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Journal Articles

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

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.

Journal Articles

Importance of root uptake of $$^{14}$$CO$$_{2}$$ on $$^{14}$$C transfer to plants impacted by below-ground $$^{14}$$CH$$_{4}$$ release

Ota, Masakazu; Tanaka, Taku*

Journal of Environmental Radioactivity, 201, p.5 - 18, 2019/05

 Percentile:100(Environmental Sciences)

$$^{14}$$CH$$_{4}$$ released from deep underground radioactive waste disposal facilities can be a belowground source of $$^{14}$$CO$$_{2}$$ owing to microbial oxidation of $$^{14}$$CH$$_{4}$$ to $$^{14}$$CO$$_{2}$$ in soils. Environmental $$^{14}$$C models assume that the transfer of $$^{14}$$CO$$_{2}$$ from soil to plant occurs via foliar uptake of $$^{14}$$CO$$_{2}$$. Nevertheless, the importance of $$^{14}$$CO$$_{2}$$ root uptake is not well understood. In the present study, belowground transport and oxidation of $$^{14}$$CH$$_{4}$$ were modeled and incorporated into an existing land surface $$^{14}$$CO$$_{2}$$ model (SOLVEG-II) to assess the importance of root uptake on $$^{14}$$CO$$_{2}$$ transfer to plants. Performance of the model in calculating the belowground dynamics of $$^{14}$$CH$$_{4}$$ was validated by simulating a field experiment of $$^{13}$$CH$$_{4}$$ injection into subsoil. The model was then applied to $$^{14}$$C transfer in a hypothetical ecosystem impacted by continuous $$^{14}$$CH$$_{4}$$ 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 $$^{14}$$CO$$_{2}$$ was significant, accounting for 80% of the $$^{14}$$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 $$^{14}$$C accumulated in the leaves was transferred by the root uptake. We found that $$^{14}$$CO$$_{2}$$ root uptake in this ecosystem depended on the distribution of methane oxidation in the soil; all $$^{14}$$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 $$^{14}$$CO$$_{2}$$ root uptake contributes significantly to $$^{14}$$CO$$_{2}$$ transfer to plants if $$^{14}$$CH$$_{4}$$ oxidation occurs at great depths and roots penetrate deeply into the soil.

Journal Articles

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

 Times Cited Count:2 Percentile:69.03(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.

Journal Articles

Updating source term and atmospheric dispersion simulations for the dose reconstruction in Fukushima Daiichi Nuclear Power Station accident

Nagai, Haruyasu; Terada, Hiroaki; Tsuzuki, Katsunori; Katata, Genki; Ota, Masakazu; Furuno, Akiko; Akari, Shusaku

EPJ Web of Conferences (Internet), 153, p.08012_1 - 08012_7, 2017/09

 Percentile:100

In 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 $$^{137}$$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.

JAEA Reports

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.

Journal Articles

Impacts of C-uptake by plants on the spatial distribution of $$^{14}$$C accumulated in vegetation around a nuclear facility; Application of a sophisticated land surface $$^{14}$$C model to the Rokkasho reprocessing plant, Japan

Ota, Masakazu; Katata, Genki; Nagai, Haruyasu; Terada, Hiroaki

Journal of Environmental Radioactivity, 162-163, p.189 - 204, 2016/10

 Times Cited Count:3 Percentile:71(Environmental Sciences)

Impacts of plant C uptake on ($$^{14}$$C) distributions around a nuclear facility were investigated by a land surface $$^{14}$$C model (SOLVEG-II). The simulation combined the SOLVEG-II with a mesoscale model and an dispersion model was applied to $$^{14}$$CO$$_{2}$$ transfer at test operations of the Rokkasho reprocessing plant (RRP) in 2007. The calculated $$^{14}$$C-specific activities in rice grains agreed with the observations. Numerical experiment of chronic $$^{14}$$CO$$_{2}$$ release from the RRP showed that $$^{14}$$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 $$^{14}$$CO$$_{2}$$ and plant growth. $$^{14}$$C accumulation in plant significantly increased when $$^{14}$$CO$$_{2}$$ releases were limited during daytime, compared with the results observed during nighttime, due to extensive $$^{14}$$CO$$_{2}$$ uptake by daytime photosynthesis. These results indicated that plant growth and photosynthesis should be considered in predictions of ingestion dose of $$^{14}$$C for long-term chronic and short-term diurnal releases of $$^{14}$$CO$$_{2}$$, respectively.

Journal Articles

Technical estimation for mass production of highly-concentrated $$^{rm 99m}$$Tc solution from $$^{99}$$Mo to be obtained by ($$n,gamma$$) reaction; A Preliminary study using inactive Re instead of $$^{rm 99m}$$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

no abstracts in English

Journal Articles

Modeling dynamics of $$^{137}$$Cs in forest surface environments; Application to a contaminated forest site near Fukushima and assessment of potential impacts of soil organic matter interactions

Ota, Masakazu; Nagai, Haruyasu; Koarashi, Jun

Science of the Total Environment, 551-552, p.590 - 604, 2016/05

 Times Cited Count:14 Percentile:18.89(Environmental Sciences)

A model for $$^{137}$$Cs transfer in forest was developed to assess behavior of Fukushima-derived $$^{137}$$Cs. The model simulation well reproduced observed 3 year migration of $$^{137}$$Cs in organic layer and mineral soil. Long-term predictions indicated that more than 90% of the deposited $$^{137}$$Cs remains in the top 5 cm soil till 30 years, suggesting that forest acts as a long-term reservoir of $$^{137}$$Cs with limited loss via groundwater pathway. Impacts of soil organic matter (SOM) on $$^{137}$$Cs dynamics were investigated by modifying parameters of $$^{137}$$Cs turnover. The results showed that SOM-induced reduction of $$^{137}$$Cs adsorption, slower fixation of $$^{137}$$Cs by clay and enhanced mobilization of the fixed $$^{137}$$Cs elevate soil-to-plant transfer of $$^{137}$$Cs by increasing fraction of dissolved $$^{137}$$Cs. A substantial proportion (27% $$sim$$ 73%) of $$^{137}$$Cs in these soils was delivered to horizons deeper than 5 cm decades later. These results suggested that SOM significantly influences behavior of $$^{137}$$Cs over long-term.

JAEA Reports

Establishment of experimental system for $$^{99}$$Mo/$$^{99m}$$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 $$^{99}$$Mo/$$^{99m}$$Tc by neutron activation method ((n, $$gamma$$) method) using JMTR has been carried out in the Neutron Irradiation and Testing Reactor Center. The specific radioactivity of $$^{99}$$Mo by (n, $$gamma$$) method is extremely low compared with that by fission method ((n,f) method), and as a result, the radioactive concentration of the obtained $$^{99m}$$Tc solution is also lowered. To solve the problem, we propose the solvent extraction with methyl ethyl ketone (MEK) for recovery of $$^{99m}$$Tc from $$^{99}$$Mo produced by (n, $$gamma$$) method. We have developed the $$^{99}$$Mo/$$^{99m}$$Tc separation/extraction/concentration devices and have carried out the performance tests for recovery of $$^{99m}$$Tc from $$^{99}$$Mo produced by (n, $$gamma$$) method. In this paper, in order to establish an experimental system for $$^{99}$$Mo/$$^{99m}$$Tc production, the R&D results of the system are summarized on the improvement of the devices for high-recovery rate of $$^{99m}$$Tc, on the dissolution of the pellets, which is the high-density molybdenum trioxide (MoO$$_{3}$$) pellets irradiated in Kyoto University Research Reactor (KUR), on the production of $$^{99m}$$Tc, and on the inspection of the recovered $$^{99m}$$Tc solutions.

Journal Articles

Detailed source term estimation of the atmospheric release for the Fukushima Daiichi Nuclear Power Station accident by coupling simulations of an atmospheric dispersion model with an improved deposition scheme and oceanic dispersion model

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:88 Percentile:0.81(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.

JAEA Reports

Fabrication technology development and characterization of irradiation targets for $$^{99}$$Mo/$$^{99m}$$Tc production by (n,$$gamma$$) 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 $$^{99}$$Mo/$$^{99m}$$Tc by (n, $$gamma$$) method using JMTR has been carried out in the Neutron Irradiation and Testing Reactor Center of the Japan Atomic Energy Agency. $$^{99}$$Mo is usually produced by fission method. On the other hand, $$^{99}$$Mo/$$^{99m}$$Tc production by the (n, $$gamma$$) method has advantages for radioactive waste, cost reduction and non-proliferation. However, the specific radioactivity per unit volume by the (n, $$gamma$$) method is low compared with the fission method, and that is the weak point of the (n, $$gamma$$) method. This report summarizes the investigation of raw materials, the fabrication tests of high-density MoO$$_{3}$$ pellets by the plasma sintering method for increasing of $$^{98}$$Mo contents and the characterization of sintered high-density MoO$$_{3}$$ pellets.

Journal Articles

Root and dissolved organic carbon controls on subsurface soil carbon dynamics; A Model approach

Ota, Masakazu; Nagai, Haruyasu; Koarashi, Jun

Journal of Geophysical Research; Biogeosciences, 118(4), p.1646 - 1659, 2013/12

 Times Cited Count:19 Percentile:26.67(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.

Journal Articles

Development of $$^{99m}$$Tc production from (n,$$gamma$$)$$^{99}$$Mo based on solvent extraction

Kimura, Akihiro; Awaludin, R.*; Shiina, Takayuki*; Tanase, Masakazu*; Kawauchi, Yukimasa*; Gunawan, A. H.*; Lubis, H.*; Sriyono*; Ota, Akio*; Genka, Tsuguo; et al.

Proceedings of 3rd Asian Symposium on Material Testing Reactors (ASMTR 2013), p.109 - 115, 2013/11

JP, 2011-173260   Patent publication (In Japanese)

$$^{99m}$$Tc is generated by decay of $$^{99}$$Mo. Production of $$^{99}$$Mo is carried out by (n,f) method with high enriched uranium targets, and the production are currently producing to meet about 95% of global supply. Recently, it is difficult to carry out a stable supply for some problems such as aging of reactors etc. Furthermore, the production has difficulties in nuclear proliferation resistance etc. Thus, (n,$$gamma$$) method has lately attracted considerable attention. The (n,$$gamma$$) method has several advantages, but the extremely low specific activity makes its uses less convenient than (n,f) method. We proposed a method based on the solvent extraction, followed by adsorption of $$^{99m}$$Tc with alumina column. In this paper, a practical production of $$^{99m}$$Tc was tried by the method with 1Ci of $$^{99}$$Mo produced in MPR-30. The recovery yields were approximately 70%. Impurity of $$^{99}$$Mo was less than 4.0$$times$$10$$^{-5}$$% and the radiochemical purity was over 99.2%.

Journal Articles

$$^{99}$$Mo-$$^{rm 99m}$$Tc production process by (n,$$gamma$$) reaction with irradiated high-density MoO$$_{3}$$ pellets

Tsuchiya, Kunihiko; Nishikata, Kaori; Tanase, Masakazu*; Shiina, Takayuki*; Ota, Akio*; Kobayashi, Masaaki*; Yamamoto, Asaki*; Morikawa, Yasumasa*; Takeuchi, Nobuhiro*; Kaminaga, Masanori; et al.

Proceedings of 6th International Symposium on Material Testing Reactors (ISMTR-6) (Internet), 9 Pages, 2013/10

no abstracts in English

Journal Articles

Development of $$^{99m}$$Tc production from (n,$$gamma$$)$$^{99}$$Mo based on solvent extraction and column chromatography

Kimura, Akihiro; Awaludin, R.*; Shiina, Takayuki*; Tanase, Masakazu*; Kawauchi, Yukimasa*; Gunawan, A. H.*; Lubis, H.*; Sriyono*; Ota, Akio*; Genka, Tsuguo; et al.

Proceedings of 6th International Symposium on Material Testing Reactors (ISMTR-6) (Internet), 7 Pages, 2013/10

JP, 2011-173260   Patent publication (In Japanese)

This research is development of $$^{99m}$$Tc production. $$^{99m}$$Tc is generated by decay of $$^{99}$$Mo. The supply of $$^{99}$$Mo in Japan depends entirely on the import from foreign countries. Thus, it is needed to supply $$^{99}$$Mo stably by the domestic manufacturing. A practical production of $$^{99m}$$Tc was tried by the method with 1 Ci of $$^{99}$$Mo produced in MPR-30. The results showed that the recovery yields were approximately 70%. The concentration of the product obtained was estimated to be corresponding to about 30 GBq (800 mCi)/ml when 150g of MoO$$_{3}$$ was irradiated for 5 days in MPR-30. Impurity of $$^{99}$$Mo was less than 4.4$$times$$10$$^{-5}$$%, which was lower than that of Japanese tentative regulation criteria. The radiochemical purity was higher than 99.8% that cleared the tentative regulation (95%) of Japan.

Journal Articles

Fabrication and characterization of high-density MoO$$_{3}$$ pellets

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

Proceedings of 2012 Powder Metallurgy World Congress & Exhibition (PM 2012) (CD-ROM), 8 Pages, 2013/02

The renewed Japan Materials Testing Reactor (JMTR) will be started from 2012, and it is expected to contribute to many nuclear fields. Especially, in case of Japan, the supplying of $$^{99}$$Mo depends on imports from foreign countries. Japan Atomic Energy Agency (JAEA) has a plan to produce $$^{99}$$Mo, which is the parent nuclide of radiopharmaceutical $$^{rm 99m}$$Tc, and JAEA has performed the R&D for $$^{99}$$Mo production by (n,$$gamma$$) method in JMTR. Generally, molybdenum oxide (MoO$$_{3}$$) is the most popular chemical form as irradiation target for the $$^{99}$$Mo production. However, the $$^{99}$$Mo production capacity is low because of low (n,$$gamma$$) cross section and isotope composition of $$^{98}$$Mo in Mo. Thus, it is necessary to fabricate the MoO$$_{3}$$ pellets with high density for the increase of the $$^{99}$$Mo production amount. In this study, MoO$$_{3}$$ pellets fabricated by a plasma activated sintering were developed and characterization of MoO$$_{3}$$ pellets was carried out.

Journal Articles

Development of $$^{rm 99m}$$Tc production from (n, $$gamma$$) $$^{99}$$Mo

Tanase, Masakazu*; Shiina, Takayuki*; Kimura, Akihiro; Nishikata, Kaori; Fujisaki, Saburo*; Ota, Akio*; Kobayashi, Masaaki*; Yamamoto, Asaki*; Kawauchi, Yukimasa*; Tsuchiya, Kunihiko; et al.

Proceedings of 5th International Symposium on Material Testing Reactors (ISMTR-5) (Internet), 9 Pages, 2012/10

$$^{rm 99m}$$Tc is used as a radiopharmaceutical and manufactured from the parent nuclide of $$^{99}$$Mo. Extraction method of $$^{rm 99m}$$Tc from (n, $$gamma$$) $$^{99}$$Mo have been developed, as a part of the industrial use expansion after JMTR will re-start. In this research, the method proposed would be applicable to a practical production of $$^{rm 99m}$$Tc obtained from (n, $$gamma$$) $$^{99}$$Mo in large quantities. The method proposed would be applicable to a practical production of $$^{rm 99m}$$Tc obtained from (n, $$gamma$$) $$^{99}$$Mo in large quantities.

Journal Articles

Atmospheric discharge and dispersion of radionuclides during the Fukushima Dai-ichi Nuclear Power Plant accident, 1; Source term estimation and local-scale atmospheric dispersion in early phase of the accident

Katata, Genki; Ota, Masakazu; Terada, Hiroaki; Chino, Masamichi; Nagai, Haruyasu

Journal of Environmental Radioactivity, 109, p.103 - 113, 2012/07

AA2011-0656.pdf:1.56MB

 Times Cited Count:112 Percentile:2.17(Environmental Sciences)

The source term of $$^{131}$$I and $$^{137}$$Cs discharged in early phase of the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident was estimated by combining environmental data with atmospheric dispersion simulations of a computer-based nuclear emergency response system, WSPEEDI-II. Major releases were estimated when air dose rates increased after the hydrogen explosion at Unit 1 during the afternoon on 12 March and during the midnight on 14 March. The high-concentration plumes discharged during these periods caused large deposition to the northwest and south-southwest directions of FNPP1, respectively. The increases of air dose rates observed at the monitoring posts were overall reproduced by WSPEEDI-II using estimated release rates. The simulation indicated that air dose rates significantly increased in the south-southwest region of FNPP1 by dry deposition of the high-concentration plume discharged from the midnight on 14 March to the morning on 15 March.

Journal Articles

Importance of root HTO uptake in controlling land-surface tritium dynamics after an-acute HT deposition; A Numerical experiment

Ota, Masakazu; Nagai, Haruyasu; Koarashi, Jun

Journal of Environmental Radioactivity, 109, p.94 - 102, 2012/07

 Times Cited Count:11 Percentile:55.19(Environmental Sciences)

To investigate the role of belowground root HTO uptake in controlling tritium (T) dynamics in a land surface ecosystem, a sophisticated numerical model predicting tritium behavior in an atmosphere-vegetation-soil system was developed and numerical experiments were conducted using the model. The developed model covered physical HT transport in multi-layered atmosphere and soil as well as microbial HT oxidation to HTO in the soil, and was incorporated into a well-established dynamical HTO transfer-OBT formation model. The model performance was tested through a simulation of an existing HT-release experiment, and the model-predicted HT deposition to the surface soil agreed with the field data at a factor of 1.1. Numerical experiments with a hypothetical acute HT exposure to a grassland field demonstrated that preferential uptake of the deposited HTO in the upper soil layers increased HTO concentrations both in the atmosphere and in the plant leaves through an enhanced soil-leaf-atmosphere HTO stream. Consequently, leaf OBT assimilation calculated with a shallow rooting depth increased by nearly an order of magnitude larger than that calculated with a deep rooting depth.

Journal Articles

A Land surface $$^{14}$$C transfer model and numerical experiments on belowground $$^{14}$$C accumulation and its impact on vegetation $$^{14}$$C level

Ota, Masakazu; Nagai, Haruyasu; Koarashi, Jun

Journal of Environmental Radioactivity, 107, p.13 - 22, 2012/03

 Times Cited Count:5 Percentile:76.71(Environmental Sciences)

A model simulating transport and exchange for $$^{14}$$C (or $$^{14}$$CO$$_{2}$$) in a land surface ecosystem was developed and the belowground $$^{14}$$C accumulation and its impact on vegetation $$^{14}$$C level at a hypothetical cultivated field was studied with the model through numerical experiments. The developed model involved physical $$^{14}$$CO$$_{2}$$ transport in surface atmosphere and soil and physiological $$^{14}$$CO$$_{2}$$ exchanges in leaves, and was incorporated into a dynamical model (SOLVEG-II) that calculates transport and exchange for heat, water and CO$$_{2}$$. The model was tested through a simulation of an existing-experiment on an acute exposure of grape-plants to $$^{14}$$CO$$_{2}$$. The calculated $$^{14}$$C amount in leaves agreed with the observations within a factor of 1.7. A hypothetical scenario used for the numerical experiments considered an annual $$^{14}$$C input into surface soil layers via $$^{14}$$C-enriched foliage or root litter under a continually-heightened atmospheric $$^{14}$$CO$$_{2}$$ environment. The specific activity of $$^{14}$$C in the surface soil layers increased with time and eventually converged to a certain value several decades after the start of accumulation. At this equilibrium state, the increased belowground $$^{14}$$CO$$_{2}$$ production enhanced the atmospheric $$^{14}$$CO$$_{2}$$ level, and consequently $$^{14}$$CO$$_{2}$$ uptake by vegetation increased to 1.1 folds of the control one calculated without belowground $$^{14}$$C accumulation. The model results also demonstrated that $$^{14}$$C accumulated in soil can retain an enhanced vegetation $$^{14}$$C level for at least several decades even after the end of accumulation.

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