Herbert's Quarry, South Wales – an analogue for host-rock alteration at a cementitious radioactive waste repository?

Abstract

Cement in radioactive waste repositories will produce leachates of pH > 10 which may alter the host rock, affecting its ability to act as a barrier to radionuclide migration. To complement the many laboratory and modelling studies of rock alteration in cement leachates, analogue sites have been investigated to understand reactions at time scales relevant to geodisposal. However, analogue site histories may be poorly constrained and these systems may be influenced by site specific factors. Therefore increasing the number of sites studied is important to minimize uncertainty in the applicability of results. Herbert's Quarry has been characterized and assessed as a potential anthropogenic analogue. Streamwater sampling at the site identified Ca-rich fluids, ≤ pH 12, analogous to cement leachates. However, rock and sediment samples exhibited extensive CaCO3 precipitation in these fluids and no reaction of silicate rock. The streamwaters were also found to be oxidizing, unlike the reducing conditions expected at a repository, and temperatures were 15–25°C below those predicted for repositories. Therefore, Herbert's Quarry is believed to have limited applicability as an analogue in this context.