Crystal Structure and Thermodynamic Stability of Ba/Ti‐Substituted Pollucites for Radioactive Cs/Ba Immobilization

Abstract

As an analogue of the mineral pollucite (CsAlSi2O6), CsTiSi2O6.5 is a potential host phase for radioactive Cs. However, as 137Cs and 135Cs transmute to 137Ba and 135Ba, respectively, through the beta decay, it is essential to study the structure and stability of this phase upon Cs → Ba substitution. In this work, two series of Ba/Ti‐substituted samples, CsxBa(1−x)/2TiSi2O6.5 and CsxBa1−xTiSi2O7−0.5x, (x = 0.9 and 0.7), were synthesized by high‐temperature crystallization from their respective precursors. Synchrotron X‐ray diffraction and Rietveld analysis reveal that while CsxBa(1−x)/2TiSi2O6.5 samples are phase‐pure, CsxBa1−xTiSi2O7−0.5x samples contain Cs3x/(2+x)Ba(1−x)/(2+x)TiSi2O6.5 pollucites (i.e., also two‐Cs‐to‐one‐Ba substitution) and a secondary phase, fresnoite (Ba2TiSi2O8). Thus, the CsxBa1−xTiSi2O7−0.5x series is energetically less favorable than CsxBa(1−x)/2TiSi2O6.5. To study the stability systematics of CsxBa(1−x)/2TiSi2O6.5 pollucites, high‐temperature calorimetric experiments were performed at 973 K with or without the lead borate solvent. Enthalpies of formation from the constituent oxides (and elements) have thus been derived. The results show that with increasing Ba/(Cs + Ba) ratio, the thermodynamic stability of these phases decreases with respect to their component oxides. Hence, from the energetic viewpoint, continued Cs → Ba transmutation tends to destabilize the parent silicotitanate pollucite structure. However, the Ba‐substituted pollucite co‐forms with fresnoite (which incorporates the excess Ba), thereby providing viable ceramic waste forms for all the Ba decay products.

The Role of Water and Hydroxyl Groups in the Structures of Stetindite and Coffinite, MSiO4 (M = Ce, U)

“…The above Rietveld refinement procedures on HT-PXRD were also stated previously elsewhere. 7,[74][75][76][77] In situ HT-PXRD of two uranothorites (x = 0.46 and x = 0.90) were collected using a Bruker D8 advance diffractometer equipped with a Lynx-eye detector and using Cu K1,2 radiation ( = 1.54184 Å).…”

Heat capacities, standard entropies and Gibbs energies of Sr-, Rb- and Cs-substituted barium aluminotitanate hollandites

“…An alternative strategy is to immobilize the radionuclides as components in the crystal structures of dense minerals, such as silicate, phosphate, and titanate [9][10][11][12][13][14][15][16]. Compared to glass waste forms, these dense minerals have robust thermal stability and aqueous durability rendering them viable candidates for hosting radionuclides.…”

Evidence of technetium and iodine release from a sodalite-bearing ceramic waste form

“…Multi-phase ceramics are candidate materials for use as nuclear waste forms and have, therefore, been extensively studied as host matrices for radionuclides (Lumpkin, 2006). Examples of multiphase ceramics include SYNROC ("synthetic rock") (Ringwood et al, 1978;Vance, 2012), pyrochlore (Icenhower et al, 2006;Zhang et al, 2013), pollucites (Xu et al, 2001;Xu et al, 2015), glass ceramics (Crum et al, 2012) and the fluidized bed steam reforming (FBSR) product (Jantzen, 2006;Neeway et al, 2012). The FBSR process has been demonstrated commercially to treat both liquid and solid low-level radioactive waste streams (Mason et al, 2003).…”