A Potential Wasteform for Cs Immobilization: Synthesis, Structure Determination, and Aqueous Durability of Cs₂TiNb₆O₁₈

Abstract

CsTiNbO is a potential ceramic wasteform for the long-term immobilization of radioactive cesium. CsTiNbO was synthesized using the aqueous precursor method and a solid-state reaction, and its crystal structure was determined from the Rietveld refinement of synchrotron X-ray and neutron powder diffraction data. The structure is a pyrochlore analogue, space group P3̅m1 with Cs in 9-fold coordination. The calculated bond valence sum from analysis of neutron diffraction data of 0.84 and high coordination number suggest that Cs has a strong bonding environment. The chemical aqueous durability was investigated using the MCC-1 and PCT-B standard test methods. The measured Cs leach rates of 3.8 × 10 and 2.1 × 10 g m day obtained via the MCC-1 and PCT-B methods, respectively, demonstrate good promise of a safe long-term immobilization material comparable to, if not better than, hollandite-the material in the multiphase titanate ceramics (Synroc) targeted for cesium sequestration.

Vitrification of wastes: from unwanted to controlled crystallization, a review

“…In one study, crystalline silicotitanate (CST) ion exchange media (which also contains Nb and a Zr-hydroxide binder) was used to separate Cs-contaminated water. HIP of these Cs-exchanged media form CsTiNb 6 O 18 (a pyrochlore analogue [Chen et al, 2016]) and Cs 2 ZrSi 6 O 15 plus mixed (Ti,Zr,Nb) dioxides and other minor phases of zircon and NaNbO 3 [Chen et al, 2018]. In another study, chabazite-zeolite-containing Cs was HIPped creating Cs-substituted leucite and residual glass along with other feldspars albite and anorthite and minor diopside [Gardner et al, 2021].…”

“…Hollandite has also been envisaged as a phase for immobilization of Cs, being stable under beta and gamma irradiation [Caurant et al, 2007a], but possibly only for ceramic waste forms. Hollandite phases for ceramic nuclear waste forms have been well-studied for nuclear waste [Caurant, 2014, Chen et al, 2016, Xu et al, 2015a and hazardous waste [Krausova et al, 2016] immobilization of large univalent and divalent (e.g., Ba 2+ , Cd 2+ , Pb 2+ ) ions.…”