; the activity concentration of
Cs in the stem wood decreases by 3% per year, mainly through radioactive decay of
Cs and partly through a dilution effect from tree growth.
Hashimoto, Shoji*; Tanaka, Taku*; Komatsu, Masabumi*; Gonze, M.-A.*; Sakashita, Wataru*; Kurikami, Hiroshi; Nishina, Kazuya*; Ota, Masakazu; Ohashi, Shinta*; Calmon, P.*; et al.
Journal of Environmental Radioactivity, 238-239, p.106721_1 - 106721_10, 2021/11
Times Cited Count:4 Percentile:33.91(Environmental Sciences)This study was aimed at analysing performance of models for radiocesium migration mainly in evergreen coniferous forest in Fukushima, by inter-comparison between models of several research teams. The exercise included two scenarios of countermeasures against the contamination, namely removal of soil surface litter and forest renewal, and a specific konara oak forest scenario in addition to the evergreen forest scenario. All the models reproduced trend of time evolution of radiocesium inventories and concentrations in each of the components in forest such as leaf and organic soil layer. However, the variations between models enlarged in long-term predictions over 50 years after the fallout, meaning continuous field monitoring and model verification/validation is necessary.
Nakasone, Shunya*; Yokoyama, Sumi*; Takahashi, Tomoyuki*; Ota, Masakazu; Kakiuchi, Hideki*; Sugihara, Shinji*; Hirao, Shigekazu*; Momoshima, Noriyuki*; Tamari, Toshiya*; Shima, Nagayoshi*; et al.
Plasma and Fusion Research (Internet), 16, p.2405035_1 - 2405035_5, 2021/02
Removal of impurities such as organic and other types of dissolved matters from environmental water samples is required for precise analysis of tritium with a liquid scintillation counting method. In general, a distillation method is a conventional one for tritium analysis in environmental water samples, but is a time-consuming process that takes 24 hours for removal of impurities. We have proposed a rapid pretreatment method for tritium analysis, that uses ion exchange resins. In this study, we performed batch experiments, to evaluate the effectiveness of the ion exchange resins on the tritium measurement. The results obtained demonstrated that removal of impurities in the sample water by ion exchange resins can be achieved during a short period of time (i.e., in 5 min).
Nakasone, Shunya*; Yokoyama, Sumi*; Takahashi, Tomoyuki*; Ota, Masakazu; Kakiuchi, Hideki*; Sugihara, Shinji*; Hirao, Shigekazu*; Momoshima, Noriyuki*; Tamari, Toshiya*; Shima, Nagayoshi*; et al.
Plasma and Fusion Research (Internet), 15, p.2405027_1 - 2405027_3, 2020/05
A quick preprocessing system for tritium analysis of environmental samples is important to judge environmental influence of tritium releases due to accident or tritium-handling facilities. Analysis of tritium in water samples with liquid scintillation counting method requires removal of impurities such as organic matter and ion species from water samples. Generally, a distillation method is adopted as a pretreatment of analysis for tritium; however, the distillation method is a time-consuming process. The aim of this study is to evaluate a rapid pretreatment method for tritium analysis with ion exchange resin. From batch and column experiments that used inland water and ion exchange resin, we confirmed removals of impurities of the water sample and that the removal of impurities was possible for a short time (by 5 minutes).
Katata, Genki*; Grote, R.*; Mauder, M.*; Zeeman, M. J.*; Ota, Masakazu
Biogeosciences, 17(4), p.1071 - 1085, 2020/02
Times Cited Count:8 Percentile:61.57(Ecology)Mountain grassland productivity is limited by cold and long winters; thus, rising temperatures and changes in snow cover expected in the future may have large impacts on the grassland yields. To investigate this, we enhanced land surface model (SOLVEG) to account for snow, freeze-thaw events, and grass growth, and the model was applied to the managed grasslands affected by extremely warm winter. The model reproduced temporal variability of observed heat fluxes, soil temperatures and snow depth throughout the 3-year simulation period. High physiological activity during the extremely warm winter led to a CO uptake of 100 g-C m
, which was, surprisingly, mainly allocated into the below-ground biomass and rarely used for plant growth during spring. This process, which is so far largely unaccounted for in global terrestrial biosphere models, may lead to carbon accumulation in the soil and/or heterotrophic respiration as a response to global warming.
Ota, Masakazu; Terada, Hiroaki; Hasegawa, Hidenao*; Kakiuchi, Hideki*
Science of the Total Environment, 704, p.135319_1 - 135319_15, 2020/02
Times Cited Count:6 Percentile:26.7(Environmental Sciences)Land-surface transfers of I are modeled and incorporated into a land-surface model (SOLVEG-II), and the model was applied to the observed transfer of
I at a vegetated field impacted by atmospheric releases of
I from Rokkasho reprocessing plant during 2007 to investigate the importance of each
I-transfer pathway. The model calculation revealed that contamination of leaves of wild bamboo grasses was mostly caused by foliar adsorption of
I (81%) induced via wet deposition of
I. Wet deposition of
I was the main
I-input to the soil, ten-fold the dry deposition of
I
; however, the deposition of
I during 2007 was only 2% of the model-assumed
I that pre-existed in the soil; indicating the importance of long-term accumulation of
I in soils. The model calculation also revealed that root uptake of
I, not methylation, control the long-term turnover of soil
I.
Yokoyama, Sumi*; Takahashi, Tomoyuki*; Ota, Masakazu; Kakiuchi, Hideki*; Sugihara, Shinji*; Hirao, Shigekazu*; Momoshima, Noriyuki*; Tamari, Toshiya*; Shima, Nagayoshi*; Atarashi-Andoh, Mariko; et al.
Plasma and Fusion Research (Internet), 14(Sp.2), p.3405099_1 - 3405099_4, 2019/06
The Large Helical Device of the National Institute for Fusion Science started D-D experiments in 2017. To ensure the safety of the facility, it is important to develop evaluation methods for environmental tritium transfer. Tritiated water (HTO) in atmosphere and soil is transferred to plants, and organically bound tritium (OBT) is formed by photosynthesis. Prediction of OBT formation is important, because OBT accumulates in plants and causes dose through ingestion. The objective of this study is to estimate environmental tritium transfer using a simple compartment model and practical parameters. We proposed a simple compartment model consisting of air-soil-plant components, and tried to validate the model by comparison with a sophisticated model, SOLVEG. In this study, we plan to add wet deposition to the model and obtain parameters from measurements of soil permeability and tritium concentrations in air, soil and plants. We also establish rapid pretreatment methods for OBT analysis.
Ota, Masakazu; Tanaka, Taku*
Journal of Environmental Radioactivity, 201, p.5 - 18, 2019/05
Times Cited Count:3 Percentile:15.43(Environmental Sciences)CH
released from deep underground radioactive waste disposal facilities can be a belowground source of
CO
owing to microbial oxidation of
CH
to
CO
in soils. Environmental
C models assume that the transfer of
CO
from soil to plant occurs via foliar uptake of
CO
. Nevertheless, the importance of
CO
root uptake is not well understood. In the present study, belowground transport and oxidation of
CH
were modeled and incorporated into an existing land surface
CO
model (SOLVEG-II) to assess the importance of root uptake on
CO
transfer to plants. Performance of the model in calculating the belowground dynamics of
CH
was validated by simulating a field experiment of
CH
injection into subsoil. The model was then applied to
C transfer in a hypothetical ecosystem impacted by continuous
CH
input from the water table (bottom of one-meter thick soil). In a shallowly rooted ecosystem with rooting depth of 11 cm, foliar uptake of
CO
was significant, accounting for 80% of the
C accumulation in the leaves. In a deeply rooted ecosystem (rooting depth of 97 cm), where the root penetrated to depths close to the water-table, more than half (63%) the
C accumulated in the leaves was transferred by the root uptake. We found that
CO
root uptake in this ecosystem depended on the distribution of methane oxidation in the soil; all
C accumulated in the leaves was transferred by the root uptake when methane oxidation occurred at considerable depths (e-folding depths of 20 cm, or 80 cm). These results indicate that
CO
root uptake contributes significantly to
CO
transfer to plants if
CH
oxidation occurs at great depths and roots penetrate deeply into the soil.
Ota, Masakazu; Kwamena, N.-O. A.*; Mihok, S.*; Korolevych, V.*
Journal of Environmental Radioactivity, 178-179, p.212 - 231, 2017/11
Times Cited Count:11 Percentile:39.46(Environmental Sciences)Environmental transfer models assume that organically-bound tritium (OBT) is formed directly from tissue-free water tritium (TFWT) in environmental compartments. Nevertheless, studies in the literature have shown that measured OBT/TFWT ratios are variable. The importance of soil-to-leaf HTO transfer pathway in controlling the leaf tritium dynamics is not well understood. A model inter-comparison of two tritium transfer models (CTEM-CLASS-TT and SOLVEG-II) was carried out with measured environmental samples from an experimental garden plot set up next to a tritium-processing facility. The garden plot received one of three different irrigation treatments - no external irrigation, irrigation with low tritium water and irrigation with high tritium water. The contrast between the results obtained with the different irrigation treatments provided insights into the impact of soil-to-leaf HTO transfer on the leaf tritium dynamics. Concentrations of TFWT and OBT in the garden plots that were not irrigated or irrigated with low tritium water were variable, responding to the arrival of the HTO-plume from the tritium-processing facility. In contrast, for the plants irrigated with high tritium water, the TFWT concentration remained elevated due to a continuous source of high HTO in the soil. Calculated concentrations of OBT in the leaves showed an initial increase followed by quasi-equilibration with the TFWT concentration. In this quasi-equilibrium state, concentrations of OBT remained elevated and unchanged despite the arrivals of the plume. These results from the model inter-comparison demonstrate that soil-to-leaf HTO transfer significantly affects OBT/TFWT ratio in the leaf regardless of the atmospheric HTO concentration, only if there is elevated HTO concentrations in the soil. The results of this work indicate that assessment models should be refined to consider the importance of soil-to-leaf HTO transfer to ensure that dose estimates are accurate and conservative.
Nagai, Haruyasu; Terada, Hiroaki; Tsuzuki, Katsunori; Katata, Genki; Ota, Masakazu; Furuno, Akiko; Akari, Shusaku
EPJ Web of Conferences, 153, p.08012_1 - 08012_7, 2017/09
Times Cited Count:3 Percentile:88.07In order to assess the radiological dose to the public resulting from the Fukushima Daiichi Nuclear Power Station (FDNPS) accident in Japan, the spatiotemporal distribution of radioactive materials in the environment are reconstructed by computer simulations. In this study, by refining the source term of radioactive materials and modifying the atmospheric dispersion model (ATDM), the atmospheric dispersion simulation of radioactive materials is improved. Then, a database of spatiotemporal distribution of radioactive materials in the air and on the ground surface is developed from the output of the simulation. This database is used in other studies for the dose assessment by coupling with the behavioral pattern of evacuees from the FDNPS accident. The ATDM simulation was improved to use a new meteorological model and sophisticated deposition scheme. Although the improved ATDM simulations reproduced well the Cs deposition pattern in the eastern Japan scale, the reproducibility of deposition pattern was decreased in the vicinity of FDNPS. This result indicated the necessity of further refinement of the source term by optimization to the improved ATDM simulations.
Katata, Genki; Ota, Masakazu
JAEA-Data/Code 2016-014, 35 Pages, 2017/01
In order to predict the impact of atmospheric pollutants (gases and aerosols) to the terrestrial ecosystem, new schemes for calculating the processes of dry deposition of gases and aerosols, and water and carbon cycles in terrestrial ecosystems were implemented in the one-dimensional atmosphere-SOiL-VEGetation model, SOLVEG. We made performance tests at various vegetation areas to validate the newly developed schemes. In this report, the detail in each modeled process is described with an instruction how to use the modified SOLVEG. The framework of "terrestrial ecosystem model" was developed for investigation of a change in water, energy, and carbon cycles associated with global warming and air pollution and its impact on terrestrial ecosystems.
Ota, Masakazu; Katata, Genki; Nagai, Haruyasu; Terada, Hiroaki
Journal of Environmental Radioactivity, 162-163, p.189 - 204, 2016/10
Times Cited Count:7 Percentile:21.3(Environmental Sciences)Impacts of plant C uptake on (C) distributions around a nuclear facility were investigated by a land surface
C model (SOLVEG-II). The simulation combined the SOLVEG-II with a mesoscale model and an dispersion model was applied to
CO
transfer at test operations of the Rokkasho reprocessing plant (RRP) in 2007. The calculated
C-specific activities in rice grains agreed with the observations. Numerical experiment of chronic
CO
release from the RRP showed that
C-specific activities of rice plants at harvest differed from the annual mean ones in the air, which was attributed to seasonal variations in atmospheric
CO
and plant growth.
C accumulation in plant significantly increased when
CO
releases were limited during daytime, compared with the results observed during nighttime, due to extensive
CO
uptake by daytime photosynthesis. These results indicated that plant growth and photosynthesis should be considered in predictions of ingestion dose of
C for long-term chronic and short-term diurnal releases of
CO
, respectively.
Tanase, Masakazu*; Fujisaki, Saburo*; Ota, Akio*; Shiina, Takayuki*; Yamabayashi, Hisamichi*; Takeuchi, Nobuhiro*; Tsuchiya, Kunihiko; Kimura, Akihiro; Suzuki, Yoshitaka; Ishida, Takuya; et al.
Radioisotopes, 65(5), p.237 - 245, 2016/05
no abstracts in English
Ota, Masakazu; Nagai, Haruyasu; Koarashi, Jun
Science of the Total Environment, 551-552, p.590 - 604, 2016/05
Times Cited Count:30 Percentile:72.63(Environmental Sciences)A model for Cs transfer in forest was developed to assess behavior of Fukushima-derived
Cs. The model simulation well reproduced observed 3 year migration of
Cs in organic layer and mineral soil. Long-term predictions indicated that more than 90% of the deposited
Cs remains in the top 5 cm soil till 30 years, suggesting that forest acts as a long-term reservoir of
Cs with limited loss via groundwater pathway. Impacts of soil organic matter (SOM) on
Cs dynamics were investigated by modifying parameters of
Cs turnover. The results showed that SOM-induced reduction of
Cs adsorption, slower fixation of
Cs by clay and enhanced mobilization of the fixed
Cs elevate soil-to-plant transfer of
Cs by increasing fraction of dissolved
Cs. A substantial proportion (27%
73%) of
Cs in these soils was delivered to horizons deeper than 5 cm decades later. These results suggested that SOM significantly influences behavior of
Cs over long-term.
Ishida, Takuya; Shiina, Takayuki*; Ota, Akio*; Kimura, Akihiro; Nishikata, Kaori; Shibata, Akira; Tanase, Masakazu*; Kobayashi, Masaaki*; Sano, Tadafumi*; Fujihara, Yasuyuki*; et al.
JAEA-Technology 2015-030, 42 Pages, 2015/11
The research and development (R&D) on the production of Mo/
Tc by neutron activation method ((n,
) method) using JMTR has been carried out in the Neutron Irradiation and Testing Reactor Center. The specific radioactivity of
Mo by (n,
) method is extremely low compared with that by fission method ((n,f) method), and as a result, the radioactive concentration of the obtained
Tc solution is also lowered. To solve the problem, we propose the solvent extraction with methyl ethyl ketone (MEK) for recovery of
Tc from
Mo produced by (n,
) method. We have developed the
Mo/
Tc separation/extraction/concentration devices and have carried out the performance tests for recovery of
Tc from
Mo produced by (n,
) method. In this paper, in order to establish an experimental system for
Mo/
Tc production, the R&D results of the system are summarized on the improvement of the devices for high-recovery rate of
Tc, on the dissolution of the pellets, which is the high-density molybdenum trioxide (MoO
) pellets irradiated in Kyoto University Research Reactor (KUR), on the production of
Tc, and on the inspection of the recovered
Tc solutions.
Katata, Genki; Chino, Masamichi; Kobayashi, Takuya; Terada, Hiroaki; Ota, Masakazu; Nagai, Haruyasu; Kajino, Mizuo*; Draxler, R.*; Hort, M.*; Malo, A.*; et al.
Atmospheric Chemistry and Physics, 15(2), p.1029 - 1070, 2015/01
Times Cited Count:204 Percentile:98.9(Environmental Sciences)We estimated a detailed time trend of atmospheric releases during the Fukushima Dai-ichi Power Station (FNPS1) accident by combining environmental monitoring data with coupling simulation of atmospheric model of WSPEEDI-II, and oceanic dispersion model of SEA-GEARN-FDM. The new scheme of dry and fogwater depositions, in-cloud scavenging, cloud condensation nuclei activity, and wet scavenging by ice phase for radioactive iodine gas and other particles was incorporated into WSPEEDI-II. The results revealed that the major releases of radionuclides occurred in the following periods: afternoon on 12 March when the wet venting and hydrogen explosion at Unit 1, morning on 13 March after the venting event at Unit 3, midnight on 14 March when three-time openings of SRV were conducted at Unit 2, morning and night on 15 March, and morning on 16 March.
Nishikata, Kaori; Kimura, Akihiro; Ishida, Takuya; Shiina, Takayuki*; Ota, Akio*; Tanase, Masakazu*; Tsuchiya, Kunihiko
JAEA-Technology 2014-034, 34 Pages, 2014/10
As a part of utilization expansion after the Japan Material Testing Reactor (JMTR) re-start, research and development (R&D) on the production of medical radioisotope Mo/
Tc by (n,
) method using JMTR has been carried out in the Neutron Irradiation and Testing Reactor Center of the Japan Atomic Energy Agency.
Mo is usually produced by fission method. On the other hand,
Mo/
Tc production by the (n,
) method has advantages for radioactive waste, cost reduction and non-proliferation. However, the specific radioactivity per unit volume by the (n,
) method is low compared with the fission method, and that is the weak point of the (n,
) method. This report summarizes the investigation of raw materials, the fabrication tests of high-density MoO
pellets by the plasma sintering method for increasing of
Mo contents and the characterization of sintered high-density MoO
pellets.
Ota, Masakazu; Nagai, Haruyasu; Koarashi, Jun
Journal of Geophysical Research; Biogeosciences, 118(4), p.1646 - 1659, 2013/12
Times Cited Count:33 Percentile:72.06(Environmental Sciences)We investigated the role of root litter input and dissolved organic carbon (DOC) transport in controlling subsurface SOC dynamics by a soil C model. The model involves C turnover by decomposition, interaction between SOC and DOC, and DOC transport along water flows, for three C pools (characterized by turnover times of years, decades and millennia). Model simulations with a range of rooting profiles demonstrated a large proportion (36-78%) of SOC is apportioned to the subsurface (below the first 30 cm) soils and a significant part (39-73%) of the subsurface SOC turns over decadally. DOC transport appeared to be dominant for distributing the C to the deeper horizons. Our results suggest soil C studies focusing on the surface alone significantly underestimate the stock of decadally cycling C and underpredict the responses of soil C to global changes.