Kinetic and thermodynamic study of the thorium phosphate–diphosphate dissolution

Hydrothermal Synthesis and Characterization of the Thorium Phosphate Hydrogenphosphate, Thorium Hydroxide Phosphate, and Dithorium Oxide Phosphate

“…In order to accelerate the kinetics of the process, leaching tests were performed in 5 M nitric acid (8,9). Under these conditions, the precipitation of a new phase was observed after 200 days at 903C; it is "rst amorphous then slowly transforms to the crystallized form.…”

“…Under these conditions, the precipitation of a new phase was observed after 200 days at 903C; it is "rst amorphous then slowly transforms to the crystallized form. The crystallized part of the product was analyzed and identi"ed as Th (PO (8,9,27).…”

“…The intermediate reactions involve the formation of an amorphous layer (thickness about 20 nm, mole ratio PO /Th" ) which seems to control the dissolution of the TPD (8,9). The global process can be presented as…”

“…Before using a matrix for the radioactive waste storage, it is necessary to investigate its resistance to water corrosion. During the leaching tests of the TPD it was noticed that thorium Th> and phosphate PO\ ions are very slowly released into the solution (8,9). Indeed, these ions are dissociated in water to give di!erent species depending on the acidity, e.g., ThOH> and HPO\ .…”

“…Because of the very slow dissolution kinetics of the TPD in water it was impossible to study this phenomenon in a short time. In order to accelerate this process it was heated at 903C with 5 M nitric acid in closed containers (8,9). In fact, the initial TPD was transformed into another solid in which the X-ray di!raction (XRD) diagram is di!erent from that of TPD.…”