Uranium and Thorium Minerals: Natural Analogues for Radioactive Waste Forms

Abstract

SYNROC (1) and Supercalcine (2) are proposed crystalline radio-waste forms that may be regarded as highly radioactive “artificial rocks.” In these crystalline radioactive waste forms, rare-earth elements (REE), U and transuranium (TRU) elements are strongly partitioned into five phases. These phases have U- and Th-bearing mineral analogues. In SYNROC, the actinide bearing phases are perovskite (CaTiO₃) and zirconolite (CaZrTi₂O₇). “BAIT” (BaAl₂Fe₈Ti₁₃O₃₈) is a phase which may occur in SYNROC formulations for wastes with large amounts of Fe and Al. “BAIT” may be structurally similar to davidite(Fe₆Ti₁₅0₃₆). Supercalcine actinide phases include silicate-apatite (CaREE₄(SiO₄)₃0), monazite ((Ce, La) PO₄ and uraninite (UO_2+x). The purpose of this paper is to summarize the relevant geologic literature on these phases. Emphasis is given to: (1) solid-solution chemistry, (2) mode of occurrence, (3) alteration and (4) radiation damage effects. A special effort is made to identify stable and unstable mineral associations, as this may be important if hydrothermal conditions occur in the waste repository. The idea of devising a radioactive waste form which is in thermodynamic equilibrium with its repository is appealing but impossible. This is apparent from the conditions for equilibrium between two or more phases, namely, for each chemical species, i, the chemical potential, U., must in each phase be equal.