Modeling the solubility of zirconia in a repository for high-level radioactive waste

The solubility of zirconia in a typical radioactive waste repository environment is examined in the light of available thermodynamic data, with particular focus on the influence of carbonate complexes. Using tetravalent actinides as chemical analogues, values for the stability constants of two Zr-carbonate complexes have been estimated. The solubility of monoclinic zirconia in a typical repository pore water has been then calculated with the help of a speciation model. Although our calculations indicate that the solubility of zirconia in a bentonite-filled repository environment will probably be between 10(-9) and 10(-8) M, there are large unavoidable uncertainties in these predictions, due to limitations in the available thermodynamic data. Similarly, although leaching experiments suggest that dissolution rates of zirconia are much smaller than for borosilicate glass, this comparison is inconclusive in view of the short leaching times involved in tests performed with zirconia. There is therefore a need for further solubility and kinetic studies of zirconia to assess the stability of this material in a radioactive waste disposal environment. (C) 1999 Elsevier Science B.V. All rights reserved.