The development of a long-term predictive model of the changes in debris properties during interaction with aqueous solutions. Thermodynamic–kinetic modeling

Abstract

This article presents a long-term predictive model of the properties’ changes of the molten core–concrete interaction (MCCI) debris material formed during the Fukushima Daiichi nuclear power plant (NPP) accident based on the available experimental data. For the development of the model, samples of MCCI debris were synthesized to represent the composition of the actual Fukushima debris. The samples' composition simulated different stages of ablation of the concrete basement beneath the NPP unit, i.e., various “concrete–corium” ratios. MCCI simulated debris samples (monoliths and powders) were exposed to aqueous solutions under the following conditions:1) continuous contact with water, i.e., no renewal of aqueous solutions (batch leaching experiments);2) periodic renewal of aqueous solutions by fresh ones (conditions simulating an open system with a constant inflow of fresh portions of aqueous solution).Experiments were performed under various conditions (temperature, pH) in two types of aqueous solutions: deaerated deionized and aerated deionized water. During the experiments, aliquots were taken to measure the concentrations of dissolved components (Ca, Si, Al, U, Zr, Fe, Ni, and Cr) in the aqueous solution.MCCI debris “aging” experiments were interpreted using kinetic curves and their relationship with physicochemical conditions (temperature, pH, type of aqueous solution). In this interpretation, the chemical processes of the MCCI debris components in water phase (dissolution and precipitation) were considered.The model was extended to 50 years and used to interpret debris aging experiments: (1) prediction of components’ releases into the aqueous phase caused by dissolution of solid phases, and (2) prediction of dusty secondary phase formation at the “debris–aqueous solution” interface.