Effects of alkaline solution and aging time on thermal conductivity of MX80 powder-granule mixtures

Abstract

In the design of high-level nuclear waste (HLW) repositories, granular bentonite is esteemed as an effective sealant for interfacing the spaces that exist between the bentonite blocks and adjacent geological bodies. When degraded cement dissolves in groundwater, it generates an alkaline solution with a high pH. Therefore, determining whether the thermal conductivity of granular bentonite changes under the long-term effect of alkaline solution is essential for the repository safety. In this study an experimental investigation was conducted on the changes in the thermal conductivity of granular bentonite with an alkaline solution (NaOH solution) over time, with the effects of aging time, particle size distribution, alkaline solution content and concentration being considered. X-ray diffraction (XRD) technique was applied for examining the variations in mineral composition, while the pores and cracks analysis system (PCAS) was utilized to process previous SEM images, revealing the change in porosity. The test results are as follows. Increasing the alkaline concentration, average particle size or aging time leads to a decline in thermal conductivity, whereas a higher alkaline solution content enhances it. In descending order of effect, the factors influencing thermal conductivity are ranked as the alkaline solution content, particle size distribution, alkaline concentration, and aging time. The interaction effects exist between these different factors. The decrease of thermal conductivity is not only related to the increase in porosity caused by the dissolution of montmorillonite, but also to the decrease in quartz content. The evolution of thermal performance can be a reference for the design and construction of HLW repositories.