Inhibiting Airborne and Groundwater-Borne Transport of Radionuclides During Decommissioning Operations with Colloidal Silica Grout

Nuclear site decommissioning involves the retrieval and handling of radioactive waste. Waste removal from nuclear reactors and/or storage facilities, such as spent fuel pools and storage silos, represents a potential hazard in terms of radiation exposure for the workforce and the surrounding environment. This may be due to the accidental release of airborne radioactive particulates during waste recovery and transport, or to the loss of radioactive debris upon retrieval due to waste corrosion and degradation. The development of innovative techniques to reduce hazard in decommissioning operations is a critical aspect of site decommissioning.<br/><br/>This study explores the suitability of colloidal silica grouting around radioactive waste, prior to its removal, to reduce radiation exposure during nuclear waste retrieval operations. Colloidal silica is an aqueous suspension of silica (SiO2) nanoparticles, with average particle size &lt;100 nm. The creation of siloxane bonds (Si – O – Si), typically triggered by the addition of an electrolyte accelerator, leads to the formation of a solid-like network of silica nanoparticles in the form of a hydrogel. Previous work on colloidal silica gel has proved its potential to form low-permeability hydraulic barriers against fluid migration, and to inhibit the diffusion of radionuclides through the gel, making it a promising material for use in retrieval operations.<br/><br/>Here we present research to determine the potential for colloidal silica to be used in a range of decommissioning operations. For example, spent fuel removal requires evidence that colloidal silica hydrogel can maintain its integrity upon exposure to temperatures higher than ambient, typical of the nuclear waste stored within pools and silos. These might be in the range 40-60 °C in standard conditions, but could exceed 100 °C during loss of cooling/loss of coolant accidents. We also consider grouting of particulate wastes prior to their retrieval, during which the grout must prevent particle release, and must not compromise formation of a suitable final wasteform.<br/><br/>Experiments were carried out to simulate colloidal silica grouting operations around objects and particles at temperatures higher than ambient, up to a maximum of 120 °C and to determine their suitability for subsequent vitrification. The effect of the temperature of the grouted object, and of the silica grout properties, on the performance of the gel was explored by a) microstructural analyses using x-ray imaging, to detect the presence and spatial distribution of temperature-induced cracks within the gel, and b) mechanical tensile and shear strength tests at different temperatures and silica concentrations. Experiments were then conducted within a furnace, using contaminated soil samples grouted with colloidal silica, to investigate the additives and processes required to produce a uniform glass and to determine its radionuclide retention. Our results confirm the strong potential of colloidal silica grout to reduce hazard during spent fuel and radioactive waste retrieval operations.