Vitrification is an internationally significant process for the disposition of nuclear waste. For several international vitrification projects, including of legacy nuclear wastes from the Hanford Site in the US, salt formation during operation of continuous melters is of substantial concern. The formation of molten salts during vitrification is detrimental due to 1) melter corrosion, 2) volatile release, 3) providing a conductive path between the melter heating elements (causing a short), and 4) segregation of waste components into nondurable water-soluble phases. As such, in-situ process monitoring is a critical technology for successful vitrification. Several in-situ process technologies for glass melters are fairly developed; however, the lack of in-situ surface salt formation detection methods presents a risk to vitrification at the Hanford Site Waste Treatment & Immobilization Plant Project (WTP). While proposed previously, millimeter wave (MMW) radiometry and interferometry are demonstrated for the first time for in-situ detection of salt formation in simulated nuclear waste glass melts. The experimental radiometer and interferometer setup uses the optical properties of the melt and a dual receiver at millimeter wavelengths to elucidate melt activity through assessment of emissivity and surface height changes. A series of previously characterized glasses designed to supersaturate sulfate (such as Na₂SO₄), chloride (such as NaCl), and fluoride (such as NaF) salts were analysed using the MMW radiometer and interferometer. providing insightinto volatile losses, fining, salt formation, salt identity, crystallization, and general emissivity properties of a heterogeneous melt. Thermal analysis, raman spectroscopy, X-ray diffraction, optical microscopy, and literature assessment of dielectric properties were utilized to verify observations from MMW radiometry and interferometry. The proposed contribution demonstrates MMW radiometery and interferometry as an useful method for in-situ salt detection to enable successful nuclear waste vitrification efforts.

Ongoing cleanup of radioactively contaminated seawater generated during the Fukushima
disaster involves the use of ion-exchange materials, which selectively adsorb radioisotopes
in solution. ⁹⁰Sr is amongst the most dangerous of these radioisotopes because its chemical
similarity to Ca promotes the bioaccumulation in bones and teeth, resulting in prolonged
internal exposure. Recyclable absorbents, such as granular sodium titanate (GST), avoid the
secondary waste generation associated with single-use adsorbents by allowing ⁹⁰Sr to be
eluted from the structure and precipitated into a desired compound, such as SrF₂, before the
adsorbent is reused. This waste precipitate requires immobilisation in a suitable wasteform
prior to final disposal. Iron-phosphate glasses are promising materials for nuclear waste
immobilisation due to their excellent chemical durability, high solubility limits, and low
melting temperatures.
In the present work, the influence of SrF₂ additions on the structure, thermal properties, and
phase formation of iron-phosphate glasses was investigated. SrF₂-loaded iron-phosphate
glasses were melted at 1100 °C, vitreous wasteforms were obtained for all compositions up
to 15 mol% SrF₂ loading, as confirmed by X-ray diffraction (XRD). Raman spectroscopy
revealed SrF₂ additions contributed to depolymerisation of the glass network. Differential
scanning calorimetry (DSC) measurements indicated increased SrF₂-loading raised the glass
transition temperature (T_g) and crystallisation temperature (T_c) of the glasses. Fluorine
retention was confirmed by compositional analysis, including electron probe micro-analysis
(EPMA) and X-ray fluorescence (XRF). Crystallisation of iron-phosphate compounds,
identified using XRD and Raman spectroscopy, was found to occur with increased SrF₂
loading. Iron-phosphate crystals were deficient in Sr with respect to the bulk glass, as
confirmed by SEM-EDS and EPMA. These findings suggest iron-phosphate glasses
represent a promising candidate for the vitrification and immobilisation of SrF₂.

Glass objects are reacted with aqueous solutions for multifarious commercial and scientific purposes. Understanding how glasses behave in solutions has been described by theory using a linear dissolution rate model, in which the rate decreases linearly as the silica concentration in solution increases. Such models are especially relevant for glasses that will be used as a waste form for disposal of nuclear waste. We melted a series of fourteen glasses in the system CaO-MgO-Al₂O₃-B₂O₃-SiO₂ in order to test how multicomponent glasses react primarily using single pass flow through (SPFT) systems. Four of these glasses contain three components, seven are four-component, and three are five-component glasses. All glasses contain 60 mol.% SiO₂, except the last five-component glass, which contains 50 mol.% SiO₂. This last composition acts as a good analogue for nuclear waste glasses. Experiments were conducted at pH 7.5 or 9.5 at 75 °C and glass structure was characterized by Solid State NMR. We also conducted flowing experiments in which the solution contained the stable isotopes ²⁶Mg, ³⁰Si, and ⁴³Ca to understand the mechanism of dissolution. After reacting in solutions at low (no or 10%), medium (50%), and high (100%) silica saturation levels for up to eight months, glass wafers were submitted for analyses by SIMS. Together, the dissolution data indicate that multicomponent glasses dissolve non-linearly with respect to dissolved silica with the steepest change occurring at the low SiO₂ concentrations. The data will be used to discuss the mechanism of reaction by a diffusion or dissolution-reprecipitation model.

The Low-Activity Waste (LAW) fraction of Hanford tank waste will be converted to glass at the Waste Treatment and Immobilization Plant (WTP) and disposed on the Hanford site. The durability of LAW glasses has been researched extensively for decades to satisfy contract requirements. To date, most LAW glass durability data has been generated via Product Consistency Test (PCT) and the Vapor Hydration Test (VHT) on non-radioactive simulant glasses fabricated via crucible melts. Non-radioactive glasses were chosen due to ALARA and cost reasons with confidence that radioactive waste glasses would have similar durability behavior through understanding of glass corrosion. To reduce the risk of significant differences in laboratory test response data between WTP melter waste glass and its associated simulant glass, PCT, VHT, and EPA1313 durability tests were performed on actual and simulated LAW glasses fabricated using identical laboratory-scaled melters and with the same procedures, equipment, and location. Actual and simulant glass durability results (including normalized B, Na, Tc, and Re releases) generated from the durability experiments are presented and statistically compared relative to experimental uncertainty.

A comprehensive understanding of the long-term durability of radioactive waste glasses is necessary to ensure the accuracy of future predictive modelling of waste glass alteration, and therefore the integrity of any future disposal solutions. The UK currently has plans to permanently dispose of its radioactive waste, including the vitrified by-products of an extensive reprocessing programme, underground in a deep geological disposal facility (GDF). The potential resumption of alteration of radioactive waste glasses in groundwater remains a source of uncertainty in the performance of any such proposed GDF and could result in a resumed rate of alteration approaching that of the most rapid initial rate. Recent research in France and the US has confirmed the prevalence of the resumption of alteration phenomenon in numerous simulant waste glass and simplified analogue compositions by ‘seeding’ glass dissolution vessels with zeolites at high pHs and temperatures to induce the effect artificially. The disparate composition of UK waste forms prevents a direct comparison with these international standards. However, this study has found not only that this phenomenon occurs in simulant UK waste glass compositions when seeded with natural analcime crystals, but also under a wide variety of conditions, including at free pH (unbuffered deionised water). No resumption was observed at 40 °C, however, at 90 °C, resumption was observed at free pH (pH_(25°C) ~ 9.7), as well as starting pH_(25°C) artificially raised with KOH to 11, 11.5, 12, and 12.5, with varied results suggesting the existence of complex buffer mechanisms. The time of seeding relative to the initiation of the experiments was varied to no effect, suggesting that the resumed rate is not closely related to alteration layer thickness. Variations in the initial seed-to-glass surface area ratio also had no effect, suggesting that a much smaller amount of the seed could induce the same effects.