Deep borehole disposal grouts – are inorganic retarders appropriate?

Abstract

Cementitious grouts are being developed for use as sealing and support matrices (SSMs) in deep borehole disposal (DBD) of high-level radioactive waste. These SSMs will seal the waste containers within a disposal zone and provide physical support during package placement. The ambient temperature and pressure will be 80–130°C and 30–50 MPa, respectively, depending on depth and local geological conditions. This will accelerate grout thickening, so retardation is required to facilitate slurry placement. Certain inorganic materials suppress cement hydration and retard thickening under ambient conditions. However, their retardation characteristics are unknown at elevated temperature and pressure. Inorganic compounds of borate, phosphate, tin and zinc were investigated as retarders, and a summary of the work is provided in this paper. All retarders were found to delay thickening, with only borate providing sufficient retardation, but only at 90°C (0·75% addition); retardation was nearly sufficient at 120°C (1% addition). The effect of all the retarders on wet paste properties was not consistent but, generally, plastic viscosity and yield stress peaked at specific addition levels. None of the retarders influenced phase composition, and each retarding ion was evenly distributed throughout the hardened microstructure. The results of this study led to the conclusion that no inorganic retarder is appropriate for use across the whole DBD temperature range.