Chemical mitigation of the transmutation problem in crystalline nuclear waste radiophases

Certain deleterious effects on a solid nuclear waste form, though not yet quantitatively defined, could occur due to transmutations of the type ¹³⁷Cs⁺ → ¹³⁷Ba²⁺ and ⁹⁰Sr²⁺ → ⁹⁰Zr⁴⁺ (of half-life, t_1/2, approximately 30 years in both cases). The relevant causes of such possible effects are the valence and size changes. In this paper, a chemical mitigation strategy is explicitly formulated: 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. A generalized scheme is: {Mathematical expression} where R is a transition metal. The present work involved chemically simulating the transmutation and then attempting to find a Cs- or Sr-bearing single-phase host which would remain single-phase after the transmutation had occurred. Of several structures investigated, perovskite appears to be promising as the A-site can accommodate the approximately 20% size change which occurs when Cs decays to Ba. Ta and Nb were used as the variable-valence ions in the B-site. The application of the results to unpartitioned and partitioned nuclear wastes is discussed.