Standard methods to assess the durability of vitrified radioactive waste were first developed in the 1980’s and, over the last 40 years, have evolved to yield a range of responses depending on experimental conditions and glass composition. Mechanistic understanding of glass dissolution has progressed in parallel, enhancing our interpretation of the data acquired. With the implementation of subsurface disposal for vitrified radioactive waste drawing closer, it is timely to review the available standard methodologies and reflect upon their relative advantages, limitations, and how the data obtained can be interpreted to support the post-closure safety case for radioactive waste disposal.

Groundwaters at nuclear sites can be characterized by low pH and high nitrate concentrations (10–100 mM). These conditions are challenging for bioremediation, often inhibiting microbial Fe(III)-reduction which can limit radionuclide migration. Here, sediment microcosms representative of the UK Sellafield site were used to study the influence of variable pH and nitrate concentrations on microbially-mediated TEAP (terminal electron accepting processes) progression. The rate of reduction through the terminal electron accepting cascade NO⁻ ₃ > NO⁻ ₂ > Mn(IV)/Fe(III) > SO²⁻ ₄ at low pH (∼5.5) was slower than that in bicarbonate buffered systems (pH ∼ 7.0), but in the low pH systems, denitrification and associated pH buffering resulted in conditioning of the sediments for subsequent Fe(III) and sulfate-reduction. Under very high nitrate conditions (100 mM), bicarbonate buffering (pH ∼ 7.0) was necessary for TEAP progression beyond denitrification and the reduction of 100 mM nitrate created alkaline conditions (pH 9.5). 16S rRNA gene analysis showed that close relatives of known nitrate reducers Bacillus niacini and Ochrobactrum grignonense dominated the microbial communities in this reduced sediment. In Fe(III)-reducing enrichment cultures from the 100 mM nitrate system, close relatives of the Fe(III)-reducing species Alkaliphilus crotonatoxidans and Serratia liquifaciens were observed. These results highlight that under certain conditions and contrary to expectations, denitrification may support bioreduction via pH conditioning for optimal metal reduction and radionuclide immobilization.

Extensive denitrification resulted in a dramatic increase in pH (from 6.8 to 9.5) in nitrate-impacted, acetate-amended sediment microcosms containing

The high-yield fission products ⁹⁹Tc and ⁹⁰Sr are found as problematic radioactive contaminants in groundwater at nuclear sites. Treatment options for radioactively contaminated land include bioreduction approaches, and this paper explores ^99mTc and ⁹⁰Sr behavior and stability under a range of biogeochemical conditions stimulated by electron donor addition methods. Dynamic column experiments with sediment from the Sellafield nuclear facility, completed at site relevant flow conditions, demonstrated that Fe(III)-reducing conditions had developed by 60 days. Sediment reactivity toward ⁹⁹Tc was then probed using a ^99mTc(VII) tracer at <10^–10 mol L^–1 and γ camera imaging showed full retention of ^99mTc in acetate amended systems. Sediment columns were then exposed to selected treatments to examine the effects of different acetate amendment regimes and reoxidation scenarios over 55 days when they were again imaged with ^99mTc. Here, partially oxidized sediments with no further electron donor additions remained reactive toward ^99mTc under relevant groundwater O₂ and NO₃^– concentrations over 55 days. Immobilization of ^99mTc was highest where continuous acetate amendment had resulted in sulfate-reducing conditions. Interestingly, the sulfate reducing system showed enhanced Sr retention when stable Sr²⁺ was added continuously as a proxy for ⁹⁰Sr. Overall, sediment reactivity was nondestructively imaged over an extended period to provide new information about dynamic iron and radionuclide biogeochemistry throughout realistic sediment redox cycling regimes.