In order to determine the durability of nuclear waste glass in a specific environment, different leaching tests have been developed over the years, aiming at investigating different types of leaching mechanisms occurring at different timescales after the first contact with water. The Single-Pass Flow-Through Test (SPFT) method is commonly used to determine the maximum glass dissolution rate, as in this setup the elements released by the glass are carried away from the sample, preventing the saturation of the solution. These rates can be considered as characteristic properties of a particular glass composition and are basic data requested to describe the properties of the reference waste glasses in the expected disposal environment. In many countries, including Belgium, this environment will be conditioned by the presence of alkaline cementitious materials, increasing the pH of the percolating ground water. To assess the chemical durability of the Belgian reference glasses SON68, SM539 and SM513 under hyper-alkaline conditions, maximum glass dissolution rates were determined at 30 °C, using a KOH solution and a synthetic young cementitious water (YCWCa) with a pH of 13.5, corresponding to young ordinary Portland concrete. In both leaching solutions, the highest dissolution rate was determined for the SM539 glass, which contains the highest amount of Al, while similar rates were found for SM513 and SON68 glasses, whose compositions are comparable. For all glasses, the maximum dissolution rates in YCWCa were lower than in KOH due to the presence of Ca, which causes the formation of a slightly protective layer. The dissolution rates in YCWCa were similar to those measured at 30 °C in static tests in which glass was altered in YCWCa in presence of Ordinary Portland Cement (OPC) with a cement to glass ratio of 1.

During the operational lifetime of a geological disposal facility, groundwater of variable composition may ingress and interact with vitrified radioactive waste, leading to leaching of elements. The rate of this dissolution process is understood to be influenced by elements dissolved in the contacting solution; however, since groundwater is a complex mix of many elements, elucidating the mechanism by which these elements influence dissolution is challenging.

The single pass flow through (SPFT) methodology was used to investigate the effect of individual groundwater cations on the forward dissolution rate of simulant UK high level waste glass. Solutions containing chloride salts of lithium, sodium, potassium, magnesium, calcium, and strontium were flowed over the glass sample until steady state conditions were reached. The forward dissolution rates were compared as a function of each cation element. A series of monolithic static dissolution tests were conducted in parallel. The same series of alkali metal and alkaline earth metal chloride salt solutions were used to study the role and behaviour of the added ions in the formation of an alteration layer during the residual rate.

The implementation of an engineered multi-barrier approach for nuclear waste disposal, to mitigate the release of radionuclides over the operational lifetime of a geological disposal facility (GDF), requires a detailed understanding of the interactions between steel canisters, engineered backfill and natural barriers. In particular, the reaction between Fe – present both in waste containers and within the vitrified waste itself – and silica in vitrified wastes is of interest as it has previously been shown that this element may enhance dissolution of glass in aqueous solutions.

Within a geological environment Fe is present trifold; within the environment in Fe-rich minerals, within the matrix of vitrified waste from aqueous HLW feedstocks, and in the cannister material. The interactions between these Fe sources and the wasteform will hold different influences and impact the long-term durability of the wasteform to varying extents.

In this study, we describe the relationship between Fe present within the glass and dissolution behaviour. A simple five oxide borosilicate glass series, with a formulation based on that of the UK high level waste sodium aluminosilicate glass, MW25, was produced to study the effect of Fe content on glass structure and glass durability, with Fe additions ranging from 0 to 5 mol %. The structure of the glasses, as a function of Fe content, was determined using Raman spectroscopy, XRF, and XAS analysis, with decreasing T_g and depolymerisation of the glass network with increasing Fe content. The dissolution of the glasses was determined using SRCA, PCT and MCC durability testing, utilising solution analysis by ICP-OES. Finally, the bioavailability of Fe within the glass network was tested in two simplified subsurface microbial systems to ascertain if glass-microbe interactions can affect the dissolution behaviour of Fe containing glasses.

Understanding the long-term behavior of nuclear waste glasses prior to storage in a near-surface disposal facility is important as this will assist in assuring that the release of radionuclides from the disposal facility will meet regulatory limits. Archeological and natural samples are of prime importance in this mission as they can be used to validate performance assessment models and will assist in public and regulatory acceptance of the disposal site. Here, we discuss five different near-surface sites where archeological and natural samples have been altered by rain and/or groundwater in the environment for hundreds to thousands of years. The selected sites offer a range of characteristics including average temperature, rainfall, and microbial activity. The chemistry of the samples also varies both in silica content, amongst other oxides, and in heterogeneity, in terms of the abundance of amorphous and crystalline fractions. In addition, we used standard laboratory tests, including the product consistency test (PCT), the vapor hydration test (VHT), and the EPA Method 1313 test, to alter archeological samples and we have compared those results with the corrosion of vitrified archeological materials excavated from one of the sites, a ~1500-year old Iron Age Swedish hillfort, Broborg. We compare characterized site samples with corrosion characteristics generated by standard laboratory durability test methods. Results show that the surficial layer of the Broborg samples resulting from VHT displays some similarities to the morphology of the surficial layer formed over longer timescales in the environment.

Glass is used in the UK, as in many other countries, to immobilise the high activity waste liquors resulting from spent fuel reprocessing. Vitrification is also under consideration for some lower activity waste streams. Understanding glass behaviour in subsurface environments is important to support the safety case for disposal of these wastes in a geological disposal facility. As borosilicate glasses have only been manufactured in the last century most experiments to understand glass dissolution rates and mechanisms have typically been conducted at elevated temperatures and increased surface areas in order to obtain measurable results in a short time period. Long-term glass alteration experiments are rare, as are those that consider glass exposed to natural environmental conditions. An experiment was established at the Ballidon limestone quarry, Derbyshire, in 1970 to investigate modern and archaeological glass alteration under mildly alkaline conditions: limestone rich sediment, pH 9.7-8.2. The study has since been extended to include US, UK and Russian nuclear waste glass compositions, samples of which were removed after 16 -18 years of burial. Here, analysis is presented from a variety of nuclear waste type glasses buried at Ballidon including UK 'Mixture Windscale' type glasses, iron phosphate glasses and US Low Activity Waste borosilicate compositions. Even after a relatively short burial time (<20 years) at low temperatures (average 8 ^oC) alteration layers were visible on most glass types. Study of these layers by electron microscopy, EPMA and microfocus X-ray absorption techniques has revealed their chemistry, morphology and interaction with the surrounding sediment. Results give insight into both the corrosion mechanisms of glasses in complex natural environments and the fate of rare earth elements (representing radionuclides) contained within these glasses.

Whilst most laboratory based tests are conducted under static, sterile, closed system conditions, studies of glasses exposed to natural conditions at Ballidon offer insight into glass behaviour in complex open systems with changing geochemistry, influence from nearfield mineralogy and geomicrobiology. Microbial community analysis conducted at the time of site excavation, supported by laboratory based experiments, shows the probable direct or indirect influence of microbiological processes on the corrosion of glasses at the Ballidon site. Similarly, studies of the adjacent sediment and glass alteration layers reveals the transfer of elements to and from the surrounding minerals.

We report the results of massive parallel molecular dynamics simulations of high-energy radiation damage in phosphate glasses. This damage is created by overlapping multiple 70 keV collision cascades. We quantify different aspects of radiation-induced structural changes including at different stages of damage development, including coordination numbers, cluster sizes and density. The overall trend is that radiation damage causes polymerisation of the phosphate network and the loss of small and isolated clusters. However, the details of this response varies with different glass compositions. This polymerisation indicates that the disparate network of strong Fe-O bonds is weakened, which will subsequently weaken the material’s resistance to radiation as phases of phosphate and iron become separated. Furthermore, the degree of recovery in these simulations is far diminished compared to simulations of low energy cascades. This qualitative difference in material response between cascade energies is an important consideration for the deployment of iron phosphate glasses for nuclear waste encapsulation.