Crystalline hosts to accommodate the transmutation of Cs and Sr

Certain deleterious effects on a solid nuclear waste form, though not yet quantitatively defined, could occur due to transmutations such as 137Cs+ → 137Ba2+ and 90Sr2+ → 90Zr4+ (t sub /1/2/ approx. = 30 yr in both cases). The relevant causes of such possible effects are the changes in ionic valence and size. This report explicitly formulates a chemical mitigation strategy: if the transmuting species can be incorporated in a multiple-cation host, in which one of the inert cations is a variable-valence transition metal, the valence-change aspect of transmutation can be mitigated by a complementary valence change of the transition metal ion. The present work consisted of chemically simulating the transmutation, the goal being to find a Cs- and Sr-bearing single-phase host that would remain single-phase after the transmutation had occurred. Of several structures tried, perovskite appears to be the most promising, as the A-site can accommodate the approx. 20% size change that occurs when Cs decays to Ba. Ta and Nb were used as the variable-valence ions in the B site. Although not explicitly studied here, magnetoplumbite seems likely to accommodate the Cs → Ba transmutation. The application of the results to unpartitioned and partitioned nuclear wastes is discussed