Plutonium Incineration in LWR's by a Once-Through Cycle with a Rock-Like Fuel

Solubility and hydrolysis of Zr oxides: a review and supplemental data

Thermodynamic data for Zr(IV) have been reviewed in the context of a recent update of the Nagra/PSI thermochemical database, which is primarily used in safety assessment calculations for radioactive waste repositories in Switzerland. In spite of the important role of Zr in nuclear reactor technology and as fission product in radioactive waste, the thermodynamic properties of this element in aqueous solutions are poorly constrained and published data are frequently contradictory. Specifically, hydrolysis constants, which are critical to any geochemical calculation of Zr speciation in aqueous solutions, may vary by several orders of magnitude.

Published solubility and hydrolysis data for Zr oxides were reviewed and evaluated. Recent solubility measurements on crystalline ZrO2, obtained in our laboratory, were integrated in this analysis. These new data are the only ZrO2 solubility measurements obtained in a pH region relevant for environmental aqueous systems, where equilibrium concentrations decrease to nanomolar levels.

All solubility data of (hydrous) Zr oxides can be reproduced satisfactorily by a single set of hydrolysis constants, provided that only data measured from undersaturation are considered. This set includes the species Zr4+, ZrOH3+, Zr(OH)4 0 and Zr(OH)5 −. However, present experimental data are still not sufficient to establish a fully reliable database.

Defect-fluorite oxides M1-yLnyO2-y/2(Ln = lanthanide; M = Hf, Zr, Ce, U, Th): Structure, property, and applications

An up-to-date summary of our coupled macroscopic (lattice parameter, a0) and microscopic (Mössbauer) studies on the entitled systems is given, shedding new light on the basic phase and structure properties of these technologically important, highly defective oxides. Through these studies, the intermediate-pyrochlore (P)-based local structure nature of the so-called stabilized-zirconia (SZ)- and stabilized-hafnia (SH)-type defect-fluorite (DF) phases in the M4+ = Zr and Hf systems seems to be almost unambiguously established with different (from the conventional) approaches: These are (1) the presence of a broad lattice parameter (a0) hump over the extended stabilized cubic DF region (y = ~0.3-~0.7); (2) the microscopic 151Eu-Mössbauer evidence of Eu3+ isomer shifts (ISs) showing characteristic V-shaped minima around the ideal-P area (y ~ 0.50) for both the M4+ = Zr and Hf systems; and (3) rich 155Gd-Mössbauer data on the Zr1-yGdyO2-y/2 system [IS, quadrupole splitting (QS), line width, peak height, and relative absorption area (RAA)], revealing detailed features of its ordered P-disordered DF phase and structure relationships. These results are discussed with reported basic-property data of these systems.