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Journal Articles

Evaluating the effectiveness of a geostatistical approach with groundwater flow modeling for three-dimensional estimation of a contaminant plume

Takai, Shizuka; Shimada, Taro; Takeda, Seiji; Koike, Katsuaki*

Journal of Contaminant Hydrology, 251, p.104097_1 - 104097_12, 2022/12

 Times Cited Count:1 Percentile:45.84(Environmental Sciences)

When assessing the risk from an underground environment that is contaminated by radioactive nuclides and hazardous chemicals and planning for remediation, the contaminant plume distribution and the associated uncertainty from measured data should be estimated accurately. While the release history of the contaminant plume may be unknown, the extent of the plume caused by a known source and the associated uncertainty can be calculated inversely from the concentration data using a geostatistical method that accounts for the temporal correlation of its release history and groundwater flow modeling. However, the preceding geostatistical approaches have three drawbacks: (1) no applications of the three-dimensional plume estimation in real situations, (2) no constraints for the estimation of the plume distribution, which can yield negative concentration and large uncertainties, and (3) few applications to actual cases with multiple contaminants. To address these problems, the non-negativity constraint using Gibbs sampling was incorporated into the geostatistical method with groundwater flow modeling for contaminant plume estimation. This method was then tested on groundwater contamination in the Gloucester landfill in Ontario, Canada. The method was applied to three water soluble organic contaminants: 1,4-dioxane, tetrahydrofuran, and diethyl ether. The effectiveness of the proposed method was verified by the general agreement of the calculated plume distributions of the three contaminants with concentration data from 66 points in 1982 (linear correlation coefficient of about 0.7). In particular, the reproduced large spill of organic contaminants of 1,4-dioxane in 1978 was more accurate than the result of preceding minimum relative entropy-based studies. The same peak also appeared in the tetrahydrofuran and diethyl ether distributions approximately within the range of the retardation factor derived from the fraction of organic carbon.

Journal Articles

Development of the evaluation method for the mobile radioactive contaminants for assessing public exposure risk in accidental events during decommissioning of nuclear power station

Sasagawa, Tsuyoshi; Shimada, Taro; Takeda, Seiji

Proceedings of 31st International Conference Nuclear Energy for New Europe (NENE2022) (USB Flash Drive), 8 Pages, 2022/12

In the risk assessment of the decommissioning phase, the inventory of radioactivity accumulated in filters and other materials changes with the progress of dismantling work under normal conditions, and a method that can evaluate the public exposure dose during an accident in which these changes are taken into account is required. The inventories (the mobile radioactive contaminants) include filters in which radioactive dust dispersed by equipment cutting work has accumulated and combustible waste generated by decontamination work. In this study, we developed a method to evaluate the accumulation of mobile contaminants in filters by calculating the amount of dust transferred into the air during equipment cutting operations using a model that evaluates the volume of the cutting kerf width and the dispersion ratio. Furthermore, the amount of the mobile contaminants that accumulates in local filters and building filters for each equipment was evaluated using this method, taking into account differences in cutting methods (underwater or in air) and work processes, and the equipment and work processes that should be focused on during regulatory inspections were studied preliminarily. It was suggested that some equipment cut in air generate the same amount of the mobile contaminants compared to reactor internals with high radioactivity that are cut in underwater. This indicates that the mobile contaminant is one of the important indicators in nuclear regulatory inspections that influence the selection of inspection targets.

Journal Articles

Experimental study on the localization and estimation of radioactivity in concrete rubble using image reconstruction algorithms

Takai, Shizuka; Namekawa, Masakazu*; Shimada, Taro; Takeda, Seiji

IEEE Transactions on Nuclear Science, 69(7), p.1789 - 1798, 2022/07

 Times Cited Count:0 Percentile:0.01(Engineering, Electrical & Electronic)

To reduce a large amount of contaminated concrete rubble stored in the Fukushima Daiichi Nuclear Power Station site, recycling low-radioactivity rubble within the site is a possible remedy. To promote recycling while ensuring safety, not only the average radioactivity but also the radioactivity distribution of concrete rubble should be efficiently evaluated because the details of rubble contamination caused by the accident remain unclear and likely include hotspots. However, evaluating inhomogeneous contamination of thick and/or dense materials is difficult using previous measurement systems, such as clearance monitors. This study experimentally confirmed the potential applicability of image reconstruction algorithms for radioactivity distribution evaluation in concrete rubble filled in a chamber. Radiation was measured using plastic scintillation fiber around the chamber (50 $$times$$ 50 $$times$$ 40 cm$$^{3}$$). Localized hotspots were simulated using standard sources of $$^{137}$$Cs, which is one of the main nuclides of contaminated rubble. The radioactivity distribution was calculated for 100 or 50 voxels (voxel size: (10 cm)$$^{3}$$ or 10 $$times$$ 10 $$times$$ 20 cm$$^{3}$$) constituting the chamber. For 100 voxels, inner hotspots were undetected, whereas, for 50 voxels, both inner and surface hotspots were reconstructible. The distribution evaluated using the maximum likelihood expectation maximization algorithm was the most accurate; the average radioactivity was estimated within 70% accuracy in all seven cases.

Journal Articles

Evaluation of clearance level for radionuclides in asbestos-containing wastes

Shimada, Taro; Nemoto, Hiromi*; Takeda, Seiji

Hoken Butsuri (Internet), 57(1), p.5 - 29, 2022/03

Of the asbestos-containing wastes arising from the dismantling activities of nuclear facilities, those with radioactive concentrations that do not need to be treated as radioactive substances will be cleared from the nuclear regulatory control. Those will be disposed of or recycled as specially controlled industrial waste based on the Waste Management and Public Cleansing Act. The authors constructed evaluation scenarios according to the treatment manual for asbestos-containing waste and evaluated public exposure doses per year for 33 radionuclides. Based on the evaluated doses, the radioactive concentration corresponding to the dose criteria of 10 $$mu$$Sv/y for clearance was calculated for each radionuclide and scenario. As a result, the evaluated concentration was equal to or higher than the current clearance level. It was confirmed that the application of the current clearance level for asbestos-containing wastes did not affect safety.

Journal Articles

Development of dose evaluation method considering radionuclides migration on the surface of the site for confirmation of completion of decommissioning

Miwa, Kazuji; Namekawa, Masakazu*; Shimada, Taro; Takeda, Seiji

MRS Advances (Internet), 7(7-8), p.165 - 169, 2022/03

We have developed evaluation method of radiocesium (RCs) migration by surface runoff and soil erosion in considering vertical distribution of RCs in initial contaminated soil and concentration of RCs in different particle size. RCs migration on ground surface during single year has been evaluated in virtual site contaminated uniformly by Cs-137. As a result, RCs has concentrated in the impoundment, and 0.18% of total inventory in the site migrated into the sea. These results suggest that surface migration of RCs effects increasing of external exposure at impoundment and internal exposure from ingestion of marine product.

Journal Articles

Determination of $$^{135}$$Cs/$$^{137}$$Cs isotopic ratio in soil collected near Fukushima Daiichi Nuclear Power Station through mass spectrometry

Shimada, Asako; Tsukahara, Takehiko*; Nomura, Masao*; Kim, M. S.*; Shimada, Taro; Takeda, Seiji; Yamaguchi, Tetsuji

Journal of Nuclear Science and Technology, 58(11), p.1184 - 1194, 2021/11

 Times Cited Count:3 Percentile:46.88(Nuclear Science & Technology)

Determining the completeness of nuclear reactor decommissioning is an important step in safely utilizing nuclear power. For example, $$^{137}$$Cs from the Fukushima Daiichi Nuclear Power Station (FDNPS) accident can be treated as background radioactivity, so determining the origin of $$^{137}$$Cs is essential. To accomplish this, measuring the $$^{135}$$Cs/$$^{137}$$Cs isotope ratio can be useful, so this study optimized a solvent extraction method, with calix[4]arene-bis(t-octylbenzo-crown-6) [BOBCalixC6] in 1-octanol, to purify radioactive Cs, radiocesium, from a solution of major environmental soil elements and mass spectrometry interference elements. This optimized method was applied to Cs purification in soil samples (40 g), and the final solutions contained a total of 10$$mu$$g/ml of the major soil elements and ng/ml concentrations at most of interfering elements. Soil samples collected near the FDNPS were then purified, and the $$^{135}$$Cs/$$^{137}$$Cs isotope ratios were measured, using both thermal ionization mass spectrometry (TIMS) and triple quadrupole induced coupled plasma mass spectrometry (ICP-QQQ). The results of each of these measurements were compared, and we found that Cs isotope ratios obtained by TIMS were more precise, by an order of magnitude, while the ICP-QQQ results possessed good abundance sensitivities. A slightly higher $$^{135}$$Cs/$$^{137}$$Cs ratio in the northwest area of the FDNPS was observed, while other areas exhibited similar values, all within the measurement error range, which indicated different origins of radiocesium. These results agreed with previously reported $$^{134}$$Cs/$$^{137}$$Cs activity distributions, suggesting that this ratio may be useful in identifying radiocesium origins for evaluating future nuclear reactor decommissions.

Journal Articles

Estimation of contaminated materials concentration by a geostatistical method with groundwater flow

Takai, Shizuka; Shimada, Taro; Takeda, Seiji; Koike, Katsuaki*

Joho Chishitsu, 32(3), P. 95, 2021/09

We received best presentation award GEOINROUM-2021 for the presentation on "Estimation of contaminated materials concentration by a geostatistical method with groundwater flow". We submit the comments of impression for getting the Award to Geoinformatics.

Journal Articles

Current status and issues for clearance, 5; Concept of restricted use of contaminated rubbles in the Fukushima Daiichi NPS

Shimada, Taro; Miwa, Kazuji; Takeda, Seiji

Nihon Genshiryoku Gakkai-Shi ATOMO$$Sigma$$, 61(7), p.531 - 534, 2019/07

Rubbles less than 5 $$mu$$Sv/h of surface dose rate, which are stored outdoor in the Fukushima Daiichi NPS (1F) site, will be recycled and applied in a restricted reuse only within 1F site in the future. However, there is no precedent for establishing the reference values such as dose and/or concentration for reuse or recycling under the existing exposure situation. In this study, we suggested a concept for establishing the reference radioactive concentration of recycling material for the restricted use in the 1F site. In addition, based on the concept, we calculated the reference radiocesium concentrations of the recycling material used for paved roads and the bases of concrete building.

Journal Articles

Study on restricted use of contaminated rubble on Fukushima Daiichi NPS site, 2; Validation of reference radiocesium concentration for recycling materials

Miwa, Kazuji; Shimada, Taro; Takeda, Seiji

Progress in Nuclear Science and Technology (Internet), 6, p.166 - 170, 2019/01

In this study, in order to validate the restricted use of recycling material at the reference radiocesium concentration (determined in series report (1)), we evaluated worker annual doses, air dose rate at the site boundary and impact of migrated radiocesium into groundwater. Firstly, we evaluated the additional annual dose for workers, on the assumption that typical workers coming in contact with the source after construction (Road: 1.2 mSv/y, Building: 1.3 mSv/y). Secondly, we evaluated the air dose rates by distance from road and building including recycling material, and investigated the distance for not exceeding 1 mSv/y (including additional dose rate by recycling and background dose rate of 0.6 mSv/y) at the site boundary (Road: 25 m, Building: 1 m). Thirdly, we evaluated the Cs migration in groundwater, and investigated the distance required for satisfying the operation target value ($$^{134}$$Cs: 1 Bq/L, $$^{137}$$Cs: 1 Bq/L) at the boundary (coastal line) (Road: 10 m, Building: 10 m).

Journal Articles

Study on restricted use of contaminated rubble on Fukushima Daiichi NPS site, 1; Estimation of reference radiocesium concentration for recycling materials

Shimada, Taro; Miwa, Kazuji; Takeda, Seiji

Progress in Nuclear Science and Technology (Internet), 6, p.203 - 207, 2019/01

Rubbles less than 5 $$mu$$Sv/h of surface dose rate, which are stored outdoor in the Fukushima Daiichi NPS (1F) site, will be recycled and applied in a restricted reuse only within 1F site in the future. In this study, we suggested a concept for establishing the reference radioactive concentration of recycling material for the restricted use in the 1F site. Reference radiocesium concentration is calculated so that increased dose rate by restricted reuse does not exceed 1 $$mu$$Sv/h which is the minimum value of dose rate map in the 1F entire site. In order to justify the restricted reuse under the reference concentration calculated, additional occupational dose, dose rate at the site boundary and groundwater concentration at the outlet to the ocean are evaluated and confirmed that the values are below 2 mSv/y, 1 mSv/y and 1 Bq/cm$$^{3}$$ of $$^{134}$$Cs and $$^{137}$$Cs, respectively. And then calculated the reference radiocesium concentrations of the recycling material used for paved roads and the bases of concrete building.

Journal Articles

Study on criticality in natural barrier for disposal of fuel debris from Fukushima Daiichi NPS

Shimada, Taro; Takubo, Kazuya*; Takeda, Seiji; Yamaguchi, Tetsuji

Progress in Nuclear Science and Technology (Internet), 5, p.183 - 187, 2018/11

After fuel debris is removed from the reactor containment vessel at Fukushima Daiichi NPS (1F) and collected in waste containers in the future, the waste containers will be disposed at a deep geological repository. The uranium inventory and uranium-235 ($$^{235}$$U) enrichment of the fuel debris are larger than those of high-level vitrified wastes which are produced from liquid waste during reprocessing of spent nuclear fuels. Therefore, there is a possibility not to be excluded that a criticality occurs in the geological media where the uranium precipitates at the far-field from the repository, after the uranium located in the repository is dissolved by groundwater. In this study, we calculated the quantity of uranium precipitated at the natural barrier, and studied dimension of uranium deposited in the natural barrier and carried out the criticality analysis.

Journal Articles

Evaluation of influence of splay fault growth on groundwater flow around geological disposal system

Takai, Shizuka; Takeda, Seiji; Sakai, Ryutaro*; Shimada, Taro; Munakata, Masahiro; Tanaka, Tadao

Nihon Genshiryoku Gakkai Wabun Rombunshi, 16(1), p.34 - 48, 2017/03

In geological disposal, direct effect on geological repositories by active faults is avoided at a stage of site characterization; however, uncertainty remains for avoidance of faults derived from the active faults, which are concealed deep under the ground and difficult to detect in advance. In this research, the influence of growth of undetected splay faults on natural barrier in a geological disposal system, which will caused by attack of the faults in the future, was evaluated. We investigated examples of splay faults in Japan and set conditions for growth of splay faults. Furthermore, we assumed a disposal site composed of sedimentary rocks and made a hydrogeological model of growth of splay faults. We carried out groundwater flow analyses, changing parameters such as location and depth of repository and growth velocity of splay faults. The results indicate that main flow path from the repository is changed into upward flow along the splay fault due to its growth and the average velocity to the ground surface becomes one or two orders of magnitude higher than that before its growth. The results also suggest that the splay fault growth leads into the possibility of downward flow of oxidizing groundwater from ground surface area.

Journal Articles

Sensitivity analysis on safety functions of engineered and natural barriers for fuel debris disposal

Shimada, Taro; Nishimura, Yuki; Takeda, Seiji

MRS Advances (Internet), 2(12), p.687 - 692, 2017/01

A disposal measure for fuel debris generated at the accident in the Fukushima Daiichi Nuclear Power Station has been studied so far. However, physical and chemical properties of the fuel debris have not yet investigated in reactor containment vessels. In order to investigate the safety function of barriers required for disposal of fuel debris, sensitivity analyses for radionuclide migration were carried out, considering with uncertainty of the properties. As a result, it is indicated that it was important for evaluation of fuel debris disposal to obtain the physical and chemical properties of $$^{14}$$C and $$^{129}$$I during release to groundwater, in addition to $$^{238}$$U.

Journal Articles

Evaluation for influence of new volcanic eruption on geological disposal site

Shimada, Taro; Takeda, Seiji; Sakai, Ryutaro*; Takubo, Kazuya; Tanaka, Tadao

MRS Advances (Internet), 1(61), p.4081 - 4086, 2016/00

Volcanic eruption which may affect geological disposal system directly depends on the regional location in Japan. It is required that the disposal site should be located far from existing volcanos. However, there are regions where it is impossible to exclude the possibility of appearance of new volcanic activity on the site even if the site is located far from existing volcanos. In order to identify the influence of volcanic eruption at disposal site to public if it occurs public exposure doses were evaluated based on the two scenarios considering types of eruption at new volcanic activity in Japan. One is the exposure by tephra widespread by Strombolian eruption and deposited on the ground surface, including radionuclides from vitrified waste forms after a volcanic conduit penetrated disposal galleries. The other is that by waste forms appeared at the surface by Merapi type pyroclastic flow. Exposure doses of the residents living on the tephra do not exceed 1mSv/y even when the eruption occurs at 1,000 years after closure of disposal site. Dose rate for the volcanic researchers temporarily approaching waste forms becomes less than 1mSv/h when the eruption occurs 100,000 years after. It indicated that attention should be paid to the impact by Merapi type pyroclastic flow on researchers approaching waste forms appeared rather than that by Strombolian eruption on residents living on the tephra widespread.

JAEA Reports

Development of ESRAD2 program for estimation of spatial radioactivity distribution based on Kriging; User's manual

Ishigami, Tsutomu; Shimada, Taro; Seki, Masaya; Mukai, Masayuki

JAEA-Data/Code 2015-019, 122 Pages, 2015/12


In ensuring compliance with the criterion of site release as the final stage of termination of decommissioning of nuclear facilities, it is supposed to confirm the radioactivity concentration obtained by measurement in the site is less than or equal to the concentration corresponding to the criterion. It is needed to estimate the distribution and mean of radioactivity concentration in the evaluation unit using a number of measured data. It is further needed to compare the estimated result with the concentration corresponding to the criterion of site release and to decide if the evaluation unit should comply with the criterion. The estimated result exhibits uncertainty depending on the number of measurement points, which results in a certain probability of the occurrence of decision error according to the uncertainty. It is important to decide the number of measurement points required by revealing a relationship of the error probability to the number of measurement points for site security. We have developed the ESRAD2 (Estimation of Spatial RadioActivity Distribution program version 2), which is an extended version of the existing ESRAD, for estimating the mean of radioactivity concentration and calculating the number of measurement points required according to the error probability. This report describes a method for ensuring compliance with the criterion of site release, structure and functions, input file format, output examples, execution method of ESRAD2, and sample run with ESRAD2.

Journal Articles

Demonstrative experiments on the migration of radiocesium from buried soil contaminated by the accident at Fukushima Daiichi Nuclear Power Station

Yamaguchi, Tetsuji; Shimada, Taro; Ishibashi, Makoto*; Akagi, Yosuke*; Kurosawa, Mitsuru*; Matsubara, Akiyoshi*; Matsuda, Yuki*; Sato, Shigeyoshi*

Genshiryoku Bakkuendo Kenkyu (CD-ROM), 22(2), p.21 - 27, 2015/12

It is predictable from previous studies that radiocesium hardly migrate into surrounding soils and groundwater from soils contaminated by the Fukushima Daiichi Nuclear Power Plant accident if they are buried and covered with indigenous soils. This study demonstrated the prediction by performing in-situ migration experiments over a year in a public park in Miho, Ibaraki prefecture and in two public parks in Misato, Saitama prefecture. Contaminated soils were buried at a depth range of 0.3 - 1.0 m or at 0.3 - 1.3 m and covered with indigenous soil layer of 0.3 m, and were sprinkled with water to accelerate the radiocesium migration. Migration of radiocesium was not observed from radiometric analyses of boring cores and soil water samples. Laboratory column and sorption experiments revealed that the radiocesium hardly leach out of the soil and even if they leach out from the contaminated soil, radiocesium is sorbed on surrounding soils and hardly migrate through the soli layer. Simulation of Cs-137 migration for 100 years by an advection-diffusion model showed that Cs-137 hardly migrate and decay out in the contaminated soil.

Journal Articles

Study on application of kriging to evaluation of radioactivity concentration for ensuring compliance with the criterion of site release

Ishigami, Tsutomu; Shimada, Taro

Journal of Nuclear Science and Technology, 52(9), p.1186 - 1204, 2015/09

 Times Cited Count:0 Percentile:0.01(Nuclear Science & Technology)

In the field of decommissioning of nuclear facilities, a reliable method for ensuring compliance with the criterion of site release is an important technical issue to be resolved in Japan. Considering that kriging can consider the spatial correlation of radioactivity concentrations, we propose a method of applying kriging to ensure compliance with the site release criterion. Estimated radioactivity concentrations exhibit uncertainty, which results in a certain probability of the occurrence of decision errors regarding site release. We describe a method for calculating the uncertainty and establish a minimum number of measurement points required. We applied the proposed method and a conventional statistical method to two sample cases. It was observed that the proposed method appropriately estimated the mean radioactivity concentration and led to an efficient measurement requiring fewer measurement points relative to the conventional method when spatial correlation existed.

Journal Articles

Nitrogen K-edge X-ray absorption near edge structure of pyrimidine-containing nucleotides in aqueous solution

Shimada, Hiroyuki*; Minami, Hirotake*; Okuizumi, Naoto*; Sakuma, Ichiro*; Ukai, Masatoshi*; Fujii, Kentaro; Yokoya, Akinari; Fukuda, Yoshihiro*; Saito, Yuji

Journal of Chemical Physics, 142(17), p.175102_1 - 175102_9, 2015/05


 Times Cited Count:10 Percentile:40.9(Chemistry, Physical)

Journal Articles

Analysis of radionuclide migration with consideration of spatial and temporal change of migration parameters due to uplift and denudation

Shimada, Taro; Takeda, Seiji; Mukai, Masayuki; Munakata, Masahiro; Tanaka, Tadao

Materials Research Society Symposium Proceedings, Vol.1744, p.229 - 234, 2015/04

Integrated safety assessment methodology which analyzes radionuclide migration reflecting the spatial and temporal changes of disposal systems was developed for a geological disposal site with uplift and denudation, and then some case analyses for an assumed site with sedimentary rocks were carried out. The combination of uniform uplift and denudation has the most effect on the radionuclide migration because the groundwater flow velocity increases with decreasing the depth from the ground surface. In the case without denudation, tilted uplift has more effect than uniform uplift because flow velocity in tilted uplift increase with increasing hydraulic gradient. The long-term change of the geological structures including the uplift and denudation, the hydraulic conditions, and the recharge and outlet of the groundwater around a candidate site should be carefully investigated to determine the appropriate the place,depth and layout of the repository.

Journal Articles

Improvement and testing of radiation source models in DecDose for public dose assessments during decommissioning of nuclear facilities

Shimada, Taro; Sukegawa, Takenori

Journal of Nuclear Science and Technology, 52(3), p.396 - 415, 2015/03

 Times Cited Count:1 Percentile:10.17(Nuclear Science & Technology)

Radiation source models in DecDose code for assessing public and worker exposure doses during the decommissioning of nuclear facilities were improved in this study. A segmentation model evaluating the length, volume, and surface area of kerfs in the object to be dismantled was improved to deal with seven shapes of objects simulating most of the components and the structures in nuclear facilities. Models for the evaluation of the external dose by direct and skyshine radiation were also improved to deal with the distribution of waste containers temporarily placed in the building and the quantity of radionuclides stored in the individual container. Good agreement was observed between actual and calculated kerf volumes in cutting some components such as the reactor pressure vessel of the Japan Power Demonstration Reactor. It is an indication of the validity of the model improved in this study. On the other hand, some discrepancies were observed between actual and calculated quantities of radionuclides discharged into the ocean, indicating the necessity of further validation of the model.

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