Implications of transmutation on the defect chemistry in crystalline waste forms

Radioactive decay within the solid state creates chemical environments which are typically incommensurate with the initial host structure. Using a combined theoretical and computational approach, we discuss this ‘transmutation problem’ in the context of the short-lived fission products Cs-137 and Sr-90. We show how a Kröger–Vink treatment is insufficient for understanding defects arising from transmutation, and present density functional theory data for chemical evolution within two prototypical hosts, CsCl and SrTiO3. While the latter has a strong driving force for phase separation with increasing Zr content, the Cs(Ba)Cl system is surprisingly stable. The sharp difference between these two findings points to the need for better understanding of novel chemistry in nuclear waste forms.