Cesium and strontium incorporation into uranophane, ca(uo₂)(sio₃oh)₂.5h₂O

The uranyl silicate solid uranophane, Ca[(UO₂)(SiO₃OH)]₂-5H₂O, is likely an important secondary solid that forms in the alteration of spent nuclear fuel. This 1:1 U(VI)-silicate structure may serve as a host for the incorporation of fission products such as ^l37Cs or ⁹⁰Sr by infstitution of Ca²⁺ in interlayer spaces in the solid. We have investigated this possibility by synthesizing 1:1 U(VI)-silicate solids where the Ca²⁺ is completely or partially replaced by either Sr²⁺ or Cs⁺. The solids obtained have been characterized by x-ray powder diffraction (XRD), scanning electron microscopy (SEM), and elemental analysis. We observe that the Sr²⁺ analog of uranophane yields a diffractogram that resembles the XRD pattern obtained for synthetic uranophane, although the morphology of the solid is different from uranophane. Conversely, infstitution of Cs⁺for Ca²⁺ in the synthesis of the 1:1 uranyl-silicate solid yields a very different diffractogram but solid phase morphology similar to uranophane. When Ca²⁺ is partially replaced in the synthesis, the diffractograms appear to be similar to uranophane for both the Sr²⁺ and Cs⁺ systems, but SEM indicates that a mixture of solid phases are formed. We describe and summarize our results, and propose additional studies to address the question of whether these cations are incorporated into the uranophane solid.