Soil dust and bioaerosols as potential sources for resuspended Cs occurring near the Fukushima Dai-ichi Nuclear Power Plant

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

https://doi.org/10.1016/j.jenvrad.2023.107198

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.

Contamination processes of tree components in Japanese forest ecosystems affected by the Fukushima Daiichi Nuclear Power Plant accident Cs fallout

Ota, Masakazu; Koarashi, Jun

Science of the Total Environment, 816, p.151587_1 - 151587_21, 2022/04

https://doi.org/10.1016/j.scitotenv.2021.151587

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.

Simple pretreatment method for tritium measurement in environmental water samples using a liquid scintillation counter

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

https://doi.org/10.1585/pfr.16.2405035

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).

Wintertime grassland dynamics may influence belowground biomass under climate change; A Model analysis

Katata, Genki*; Grote, R.*; Mauder, M.*; Zeeman, M. J.*; Ota, Masakazu

Biogeosciences, 17(4), p.1071 - 1085, 2020/02

https://doi.org/10.5194/bg-17-1071-2020

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.

Development of field estimation technique and improvement of environmental tritium behavior model

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

https://doi.org/10.1585/pfr.14.3405099

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.

Role of soil-to-leaf tritium transfer in controlling leaf tritium dynamics; Comparison of experimental garden and tritium-transfer model results

Ota, Masakazu; Kwamena, N.-O. A.*; Mihok, S.*; Korolevych, V.*

Journal of Environmental Radioactivity, 178-179, p.212 - 231, 2017/11

https://doi.org/10.1016/j.jenvrad.2017.09.002

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.

A Terrestrial ecosystem model (SOLVEG) coupled with atmospheric gas and aerosol exchange processes

Katata, Genki; Ota, Masakazu

JAEA-Data/Code 2016-014, 35 Pages, 2017/01

JAEA-Data-Code-2016-014.pdf:1.64MB

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.

Technical estimation for mass production of highly-concentrated Tc solution from Mo to be obtained by () reaction; A Preliminary study using inactive Re instead of Tc

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

https://doi.org/10.3769/radioisotopes.65.237

no abstracts in English

Establishment of experimental system for Mo/Tc production by neutron activation method

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

JAEA-Technology-2015-030.pdf:4.82MB

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.

Fabrication technology development and characterization of irradiation targets for Mo/Tc production by (n,) method

Nishikata, Kaori; Kimura, Akihiro; Ishida, Takuya; Shiina, Takayuki*; Ota, Akio*; Tanase, Masakazu*; Tsuchiya, Kunihiko

JAEA-Technology 2014-034, 34 Pages, 2014/10

JAEA-Technology-2014-034.pdf:3.26MB

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.