Colloids in the mortar backfill of a cementitious repository for radioactive waste

Colloids are present in groundwater aquifers and water-permeable engineered barrier systems and may facilitate the migration of radionuclides. A highl y permeable mortar is foreseen to be used as backfill for the engineered ba rrier of the Swiss repository for low- and intermediate-level waste. The ba ckfill is considered to be a chemical environment with some potential for c olloid generation and, due to its high porosity, for colloid mobility. Coll oid concentration measurements were carried out using an in-situ liquid par ticle counting system. The in-house developed counting system with three co mmercially available sensors allowed the detection of single particles and colloids at low concentrations in the size range 50-5000 nm. The counting s ystem was tested using suspensions prepared from certified size standards. The concentrations of colloids with size range 50-1000 nm were measured in cement pore water, which was collected from a column filled with a highly p ermeable backfill mortar. The chemical composition of the pore water corres ponded to a Ca(OH)(2)-controlled cement system. Colloid concentrations in t he backfill pore water were found to be typically lower than similar to0.1 ppm. The specific (geometric) surface areas of the colloid populations were in the range 240 m(2) g(-1) to 770 m(2) g(-1). The low colloid inventories observed in this study can be explained by the high ionic strength and Ca concentrations of the cement pore water. These conditions are favourable fo r colloid-colloid and colloid-backfill interactions and unfavourable for co lloid-enhanced nuclide transport. (C) 2001 Elsevier Science Ltd. All rights reserved.