Analytical models for predicting the behavior of the Fukushima fuel debris during laboratory tests and long-term storage

The results of the laboratory-based aging experiments with synthesized fuel debris (simulated corium–concrete compound) are discussed. The tests included dynamic ( i.e. with periodic solution renewal) experiments with monoliths of the synthesized fuel debris. At the time scale of the experiments, at a temperature of 25 °C, no evolution in the composition of the leachate debris was observed. At the elevated temperature regime, T = 50, 90 and 120 °C and pH ranges from acid to alkaline condition (pH=4, pH=7, and pH=9), the major chemistry changes that occurs during the leaching were the increase of Si, Al and Ca, as structural elements of the concrete fraction of the debris, while the major elements of the melted fuel matrix, U and Zr, were found in the leachate in trace quantities. The release of Si, Al and Ca, was proved to be non-congruent (Ca and Al releases much faster compared to Si) that corresponds to the basic concept of multioxide silicate dissolution. Such system behavior as well as a number of the chemical interactions between the elements limits the classical chemical thermodynamic and kinetic methods and forced us to use a simplified, semi-empirical, effective approach. The component release rates measured in batch tests were fitted to a proposed diffusion and kinetics model which accounts also for the various pH and temperature conditions. Long-term prediction of the glassy phase dissolution and surface enrichment with a poorly dissolved fuel fraction, (U, Zr)O ₂ , is based on a first-order kinetic model inherited the significant properties of the more detailed lab-scale model. To evaluate the growth kinetics of (U, Zr)O ₂ layer on debris surface, dissolution rate coefficients from the aging laboratory experiments were used. These coefficients calculated for high-temperature ( T = 50 and 90 °C) surface reactions, were extrapolated to low-temperature debris storage conditions via the Arrhenius equation. It was shown, that the aging of debris in a water environment at room temperature and neutral pH under no-flow storage conditions results in the density accumulation of the (U, Zr)O ₂ product up to several g/m ² . At the alkaline condition (pH=9) these values increase about five times. The further rise in temperature results in further intensification of the silica leaching process that may dramatically increase the accumulation amount of (U, Zr)O ₂.