Abstract Radioactive waste vitrification has been carried out industrially in several countries for nearly 40 years. Research into the formulation and long term behavior of high and intermediate level waste glasses, mainly borosilicate compositions, is still continuing in order to (i) safely condition new types of wastes and (ii) design and demonstrate the safety of the disposal of these long-lived waste forms in a deep geological repository. This article presents a summary of current knowledge on the formulation, irradiation resistance and the chemical durability of these conditioning materials, with a special focus on the fate of radionuclides during glass processing and aging. It is shown that, apart from the situation for certain elements with very low incorporation rate in glass matrices, vitrification in borosilicate glass can enable waste loadings of up to ~20 wt% while maintaining the glass homogeneity for geological time scales and guaranteeing a high stability level in spite of irradiation and water contact.

Radionuclides containment in nuclear glasses: an overview

Results from durability drip tests designed to simulate the unsaturated conditions in the proposed Yucca Mountain Repository are reported for two actinide-doped glasses used as model waste forms. These tests are being conducted with reference glass compositions doped with neptunium, plutonium, and americium from the Defense Waste Processing Facility (DWPF) and the West Valley Demonstration Project (WVDP), and have been ongoing for over 8 years. Solution compositions, including transuranics, have been periodically determined, and selected analyses of colloid formation and composition, glass corrosion layers, and solid alteration phases have been obtained by scanning and analytical transmission electron microscopies. The importance of integrated testing has been demonstrated, as complex interactions among the glass, the groundwater, and the sensitized stainless steel have been observed. The cumulative releases of both glass-forming and dopant elements are presented along with identification of reaction phases and their partitioning between solution and solid phases. Alteration phases, including smectite clay, iron silicates, uranium silicates, and calcium thorium phosphate, have been observed forming on the glass and stainless steel and have occasionally been found suspended in solution as colloids. Actinides, except neptunium, concentrate into alteration phases or sorb onto the stainless steel. The subsequent transport of the actinides is then controlled by these phases.

Long-Term Results from Unsaturated Durability Testing of Actinide-Doped DWPF and WVDP Waste Glasses

Effects of Al:Si and (Al+Na):Si ratios on the static corrosion of sodium-boroaluminosilicate glasses

In the alteration layer of HLW glasses corroded in aqueous solutions, less soluble elements like REE, actinides and other elements are retained by sorption or by formation of secondary alteration products controlling the solution concentrations. In order to quantify the sorption effect on the retention of metal ions, first sorption tests with U, Th and Eu have been performed using precorroded, simulated HLW glass as substrate. To demonstrate that the sorption behaviour of Eu is also representative for homologue trivalent actinides additional sorption tests were performed with Am 2 4 1 in deionized water, NaCI and MgCl 2 brine. Distribution ratios R, (R s = Am s o r b /Am s o l x V/m) for Am were found independent of the starting concentration in the pH range 2 to 6. At higher pH, a high amount of Am-containing colloidal particles were detected. The R s values of Am show a strong increase with pH, quite similar to the R s values of Eu. In general, the sorption behaviour of Eu and Am is very similar in deionized water and in brines. Clay minerals formed in the gel layer as secondary phases during long-term glass corrosion are assumed to act primilary as strong sorbents. Therefore, the sorption of REE and Am onto synthetic poor crystalline smectite precursors and a "well crystallized" natural smectite is compared with the findings obtained from precorroded glass. The sorption of Am on the precorroded glass is found to be much stronger than the retention on the investigated pure clay phases, indicating a different number of active sites or occurance of a different sorption mechanism.

Sorption behaviour of Am on precorroded HLW glass in water and brines

We considered a sorption model containing the minimal dynamic features of the system to fit plutonium adsorption data similar to that developed by Painter et al. (2002). Global fits to recent data favored nonzero values of reversible sorption, allowing the definition of equilibrium distribution coefficients in all cases except the synthetic form of montmorillonite. In most cases, the two-site model was adequate to fit the data. The model represents a mathematic simplification of the time-dependent sorption process and takes no account for pH-dependent surface charge changes and actinide-mineral surface interfacial chemistry. However, this allows the model to be readily incorporated into existing performance assessment codes when applied to repository environment relevant data sets.

/pdf/data-analysis-of-plutonium-sorption-on-colloids-in-a-minimal-3tq3e0emqo.pdf

Data Analysis of Plutonium Sorption on Colloids in a Minimal Kinetics Model

Waste package assemblies consisting of actinide-doped West Valley ATM-10 reference glass and sensitized 304L stainless steel have been reacted with simulated repository groundwater using the Unsaturated Test Method Analyses of surface corrosion and reaction products resulting from tests that were terminated at scheduled intervals between 13 and 52 weeks are reported Analyses reveal complex interactions between the groundwater, the sensitized stainless steel waste form holder, and the glass Alteration phases form that consist mainly of smectite clay, brockite, and an amorphous thorium iron titanium silicate, the latter two incorporating thorium, uranium, and possibly transuranics The results from the terminated tests, combined with data from tests that are still ongoing, will help determine the suitability of glass waste forms in the proposed high-level repository at the Yucca Mountain Site

/pdf/analysis-of-components-from-drip-tests-with-atm-10-glass-1kewuxvf14.pdf

Analysis of components from drip tests with ATM-10 glass

Results from drip tests designed to simulate unsaturated conditions in the proposed Yucca Mountain Repository are reported for an actinide-doped glass (reference glass ATM-10) used as a model waste form. These tests have been ongoing for nearly 7 years, with data collected on solution composition (including transuranics), colloid formation and disposition, glass corrosion layers, and solid secondary phases. This test is unique because of its long elapsed time, high content of thorium and transuranics, use of actual groundwater from the proposed site area, use of contact between the glass and sensitized stainless steel in the test, and the variety of analytical procedures applied to the components. Some tests have been terminated, and scanning electron microscopy (SEM) and analytical transmission electron microscopy (AEM) were used to directly measure glass corrosion and identify secondary phases. Other tests remain ongoing, with periodic sampling of the water that had contacted the glass. The importance of integrated testing has been demonstrated, as complex interactions between the glass, the groundwater, and the sensitized stainless steel have been observed. Secondary phases include smectite clay, iron silicates, and brockite. Actinides, except neptunium, concentrate into stable secondary phases. The release of actinides is then controlled by the behavior ofmore » these phases.« less

/pdf/long-term-test-results-from-a-west-valley-actinide-doped-qxlg4gg80t.pdf

Long-term test results from a West Valley actinide-doped reference glass

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Analysis of components from drip tests with ATM-10 glass

Long-term test results from a West Valley actinide-doped reference glass