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Taniguchi, Naoki; Kawakami, Susumu; *
JNC TN8400 2001-025, 27 Pages, 2002/03
JNC-TN8400-2001-025.pdf:1.16MB
It is essential to understand the corrosion type of carbon steel under the repository conditions for the lifetime assessment of carbon steel overpack used for geological isolation of high-level radioactive waste. According to the previous study, carbon steel is hard to passivate in buffer material assuming a chemical condition range of groundwater in Japan. However, concrete support will be constructed around the overpack in the case of repository in the soft rock system and groundwater having a higher pH may infiltrate to buffer material. There is a possibility that the corrosion type of carbon steel will be influenced by the rise of the pH in groundwater. In this study, anodic polarization experiments were performed to understand the passivation condition of carbon steel in buffer material saturated with water contacted with concrete. An ordinary concrete and a low-alkalinity concrete were used in the experiment. The results of the experiments showed that the carbon steel can passivate under the condition that water having pH 
13 infiltrate to the buffer material assuming present property of buffer material. If the low-alkalinity concrete is selected as the support material, passivation can not occur on carbon steel overpack. The effect of the factors of buffer material such as dry density and mixing ratio of sand on the passivation of carbon steel was also studied. The results of the study showed that the present property of buffer material is enough to prevent passivation of carbon steel.
Suzuki, Satoru; ; *
JNC TN8400 2000-020, 25 Pages, 2000/04
JNC-TN8400-2000-020.pdf:0.94MB
Nature of porewater in bentonite plays important roles on the mass transport in the compacted bentonite used as a physical and chemical buffer material of the multi-barrier system in the high level radioactive waste manegement Higher activation energies of diffusion in the compacted bentonite than those in the aqueous solution is due probably to change in molecular structure of water in the porewater. The Raman spectroscopy was applied to studying the structure of porewater in bentonite at room temperature. Bentonite (Kunipia F, 98-99wt% of Na-smectite) was mixed with ion-exchanged water by water content of 75, 80, 90, 95 and 98wt% of water or with 0.5M NaCl aqueous solution by 75 and 80wt% of NaCl solution. Intensity maxima of the spectra of ion exchanged water, NaCl solution and their porewater were observed near 3200 to 3250, 3400, 3630cm
. These bands can be attributed to water molecules forming stronger hydrogen bond in this manner. Ratio of intensity, 3250cm/3400cm, increased from 0.97 to 1.1 with a decrease in water content of 100wt% (water) to 75wt%. On the other hand, intensity ratio of 3400cm/3250cm of NaCl aqueous solution, 80wt%and 75wt% were 0.92, 1.2 and 1.3, respectively. Since the Raman scattering near 3250cm was attributed to water molecule forming the strongest hydrogen bonding in the three bands, those changes in intensity ratio suggests an increase in number of water molecule forming strong hydrogen bond in porewater of the bentonite. The constrained porewater possibly results in the high activation energy of diffusion in the compacted bentonite.
Taniguchi, Naoki; ; Kawasaki, Manabu*; *; *
JNC TN8400 99-067, 119 Pages, 1999/10
Carbon steel is one of the candidate materials for overpacks for high level radioactive waste disposal in Japan. The estimation of corrosion allowance needs to clarify the localization of corrosion. One of the most extreme case of the localization under oxidizing condition at the initial stage of repository is considered to be localized corrosion such as pitting and crevice corrosion due to the break down of passive film formed on the metal surface. In this study, the critical condition for the initiation and the propagation behavior of pitting and crevice corrosion were assessed by the experimental study. The results of the electrochemical experiments in carbonate-chloride aqueous solution showed that carbon steel have high sensitivity for the pitting and crevice corrosion in passive region. The propagation behavior of pitting and crevice corrosion was studied by the immersion tests under aerated condition. The results of the immersion tests showed that the degree of localization became small and seemed to be like general corrosion in appearance as the localized corrosion propagate. The extreme value statistical analysis were applied to the corrosion depth measured in the immersion tests. Based on the analysis, it was concluded that the equation for maximum corrosion depth due to oxygen obtained in our previous study would give conservative assessment even in the assumption of propagation of pitting corrosion or crevice corrosion. The localization of corrosion for carbon steel under reducing condition after consumption of oxygen was also estimated. The external current was supplied to the carbon steel specimens in compacted bentonite to accelerate the corrosion and corrosion depth were measured for each specimens and extreme value statistical analysis were applied to the results. The pitting factor became small as the average corrosion depth became large. Assuming that the average corrosion depth in 1000 years is 5 
10mm, the pitting factor is ...