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-independent Kd of 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 the 5D0-7F2 transition is 350 μs for an incorporated Eu(III) species, suggesting also intrinsic quench effects, due to the local crystal field or impurities. (orig.)