• Courtois, Nicolas

Abstract

Electricity production from nuclear power plants generates radioactive wastes, the management of which represents a major industrial and environmental concern. Thus, low -- or intermediate -- level radioactive aqueous waste streams may be concentrated by evaporation, and immobilized with a hydraulic binder (Portland cement, usually), before being sent to disposal. Nevertheless, interactions may occur between some components of the waste and the cement phases or aggregates, and decrease the stability of the final waste forms. Thereby, the formation of a gel-like product has been recently observed on the surface of some cemented drums of evaporator concentrates, which were produced in the 80’s at Doel nuclear power plant in Belgium. This product may result from a reaction between silica from the aggregates and the very alkaline pore solution of the concrete. However, its properties seem to differ from those reported for alkali-silica gels in civil engineering. Thus, this work aims at better understanding the processes involved in the gel formation within the cement-waste forms and at characterizing its properties, linked to its composition and structure.The first objective is to reproduce the gel in the laboratory. Two methods are successfully developed. Concrete specimens comprising a surrogate waste and a reactive siliceous aggregate (flint) are designed and submitted to a thermal cycle representative of the early-age thermal history of the cemented drums produced at Doel. Synthetic gels are also directly prepared by mixing concentrated saline solutions with a sodium silicate solution.Characterizations achieved on the resulting gels highlight their hygroscopic and efflorescent behaviors, as well as their high tendency to undergo carbonation, requiring a careful handling while analyzing them. These gels, structurally representative of real gels, exhibit an amorphous or nanocrystalline structure that remains weakly influenced by their chemical composition in the studied domain. They exhibit a Newtonian fluid behavior, their viscosity depending mainly on their water content and on the temperature.The second objective is to refine the influence of the cemented concentrates composition and concrete thermal history on the gel formation rate, as well as on the damage caused to the cementitious material. The incorporation of alkaline concentrates promotes the expansion and cracking of the concrete. Reversely, a temperature rise at early age, by allowing the gel exudation, improves the subsequent stability of the material.Finally, the aggregate degradation by alkali-silica reaction is investigated on the first hand by looking at the aggregate behavior in ideal alkaline medium, and on the second hand by characterizing aggregates extracted from artificially aged concrete specimens. Two competitive behaviors are outlined for the aggregate degraded in ideal medium: dissolution, and creation of flaws in its siliceous network (amorphisation), which decreases its volumetric mass density and increases its specific surface area. The ideal medium is shown to be an interesting approach to reach high degrees of degradation within a short period.

Topics

• density
• impedance spectroscopy
• pore
• surface
• amorphous
• phase
• reactive
• Sodium
• cement
• viscosity
• chemical composition
• evaporation