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Redox chemistry, solubility and hydrolysis of uranium in dilute to concentrated salt systems

Nese Cevirim-Papaioannou
Uranium is the main element present in spent nuclear fuel and accordingly contributes with the largest mass inventory to the nuclear waste. In spite of uranium being a relatively minor contributor to the overall radiological dose of the waste, it is certainly required to have an accurate knowledge on the solution chemistry and solubility phenomena of this key element. Uranium is also a redox-sensitive actinide, and accordingly its chemical behavior is strongly dependent on the redox boundary
more » ... e redox boundary conditions of the system. Disposal of spent fuel in deep geological formations such as crystalline/granite, clay and rock salt is the option favored by international consensus. Water intrusion is a possible scenario that needs to be accounted for in the context of the long-term Safety Assessment of these repositories. The composition of the pore water contacting the waste will largely vary depending upon host-rock, backfill and other technical barriers, as well as the waste itself. Although a vast number of studies have previously investigated the solution chemistry of uranium, a number of key uncertainties remain. These affect to redox behavior, solid phases controlling solubility and hydrolysis, especially in the alkaline to hyperalkaline pH conditions of relevance in the context of nuclear waste disposal. U(IV) and U(VI) are the most stable oxidation states of uranium controlling its solution chemistry and solubility within the stability field of water and in the absence of strong complexing ligands. The study of this redox couple in the alkaline reducing conditions relevant in certain concepts for waste disposal (e.g. cementitious) is challenged by the further stabilization of U(VI) in the hyperalkaline pH-region, and the high sensitivity of U(IV) towards oxidation in the presence of traces of oxygen. Accordingly, an adequate knowledge of uranium redox chemistry in the aqueous and solid phases under geochemical boundary conditions (pH, pe, ionic strength, etc.) relevant in the context of nuclear waste disposal is import [...]
doi:10.5445/ir/1000084978 fatcat:lh4yxrhcbrbmjl6zeyt5hnazwu