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Takubo, Yusaku*; Takayama, Yusuke; Ishida, Keisuke*
no journal, ,
The preliminary results of coupled TH analysis for laboratory infiltration test data using FEBEX bentonite reported by Takubo et al., (2022) showed that some input parameters of bentonite, such as an intrinsic permeability and parameters related to the retention function, are key parameters that have large effects on the coupled TH analysis results. However, it is known that bentonite has different hydraulic properties such as permeability and water retention curves depending on its origin and properties, and the key parameters for evaluating coupled TH phenomena may be different for other bentonites. In this study, we investigated parameter sensitivity of coupled TH analysis model for Kunigel V1 using experimental data measured in previous laboratory tests in order to identify the key parameters that should be focused on when evaluating coupled TH phenomena in Kunigel V1. The large and small influential parameters on the analytical evaluation of hydraulic fields were consistent with the key parameters of the FEBEX bentonite. Although there are only limited comparative examples, this result indicates that key parameters related to coupled TH phenomena may be common to all bentonites. The information of the key parameters can be used to prioritize the parameters that need to be precisely obtained from experiments, and it is expected to promote efficiency in future research and development about bentonite.
Anraku, Sohtaro; Kawakita, Ryohei; Hanamachi, Yuji*; Mitsui, Seiichiro; Sasamoto, Hiroshi; Mihara, Morihiro
no journal, ,
To evaluate the observed alteration of bentonite and secondary mineral formation by OPC or HFSC near the interface, 1D reactive transport models were constructed using the Cement And Bentonite Alteration due to REactive Transport (CABARET) computer modelling code. Supporting calculations for initial hydration of OPC or HFSC were conducted using PHREEQC to generate the initial porewater compositions. JAEA's Thermodynamic DataBase for geochemical reaction was used in all calculations. Modeling of OPC and Kunigel V1 interaction resulted in depletion of Ca in OPC by portlandite dissolution and depletion of Si in Kunigel V1 by chalcedony dissolution leading to C-S-H gel precipitation at the interface, which were confirmed by XRD. Clogging of the interface by the precipitation of C-S-H gel at 80
C, however, limited diffusion and therefore the alteration of Kunigel V1. Coupling between diffusion coefficients and low porosities requires further data and validation to improve the simulation. The temperature dependence of dissolution rates of the C-S-H gel and chalcedony also needs to be confirmed. Modeling of HFSC and Kunigel V1 interaction showed significantly less alteration of the Kunigel V1 from the significantly less alkaline HFSC porewater, which is also consistent with the experiments. Modelled changes in HFSC resulted in an increase in porosity at the interface by the dissolution of C-A-S-H gel and ettringite, and in Kunigel V1 by the slight dissolution of chalcedony. To evaluate the elevated temperature effect in HFSC hydration, it is important to use a C-A-S-H gel model and to confirm the extents of pozzolanic reaction of silica fume and fly ash, which are currently based on measurements at room temperature.
Matsushima, Kanato*; Ishidera, Takamitsu; Fukatsu, Yuta; Eguchi, Ayano; Yotsuji, Kenji*; Tachi, Yukio; Okubo, Takahiro*
no journal, ,
Kawakita, Ryohei; Anraku, Sohtaro; Hanamachi, Yuji*; Mitsui, Seiichiro; Sasamoto, Hiroshi; Mihara, Morihiro
no journal, ,
Cement and bentonite materials are to be used in a repository for the geological disposal of radioactive wastes in Japan. Cement will provide structural support to the repository and will be in close contact with bentonite. Understanding the interactions between these two materials is necessary to improve repository design and ensure long-term safety. Ordinary Portland Cement (OPC) is widely used for construction, however, there are concerns that its high pH 
13 porewater could cause significant alteration of bentonite. JAEA has therefore developed a High-content Fly ash Silica fume Cement (HFSC) to lower the pH of cement porewater. Studies on HFSC and bentonite interactions are, however, limited. In addition, elevated temperatures can be expected in the repository by the combination of radiolytic heating and geothermal gradients. Temperature affects mineral reaction rates and their overall stability, but such thermal effects have not yet been studied in detail in the context of cement and bentonite interactions. Laboratory scale experiments were therefore conducted to investigate the interactions between OPC or HFSC and compacted bentonite (Kunigel V1; dry density = 1.37 Mg/m
) at repository relevant temperatures of 50C and 80C.