Retention mechanisms of uranium in calcite veins from deep subsurface sedimentary formations of the Horonobe area, Hokkaido, Japan

Dei, Shuntaro  ; Sugiura, Yuki  ; Amano, Yuki   ; Francisco, P. C. M.  ; Takahashi, Yoshio*; Tachi, Yukio  

Uranium (U) is considered one of the important radionuclides for safety assessment of the geological disposal because U(VI) can form soluble carbonate complexes. Calcite is expected to precipitate as a secondary mineral in sedimentary rocks, and radionuclides migration may be retarded by incorporation into the calcite crystal structure. Although some laboratory experiments have reported U uptake by calcite, the distribution and chemical form of U in natural calcite is difficult to determine due to interference from coexisting elements. In this study, micro X-ray analyses were carried out on deep sedimentary rock samples to identify the distribution and chemical form of U in natural calcite.