PNNL

Abstract

Cs3Bi2I9, a defect perovskite derivative, is a potential host phase to immobilize iodine and cesium with high waste loadings. In this work, two strategies are explored to form Cs3Bi2I9-silica composites and core-shell structures 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 at low temperatures and high pressure. Semi-dynamic leaching experiments were performed to obtain the release rates of Cs and I in aqueous solutions and the release mechanisms of radionuclides were investigated. Cs and I were incongruently released from waste form matrices. A BiOI alteration layer formed during dissolution, 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 70 wt.% Cs3Bi2I9–silica composites 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. The release rates of the waste forms with significantly higher waste loadings are comparable or orders of magnitude lower than previous waste forms.