Elemental analysis and radioactivity evaluation of aerosols generated during heating of simulated fuel debris; The Urasol project in the framework of Fukushima Daiichi fuel debris removal

Tsubota, Yoichi; Porcheron, E.*; Journeau, C.*; Delacroix, J.*; Suteau, C.*; Lallot, Y.*; Bouland, A.*; Roulet, D.*; Mitsugi, Takeshi

Proceedings of International Conference on Environmental Remediation and Radioactive Waste Management (ICEM2023) (Internet), 6 Pages, 2023/10

https://doi.org/10.1115/ICEM2023-110064

In order to safely remove fuel debris from the Fukushima Daiichi Nuclear Power Station (1F), it is necessary to quantitatively evaluate radioactive airborne particulate generated by the cutting of nuclear fuel debris. We fabricated Uranium-bearing simulated fuel debris (SFD) with In/Ex-Vessel compositions and evaluated the physical and chemical properties of aerosols generated by heating the SFDs. Based on these results, we estimated the isotopic composition and radioactivity of aerosols produced when 1F-Unit2 fuel debris is laser cut, which is a typical example of a heating method. Plutonium, mainly Pu,Am, and Cm were found to be the alpha nuclide, and Pu, Cs-Ba, and Sr-Y were found to be the beta nuclide of interest.

Development of risk assessment code for dismantling of radioactive components in decommissioning stage of nuclear reactor facilities

Shimada, Taro; Sasagawa, Tsuyoshi; Miwa, Kazuji; Takai, Shizuka; Takeda, Seiji

Proceedings of International Conference on Environmental Remediation and Radioactive Waste Management (ICEM2023) (Internet), 7 Pages, 2023/10

https://doi.org/10.1115/ICEM2023-109848

Nuclear regulatory inspection should be performed on the basis of the risk information during the decommissioning phase of the nuclear power plant. However, it is difficult because the methodology for quantitatively assessing the radiation exposure risk during decommissioning activities has not been established. Therefore, a decommissioning risk assessment code, DecAssess-R, has been developed based on the decommissioning safety assessment code, DecAssess, which creates event trees from initiating events and evaluates the radiation risk resulting from public exposure dose for each accident sequence. The assessment took into account that mobile radioactive inventories that can be easily dispersed in the work area, such as radioactive dust accumulated in HEPA filters attached to a contamination control enclosure, will fluctuate with the progress of the decommissioning work. Initiating events were selected based on the investigation of accidents and malfunctions during dismantling, disassembly, and component replacement activities around the world, and event trees were created from the initiating events to indicate the progress scenario. The frequencies of occurrence were determined with reference to general industry data in addition to the above accidents and malfunctions, and the probabilities of event progression were determined with reference to failure data during the operation phase. The exposure risks during dismantling of components in the reference BWR were evaluated. As a result, the public exposure dose was maximum in case of fire during dismantling of reactor internals and fire spread to combustibles and filters, including radioactivity temporarily stored in the work area. The exposure risk was also maximum because the probability of occurrence of this accident sequence was greater than that of other scenarios.

Analysis of factors contributing to the increase in Be activity concentrations in the atmosphere

Narazaki, Yukinori*; Sakoda, Akihiro; Akata, Naofumi*; Ito, Hisanori*; Momoshima, Noriyuki*

International Journal of Environmental Research and Public Health, 19(16), p.10128_1 - 10128_9, 2022/08

https://doi.org/10.3390/ijerph191610128

In March 2013, increased Be activity concentrations in the atmosphere were observed for successive days in Dazaifu, western Japan. The daily Be activity concentration averages ranged from 0.93 to 14 mBq/m, with a monthly average of 8.3 mBq/m. This average was the highest among the monthly averages observed between 1999 and 2015 and higher than the monthly average over this period (4.7 mBq/m) plus twice the standard deviation (1.7 2 mBq/m = 8.1 mBq/m). Also, this exceeded the monthly average (6.0 mBq/m) only for March 1999-2015, excluding 2013, where the cosmic-ray intensity, a component producing Be, decreased. Based on the backward trajectory analysis results, the inflow of air from the stratosphere and upper troposphere at high latitudes that frequently occurred in March 2013 was considered the reason for the Be activity concentration increase.

Discriminative measurement of absorbed dose rates in air from natural and artificial radionuclides in Namie Town, Fukushima Prefecture

Ogura, Koya*; Hosoda, Masahiro*; Tamakuma, Yuki*; Suzuki, Takahito*; Yamada, Ryohei; Negemi, Ryoju*; Tsujiguchi, Takakiyo*; Yamaguchi, Masaru*; Shiroma, Yoshitaka*; Iwaoka, Kazuki*; et al.

International Journal of Environmental Research and Public Health, 18(3), p.978_1 - 978_16, 2021/02

https://doi.org/10.3390/ijerph18030978

Empirical equations for tensile properties and stress-strain curves of neutron irradiated stainless steels in LWR conditions

Fukuya, Koji*; Fujii, Katsuhiko*; Chimi, Yasuhiro; Hata, Kuniki

Proceedings of 19th International Conference on Environmental Degradation of Materials in Nuclear Power Systems - Water Reactors (Internet), p.523 - 531, 2019/08

For structural integrity assessment on reactor internals of light water reactors, empirical equations of tensile properties as a function of neutron dose, and trend curves of stress-strain relations of neutron-irradiated austenitic stainless steels was proposed by fitting to recently developed database. The data in the database were obtained from reports of national projects in Japan and open literature, which was summarized in the form of data sheets. The empirical equations for tensile properties were formulated by using a saturation-type formulae. The equations were for CW 316 and SA 304/316 stainless steels in the temperature range of 280-350C and the dose range up to 80 dpa. Stress-strain relation curves were reproduced based on the Swift model. Obtained calculated results by the empirical equations and stress-strain relations were reasonably well fitted to experimental data. The effects of composition and cold-working, etc. on tensile properties were discussed.

Large-eddy simulation studies for predicting plume concentrations around nuclear facilities using an overlapping technique

Nakayama, Hiromasa; Takemi, Tetsuya*

International Journal of Environment and Pollution, 64(1/3), p.125 - 144, 2018/00

https://doi.org/10.1504/IJEP.2018.10019970

We have attempted to develop a practical and quick local-scale atmospheric dispersion calculation method using an overlapping technique for plume concentration distributions in an emergency response to nuclear accidents. In order to evaluate the overlapping approach, we performed LESs of turbulent flows and plume dispersion under varying mean wind directions using the meteorological data as the model input. When compared with the simulated results under the real meteorological condition, it is shown that the concentration distribution patterns are reasonably simulated by the overlapping method. It can be concluded that the atmospheric dispersion calculation method using the overlapping technique has potential performance for emergency responses to nuclear accidents.

In situ electrochemical study on crevice environment of stainless steel in high temperature water

Soma, Yasutaka; Kato, Chiaki; Ueno, Fumiyoshi

Proceedings of the 18th International Conference on Environmental Degradation of Materials in Nuclear Power Systems - Water Reactors, Vol.2, p.509 - 521, 2018/00

https://doi.org/10.1007/978-3-030-04639-2_116

In-situ electrochemical measurement within crevice of stainless steel in 288C water has been conducted to analyze crevice water chemistry. Small sensors ( 250m) measured local solution electrical conductivity, , polarization resistance, and electrochemical corrosion potential. Real-time response of the as functions of bulk water conductivity, dissolved oxygen (DO) concentration has been quantitatively analyzed. The effect of geometrical factors on the crevice environment was also studied. The differ more than an order of magnitude depending on the oxygen potential inside the crevice. The increased by small amount of bulk DO (e.g. 30 ppb). Maximum was observed with DO of 32000 ppb and became more than 100 times higher than that of bulk water. Crevice geometry affected significantly on the water chemistry inside.

Estimation of the vertical distribution of radiocesium in soil on the basis of the characteristics of -ray spectra obtained via aerial radiation monitoring using an unmanned helicopter

Ochi, Kotaro; Sasaki, Miyuki; Ishida, Mutsushi*; Hamamoto, Shoichiro*; Nishimura, Taku*; Sanada, Yukihisa

International Journal of Environmental Research and Public Health, 14(8), p.926_1 - 926_14, 2017/08

https://doi.org/10.3390/ijerph14080926

After the Fukushima Daiichi Nuclear Power Plant accident, the vertical distribution of radiocesium in soil has been investigated to better understand the behavior of radiocesium in the environment. The typical method used for measuring the vertical distribution of radiocesium is troublesome because it requires collection and measurement of the activity of soil samples. In this study, we established a method of estimating the vertical distribution of radiocesium by focusing on the characteristics of -ray spectra obtained via aerial radiation monitoring using an unmanned helicopter. In this method, the change in the ratio of direct rays to scattered rays at various depths in the soil was utilized to quantify the vertical distribution of radiocesium. The results show a positive correlation between the abovementioned and the actual vertical distributions of radiocesium measured in the soil samples.

Determination of detrapping and trapping rate constants for hydrogen based on experimental thermal desorption spectra

Ebihara, Kenichi; Saito, Kei*; Takai, Kenichi*

Proceedings of 2016 International Hydrogen Conference (IHC 2016); Materials Performance in Hydrogen Environments, p.470 - 477, 2017/00

http://doi.org/10.1115/1.861387_ch53

For understanding hydrogen (H) embrittlement of steels, it is necessary to infer the state that defects trap H in the steels. Thermal desorption spectra of H obtained by the thermal desorption spectrometry (TDS) are used for inferring such a state. Because the thermal desorption spectra include the influence of experimental conditions and hydrogen diffusion as well as information of the defects trapping H, it is necessary to interpret the spectra using the numerical simulation. In the presentation, we determined the detrapping and the trapping rate constants which are necessary for the simulation from the experimental spectra obtained for plate specimens which is so small that H diffusion is ignorable. Then we confirmed that the model using the obtained rate constants can simulate the spectra of larger cylindrical specimens, so that it was found that the rate constant for small specimens can be used for the simulation of the spectra for specimens of different shape or size.

Formative mechanism of inhomogeneous distribution of fractures, an example of the Toki Granite, Central Japan

Sasao, Eiji; Yuguchi, Takashi*; Ito, Yasuto*; Inoue, Takashi*; Ishibashi, Masayuki

Proceedings of 10th Asian Regional Conference of International Association for Engineering Geology and the Environment (IAEG ARS 2015) (USB Flash Drive), 6 Pages, 2015/09

Understanding of the fracture network is important for disposal of high-level radioactive waste. We present our hypothesis on the formative mechanism of inhomogeneous distribution of fracture in the Toki Granite. In the Toki Granite, low- and high-angle fractures are abundant at the shallower part, while less at the deeper part where high-angle fracture is dominant. Distribution of the high-angle fracture is inhomogeneous. Thermochronological study revealed that the rapid cooling occurred at the early stage of granite formation. Paleomagnetic directions of the intact granite were dispersed. This suggests granite was plastically deformed during rapid cooling period. The rapid cooling might cause inhomogeneous distribution of cooling strain. When the granite reached to brittle deformation field, inhomogeneous fracture distribution was formed by the inhomogeneous strain. If so, recognition of the cooling history is essential to understand the distribution of the fracture network.