Immobilization of cesium and iodine into Cs₃Bi₂I₉ perovskite-silica composites and core-shell waste forms with high waste loadings and chemical durability

Cs₃Bi₂I₉, a defect perovskite derivative, is a potential host phase to immobilize iodine and cesium with high waste loadings. In this work, two strategies were explored to form Cs₃Bi₂I₉-silica composites and a core-shell structure in order to improve chemical durability of waste form materials meanwhile maintaining high waste loadings. Cs₃Bi₂I₉ loadings as high as 70 wt.% were incorporated into a silica matrix to form silica-ceramic composites, and 20 wt.% Cs₃Bi₂I₉ was encapsulated into silica to form a core–shell structure by low temperature spark plasma sintering. Chemical durability of the composite and core-shell waste forms was evaluated by semi-dynamic leaching experiments, and Cs and I were incongruently released from waste form matrices. A BiOI alteration layer formed, acting as a passivation layer to reduce the release of radionuclides. The long-term iodine release rate was low (30 mg m⁻² day^-1) for the 70 wt.% Cs₃Bi₂I₉–silica composite leached in deionized water at 90 °C, which can be further reduced to 5 × 10⁻³ mg m⁻² day⁻¹ for the 20 wt.% core-shell structure. This work highlights a robust way to immobilize the highly mobile radionuclides with high waste loadings through encapsulation into durable matrices and a surface passivating mechanism that can greatly reduce the elemental transport from waste form materials and significantly enhance their chemical durability.