We analyzed the mineral composition of compacted calcium bentonite (GJ-I) and uncompressed sodium bentonite (MX80), both of which were exposed for two years in the YS03 borehole. The YS03 borehole is characterized by a high concentration of anaerobic microorganisms, including sulfate-reducing bacteria, elevated levels of hydrogen sulfide, and high pH conditions. The compacted Ca bentonite showed minimal alteration, with a small amount of new magnetite formation. However, an X-ray diffraction (XRD) analysis revealed that the uncompressed Na bentonite underwent a complete transformation from montmorillonite to muscovite, goethite, and magnetite. Therefore, it is suspected that the compactness of the bentonite significantly impacts the rate of alteration. Furthermore, an X-ray fluorescence (XRF) analysis demonstrated a marked increase in iron oxide in the Na bentonite, whereas key elements of montmorillonite such as alumina (Al2O3), silica (SiO2), and magnesium oxide (MgO) showed substantial decreases. The presumed cause of the alteration in the uncompressed MX80 bentonite is the presence of Fe cations coupled with a high pH environment. We believe that Fe cations, which were likely released from the corrosion of cast iron, played a significant role in altering the montmorillonite lattice.

1. Introduction
2. Experiment
    2.1 YS03 Borehole
    2.2 Test Module
    2.3 Microbial Activity
    2.4 Test Module Recovery
3. Experimental Result
    3.1 Compact Ca Bentonite
    3.2 Uncompact Na Bentonite
4. Discussions
5. Conclusions
Conflict of Interest
Acknowledgements
REFERENCES

• Minsoo Lee(Korea Atomic Energy Research Institute, 111, Daedeok-daero 989beon-gil, Yuseong-gu, Daejeon 34057, Republic of Korea)
• Jinseop Kim(Korea Atomic Energy Research Institute, 111, Daedeok-daero 989beon-gil, Yuseong-gu, Daejeon 34057, Republic of Korea)
• Mihye Kong(Chonnam National University, 77, Yongbong-ro, Buk-gu, Gwangju 61186, Republic of Korea) Corresponding author