Simulated Nuclear Waste Glass Research Articles

Wet Chemical and UV-Vis Spectrometric Iron Speciation in Quenched Low and Intermediate Level Nuclear Waste Glasses

ABSTRACTIn this study wet chemical methods combined with UV-Vis spectroscopy were performed to quantify Fe(II)/Fe(III) ratios and total iron content of quenched alkali alumino-boro-silicate (simulated nuclear waste) glasses, applying a colorimetric method. We report lessons learned from experimental challenges encountered associated with the colorimetric method, where 1,10 phenanthroline method is complexed with dissolved glass powder and the resulting solution measured for absorbance at 520 nm to determine Fe(II). To obtain total iron, the solution was then equilibrated with a mild reducing agent to chance all Fe to Fe(II), and the absorbance measured again at 520 nm. These absorbance values allowed for calculation of the Fe(II)/Fe(III) ratio, and the total iron content in the glasses. Total Fe measured is somewhat higher than as-batched target values for waste glasses, but very accurate for reference BCR-2G glass. All quenched alumino-boro-silicate glasses analyzed showed a Fe(II)/Fe(III) ratio between 0.06 (± 0.01) and 0.04 (± 0.01). These values are consistent with those obtained for similar glass compositions melted under analogous conditions, indicating a composition of ca. 94-96% Fe(III).

A Sampling Method for Semi-Quantitative and Quantitative Electron Microprobe Analysis of Glass Surfaces

ABSTRACTThe determination of the long-term stability and corrosion of vitrified nuclear waste is an important aspect of research for the U.S. Department of Energy (DOE). It is necessary to understand the rate and mechanisms of Nuclear Waste Glass (NWG) corrosion to determine whether or not the glassy matrix will be able to retain radionuclides for the required repository performance time period. Glass corrosion and the rate of glass corrosions is determined by both chemical and microscopy. Electron Microprobe Analysis (EPMA) is a common and powerful method utilized in the examination of the chemographic difference between corroded and uncorroded NGWs. In this work, two forms of quantitative and semi-quantitative EPMA methods are defined by optimizing the instruments counting statistics against a standard glass and NIST minerals that have compositions similar to the glasses under examination. Data collected on both the planar and cross-sectioned surfaces of an unaltered simulated NWG by Standard based Wavelength Dispersive Spectroscopy (WDS) was found to be comparable to the theoretical composition of the glass. Conventional standardless Energy Dispersive Spectroscopy (EDS) data collected on the same surfaces was not comparable. However, standard-based EDS analysis is shown to be able to discriminate between unaltered and corroded glass surfaces.

Low-temperature lithium diffusion in simulated high-level boroaluminosilicate nuclear waste glasses

Ion exchange is recognized as an integral, if underrepresented, mechanism influencing glass corrosion. However, due to the formation of various alteration layers in the presence of water during the corrosion process, it is difficult to conclusively deconvolute the mechanisms of ion exchange from other simultaneously occurring processes. In this work, an inert non-aqueous solution was used as an alkali source material to isolate ion exchange and study the solid-state diffusion of lithium. Specifically, the experiments involved contacting simulated nuclear waste glass coupons, the 6-oxide CJ6 and the 26-oxide SON68, with a non-aqueous solution of 6LiCl dissolved in dimethyl sulfoxide at 90°C for various time periods. The depth profiles of major elements in the glass coupons were measured using time-of-flight secondary ion mass spectrometry (ToF-SIMS). Lithium interdiffusion coefficients, DLi, were then calculated based on the measured depth profiles. The results indicate that the penetration of 6Li is faster in the simplified CJ6 (D6Li≈4.0–8.0×10−21m2/s) compared to the more complex SON68 (D6Li≈2.0–4.0×10−21m2/s). These values are roughly an order of magnitude greater than measured water diffusion in glasses at similar temperatures. Additionally, sodium ions present in the glass were observed to participate in ion exchange reactions with lithium from the solution. Implications of the diffusion coefficients obtained in the absence of alteration layers to the long-term performance of nuclear waste glasses in a geological repository system are also discussed.

Effect of Groundwater on the Dissolution Rate of the Simulated Nuclear Waste Glass SON68 under Gamma Irradiation

The leaching behaviour of the inactive reference of the R7T7 glass is investigated here in a synthetic clayey groundwater solution, focusing on the effect of gamma irradiation on this residual alteration rate, under static conditions, at 90¡C. Results don’t underline any significant effect of gamma irradiation on the residual alteration rate up to 5kGy/h, compared to experiments performed out of irradiation in groundwater or in pure water. Moreover, the evolution of boron releases in groundwater versus time is similar whatever the dose rate. These observations are consistent with SEM characterizations.

Effect of clayey groundwater on the dissolution rate of the simulated nuclear waste glass SON68

The influence of clayey groundwater on the dissolution rate of SON68 glass was investigated under rate drop conditions. Leaching in contact with groundwater resulted in larger amounts of altered glass than obtained with initially pure water. Clayey groundwater delays the rate drop and subsequently results in dissolution rates higher than in pure water. This effect is due to the presence of magnesium, which precipitates in secondary phases with silicon while the other elements found in clayey water have no effect on the glass kinetics. Modeling the test results showed that the precipitation of secondary magnesium phases sustains the dissolution of the passivating layer of the glass and lowers the leachate pH, thereby increasing the diffusivity of this layer. The simulations also showed that the precipitation of magnesium phases is not limited by their precipitation kinetics, but is controlled by their solubility and the flow of silicon from glass dissolution. When all the magnesium in solution has precipitated, the pH slowly returns to the values usually measured during leaching of glass in initially pure water. Then the dissolution rate reflects the values measured in pure water. This study demonstrates that the reactivity of magnesium phases in the geological environment and the transport of magnesium in solution could have a significant impact on the long-term behavior of the glass.

A micro-XAS/XRF and thermodynamic study of CeIII/IV speciation after long-term aqueous alteration of simulated nuclear waste glass: Relevance for predicting Pu behavior?

Abstract In the present study, the dissolution and mobilization of Ce introduced in a simulated nuclear waste glass (MW) as a surrogate of Pu was investigated after leaching in pure water over 12 a at 90 °C and pH ∼ 9.6. The microscopic distribution and oxidation state of Ce in the altered glass were studied using micro-X-ray fluorescence (micro-XRF) mapping techniques and micro-X-ray near-edge absorption spectroscopy (micro-XANES). Distribution maps of Ce III and Ce IV were obtained by recording the L α fluorescence emission at two different incident X-ray energies, coinciding with the maximum contrast between Ce III and Ce IV fluorescence intensities. The micro-XRF maps revealed that Ce was dominantly present as oxidized species (Ce IV ) in the original glass. After dissolution from the glass matrix, Ce IV was partly reduced and re-immobilized as Ce III at grain boundaries or in the interstitial spaces between the glass particles. The concentration of Ce III was found to correlate with the spatial distribution of secondary Mg-clay formed during the aqueous corrosion as the main glass alteration product. Micro-XANES spectra collected at locations representative of both altered and non-altered glass domains confirmed the findings obtained by the redox mapping. Because redox-sensitive elements in the pristine MW glass (Fe, Cr, Se) occur almost exclusively as oxidized species, reduction of Ce IV was probably mediated by an external source of reductants, such as Fe(0) from the steel reaction vessel. These results, in conjunction with an Eh–pH stability diagram of the Ce III/IV –O–H–C IV –P–S VI –Na–Cl system, indicate that the glass was leached at relatively low oxidation potentials (Eh IV to Pu III would require considerably more reducing conditions (Eh

Nepheline Crystallization in Nuclear Waste Glasses: Progress Toward Acceptance of High‐Alumina Formulations

Historical data have been critically compiled and analyzed for investigating the quantity of nepheline (NaAlSiO4) precipitated as a function of composition in simulated nuclear waste glasses. To understand compositional effects two primary methods were used: (1) investigating the Al2O3–SiO2–Na2O ternary while filtering for different B2O3 levels and (2) creating a quadrant system consisting of compositions reduced to two representations: (i) the nepheline discriminator (ND) which depends only on the SiO2 content by weight normalized to the total weight of the Al2O3–SiO2–Na2O submixture and (ii) the optical basicity (OB) which contains contributions from all constituents in the glass. Nepheline precipitation is expected to be suppressed at high SiO2 levels (ND > 0.62) or at low basicities (OB < 0.55–0.57). Changes in sodium aluminosilicate glass OB values due to additions of CaO and B2O3 correlate with the observed effects on nepheline formation. It is proposed that additional composition space is available for formulating high waste loading, high‐Al2O3 nuclear waste glasses when consideration is given to location on the Al2O3–SiO2–Na2O submixture as well as OB.

Na, Mg, Ni and Cs distribution and speciation after long-term alteration of a simulated nuclear waste glass: A micro-XAS/XRF/XRD and wet chemical study

Microscopic distribution and speciation of Na, Mg, Ni and Cs in a simulated (inactive) nuclear waste glass were studied using micro X-ray fluorescence (μ-XRF) and micro X-ray absorption spectroscopy (μ-XAS), after aqueous leaching during 12 years at 90 °C. Na and Mg are major constituents of the glass that can be used to determine the progress of the glass corrosion process and the nature of secondary alteration phases. Ni and Cs represent dose determining long-lived radionuclides ( 59Ni, 135Cs) in vitrified nuclear waste. The Na-Mg μ-XRF maps revealed that the core regions of the glass fragments are apparently unaltered and compositionally homogeneous, whereas rims and interstitial spaces are enriched with Mg-rich smectite formed during the leaching process. The micro X-ray absorption near edge structure (μ-XANES) spectra collected at the Mg K-edge in the altered zones show three sharp resonances typical for crystalline Mg-silicates. These resonances are distinctive of Mg occupying undistorted octahedral positions. In contrast, the μ-XANES spectra collected in the core zones of the glass fragments lack this resonance pattern and are identical to the spectra measured on the pristine (unleached) MW glass. Micro extended X-ray absorption fine structure (μ-EXAFS) and μ-XANES analyses at the Ni K-edge revealed three distinct Ni(II) species: (a) Ni uniformly distributed in the glass matrix, (b) micro-inclusions with high Ni concentrations and (c) Ni associated to the Mg-clay. The comparison with reference spectra of unleached MW and other Ni-bearing silicate glasses indicated that species (a) represents the original coordinative environment of Ni in the glass. The μ-EXAFS analyses revealed that species (b) is structural Ni in trevorite (NiFe 2O 4), which probably formed through unmixing processes during the cooling of the glass melt. The μ-EXAFS of species (c) could be successfully modeled assuming specific adsorption or incorporation of Ni into the lattice of trioctahedral Mg-clay minerals. Alternative models assuming other elements (Ni, Al, Fe) in addition to Mg in the second shell could not be fitted successfully. Aqueous concentration data were used to calculate the speciation of the leaching solutions. Saturation index (SI) calculations indicate undersaturation with respect to NiCO 3 and NiSO 4·7H 2O, but oversaturation with respect to β-Ni(OH) 2. The latter result is probably due to the omission of Ni borate and Ni silicate complexes in the speciation calculations, for which formation constants are not available. With the help of estimation techniques, we could infer that such complexes would dominate the Ni speciation and consequently reduce the SI below the saturation of β-Ni(OH) 2. The μ-XRF maps show that Cs is uniformly distributed in the MW glass, since no region with high Cs concentration could be detected. The Cs L III-edge μ-XAS spectra were all very similar independently of the degree of alteration, indicating similar coordination environments of Cs in the core regions of the glass as well as in the secondary clays. These spectra largely differ from that measured for pollucite (a potential secondary Cs-phase in altered glasses) implying that the coordination environments of Cs in the MW glass and in pollucite are fundamentally different. The present study shows that μ-XRF and μ-XAS are essential tools in determining the fate and the retention mechanisms of radionuclides released from nuclear waste during aqueous alteration. Our spectroscopic analyses allowed us to exclude formation of specific Ni and Cs secondary solids (e.g. nepouite, β-Ni(OH) 2, pollucite) during the aqueous alteration. Ni and Cs are instead distributed as trace elements in the alteration phases formed by major elements during the leaching process. Our results imply that solid solution and/or adsorption equilibria, rather than pure phase solubility equilibria, are the adequate chemical models to determine Ni and Cs aqueous concentrations in performance assessments for radioactive waste repositories.

Property–Composition–Temperature Modeling of Waste Glass Melt Data Subject to a Randomization Restriction

Properties such as viscosity and electrical conductivity of glass melts are functions of melt temperature as well as glass composition. When measuring such a property for several compositions, the property is typically measured at several temperatures for one composition, then at several temperatures for the next composition, and so on. This data collection process involves a restriction on randomization, which is referred to as a split‐plot experiment. The split‐plot data structure must be accounted for in developing property–composition–temperature models and the corresponding uncertainty equations for model predictions. Instead of ordinary least squares (OLS) regression methods, generalized least squares (GLS) regression methods using restricted maximum likelihood (REML) estimation must be used. This article summarizes the methodology for developing property–composition–temperature models and corresponding prediction uncertainty equations using the GLS/REML regression approach. Viscosity data collected on 197 simulated nuclear waste glasses are used to sequentially develop a viscosity‐composition‐temperature model. The final model has 29 terms in 15 components, reduced from the initial model of 44 terms in 22 components. For the initial model, the correct results using GLS/REML regression are compared with the incorrect results obtained using OLS regression.

A Component Slope Linear Model for Mixture Experiments

The Scheffé linear model and the Cox linear model are two first-order model forms commonly used for analyzing mixture experiment data. In a fitted Scheffé linear model, the coefficient for a mixture component is the predicted response at that pure component (i.e., single-component mixture). In a fitted Cox linear model, the coefficient for a mixture component is the predicted difference in response at that pure component and at a pre-specified reference mixture. An alternative first-order model for mixture experiments, the component slope linear mixture (CSLM) model, is proposed in this article. In a fitted CSLM model, the coefficient for a mixture component is the predicted slope of the linear response surface along the Cox effect direction for that component (where the direction is determined by a pre-specified reference mixture). The CSLM model and methods for fitting it are presented. The advantages and disadvantages of the CSLM model, Scheffé linear model, and Cox linear model are discussed. The three models are illustrated and interpreted for an example involving the liquidus temperature of spinel crystals in simulated nuclear waste glass.

EELS Spectrum Imaging and Tomography Studies of Simulated Nuclear Waste Glasses

AbstractElectron energy loss spectroscopy (EELS) has been widely used in analysis of ceramic, minerals and semiconductors. It can provide the unique advantage of sensitivity to composition, coordination and valency, while providing nano-scale spatial resolution. Due to the electron irradiation sensitivity, EELS studies on glass are quite rare. However, using special care in glass-composition selection and by recording time series of EELS-spectra to monitor any structural changes, EELS then becomes a very useful technique to study glass-chemistry and glass structure with the highest spatial resolution of all chemically sensitive techniques. Alkali borosilicate glasses (ABS) doped with Cr2O3 (2 mol%), CeO2 (4 mol%) and ZrO4 (4 mol%) were melted, cooled and annealed in the context of simulating radionuclide immobilisation glasses. Precipitates with diameter in the range of ∼20 nm to ∼200 nm were found homogeneously distributed in the glass. In preliminary studies we measured the oxidation state in the glass and in precipitates via evaluation of the Ce-M-edge double white line ratio, confirming the crystals as Ce(IV) oxide. We also found Boron K-edge ELNES spectra as a sensitive signal to boron coordination (N4 = BO4/(BO3+BO4)), which is found at N4 ratios of around 35-45%.In the present study we have extended this work to the acquisition of dense line scans (spectrum imaging) across an area of interest on our composite glass with emphasis on three questions:(i) Change of cerium valence upon transition from the glass to the precipitates?(ii) Possible oscillation of boron N4-ratio in the glass within a line scan as a consequence of random medium-order related glass fluctations when scanning the <5nm probe across a seemingly homogeneous area?(iii) N4-changes in the immediate glass layer surrounding the precipitates?Our ongoing research has produced results which prove a systematic change of the Ce oxidation state from precipitates (+IV) to glass (mixed +III/+IV). It is found that boron coordination is constant within the sensitivity given by signal-to-noise in the glass matrix. A change of the Boron K-edge fine structure upon crossing a nanoparticle during one particular linescan has been observed, which could hint to a coordination change of the glass layer surrounding the particle. Other linescan measurements have not shown any change and acquisition of an enlarged better statistic is work in progress.The ABS-glass with distributed nanoparticles provides an ideal example of a 3D nanocomposite. The only proven technique for the three-dimensional reconstruction of such materials on the nanoscale is electron tomography. We present the first results of a 3D reconstructed nuclear waste glass by using a tilt series of ADF STEM images covering a glass fragment of 2000nm field of view containing several tens of nanoparticles distributed around its volume.

Mechanical Behavior of a Borosilicate Glass Under Aqueous Corrosion

In France, fission products are being vitrified for a possible final geological disposal. Under disposal conditions, corrosion of the glass by groundwater as well as stress corrosion because of stresses occurring at surface flaws cannot be excluded. Within this framework, the mechanical behavior of the French simulated nuclear waste glass SON68 was studied by Vickers indentation and fracture experiments in air and in a corrosive solution. The glass was corroded at 90°C in a solution enriched with Si, B, and Na. The results showed that the glass corrosion enhances the cracks propagation relative to experiments in air. The indentation fracture toughness (KIC) obtained using a four‐point bending test showed that the KIC of the glass decreased with increasing corrosion time.

Self-Diffusion of <SUP>22</SUP>Na and <SUP>137</SUP>Cs in Simulated Nuclear Waste Glass

The knowledge of self-diffusion of various elements is important for understanding and elucidating the long-term dissolution of the nuclear waste glass. The self-diffusion coefficients of Na and Cs in P0798 simulated nuclear waste glass have been measured by an ion beam sputter-sectioning technique using the radioactive isotopes 22 Na and 137 Cs. The temperature dependence of the diffusion coefficients in each temperature range in P0798 glass below the glass transition temperature Tg can be expressed by the following equations: (428-574K) : D Na-belowTg = 2.7 x 10 -6 exp(-113±4kJ.mol -1 /RT)m 2 .s -1 (713-758K): D Cs-belowTg = 7.1 x 10 -5 exp(-241±10kJ.mol -1 /RT)m 2 .s -1 The diffusion coefficient of 22 Na was about ten orders of magnitude larger than that of 137 Cs at 573 K. The large difference in the diffusion coefficients is explained in terms of the ion size effect, the mixed-alkali effect and lower concentration of Cs compared with Na in P0798 glass.

Long Term Durability Testing of Simulated Iron-Phosphate Nuclear Waste Glass

ABSTRACTAn iron phosphate base glass and a simulated Hanford Tank Farm B (TFB) waste loaded iron phosphate glass were reacted under both Product Consistency Test (PCT) and Vapor Hydration Test (VHT) conditions. Solution aliquots were collected following reaction of the TFB glass after the completion of a PCT at 90°C for time periods of 7, 49, 185, 274, and 365 days. Normalized element release patterns for sodium were highest of all the elements present, with values initially decreasing between 7 and 182 days and then increasing thereafter. Normalized release values for phosphorous declined between 49 and 182 days but increased thereafter, whereas calcium contents declined to and remained below the analytical detection limit after 182 days. Normalized element release patterns for Ce, Nd, and Fe were below 0.00045 g/m2 for all time periods tested. Amorphous iron oxide and elongate crystals (possibly apatite?) were noted as alteration products in both long-term PCT and short-term VHT formats.The VHT samples were reacted at 150°C for time periods of 3, 7, and 35 days for the base glass and 3, 7, 35, 91, 185, and 300 days using the TFB glass. Reacted samples were examined for alteration products, with an emphasis on identifying phases that could potentially occur during geologic disposal. The base glass reacted quickly under the VHT conditions. Phosphorous released from the base glass apparently formed phosphoric acid, which quickly lowered the solution pH to a value of one. The acidic nature of the fluids, in turn, led to an accelerated rate of glass corrosion. The addition of alkali and alkaline earth elements in the TFB glass prevented the low pH excursion seen in the base glass; possibly by ion exchange processes that remove hydrogen ions from solution, sequestering of phosphorous in alteration phases to prevent phosphoric acid formation, or both. This change led to a dramatic decrease in TFB glass reaction rates relative to the base glass composition. Alteration products included an amorphous iron precipitate and several crystalline phases.

高レベル放射性廃棄物模擬ガラス固化体中のNaとCsの自己拡散

高レベル放射性廃棄物模擬ガラス固化体中のNaとCsの自己拡散

Conductivity - Chemistry Relationship in Simulated Nuclear Waste Glass Melts

Electrical conductivity of three simulated nuclear waste glass melts was measured over a wide range of temperatures, using a high-accuracy, calibration-free, coaxial-cylinders technique. Glass chemistries representing both low-activity and high-level wastes at Hanford and Savannah River sites were used. The temperature dependence of the conductivity of these glasses was presented. The conductivity values measured at 1150°C were compared to the conductivity values predicted from first-order mixture model.

Europium complexes investigations in natural waters by time-resolved laser-induced fluorescence

Time-resolved laser-induced fluorescence (TRLIF) has been used to investigate Eu complexes formed with inorganic and organic ligands encountered in natural waters as well as Eu complexes formed during the interaction of simulated nuclear waste glass with claywater. By comparing spectral and temporal data obtained by TRLIF on various europium solutions containing humic acids (HA) and carbonates (CO 3), it has been possible to confirm the presence of mixed europium complexes such as Eu–CO 3–HA. These results are quite important for speciation purposes, in particular in the framework of environmental studies.

Quantification of Water-Content of Simulated Nuclear Waste Glasses Using Nuclear Reaction Measurements

AbstractWater content of glasses is an important factor in glass manufacture. In the case of slurryfed processing of waste glass melters, the water content of waste glasses may change the waste form properties significantly. A reliable, non-contact and quantitative measurement technique, with a spatial resolution of about 2-mm was used to determine the concentration of dissolved water molecules in simulated nuclear waste glasses. The 19F resonant reaction was used. The 1H (19F, αγ)16O reaction requires a resonance energy that is available by use of a 6.471 MeV19F beam. A series of ten segments of slurry-fed, joule-heated, research-scale melter tests were conducted to determine the redox effects on processing of various feeds. Representative glass samples were collected at the middle of each segment. Their hydrogen contents were measured. The hydrogen content was correlated to the feed processing and redox of the glasses.

Chemical interaction between a simulated nuclear waste glass and different backfill materials under a thermal gradient

The initial stage of a HLW disposal will be dominated by a strong thermal gradient that will exist between the hot canister and the “cold” geological medium. In the case of hot wastes (MOx, direct disposal of used spent fuel, short interim storage), nuclear glass can begin to dissolve in groundwater and to interact with other materials in the presence of a thermal gradient. Moreover, it has already been demonstrated that thermal gradients strongly influence the hydrothermal corrosion of nuclear glass by creating large elemental segregation and subsequent mass transport processes. The aim of this article is to experimentally test whether the presence of a simulated engineered clay barrier affects these mass transport processes. In parallel, experiments were performed with different clays of various Si/Al ratios (smectite, kaolinite and allophane) as Si and Al are often invoked as possible rate limiting species for nuclear glass aging. A simulated nuclear glass was disposed between two layers of various clays, enclosed in a sealed gold tube filled with deionised water. The gold tube was put in a cold-seal vessel and submitted to a thermal field under high temperatures (300–200°C, 140 bars, 100 days). High temperatures were used in order to enhance transformation kinetics as mass transport is supposed to be independent of the temperature. At the end of the experiments, the newly formed phases along the tube as well as the mineralogical transformations of the reactants were studied by integrated in-situ analyses (SEM, TEM, EDS and XRD). Precise descriptions of the mineralogical transformations are presented. The major result is a systematic re-crystallization of the initial clay under the influence of the leached elements from the glass (in particular Si and alkalies). The nuclear glass aging is strongly affected by the composition of the surrounding clay and consumes Al from the clay. The aluminous-rich medium seems to limit the glass corrosion and the crystallinity of the newly formed phases. Furthermore, the cold end of the system is devoted to almost pure Al–Si bearing minerals associated with borate. This finding attests to (i) the migration capacities of Si, Al and B through the clay barrier and (ii) the confining properties of the clay barrier for the heaviest elements (Fe, Zn, Zr) which were mineralogically trapped at the clay–glass interface. In conclusion, the study reveals that interactions between the nuclear glass and the backfill materials are important. Thus, these interactions have to be taken into account in order to predict the long term evolution of the system. Furthermore, backfill materials are an efficient chemical barrier which favors the immobilization of the heaviest elements (Fe, Zn and Zr in our experiments).

Effects of Metals, Metal Oxides and Metal Hydroxide on the Leaching of Simulated Nuclear Waste Glass

Effects of Metals, Metal Oxides and Metal Hydroxide on the Leaching of Simulated Nuclear Waste Glass