Hydrothermal interactions of basalts with Cs and Sr of spent fuel elements. Implications to basalt as a nuclear waste repository

The immobilization reactions of Cs, Sr, U and Mo from spent fuel elements (SFE) in the presence of basalts and basalt phases were investigated under hydrothermal, closed-system conditions. The exact Cs and Sr phases in SFE are not known with certainty, but Cs₂MoO₄, CsI, Cs₂O, β-Cs₂U₂O₇ and SrZrO₃ are some of the predicted possibilities. These phases were mixed with basalts and basalt phases along with water, sealed in gold capsules and reacted at 100, 200 and 300°C under a confining pressure of 300 bars. The Cs and Sr concentrations of the product solutions were measured to determine the partitioning of these elements between the liquid and solid phases. Results varied substantially, depending on the particular basalt or basalt phase or Cs phase, the temperature and the duration of run. For example, a Deep Drill Hole (DDH-3) basalt-Cs₂MoO₄ reactions resulted in 15, 78 and 99% fixation of added Cs at 100, 200 and 300°C respectively. Fixation of up to 99.7 and 99.8% of the added Cs and Sr respectively were observed in some cases. X-Ray diffraction (XRD) analysis of the solid products revealed that Cs was immobilized by the formation of pollucite, (Cs, Na)AlSi₂O₆ in most cases and the formation of CsAlSiO₄ in some cases. The interactions of basalts and basalt phases with Cs₂MoO₄ also resulted in the immobilization of Mo by the formation of powellite, CaMoO₄ because Ca is available in the reaction mixtures. The U⁶⁺ in β-Cs₂U₂O₇ was reduced to form uraninite, UO₂ by the divalent iron component of basalts. The exact nature of Sr reaction with basalts is not clear because no new Sr phases could be detected by XRD. The reaction products, pollucite, CsAlSiO₄, powellite and uraninite from SFE-basalt interactions serve to immobilize Cs, Mo and U because Cs in pollucite and CsAlSiO₄ is difficult to exchange and Mo and U in powellite and uraninite are quite insoluble.