A Structural and Spectroscopic Investigation of Hydrous and Anhydrous Rare-Earth Phosphates

Abstract

Nuclear power plants provide clean energy for generating electricity via neutron induced fission reactions of nuclear fuels and at the end of this energy generation process, radioactive waste is produced. Currently, the waste is chemically incorporated into a glass matrix and the resulting wasteform is destined for storage in geological repositories. Glass based wasteforms, however, might corrode under repository conditions and could potentially release radionuclides to the biosphere. Hence, crystalline wasteforms were proposed as an alternative to glass based wasteforms and among the many materials studied, materials adopting the monazite- (REPO4; RE = La to Gd) and xenotime- (RE’PO4; RE’ = Tb to Lu and Y) type structures were suggested as a potential wasteform. Both monazite and xenotime are naturally abundant rare-earth minerals containing significant amounts of U and Th and have remained stable on a geological time-scale. Hydrous rare-earth phosphates adopting the rhabdophane-type structure (REPO4.nH2O; RE = La to Dy) also exist in nature and are present on the surface of anhydrous rare-earth minerals (e.g., monazite). The hydrous phase may act as a secondary barrier by preventing the release of actinides from reaching the biosphere. This thesis aims to provide an atomic level understanding of hydrous and anhydrous rare-earth phosphates using X-ray based diffraction and spectroscopic techniques.

A comprehensive account of the rich structural chemistry of rare-earth phosphates are provided in Chapters 2 and 3 using X-ray diffraction (XRD) and X-ray absorption near-edge spectroscopy (XANES). Crystalline materials containing radioactive wastes are prone to undergo radiation-induced structural damage and, in Chapter 4, radiation damage studies on monazite- and xenotime-type materials were conducted by simulating radiation damage events using high-energy ion implantation. The results from this study depict the ability of these materials to recover from the structural damage inflicted by high energy ion-implantation. In Chapter 5, the chemical durability of rare-earth phosphates was studied by investigating leaching behaviour of these materials in deionized water. Preliminary results suggest a faster leaching of hydrous rare-earth phosphates when compared to their anhydrous counterparts. The information presented in this thesis will contribute to the growing body of work on crystalline wasteforms for nuclear waste immobilization.