Equilibrium leach tests: colloid generation and the association of radionuclides with colloids under simulated repository conditions

Equilibrium leach tests (ELTs) are designed to simulate the situation in a radioactive waste repository after groundwater has penetrated the engineered barriers (i.e. the vault backfill, the waste containers and the waste-form). Two ELTs using real intermediate level waste were assembled and operated for nearly 7 years. In these tests, the principal near-field components (waste, encapsulation matrix, container and backfill) were equilibrated with the aqueous phase under static conditions to simulate the low flow rate in a repository. The principal objective of the tests was to measure the resulting solution concentrations of key radionuclides. In addition, the tests provided a unique opportunity to investigate the formation of colloids and the association of radionuclides with colloids under simulated repository conditions. Colloid populations in leachates and fines samples from the tests have been characterised by high-resolution scanning electron microscopy (SEM). The colloid concentrations in the 0.1–5.0 μm filtered fraction of the equilibrium leachate at the end of each test are estimated to be about 1010 dm−3. The predominant particles are composed of calcium carbonate, a major component of the backfill. Very low α-activity is associated with the colloidal phase. Results of SEM/X-ray analysis suggest that the uranium is associated with the colloidal phase predominantly by sorption.