Geochemical behaviour of host phases for actinides and fission products in crystalline ceramic nuclear waste forms

Abstract

A number of polyphase or single-phase ceramic waste forms have been considered as options for the disposal of nuclear waste in geological repositories. Of critical concern in the scientific evaluation of these materials is their performance in natural systems over long periods of time (e.g., 103 to 106 years). This paper gives an overview of the aqueous durability of the major titanate host phases for actinides (e.g., Th, U, Np, Pu, Cm) and important fission products (e.g., Sr and Cs) in alternative crystalline ceramic waste forms. These host phases are compared with reference to some basic acceptance criteria, including the long-term behaviour determined from studies of natural samples. The available data indicate that zirconolite and pyrochlore are excellent candidate host phases for actinides. These structures exhibit excellent aqueous durability, crystal chemical flexibility, high waste loadings, and well-known processing conditions. Although both pyrochlore and zirconolite become amorphous due to alphadecay processes, the total volume swelling is only 5-6% and there is no significant effect of radiation damage on aqueous durability. Hollandite also appears to be an excellent candidate host phase for radioactive Cs isotopes. Brannerite and perovskite, on the other hand, are more prone to alteration in aqueous fluids and have a lower degree of chemical flexibility. With the exception of hollandite, many of the properties of these potential host phases have been confirmed through studies of natural samples.

Thermal annealing of natural, radiation-damaged pyrochlore

“…Therefore, important properties like the critical amorphization dose [18,36,[44][45][46] and the leaching behavior [18,[47][48][49][50][51][52][53] have been studied in detail. To obtain better understanding of the behavior of pyrochlore under extreme conditions, combined swift heavy-ion irradiation and high-pressure studies have been performed (see [54]).…”

Nature of the chemical bond and prediction of radiation tolerance in pyrochlore and defect fluorite compounds

“…Pyrochlore is also one of the principal actinide host phases in oxide ceramics designed for the safe disposal of actinide-rich wastes derived from nuclear power generation and the nuclear weapons programs [4][5][6]. The pyrochlore structure type also has a high tolerance for impurities, is capable of very high waste loadings, and shows excellent chemical durability in aqueous fluids [7][8][9].…”

Actinides and radiation effects: impact on the back-end of the nuclear fuel cycle

“…The 'scrap' or less pure Pu would be immobilized in a titanate ceramic, the dominant phase being a Hfpyrochlore, (U,Pu,Hf,Gd) 2 Ti 2 O 7 . A considerable amount of research has been completed on phases suitable for Pu-immobilization, including pyrochlore and related structure types (Lutze and Ewing, 1988;Donald et al, 1997;Lumpkin et al, 2004). In April of 2002, the United States stopped almost all work on the Pu-immobilization strategy in favour of accelerated conversion of Pu into MOX fuel.…”