Iron Redox Reactions in Model Nuclear Waste Glasses and Melts

The influence of boron on the kinetics of oxidation of iron in silicate melts relevant to nuclear waste storage has been investigated by XANES experiments. The measurements have been performed isothermally as a function of time at the iron K-edge. The redox kinetics become slower with increasing B 2 O 3 content either close to the glass transition range, where the redox kinetics are controlled by diffusion of network-modifying cations, or at superliquidus temperatures where oxygen diffusion is the rate-limiting factor. In both ranges the kinetics can be interpreted in terms of boron speciation and interaction with alkali cations. Below the liquidus, however, the long times needed to reach redox equilibrium allow sintering of the powders investigated to take place so that the resulting changes in sample geometry prevent determinations of oxidation kinetic parameters from being made.