A review of modeling concepts for sodium-concrete reactions and a model for liquid sodium transport to the unreacted concrete surface

A review of existing modeling concepts and studies of sodium-concrete reactions is presented. Consistent with experimental observations, the current modeling study assumes for hydrated concretes the presence of liquid layer of reaction products intervening between sodium pool and concrete surface. Primary liquid component in this layer is NaOH which has a low melting point. This liquid component dissolves the reaction products such as silicates, aluminates and forms a very viscous liquid more dense than sodium. As this layer assumes a significant thickness, the only mechanism available for transport of sodium to fresh concrete surface is the motion and agitation induced by gas bubbles consisting of hydrogen, water vapor, CO2, and sodium vapor. However, to date there exists no satisfactory model that describes this transport mechanism. To rectify this shortcoming, we propose a mass ''diffusion'' model for sodium transport. The model reduces the sodium transport process by bubble motion to a single unknown parameter which has the appearance of a diffusion coefficient and consequently can be determined by solving an inverse problem in conjunction with measured ''concentration'' distributions in simulant material experiments. (orig./HP)