High level waste (HLW) in the UK is immobilised in a borosilicate glass matrix prior to disposal within a geological disposal facility (GDF). This work aims to support the UK’s safety case for geological disposal by further understanding the role of Fe on the durability of vitrified wasteforms through laboratory dissolution tests.
The UKs HLW waste glass, typically with a wasteloading of 25 %, denoted MW25, was, alongside its base glass, subjected to a variety of monolithic and powder dissolution tests. These tests were conducted across varying time scales ranging from 14 to 274 days and provided insight into the effect of Fe on the dissolution. With high dissolution rates observed for the base glasses doped with 0, 0.5 and 1 mol% Fe2O3, and significantly reduced dissolution rates for samples doped with 2.5 and 5 mol% Fe2O3.
This split behaviour in the dissolution rates is mirrored by the changes within the structure as Fe2O3 concentrations increase. Where the lower dissolution rates are noted, the proportion of BO4 present within the network is significantly higher than that of the glasses of poor durability and the overall connectivity of the glass is increased.
The role of microbial activity within a repository is poorly understood and work was conducted to determine whether the macro-nutrients present within the vitrified wasteforms are bioavailable. Dissolution tests were performed with the inclusion of a mixed culture, from a high pH sediment, and a single strain Fe-reducing bacteria. Slight differences in dissolution rates were noted in biotic tests, with the production of extracellular polysaccharides (biofilms) being observed consistently across the biotic tests. Microbial communities, through the production of respiration products and Fe-chelating compounds, may greatly alter the local chemistry at the glass-solution interface. Preliminary data suggests that microbially mediated Fe reduction reduces the glass dissolution rate.
