Synthesis and characterization of novel lanthanide- and actinide-containing titanates and zircono-titanates; relevance to nuclear waste disposal

Because working with actinide elements requires special facilities for handling them, many compounds of these elements are less-well studied than those of non-radioactive elements. Thus, augmenting the knowledge about actinide compounds has intrinsic scientific and technological values. For example, investigations of several actinide compounds, which are discussed here, have potential applications such as immobilization matrices for nuclear wastes.

However, before experiments using actinide elements are performed, synthetic routes are tested using lanthanides of comparable ionic radii as surrogates. Compound and solid solution formation in several lanthanide-containing titanate and zircono-titanate systems have been established using X- ray diffraction (XRD) analysis, which helped to define interesting and novel experiments, some of which have been performed and are discussed, for selected actinide elements. The aqueous solubilities of several lanthanide- and actinide-containing compounds, representative of the systems studied, were tested in several leachants, including the WIPP "A" brine, following modified Materials Characterization Center procedures (MCC-3). The WIPP "A" brine is a synthetic substitute for that found in nature at the Waste Isolation Pilot Plant (WIPP) in New Mexico.

The concentrations of cerium, used as a surrogate for plutonium, leached by the WIPP "A" brine from all the cerium-containing compounds and solid solutions tested were below the Inductively Coupled Plasma (ICP) atomic emission spectrometry limit of detection (10 ppm) established for cerium in this brine. The concentrations of plutonium leached from the two plutonium-containing solid solutions were less than 1 ppm as determined by gross alpha counting and alpha pulse height analysis. Concentrations of strontium leached by the WIPP brine from stable strontium- containing titanate compounds, studied as possible immobilizers of both ⁹⁰;Sr and actinide elements, were also quite low.

These compound and solid solution formation investigations and the aqueous solubility studies suggest that the types of titanate and zirconate-titanate compounds and solid solutions studied in this work appear to be useful as host matrices for nuclear waste immobilization.