Distribution and Solubility of Radionuclides in Waste Forms for Disposition of Plutonium and Spent Nuclear Fuels: Preliminary Results

Successful immobilization of actinides (Am, Cm, Th, Pu, U, Np, etc.) in crystalline and amorphous host matrices with appropriate neutron absorbers (B, Hf, Gd, etc.) requires sound scientific knowledge of the local chemical environments of both the actinides (An) and the neutron absorbers and their interaction with the host matrices. This understanding leads to a more systematic and efficient approach to predicting solubilities than the strictly empirical approach currently used. The goals of this ongoing research are to determine solubility limits of representative actinides (Pu and U) and neutron absorbers (Hf and Gd) in crystalline and amorphous matrices and to determine solution mechanisms of these species in the two types of systems. Understanding of the chemical systematics of in the two systems will make it possible to calculate the solubility of actinides, neutron absorbers, and, we would suggest, other metal species in glass and ceramic waste form, which have been determined empirically for the past. The research results of the ongoing project will be presented, including solubility of neutron absorbers, Hf(IV) and Gd(III), and local structures of these species in comparison to those of Pu(IV) and Pu(III) in glass. Effect of glass composition, in terms of peralkallinity, Na2O/(Na2O+Al2O3), on dissolution of Hf(IV) and Gd(III)and glass microstructure development will be also presented