A review and interpretation of iodine-129 in Earth's surface layers: Towards evaluating the long-term stability of geological environments

Abstract

  Geological disposal of high-level radioactive waste requires not only selecting sites appropriate for the waste repository, where its isolation ability would not be damaged by natural phenomena for several tens of thousands of years, but also rationally constructing the disposal system depending on site-specific geological environments and their anticipated long-term variability. Recently, elemental/isotopic compositions of underground fluids (deep groundwaters, hot/cold spring waters, brines associated with oil and natural gas reservoirs, and so on) in Japan have been studied for evaluating the long-term stability of the geological environments of this country. Iodine and its radioactive isotope ¹²⁹I (half-life = 15.7 million years) are included in the subjects of the study. The current review paper provides overviews of (i) the iodine content and iodine-129/127 ratio (¹²⁹I/¹²⁷I ratio) of various materials in Earth's surface layers, (ii) relevant sample pretreatments and measurements, and (iii) ¹²⁹I/¹²⁷I data of the underground fluids in Japan, then gives (iv) some interpretations of the fluid ¹²⁹I/¹²⁷I data, along with their problems and uncertainties, and (v) some implications towards evaluating the long-term stability of geological environments.