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Journal Articles

Evaluation of the radiation protection capability in shelter facilities with positive pressure ventilation

Ishizaki, Azusa; Nakanishi, Chika*; Takubo, Kazuya*; Munakata, Masahiro

Proceedings of 27th International Conference on Nuclear Engineering (ICONE-27) (Internet), 9 Pages, 2019/05

JAEA Reports

Photon transmittance data collection of building materials for evaluating radiation protection capability of buildings (Contract research)

Ishizaki, Azusa; Futemma, Akira; Takubo, Kazuya*; Nakanishi, Chika*; Munakata, Masahiro

JAEA-Data/Code 2018-022, 20 Pages, 2019/03

JAEA-Data-Code-2018-022.pdf:2.05MB

If a nuclear disaster occurs, we may evacuate indoor escape facilities and buildings such as houses as avoid extra exposure doses. In order to evaluate exposure doses, it is necessary to estimate shielding capabilities of the building materials constituting the sheltering facility. Therefore, photon irradiation tests with three kinds of photon energy were carried out for Japanese familiar building materials in Japan, and photon transmittance of each building material is acquired and summarized. As a result, it was found that the shielding capabilities of composite walls and roofs which are widely used in a tree structure and a steel structure were relatively low. And, difference of materials used for composite walls and roofs resulted in a difference in shielding capabilities. For example, in the case of composite walls, compared with the photon transmittance of wall with ceramic-based siding materials, those of wall with lightweight concrete were lower. Furthermore, photon transmittance was also measured for building materials with relatively low shielding performance added shielding materials as additional measures to enhance shielding capabilities.

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 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.

Oral presentation

Development of safety assessment methodology considering the evolution of geological disposal system, 3; Analysis of radionuclide migration with consideration of spatial and temporal change of migration parameters

Shimada, Taro; Takubo, Kazuya; Takeda, Seiji; Tanaka, Tadao

no journal, , 

In long-term safety assessment of geological disposal system, it is necessary to evaluate the impact on the nuclides migration where groundwater flow and water composition are changed with decreasing a depth of the repository by uplifts and denudation. Therefore, we developed an integrated safety assessment methodology for uplifts and denudation where nuclides migration was evaluated reflecting temporal and spatial changes of parameters of natural and engineered barriers such as distribution coefficient, porosity and groundwater velocity, which were obtained by calculation of groundwater flow, salt water distribution and long-term transition of engineered barriers. The methodology was applied to an assumed disposal site composed of sedimentary rocks with uplift and denudation. Migration parameters of engineered barrier such as coefficient of water permeability and porosity of buffer material were evaluated considering the interaction between overpack and surrounding bentonite clay under the condition of water composition. Migration parameters of the natural barrier such as distribution coefficient were determined by the combination of the geology section and water composition section on the migration pathway from the repository to the outlet of the natural barrier at the depth of 40m from the surface. The migration fluxes of some radionuclides at the outlet of natural barrier were evaluated and then the impacts on nuclides migration by uplifts and denudation were compared.

Oral presentation

Evaluation methodology for influence of splay fault growth on geological disposal system, 2; Analysis of radionuclide migration with consideration of fault growth

Shimada, Taro; Takubo, Kazuya; Takai, Shizuka; Takeda, Seiji; Tanaka, Tadao

no journal, , 

There is a residual uncertainty not able to avoid geological and climate events because some indications of them are difficult to be detected, while direct damage by the events such as active fault is avoided at site selection. Therefore radionuclide migration analysis was carried out considering safety functions of engineered barrier decreased or lost by intersection of fault and temporal and spatial changes of nuclide migration pathway under the assumption that concealed spray fault reaches the disposal system. And we evaluated the affects by inflow of fluid from deep region and of acidic groundwater from the surface along the grown spray fault. Based on the results of groundwater flow analysis, we calculated the nuclides migration fluxes at the outlet of engineered and natural barriers using nuclides migration parameters such as migration pathway, its length and groundwater velocity and sorption distribution coefficients estimated by combination of groundwater composition and geology. And important factors to affect the fluxes were extracted.

Oral presentation

Development of internal-dosimetry code based on the ICRP 2007 recommendations; Overview of the code

Takahashi, Fumiaki; Manabe, Kentaro; Takubo, Kazuya*; Sato, Kaoru

no journal, , 

JAEA has developed an internal dosimetry code based on the ICRP 2007 recommendation in a four-year plan from FY 2017 to FY 2020 through a project commissioned by the Nuclear Regulation Authority. In this year, we heard comments on operability and functions for the prototype code developed up to last from experts of institutions with knowledge and experience in radiation monitoring. After that, the GUI screen for input was improved and a new output function was added by reflecting the result of hearing. In addition, the new dose evaluation models released as ICRP Publ.141 in 2020 were investigated and implemented in the code. The improved code was verified to calculate effective dose coefficient correctly for the newly added models. In this presentation, we report the overall of the code and its utilization.

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