Selenide [Se(-II)] immobilization in anoxic, Fe(II)-rich environments; Coprecipitation and behavior during phase transformations

Francisco, P. C. M.  ; Matsumura, Daiju   ; Kikuchi, Ryosuke*; Ishidera, Takamitsu ; Tachi, Yukio  

The radionuclide selenium-79 (Se-79) is predicted to be a key contributor to the long-term radiologic hazards associated with geological high-level waste (HLW) repositories, hence its release is of pertinent concern in the safety assessment of repositories. In this study, we examined the immobilization mechanisms of Se(-II)-prevalent under anoxic and reducing repository conditions-during interaction with aqueous Fe(II) and freshly precipitated Fe(OH) at circumneutral and alkaline conditions, respectively, its response to changes in pH, and its behavior during aging at 90C. Using microscopic and spectroscopic techniques, we observed that Se(-II) was readily immobilized via iron selenide precipitation regardless of whether Se(-II) reacts with aqueous species or solid phases and that modifying the pH following initial immobilization did not immediately remobilize Se(-II). These observations indicate that Se(-II) migration beyond the overpack can be effectively and rapidly retarded via interaction with Fe(II) species arising from overpack corrosion. Thermodynamic calculations, however, showed that at alkaline conditions, iron selenides immediately became metastable and will likely dissolve in the long term. Aging experiments at 90C showed that Se(-II) can be completely retained via the crystallization of ferroselite at circumneutral conditions, while it will be largely remobilized at alkaline conditions. Our results show that Se(-II) mobility can be significantly influenced by its interactions with the corrosion products of the steel overpack and that these behaviors will have to be considered in repository safety assessments.