Experimental study and kinetic analysis on sodium oxide-silica reaction

In a sodium-cooled fast reactor (SFR), if considering hypothetical severe accidental condition such as the steel liner failure of structural concrete caused by intensive leakage of liquid sodium (Na) coolant, the liquid sodium-concrete reaction (SCR) may take place. The major consequences of SCR are hydrogen release, energy release and concrete ablation. Thus, it is important to understand the phenomenology and kinetic behavior of SCR. As a part of a series of studies on SCR, this study focused on the reaction between sodium oxide (Na2O) and silica (SiO2), which is dealt with possible formation of Na2O as the result of sodium-water reaction in the initial stage of SCR. Through thermoanalytical and X-ray diffraction measurements, it was revealed that Na2O-SiO2 reaction to form sodium orthosilicate (Na4SiO4) occurs at significantly lower temperature in comparison with Na-SiO2 reaction. The reaction is kinetically characterized as a largely overlapping multistep reaction, which is composed of at least three reaction steps. On the basis of the observations, the impact of Na2O-SiO2 reaction in the overall SCR and the significance of the conventional kinetic analysis using the Kissinger method are discussed.