An investigation of the chemical durability of hydrous and anhydrous rare-earth phosphates

Geological disposal of nuclear waste relies upon a multiple barrier approach in which canisters containing nuclear wasteforms are proposed to be stored in geological repositories. In the event of a failed container situation, the groundwater present near the repository may penetrate the waste containment system and eventually come in contact with the nuclear wasteform. Assessing the chemical durability of nuclear wasteforms is of utmost importance and in this study, leaching experiments were conducted on rare-earth phosphate materials adopting the monazite- (LaPO₄), xenotime- (YbPO₄), and rhabdophane- (GdPO₄.H₂O) type structures. Monazite and xenotime are abundant rare-earth minerals containing varying amounts of actinides incorporated within their crystal structure and are proposed as a potential host matrix for the immobilization of actinides. Rhabdophane is a hydrous rare-earth mineral that forms on the surface of chemically altered monazite mineral and is proposed to act as a protective barrier by preventing the release of actinides to the biosphere. The as-synthesized materials were exposed to deionized water for a total period of seven months and the concentration of the leached elements in the water solution were determined using inductively coupled plasma – mass spectrometry (ICP-MS). In this study, the normalized leach rates of LaPO₄, YbPO₄, and GdPO₄.H₂O materials were found to be low and indicate the chemical durability of these materials. Structural characterization of materials before and after leaching was performed using powder X-ray diffraction (XRD) and X-ray absorption near-edge spectroscopy (XANES). Analysis of the powder XRD diffractograms and XANES spectra has shown that the long-range and local structures of monazite-, xenotime-, and rhabdophane-type materials remain unaffected after exposure to water for seven months.