Determination of Molybdate Mean Ionic Activity Coefficients for the Assessment of Radionuclide Mobility

Solubility phenomena in science and education: Experiments, thermodynamic analyses, and theoretical aspects

Solubility equilibria between solid salts, salt hydrates, and water play an important role in fundamental and applied branches of chemistry. The continuous interest in this field has been reflected by the 15th International Symposium on Solubility Phenomena as well as by the ongoing IUPAC-NIST Solubility Data Series (SDS), which by now comprises close to 100 volumes. Three typical examples concerning solubility phenomena of ionic solids in aqueous solutions are discussed: (1) sparingly soluble, simple molybdates; (2) sparingly soluble ionic solids with basic anions; and (3) hydrolysis of inert hexa-aqua-M(III) ions, where M is Ir, Rh, or Cr. In the first two cases, essential experimental details are discussed, an outline of thermodynamic analyses is given, and theoretical aspects are emphasized. In the third case, an educational suggestion is made.

Formation of secondary phases after long-term corrosion of simulated HLW glass in brine solutions at 190 °C

The behavior of rare earth elements (REE) as chemical analogues for actinides during glass corrosion was studied with static long-term batch experiments (7.5 years) at 190 °C. Corrosion tests were carried out using a simulated inactive high level waste (HLW) glass powder. Two different highly concentrated salt solutions (NaCl-rich and MgCl2-rich) have been used to simulate the potential corrosion by aqueous solutions in geological salt formations. Samples with surface-to-volume ratios (S/V) of 1000 m−1and 10000 m−1were investigated. The chemical composition of the aqueous solution and the formation of secondary phases at the glass/solution interface as well as their elemental composition, especially with respect to the behavior of REE was studied. The glass corrosion behavior and secondary phase formation was significantly different in the two brines. In the MgCl2-rich salt solution, with a final pH < 3.5, hydrous Mg-silicates and (Ba,Sr)SO4solid solutions with considerable amounts of Sr (up to 15 wt.%) have been observed. Further, REE-sulfates and REE-molybdates with an indication of dissolution (etch pits) as secondary phases could be identified. In the NaCl-rich solution with a final high pH, REE-containing hydrous silicates (up to 35 wt.%) and (Ba,Sr)SO4solid solutions with small amounts of Ce (up to 5 wt.%) have been observed. Frequently observed Ca-molybdates (powellite) did not contain detectable amounts of REE.

Trivalent actinide coprecipitation with powellite (CaMoO4): Secondary solid solution formation during HLW borosilicate-glass dissolution

Powellite (CaMoO4) is one of the various crystalline secondary alteration phases which form during the corrosion of high level waste (HLW) glasses. Due to their structural variability, powellite can accommodate considerable chemical substitutions including trivalent actinides. Batch adsorption and coprecipitation experiments in mixed flow reactors have been used to study quantitatively the uptake of Cm(III) and Eu(III) (as a nonradioactive chemical homologue for trivalent actinides) from aqueous solution under repository relevant conditions. A metal ion concentration-independentKdof 1200 ml/g has been determined for adsorption above pH 4 up to 3 μmol/L Eu(III). The partition coefficient for coprecipitation varies between 10 and 800 depending on the precipitation rate. Time-resolved laser fluorescence spectroscopy has been used to study the aqueous complexation of Cm(III)/Eu(III) and MoO42-as well as their incorporation into the powellite crystal lattice during coprecipitation. A red shift of the Cm(III) fluorescence emission of the transition indicates the formation of aqueous CmMoO4+(598 nm) and Cm(MoO4)22-(601 nm) complexes. The incorporation of Cm(III) is indicated by a significant red shift. The corresponding lifetime of 165 μs indicates quench effects. A similar behaviour has been observed for Eu(III). The life time of the5D0-7F2transition is 350 μs for an incorporated Eu(III) species, suggesting also intrinsic quench effects, due to the local crystal field or impurities.

Waste form corrosion modeling: comparison with experimental results

Results of leaching experiments using active and inactively simulated HLW glass in concentrated NaCl solution are described. Measured solution concentrations of glass components, fission products and actinides are compared with computed data. The computed pH value corresponds with the findings from experiments with inactively simulated glass samples. Moreover, the concentrations of silica and strontium can be described adequately by reaction path modelling. The computed U concentration is explained by the precipitation of schoepite or Na2U2O7. The computed Am concentration significantly exceeds the measured data. This may be attributed to sorption processes on corrosion products of the glass, which are relevant also for lanthanide elements under the conditions of the experiments. This hypothesis is tested by solid solution approaches and by computing sorption of Am onto SiO2 precipitates.