Simulated High-level Waste Glass Research Articles

The leaching behavior of simulated HLW glass under repository condition

As the multibarrier system has been adopted to dispose HLW glass in geological formation in many countries, it was important to study the leaching behavior of vitrification under geological formation. This article describes the leaching behavior of simulated high level waste glass (90Nd/10), which can incorporate 16 wt.% simulated HLW in five kinds of geological media, such as granite, cement, bentonite, Fe 3O 4, etc. The durable experimental results show that the glass had less mass loss in granite and more mass loss in bentonite after a two-year leaching test. The SEM/XEDS analysis shows some element distributions on the leached specimen's surface, i.e., Na, Si and Mg elements were reduced on the specimen's surface, whereas Ba, Al, and Fe were enriched on the specimen's surface.

Degradation of high-level waste glass under simulated repository conditions

The internal waste package environment that contains corrosion products from the containers may have a significant influence on waste form degradation. Leaching experiments of two simulated high-level waste glasses (WVDP Ref. 6 and DWPF Blend 1) were carried out in aqueous solutions of FeCl 2 and FeCl 3 at temperatures of 40, 70, and 90 °C that simulate the possible aqueous environment inside the breached waste packages. Both solution pH and the presence of corrosion species such as iron chloride were found to be major contributing factors to the net glass durability. Model abstraction and performance assessment analyses using rate expressions showed that the presence of corrosion products enhances glass dissolution and increases the subsequent release of radionuclides to the environment.

Augmenting a Waste Glass Mixture Experiment Study with Additional Glass Components and Experimental Runs

A glass composition variation study (CVS) for high-level waste (HLW) stored in Idaho is being statistically designed and performed in phases over several years. The purpose of the CVS is to investigate and model how HLW-glass properties depend on glass composition. The resulting glass property–composition models will be used to develop desirable glass formulations and for other purposes. Phases 1 and 2 of the CVS have been completed and are briefly described. This paper focuses on the CVS Phase 3 experimental design, which was chosen to augment the Phase 1 and 2 data with additional data points, as well as to account for additional glass components not studied in Phases 1 and/or 2. In total, 16 glass components were varied in the Phase 3 experimental design. The paper describes how these Phase 3 experimental design augmentation challenges were addressed using the previous data, preliminary property–composition models, and statistical mixture experiment and optimal experimental design methods and software. The resulting Phase 3 experimental design of 30 simulated HLW glasses is presented and discussed.

Formation of secondary phases after long-term corrosion of simulated HLW glass in brine solutions at 190 °C

SummaryThe behavior of rare earth elements (REE) as chemical analogues for actinides during glass corrosion was studied with static long-term batch experiments (7.5 years) at 190 °C. Corrosion tests were carried out using a simulated inactive high level waste (HLW) glass powder. Two different highly concentrated salt solutions (NaCl-rich and MgCl2-rich) have been used to simulate the potential corrosion by aqueous solutions in geological salt formations. Samples with surface-to-volume ratios (S/V) of 1000 m−1and 10000 m−1were investigated. The chemical composition of the aqueous solution and the formation of secondary phases at the glass/solution interface as well as their elemental composition, especially with respect to the behavior of REE was studied. The glass corrosion behavior and secondary phase formation was significantly different in the two brines. In the MgCl2-rich salt solution, with a final pH < 3.5, hydrous Mg-silicates and (Ba,Sr)SO4solid solutions with considerable amounts of Sr (up to 15 wt.%) have been observed. Further, REE-sulfates and REE-molybdates with an indication of dissolution (etch pits) as secondary phases could be identified. In the NaCl-rich solution with a final high pH, REE-containing hydrous silicates (up to 35 wt.%) and (Ba,Sr)SO4solid solutions with small amounts of Ce (up to 5 wt.%) have been observed. Frequently observed Ca-molybdates (powellite) did not contain detectable amounts of REE.

Liquidus temperature–composition model for multi-component glasses in the Fe, Cr, Ni, and Mn spinel primary phase field

We developed an extensive T L database of simulated high-level waste (HLW) glasses within the spinel primary phase field. Partial-molar T Ls, T i , were determined for all components that were systematically varied in database glasses ( i=Al, B, Ca, Cr, Fe, K, Li, Mg, Mn, Na, Ni, Si, Ti, U, and Zr). A clear relationship was found between the T i values and ion potential. This led to a new model that can accurately predict the T L of glasses within component concentration ranges of the database. The model gives slightly better predictability than a first-order expansion of T L in composition while using only half of the fitted parameters and offers an improvement in predictability over previously published models. The success of this model gives insight to the nature of component effects on T L, which warrants further investigation. Specifically, the concentrations of glass components appear to be influential on T L in proportion to the character of their bonds or their ionic potential.

The Effect of Adding Crystalline Silicotitanate on the Durability, Liquidus, and Viscosity of Simulated High-Level Waste Glasses at Savannah River Site

This report provides a summary of the results obtained for a limited variability study for glasses containing Crystalline Silicotitanate (CST), Monosodium Titanate (MST), and either simulated Purex or HM sludge. Twenty-two glasses containing Purex sludge and three glasses containing HM sludge were fabricated and tested. The fabricated glasses were tested for durability using the 7-day Product Consistency Test (PCT) and characterized by measuring the viscosity at 1,150 C and by determining an approximate, bounding liquidus temperature. The current models used by Defense Waste Processing Facility (DWPF) for predicting durability, viscosity, and liquidus temperature were applied to all 25 glasses. The goal of this work was to identify any major problems from a glass perspective, within the scope of this effort, which could potentially preclude the use of CST at DWPF.

Temperature Effects on the Leaching Behavior of a High-Level Waste Glass Form

Temperature has strong effects on the leaching behavior of the 90-19/U simulated high-level waste glass form. The Arrhenius equation is used to determine the change in the glass corrosion mechanism throughout the different temperature ranges. The apparent activation energies have been obtained for different leaching conditions. In deionized water, the glass corrosion mechanism is similar whether the leaching condition is static or dynamic. The glass corrosion process is dominated by the ion exchange reactions at lower temperatures (≤60 to ~70°C); however, the glass corrosion process is dominated by the network hydrolysis reactions at higher temperatures (>70°C). The apparent activation energy in the lower temperature range is larger than that of the higher temperature range. In simulated underground water, the ion exchange reactions dominated the glass corrosion mechanism at temperatures from 50 to 150°C, and the large amount of ions in simulated underground water would participate in the ion exchange reactions with the glass.

Structure of Pd–Te precipitates in a simulated high-level nuclear waste glass

Structural and bonding characteristics of simplified (Pd, Te) precipitates have been determined in a simulated nuclear French glass using extended x-ray absorption fine structure (EXAFS) and x-ray diffraction. In this sample, these precipitates have a homogeneous composition, with about 10 wt.% Te. They retain a face-centered cubic structure as in pure Pd with a cell parameter which obeys Vegard's law. Pd K-edge EXAFS shows the presence of Te in the Pd coordination shell, with (Pd–Te) distances of 2.80 Å. These distances, higher by 0.05 Å than the (Pd–Pd) distances, may result in a lower packing efficiency of the CFC lattice. The comparison with the average distances derived from x-ray diffraction shows the nonmetallic character of the Pd–Te bond in these precipitates. These bonding modifications may cause the limited solubility of Te in metallic Pd.

Isothermal Crystallization Kinetics in Simulated High-Level Nuclear Waste Glass

ABSTRACTCrystallization kinetics of a simulated high-level waste (HLW) glass were measured and modelled. Kinetics of acmite growth in the standard HW39–4 glass were measured using the isothermal method. A time-temperature-transformation (TTT) diagram was generated from these data. Classical glass-crystal transformation kinetic models were empirically applied to the crystallization data. These models adequately describe the kinetics of crystallization in complex HLW glasses (i.e., RSquared = 0.908). An approach to measurement, fitting, and use of TTT diagrams for prediction of crystallinity in a HLW glass canister is proposed.

Apatite- and Monazite-Bearing Glass-Crystal Composites for the Immobilization of Low-Level Nuclear and Hazardous Wastes

AbstractThis study demonstrates that glass-crystal composite waste forms can be produced from waste streams containing high proportions of phosphorus, transition metals, and/or halides. The crystalline phases produced in crucible-scale melts include apatite, monazite, spinels, and a Zr-Si-Fe-Ti phase. These phases readily incorporated radionuclide and toxic metals into their crystal structures, while corrosion tests have demonstrated that glass-crystal composites can be up to 300-fold more durable than simulated high-level nuclear waste glasses, such as SRL 202U.

Durability of High-Level Waste Glass in Flowing Groundwater Under Gamma-Irradiation

AbstractDurability of simulated high-level waste glass in continuously-flowing J-13 tuff groundwater has been examined at 90°C under gamma-irradiation. The results obtained are compared with those without gamma-irradiation. The effects of groundwater radiolysis on the glass durability are discussed based on the Eh-pH excursion obtained in the present system. It is found that the groundwater tends to be reduced under gamma-irradiat ion, however, this may not influence the solubility of multivalent cations leached from the glass.

Characterization of High-Level Nuclear Waste Glass Using Magnetic Measurements

ABSTRACTMagnetic measurements constitute a promising method for the characterization of nuclear waste glasses in view of their simplicity and small sample weight requirements.Initial studies of simulated high-level waste glasses show that the Curie constant is generally a useful indicator of the Fe2+:Fe3+ ratio. Glasses produced by air-cooling in large vessels show systematic deviations between experimental and calculated values, which are indicative of the presence of small amounts of crystalline iron-containing phases. Most of the iron in these phases becomes dissolved in the glass upon re-heating and more rapid quenching. The studies further show that upon leaching the glass in water some of the iron in the surface regions of the glass is converted to a form which has high temperature-independent magnetic susceptibility.

The compared aqueous corrosion of four simple borosilicate glasses: Influence of Al, Ca and Fe on the formation and nature of secondary phases

Abstract The aqueous corrosion of a series of four glasses under static conditions at 65 and 90°C was studied by means of solid and solution analyses. Various glass compositions were studied by successive addition of Al 2O 3, CaO and Fe 2O 3 to a simple sodium borosilicate glass in order to maintain a compositional and structural analogy to a simulated high-level radioactive waste glass. The comparison of leaching behaviours shows that Al has the greatest influence on the chemical durability, probably for two main reasons: (i) a structural effect on the glass network and on the compound forming the residual alteration layer (ii) a solubility constraint on the residual layer. The identification of the phase composing the alteration layer was only possible for the simplest glass. On the basis of chemical analysis, infrared spectroscopy and solubility constraints, it was identified as amorphous silica. For the other glasses, the alteration process led to the growth of an alteration layer made up of an amorphous hydroaluminosilicate. This material cannot be defined as a phase with a fixed chemical composition ad known thermodynamic properties. The chemical composition of this compound, initially controled by the glass composition, adapts to changes in the solution chemistry, especially pH.

Volatility of Simulated High‐Level Nuclear Waste Glass by Thermogravimetric Analysis

The volatilities of simulated, high‐level nuclear waste glasses have been measured using thermogravimetric analysis (TGA). These volatilities were measured in the region of the glass transition temperature (Tg) of the waste glasses, which is between 450° and 500°C. These data were obtained because the Waste Acceptance Preliminary Specifications require that no foreign materials be released into the canistered waste form upon heating of the canister to this glass transition temperature. In fact, all of the waste glass samples studied actually exhibited a net weight gain upon heating. This weight gain was shown to be due to oxygen uptake through oxidation of FeO. Powdered glass samples did show a small weight loss which was smaller in magnitude than the weight gain and was associated with water desorption. No true volatility was detected to the level of sensitivity (0.01 wt%) of the TGA instrument. This converts to a sensitivity of 330 μg/m2 of glass surface and a corresponding minimum value of 41 mg of volatiles for each Defense Waste Processing Facility conistered waste form. TGA experiments carried out at higher temperatures (800°C) revealed that organic concentrations in the waste glasses are less than 0.01 wt%. Thus, these results demonstrate that the Defense Waste Processing Facility will be able to comply with the Waste Acceptance Preliminary Specifications on the exclusion of foreign materials from the canistered waste forms, after exposure to Tg.

Microstructure of Simulated High-Level Waste Glass Doped with Short-Lived Actinides, 238Pu and 244Cm

ABSTRACTThe microstructural changes of a simulated waste glass irradiated with doping of 238pu and 244Cm were observed by use of a preshadowed carbon replica technique in combination with scanning electron microscopy (SEM). The irradiated glass was annealed and its microstructural changes after annealing were observed by use of the sametechnique.In the glass irradiated at a dose of 2.75x1025 α-decays/m3, bubbles with a radius from 0.15μrm to 0.35μm were observed. The average bubble radius and the bubble density were 0.23μm and 1x1017 bubbles/m3, respectively. Using these observed values, the volume change of the glass resulting from the bubble formation was estimated to be + 0.51 %. This valuoof + 0.51 % was close to the volume change of the irradiated glass measured in our previous study, which suggests a large portion of the volume change by α-irradiation results from bubble formation. In the glass annealed after irradiation the average bubbleradius was observed to decrease with annealing time. The bubble radius as a function of annealing time was analyzed on the basis of the helium diffusion model with two chemicalprocesses, i.e. trapping at bubbles and re-solution from bubbles into glass matrix. The values of the diffusion coefficient of helium, the trapping parameter and the re-solution parameter, which had been obtained experimentally in our previous study on the helium release, were applied to the calculation. The calculated curve was in good agreement with the observed data.

Congruent dissolution of high-level waste glass in synthetic groundwater

Four kinds of synthetic groundwater which resemble one type of Japanese groundwater were prepared. Simulated high-level waste glass was leached in the synthetic groundwater at 70°C for up to 49 days and at 20°C for one year. It was found that the leaching of the high-level waste glass could be characterized as congruent in the synthetic groundwater and incongruent in deionized water. The chemistry of the synthetic groundwater causes the congruent leaching which is likely to occur in deep geologic disposal sites.

Behavior of helium release from simulated radioactive waste glasses.

Rates of He release from two kinds of simulated high-level radioactive waste glasses were measured in the temperature range from 573 to 753K by a mass spectrometric method. Diffusion coefficients of He in the glasses were determined from the fractional release data. The diffusion coefficients were given as a function of temperature by the equations: D/(m2·s−1) = 2.3 × 10−6 exp (−71.1±2.1 kJ/RT) for P0798 glass =9.8 × 10−7 exp (−60.2±2.1 kJ/RT) for P0504 glass. In addition, solubilities of He in the glasses were determined from the measurement of the total amount of released He and found to be S/(atoms/m3) = 2.0 × 1022 for P0798 glass =2.2 × 1022 for P0504 glass. at 693 K and 1.7 × 105 Pa He. The diffusivities and solubilities of He in the glasses were, respectively, several and an order of magnitude lower than silica, alkali silicate and Pyrex glasses, which properties are notable for the radioactive waste glasses.

Continouos-flow leach tests of simulated high-level waste glass in synthetic basalt groundwater

Continuous-flow leach tests have been carried out on simulated high-level waste glass in synthetic Grande Ronde basalt groundwater at 90 °C for up to 180 d. Preliminary results obtained without irradiation are described here as reference data for later comparison with those from future experiments with irradiation. The time-dependence of the concentrations of silicon is traced, and the maximum concentration at the plateau is estimated. It is concluded from the results that, above a linear flow rate of 0.1 cm/d, the normalized leach rates of silicon tend to level off at about 6 μg · cm −2 · d −1 which is the maximum value under the present flow conditions.

The Long-Term Corrosion and Modelling of Two Simulated Belgian Reference High-Level Waste Glasses - Part II

ABSTRACTThe long term corrosion of two reference Belgian high-level waste glasses (SAN60 and SM58) were investigated in pure water. The corrosion was studied using powdered glass at a high surface area to volume ratio and temperatures of 90 and 120°C. The experimental data at 90°C reveal a “final” leach rate which decreases with time. At 120°C this “final” state is transient and followed by an enhanced dissolution, which was correlated with extensive surface crystallization. Modelling using the PHREEQE and GLASSOL computer codes described the initial corrosion, but was unable to account for the enhanced dissolution at 120°C.

Continuous-Flow Leach Tests of Simulated High-Level Waste Glass in Synthesized Groundwater and Deionized Water

Simulated high-level waste glass was leached at 70°C for 28 d in one type of synthesized groundwater in Japan and in deionized water. The MCC-4 low-flow-rate leach test method was used for producing continuous-flow leach conditions with flow rates of 0.1, 0.01 and 0.001 ml/min. The MCC-1 static leach tests were also carried out for a comparison. First, the increase of the normalized elemental mass losses (NL) for Si and B was observed for all flow rates studied using synthesized groundwater and for flow rates of more than 0.01 ml/min in deionized water. This was in line with the results of several other previous studies. Second, in the deionized water, the values of NL by the MCC-1 method were larger than those obtained by the MCC-4 method with flow rates of 0.001 and 0.01 ml/min. Since the deionized water does not have buffer capacity, the pH-value in the MCC-1 leach tests was higher than those in the MCC-4 leach tests with flow rates of 0.001 and 0.01 ml/min, thereby helping the glass to dissolve progressively in the MCC-1 static conditions.