Specification of Np(V) and Np(IV) in Brine

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Internationally, agencies which have responsibility for used nuclear fuel management are considering deep underground repositories in either crystalline or sedimentary rock formations as a safe and long term solution to the growing used nuclear inventory. These agencies will have to prepare a safety assessment to demonstrate that used nuclear fuel will be safely managed over hundreds of thousands of years. The dissolution and the sorption behaviors of fission products and actinides significantly depend on their speciation. An improved scientific understanding of these behaviors is needed to reduce associated uncertainties in safety assessment calculations. One of the typical characteristics of deep groundwater in southern Ontario, where several municipalities have expressed interest in learning about hosting a used nuclear fuel repository, is that the waters are highly saline brines. However, the speciation of neptunium (Np) in brine is not yet understood. In this proposed research, we will (1) identify the speciation of Np(V) and Np(IV) in brine, (2) estimate the equilibrium constants of the Np complexes in brine, and (3) develop the methodology to study the speciation in brine. Firstly, Np speciation in brine will be predicted using thermodynamic databases. Secondly, Np(V) and Np(IV) speciation will be elucidated by combining spectroscopy, factor analysis, solvent extraction method and quantum chemistry simulation and by combining solubility method and quantum chemistry simulation, respectively. Finally, the equilibrium constants of complexation reactions of Np(V) and Np(IV) in brine will be estimated. Our proposed research will establish a cutting-edge research core of actinides at McMaster University. This proposed research will contribute significantly to improved understanding and predictions of Np sorption and to reducing uncertainty in safety assessment calculations in brines. Importantly, it will also contribute to the development of highly qualified personnel in the fields of actinide chemistry and engineering of used nuclear fuel management. Furthermore, this research will contribute to establishing a new frontier of actinide chemistry and engineering in the brine environment.