Rock alteration at high pH relevant to the geological disposal of radioactive waste

Dissolution of the cement used in radioactive waste repositories will produce a high pH leachate (pH 10.5-13.1) that will evolve in pH and composition over time, remaining hyperalkaline for 106 years. This will migrate into the repository host rock potentially altering the rock’s physical and chemical properties, and its function as a barrier to radionuclide migration. To investigate the possible alteration over timescales relevant for geological disposal (104 to 105 years), previous studies included short term (< 2 years) laboratory experiments, natural analogue studies (100s to 105 years) and predictive modelling. However, the geochemical and mineralogical processes which may occur as such systems evolve remain poorly constrained.
Here, rock alteration in high pH solutions, analogous to cement leachates, has been investigated on short (1 year), medium (15 years) and long (100s years) timescales. The short term investigation agreed with previous studies finding that silicate mineral dissolution and C-S-H precipitation were the predominant alteration processes. In the medium term study, although initially silicate mineral dissolution and C-S-H precipitation occurred, by 15 years of reaction neo-formed Mg-silicates replaced the C-S-H. To investigate rock alteration on a longer timescale Herbert’s Quarry, where high pH fluids have existed for 100s years, was characterised and assessed as a potential analogue site. However, the site was found to have limited applicability in the context of the high pH rock alteration expected at a radioactive waste repository.
The impact of rock alteration on U(VI) behaviour was also investigated in the short and medium term. In the short term, U(VI) behaviour during rock alteration varied between solutions representative of different periods of cement leachate evolution, with potential impacts for radionuclide transport. The sorption of U(VI) to unaltered rock and rock altered for 15 years was also investigated. This suggested alteration may increase a rock’s sorption capacity for U(VI).