(162c) Rheology of Concentrated Nuclear Waste Suspensions: Reduced Yield Stress By Additive Blending

A significant activity of the UK nuclear sector is decommissioning, which involves removing and packaging legacy wastes for their long-term safe storage. Intermediate level waste materials include magnesium alloy fuel cladding from Magnox reactors that has corroded over time in underwater storage to form a highly complex suspension. The aim is to encapsulate the suspension waste in cement, but to do this there is a compromise between the suspension rheology and total waste volume, i.e. to reduce total waste volume the solids content should be high. However, higher solids content produces undesirable rheological properties such as high yield stress and shear thickening. To mitigate these issues, the current research has explored the use of nano-silica (SiO₂), an inorganic additive, to lower the yield stress of concentrated magnesium hydroxide (Mg(OH)₂) suspensions.

The yield stress of 27 vol% Mg(OH)₂ suspension was 75 Pa, but when blended with 3 vol% SiO₂, so that the total solids content was 30 vol%, the yield stress decreased by 40% (Fig. 1a). Samples from the blended suspensions were imaged by scanning electron microscopy (SEM) and x-ray computed tomography (X-ray CT) (Figs. 1b and c) to determine the distribution and dispersion of the nano-silica throughout the Mg(OH)₂ network. Initially, good distribution but poor dispersion of SiO₂ was seen with large amounts of particle clustering. The drop in yield stress is attributed to the distribution of SiO₂ which modifies the contact mechanics between Mg(OH)₂ aggregates and promotes a lubrication effect. With aging, the yield stress increased to >200 Pa after 7 days (Fig. 1a) at ambient conditions, with the onset for yield stress growth occurring after 1 day. This gradual stiffening of the particle network was attributed to the slow dissolution of SiO₂ and its reaction with soluble Mg²⁺ ions forming a magnesium silicate hydrate precipitate between particles, as seen in high-resolution SEM images.

The study demonstrates the use of nano-additives to lower the yield stress of concentrated suspensions and explores the timescales of physical lubrication in competition with the onset of chemical gelation. Practical application of the method can lead to significant cost savings for the UK nuclear decommissioning program by reducing the total number of waste packages needed for long-term storage.