Leaching mechanisms in polyphase ceramic high-level nuclear waste forms

The crystalline phases of ceramics developed for defense high-level nuclear waste dissolve by surface reaction-controlled mechanisms that, at high undersaturations, give dissolution rates linear in time. The effective back reactions are the precipitation of less-soluble reaction products. Reaction rates in acid regimes increase proportionally to hydrogen ion activity and are slowest in near-neutral regimes. In general the dissolution rates follow the Arrhenius equation, most with similar activation energies. Ceramic phases such as spinel, zirconolite, and magnetoplumbite are highly insoluble and dissolution rates can be measured only in strongly acid regimes. Nepheline is the common host phase for silica and alkali in most defense waste ceramics and is by far the least resistant mineral. Nepheline and some amorphous phases are preferentially leached from grain boundaries. In some ceramics this process continues until all nepheline has been removed. In others, such as the RSC-S29 defense waste ceramic, nepheline dissolution slows after a time, indicating that physical microencapsulation by the more resistant phases in the ceramic can limit leaching