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土屋 晴文; 日比野 欣也*; 川田 和正*; 大西 宗博*
no journal, ,
Recent observations related to thunderstorms show that there are three types of radiation enhancements based on their duration; terrestrial gamma-ray flashes (TGFs) with duration of 1 ms or less, short bursts (or TGF afterglows) with duration of a few tens of ms to a few hundred ms, and long bursts (or gamma-ray glows) with duration of a few minutes to a few tens of minutes. Such a thunderstorm-related radiation consists of not only bremsstrahlung emissions due to accelerated electrons, but also neutrons and/or the subsequent prompt gamma rays. It is currently debated how those radiation enhancements with different durations and/or different particles are generated in lightning and thunderclouds. A high-mountain observatory, located at an altitude of 4300 m (Yangbajing, Tibet), has neutron monitors with an area of 32 m
. Neutron monitors are mainly sensitive to hadrons of neutron and protons, but has a small detection efficiency of gamma rays and other leptons. Therefore, neutron monitors can give us insights into production mechanism of radiation increases due to thunderstorms. During rainy seasons from 1998 to 2017, the Tibet neutron monitors detected around 140 significant enhancements of long bursts. Observed duration ranged from 5 minutes to 50 minutes, with the peak of around 10-20 minutes. In addition, electric-filed measurements, which started from 2010 at the site, showed that the observed increases correlate with electric-field variations caused by thunderclouds and/or lightning. Using the results observed by the Tibet neutron monitors and other meteorological information, we discuss in this presentation how the observed long bursts are produced and what kind of particles mainly contribute to them. This work is partly supported by JSPS/MEXT KAKENHI Grants No. 18H01236.
Francisco, P. C. M.; 松村 大樹; 菊池 亮佑*; 石寺 孝充; 舘 幸男
no journal, ,
The radionuclide Selenium-79 (Se-79) is predicted to be a key contributor to the long-term radiologic hazards associated with geological high-level waste (HLW) repositories. Se exists predominantly as selenide Se(-II) under anoxic conditions in the deep subsurface, and is likely immobilized via precipitation with, or adsorption on, Fe(II) minerals. However, the initial immobilization mechanisms with metastable Fe(II) minerals, and Se(-II)s subsequent response to iron phase transformation in the long term remain poorly understood. In this work, we investigated the retention and mobilization behavior of Se(-II) as a function of pH and the mode of its initial interaction with aqueous or solid Fe(II) phases. We carried out batch precipitation and transformation experiments under N
atmosphere and reducing conditions. We examined two cases: in the first, Se(-II) was reacted with aqueous Fe(II), while in the second, Fe(OH) was first precipitated before reaction with Se(-II); both experiments were carried out at pH 8 and 12. In both cases, Se(-II) introduction resulted in the immediate precipitation of black particles. EXAFS and TEM characterization showed that Se(-II) precipitated as iron selenide nanoparticles in both cases regardless of pH. Aging of these initial precipitates at 
C resulted in magnetite crystallization. Regardless of whether Se(-II) was reacted with aqueous Fe(II) or solid phases, it was completely retained as discrete, crystalline FeSe at pH8. At pH12, Se(-II) was mostly remobilized; however, we observed evidence of partial retention via the precipitation of nanocrystalline iron selenide (Fe
Se
or Fe
Se
) on the surface of magnetite, as well as by incorporation in defects on the edges of magnetite crystals. These results show that pH controls long-term Se(-II) behavior and that magnetite crystallization may play a role in the retention of Se(-II), particularly at high pH.
深津 勇太; 舘 幸男
no journal, ,
Crystalline rocks such as granodiorites have been investigated as potential host rocks for the geological disposal of radioactive waste in many countries. Radionuclide transport in fractured crystalline rocks can be conceptualized by a dual-porosity model where radionuclides are transported by advective water flow through a fracture and are retarded by both diffusion and sorption into the surrounding rock matrix. Aperture variability within a fracture determines flow in a channel, whereas other areas of the fracture have almost stagnant water. The effect of different degrees of heterogeneity in fracture aperture distributions is critically important to quantify solute transport. In the present study, we identified flow channels and stagnant water zones in a fractured granodiorite by mean of a flow-through test, using imaging tracer coupled with micro X-ray CT measurements.
Hu, Q.*; Wang, Q.*; Oware, P.*; 舘 幸男; 深津 勇太; Ilavsky, J.*; Almer, J.*
no journal, ,
Pore connectivity is important in controlling fluid flow and mass transport in porous natural rocks. A different extent of pore connectivity can be reflected in the proportion of isolated pore space not connected to the surface of natural rocks. This work presents the multi-approach and multi-scale laboratory studies to investigating the proportion of isolated pore space of, and its resultant anomalous fluid flow and radionuclide movement in, generic geological barrier materials. The samples include mudstone from Wakkanai formation, Opalinus clay from Mt. Terri as well as granodiorite from Grimisel, salt rock from Waste Isolation Pilot Plant in New Mexico, and welded tuff in Yucca Mountain. The independent quantification of both (1) surface-accessible pore space with various probing fluids (e.g., micron-scale tracer mapping using laser ablation-ICP-MS); and (2) total porosity by small angle X-ray scattering. Our complementary approaches provide a rich toolbox for tackling the pore structure characteristics in geological barrier materials, and associated fluid flow and radionuclide transport.
舘 幸男; 伊藤 剛志*; 深津 勇太; 赤木 洋介*; 佐藤 久夫*; Hu, Q.*; Martin, A. J.*
no journal, ,
In order to develop a realistic model and reliable parameters for long-term safety assessments of geological disposal, it is necessary to understand and quantify the effects of heterogeneities found around the fractures on RN transport processes in fractured crystalline rocks. This paper presents a comprehensive approach developed for coupling laboratory tests, microscopic observations and modeling in order to understand and quantify tracer transport processes occurring in natural fracture, using different types of fractured granodiorite sample from the Grimsel Test Site (GTS), Switzerland. Laboratory tests including through-diffusion, batch sorption and flow-through tests using five tracers with different retention properties indicated that tracer retention was consistently in the sequence of HDO 
Se Cs Ni Eu. Microscale heterogeneities around the fracture were clarified and quantified by coupling X-ray computed tomography and electron probe microanalysis. Realistic model incorporating heterogeneities around the fracture and their properties provided a much better interpretation for breakthrough curves of all tracers.