Speaker
Description
Legacy nuclear sites typically feature subterranean volumes of radiologically contaminated soil, and concrete containment structures that have deteriorated over time. Disturbances caused by surface-level decommissioning operations exacerbate the risk of radionuclide release into the environment via contaminated groundwater. Sub-surface hydraulic barriers are needed to mitigate this, but are conventionally installed by excavation, which would require safe handling and disposal of large quantities of contaminated soil, and increase the radiation exposure of workers. Cementitious grouts can instead be injected into the ground to form barriers in situ, but the high viscosity of cements requires injection pressures that risk inducing ground heave, which can further damage weakened containment structures, creating additional leakage pathways. Colloidal silica (CS), however, is a novel grouting material with water-like viscosity, allowing it to be injected at minimal pressure to fill free porosity in the soil without any ground disturbance; after a time it will then transition into a rigid hydrogel with no change in volume. Unlike cement, CS is non-toxic and environmentally-friendly, has nano-scale particle size, allowing superior penetration, and has precisely controllable gelation time. CS gel’s very low hydraulic conductivity facilitates its use as a hydraulic barrier, but CS also strengthens weak soils and has potential as a chemical barrier, being able to trap radioactive ions of Cs-137 and Sr-90 via chemical sorption. Additionally, CS may be suitable for other decommissioning applications, such as encapsulating radioactive waste canisters, or as a spray coating for contaminated dust.
These advantages give CS potential to make aspects of nuclear decommissioning cheaper, faster, and safer, but its application has thus far been limited. This research contributes to the case for CS’s use in nuclear decommissioning by characterising the behaviour of the grout, working with industry to identify applications for it in decommissioning, and identifying ways of modifying and improving its properties to best fit these and to reduce its cost. Here, the results of compression tests are presented, showing how a range of factors and additives improve the mechanical properties of CS-grouted soil: compressive strength increases continuously with curing time, and samples cured in sea water (0.6M NaCl) are stronger than those in fresh water, with further improvement on increased concentrations and valences of salt (e.g. CaCl2). Kaolin clay additive also increases strength and may be used to partially replace silica mass to reduce cost. Combining CS with hydrophilic polymers also greatly increases flexibility. Shear vane tests show how CS is able to re-heal after damage, and pH, salinity and the presence of dissolved silica are used to enhance this. Increasing compressive strength improves CS’s ability as a soil stabiliser, and increasing flexibility and re-healability gives it resistance to cracking and other damage. Ways to improve CS’s water retention, hydraulic conductivity, and sorption capacity are also under investigation.
| Participation | In-Person |
|---|---|
| Country | United Kingdom |
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