Vapor Hydration Research Articles

Unveiling the effect of composition on nuclear waste immobilization glasses’ durability by nonparametric machine learning

Ensuring the long-term chemical durability of glasses is critical for nuclear waste immobilization operations. Durable glasses usually undergo qualification for disposal based on their response to standardized tests such as the product consistency test or the vapor hydration test (VHT). The VHT uses elevated temperature and water vapor to accelerate glass alteration and the formation of secondary phases. Understanding the relationship between glass composition and VHT response is of fundamental and practical interest. However, this relationship is complex, non-linear, and sometimes fairly variable, posing challenges in identifying the distinct effect of individual oxides on VHT response. Here, we leverage a dataset comprising 654 Hanford low-activity waste (LAW) glasses across a wide compositional envelope and employ various machine learning techniques to explore this relationship. We find that Gaussian process regression (GPR), a nonparametric regression method, yields the highest predictive accuracy. By utilizing the trained model, we discern the influence of each oxide on the glasses’ VHT response. Moreover, we discuss the trade-off between underfitting and overfitting for extrapolating the material performance in the context of sparse and heterogeneous datasets.

A Six-Year Hydration Evaluation of Cs-Bearing Materials at Room Temperature and 55% Relative Humidity Simulating Radioactive Waste with Different Crystallinities.

Radioactive wastes often contain amorphous and crystalline phases, and vapor hydration can affect their durability. In this study, Cs-clinoptilolite was heated (at 1100 °C and for 2-36 h) to prepare the samples that were composed mainly of an amorphous phase (AmP) and CsAlSi5O12 (≥94%) with minor CsAlSi2O6. Six samples with an AmP/CsAlSi5O12 ratio from 26.5 to 0.1 were kept at 21 °C and 55% relative humidity, and their hydration was measured via thermogravimetry (TG) over a period of almost six years. The hydration that resulted was directly related to the AmP quantity. The increase in water content followed a logarithmic trend over time. It reached 1.95% in the AmP-richest material, while it attained only 0.07% in the most crystalline sample. The hydrolysis of the AmP led to an increase over time in the tightly bound water. Samples with an AmP of ≤19% demonstrated slightly higher durability due to the lower Cs content in the AmP.

Surfactant-Mediated Structural Modulations to Planar, Amphiphilic Multilamellar Stacks.

The hydrophobic effect, a ubiquitous process in biology, is a primary thermodynamic driver of amphiphilic self-assembly. It leads to the formation of unique morphologies including two highly important classes of lamellar and micellar mesophases. The interactions between these two types of structures and their involved components have garnered significant interest because of their importance in key biochemical technologies related to the isolation, purification, and reconstitution of membrane proteins. This work investigates the structural organization of mixtures of the lamellar-forming phospholipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and two zwitterionic micelle-forming surfactants, being n-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (Zwittergent 3-12 or DDAPS) and 1-oleoyl-2-hydroxy-sn-glycero-3-phosphocholine (O-Lyso-PC), when assembled by water vapor hydration with X-ray diffraction measurements, brightfield optical microscopy, wide-field fluorescence microscopy, and atomic force microscopy. The results reveal that multilamellar mesophases of these mixtures can be assembled across a wide range of POPC to surfactant (POPC:surfactant) concentration ratios, including ratios far surpassing the classical detergent-saturation limit of POPC bilayers without significant morphological disruptions to the lamellar motif. The mixed mesophases generally decreased in lamellar spacing (D) and headgroup-to-headgroup distance (Dhh) with a higher concentration of the doped surfactant, but trends in water layer thickness (Dw) between each bilayer in the stack are highly variable. Further structural characteristics including mesophase topography, bilayer thickness, and lamellar rupture force were revealed by atomic force microscopy (AFM), exhibiting homogeneous multilamellar stacks with no significant physical differences with changes in the surfactant concentration within the mesophases. Taken together, the outcomes present the assembly of unanticipated and highly unique mixed mesophases with varied structural trends from the involved surfactant and lipidic components. Modulations in their structural properties can be attributed to the surfactant's chemical specificity in relation to POPC, such as the headgroup hydration and the hydrophobic chain tail mismatch. Taken together, our results illustrate how specific chemical complexities of surfactant-lipid interactions can alter the morphologies of mixed mesophases and thereby alter the kinetic pathways by which surfactants dissolve lipid mesophases in bulk aqueous solutions.

Multi-stage Transformations of a Cluster-Based Metal-Organic Framework: Perturbing Crystals to Glass-Forming Liquids that Re-Crystallize at High Temperature.

Glassy and liquid state metal-organic frameworks (MOFs) are emerging type of materials subjected to intense research for their rich physical and chemical properties. In this report, we obtained the first glassy MOF that involves metal-carboxylate cluster building units via multi-stage structural transformations. This MOF is composed of linear [Mn3 (COO)6 ] node and flexible pyridyl-ethenylbenzoic linker. The crystalline MOF was first perturbed by vapor hydration and thermal dehydration to give an amorphous state, which can go through a glass transition at 505 K into a super-cooled liquid. The super-cooled liquid state is stable through a wide temperature range of 40 K and has the largest fragility index of 105, giving a broad processing window. Remarkably, the super-cooled liquid can not only be quenched into glass, but also recrystallize into the initial MOF when heated to a higher temperature above 558 K. The mechanism of the multi-stage structural transformations was studied by systematic characterizations of in situ X-ray diffraction, calorimetry, rheological, spectroscopic and pair-distribution function analysis. These multi-stage transformations not only represent a rare example of high temperature coordinative recognition and self-assembly, but also provide new MOF processing strategy through crystal-amorphous-liquid-crystal transformations.

Multi‐stage Transformations of a Cluster‐Based Metal‐Organic Framework: Perturbing Crystals to Glass‐Forming Liquids that Re‐Crystallize at High Temperature

AbstractGlassy and liquid state metal–organic frameworks (MOFs) are emerging type of materials subjected to intense research for their rich physical and chemical properties. In this report, we obtained the first glassy MOF that involves metal‐carboxylate cluster building units via multi‐stage structural transformations. This MOF is composed of linear [Mn3(COO)6] node and flexible pyridyl‐ethenylbenzoic linker. The crystalline MOF was first perturbed by vapor hydration and thermal dehydration to give an amorphous state, which can go through a glass transition at 505 K into a super‐cooled liquid. The super‐cooled liquid state is stable through a wide temperature range of 40 K and has the largest fragility index of 105, giving a broad processing window. Remarkably, the super‐cooled liquid can not only be quenched into glass, but also recrystallize into the initial MOF when heated to a higher temperature above 558 K. The mechanism of the multi‐stage structural transformations was studied by systematic characterizations of in situ X‐ray diffraction, calorimetry, rheological, spectroscopic and pair‐distribution function analysis. These multi‐stage transformations not only represent a rare example of high temperature coordinative recognition and self‐assembly, but also provide new MOF processing strategy through crystal‐amorphous‐liquid‐crystal transformations.

Carbon-concentrating mechanisms are a key trait in lichen ecology and distribution.

Carbon-concentrating mechanisms (CCMs) are a widespread phenomenon in photosynthetic organisms. In vascular plants, the evolution of CCMs ([C44-carbon compound] and crassulacean acid metabolism [CAM]) is associated with significant shifts, most often to hot, dry and bright, or aquatic environments. If and how CCMs drive distributions of other terrestrial photosynthetic organisms, remains little studied. Lichens are ecologically important obligate symbioses between fungi and photosynthetic organisms. The primary photosynthetic partner in these symbioses can include CCM-presenting cyanobacteria (as carboxysomes), CCM-presenting green algae (as pyrenoids) or green algae lacking any CCM. We use an extensive dataset of lichen communities from eastern North America, spanning a wide climatic range, to test the importance of CCMs as predictors of lichen ecology and distribution. We show that the presence or absence of CCMs leads to opposite responses to temperature and precipitation in green algal lichens, and different responses in cyanobacterial lichens. These responses contrast with our understanding of lichen physiology, whereby CCMs mitigate carbon limitation by water saturation at the cost of efficient use of vapor hydration. This study demonstrates that CCM status is a key functional trait in obligate lichen symbioses, equivalent in importance to its role in vascular plants, and central for studying present and future climate responses.

Effects of vapor hydration and radiation on the leaching behavior of nuclear glass

During the long-term disposal of high-level nuclear waste glass in the geological disposal facility, the nuclear glass could be exposed to an unsaturated vapor environment prior to the direct contact with groundwater. Additionally, glass will experience damages due to self-irradiation. In this study, experiments were performed to investigate the chemical durability of glass after being hydrated and irradiated. This study proposes an attempt to investigate the impact of ionizing effect by applying alpha or gamma radiations to the pre-hydrated glass. The impact on chemical durability is evaluated by leaching the hydrated and irradiated glass samples. With the studied doses, 8 Giga Gy of alpha radiation or 100 kGy of gamma radiation, no significant modification of chemical durability is observed upon irradiation. On the other hand, leaching of pre-hydrated glass shows the increase of the initial release of Si, which is considered to be related to the dissolution of secondary phases. Furthermore, the total dissolution of hydrated layer is observed, which would result in the release of radionuclides at the same rate as Si does and thus needs further investigation.

Hydrogen isotope behavior during rhyolite glass hydration under hydrothermal conditions

The diffusion of molecular water (H2Om) from the environment into volcanic glass can hydrate the glass up to several wt% at low temperature over long timescales. During this process, the water imprints its hydrogen isotope composition (δDH2O) to the glass (δDgl) offset by a glass-H2O fractionation factor (ΔDgl-H2O = δDgl – δDH2O) which is approximately −33‰ at Earth surface temperatures. Glasses hydrate much more rapidly at higher, sub-magmatic temperatures as they interact with H2O during eruption, transport, and emplacement. To aid in the interpretation of δDgl in natural samples, we present hydrogen isotope results from vapor hydration experiments conducted at 175–375 °C for durations of hours to months using natural volcanic glasses. The results can be divided into two thermal regimes: above 250 °C and below 250 °C. Lower temperature experiments yield raw ΔDgl-H2O values in the range of −33 ± 11‰. Experiments at 225 °C using both positive and negative initial ΔDgl-H2O values converge on this range of values, suggesting this range represents the approximate equilibrium fractionation for H isotopes between glass and H2O vapor (103lnαgl-H2O) below 250 °C. Variation in ΔDgl-H2O (−33 ± 11‰) between different experiments and glasses may arise from incomplete hydration, analytical uncertainty, differences in glass chemistry, and/or subordinate kinetic isotope effects. Experiments above 250 °C yield unexpectedly low δDgl values with ΔDgl-H2O values of ≤–85‰. While alteration alone is incapable of explaining the data, these run products have more extensive surface alteration and are not interpreted to reflect equilibrium fractionation between glass and H2O vapor. Fourier transform infrared spectroscopy (FTIR) shows that glass can hydrate with as much as 5.9 wt% H2Om and 1.0 wt% hydroxl (OH−) in the highest P-T experiment at 375 °C and 21.1 MPa. Therefore, we employ a 1D isotope diffusion–reaction model of glass hydration to evaluate the roles of equilibrium fractionation, isotope diffusion, water speciation reactions internal to the glass, and changing boundary conditions (e.g. alteration and dissolution). At lower temperatures, the best fitting model results to experimental data for low silica rhyolite (LSR) glasses require only an equilibrium fractionation factor and yield 103lnαgl-H2O values of −33‰ ± 5‰ and −25‰ ± 5‰ at 175 °C and 225 °C, respectively. At higher temperatures, ΔDgl-H2O is dominated by boundary layer effects during glass hydration and glass surface alteration. The modeled bulk δDgl value is highly responsive to changes in the δDgl boundary condition regardless of the magnitude of other kinetic effects. Observed glass dissolution and surficial secondary mineral formation are likely to impose a disequilibrium boundary layer that drives extreme δDgl fractionation with progressive glass hydration. These results indicate that the observed ΔDgl-H2O of ∼−33 ± 11‰ can be cautiously applied as an equilibrium 103lnαgl-H2O value to natural silicic glasses hydrated below 250 °C to identify hydration sources. This approximate ΔDgl-H2O may be applicable to even higher temperature glasses hydrated on short timescales (of seconds to minutes) in phreatomagmatic or submarine eruptions before H2O in the glass is primarily affected by boundary layer effects associated with alteration on the glass surface.

Synergistic Stimulation of Metal-Organic Frameworks for Stable Super-cooled Liquid and Quenched Glass.

Metal-organic framework (MOF) glasses are a fascinating new class of materials, yet their prosperity has been impeded by the scarcity of known examples and limited vitrification methods. In the work described in this report, we applied synergistic stimuli of vapor hydration and thermal dehydration to introduce structural disorders in interpenetrated dia-net MOF, which facilitate the formation of stable super-cooled liquid and quenched glass. The material after stimulus has a glass transition temperature (Tg) of 560 K, far below the decomposition temperature of 695 K. When heated, the perturbed MOF enters a super-cooled liquid phase that is stable for a long period of time (>104 s), across a broad temperature range (26 K), and has a large fragility index of 83. Quenching the super-cooled liquid gives rise to porous MOF glass with maintained framework connectivity, confirmed by EXAFS and PDF analysis. This method provides a fundamentally new route to obtain glassy materials from MOFs that cannot be melted without causing decomposition.

Impact of initial states on the vapor hydration of iodine-bearing borosilicate glass

Under the context of long-term geological disposal of nuclear waste glass, the present work investigates the effect of initial states on the glass chemical durability in the vapor phase and during subsequent leaching. Glass with different finishing steps of polishing and thermal annealing were prepared, vapor-hydrated, and aqueous-altered. Multiple characterizations of mechanical properties and structural modifications were performed to understand the reduction of vapor hydration rates with annealed glasses. Discussions on the degradation of mechanical properties provide insights to understanding the variations of glass chemical durability, which shows a feasible approach to link mechanical and chemical studies together. By looking into the release of boron in the vapor phase, similarities between glass vapor hydration and glass aqueous alteration at residual rate are noticed, which allow the proposition of mechanisms of boron release. Characterizations of the hydrated-then-altered glass reveal the transport-limiting property of the vapor hydrated layer.

Impact of Initial States on the Vapor Hydration of Iodine-Bearing Borosilicate Glass

Impact of Initial States on the Vapor Hydration of Iodine-Bearing Borosilicate Glass

Polymorphic transitions of V-type amylose upon hydration and dehydration

In the current study, V6, V7 and V8 amyloses were prepared, their crystalline polymorphic transitions upon hydration at different moisture conditions examined by wide-angle X-ray diffraction (XRD), and the relative degrees of crystallinity calculated. Upon the formation of V7 or V8 crystals, the XRD peaks shifted to lower Bragg angles as compared to V6h due to the expansion of the amylose helices. Conditioning at 75% equilibrium relative humidity (ERH) transformed V6a and V7 amyloses to the V6h pattern. Long-term vapor hydration at 100% ERH or total hydration by immersion in water transformed all V-type XRD patterns to the B-type with decreased degrees of crystallinity. Upon dehydration by ethanol wash, the relative crystallinities of all V-type amyloses were increased, while the crystalline patterns remained mostly unchanged. Such explicit study of polymorphic transitions revealed by XRD can help establish optimal conditions for the encapsulation of bioactive compounds with V-type amyloses and provide insights for novel food product developments.

Impacts of constraints and uncertainties on projected amount of Hanford low-activity waste glasses

A systematic evaluation of Hanford low-activity waste (LAW) loading in alkali-aluminoborosilicate glasses was performed. A set of 22 waste compositions were selected to represent the range of wastes anticipated across the Hanford mission. Glass composition was formulated for each waste to simultaneously satisfy a full cadre of property and composition constraints while maximizing waste loading. Glass properties were estimated using property models recently developed for Hanford LAW Facility operation. The impacts from the property and composition constraints on the glass mass to be produced were calculated and compared. Electrical conductivity, Vapor Hydration Test and SO3 melter tolerance, and canister-centerline cooling crystal constraints have a large effect on reducing total glass mass, while the impacts from Product Consistency Test, viscosity, and isothermal crystal constraints are relatively small. Additionally, effects of model prediction and process/composition uncertainties were quantified. The results identified areas of future research that could be useful to improve the efficiency of plant operation.

Applying laboratory methods for durability assessment of vitrified material to archaeological samples

Laboratory testing used to assess the long-term chemical durability of nuclear waste forms may not be applicable to disposal because the accelerated conditions may not represent disposal conditions. To address this, we examine the corrosion of vitrified archeological materials excavated from the near surface of a ~1500-year old Iron Age Swedish hillfort, Broborg, as an analog for the disposal of vitrified nuclear waste. We compare characterized site samples with corrosion characteristics generated by standard laboratory durability test methods including the product consistency test (PCT), the vapor hydration test (VHT), and the EPA Method 1313 test. 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. This work provides improved understanding of long-term glass corrosion behavior in terms of the thickness, morphology, and chemistry of the surficial features that are formed.

Investigation on boron and iodine behavior during nuclear glass vapor hydration

Understanding the durability of nuclear glass in the vapor phase is essential for evaluating the long-term safety of the nuclear glass. This work studies the vapor hydration of borosilicate and iodine-bearing glass at 90 °C and relative humidity at 99%. Hydration kinetics was monitored by analyzing the boron and iodine released during the vapor hydration process. It is found that boron becomes more easily released after iodine being added to the glass matrix. The results related to boron release and retention indicate the possible transformation of tetrahedral [4]B to trigonal [3]B on the hydrated glass surface, which may enhance the release of boron in the form of boric acid afterward. Release of iodine occurred but at a far lesser extent than boron with 92% of iodine retained in the alteration layer, thus opening good perspectives with regard to the disposal of high-level waste containing iodine.

Preparation and characterization of multiphase ceramic designer waste forms

The long-term performance, or resistance to elemental release, is the defining characteristic of a nuclear waste form. In the case of multiphase ceramic waste forms, correlating the long-term performance of multiphase ceramic waste forms in the environment to accelerated chemical durability testing in the laboratory is non-trivial owing to their complex microstructures. The fabrication method, which in turn affects the microstructure, is further compounding when comparing multiphase ceramic waste forms. In this work, we propose a “designer waste form” prepared via spark plasma sintering to limit interaction between phases and grain growth during consolidation, leading to monolithic high-density waste forms, which can be used as reference materials for comparing the chemical durability of multiphase waste forms. Designer waste forms containing varying amounts of hollandite in the presence of zirconolite and pyrochlore in a fixed ratio were synthesized. The product consistency test (PCT) and vapor hydration test (VHT) were used to assess the leaching behavior. Samples were unaffected by the VHT after 1500 h. As measured by the PCT, the fractional Cs release decreased as the amount of hollandite increased. Elemental release from the zirconolite and pyrochlore phases did not appear to significantly contribute to the elemental release from the hollandite phase in the designer waste forms.

Influence of temperature and relative humidity on vapor hydration of an AVM nuclear waste glass

The safety assessment of the geological disposal of nuclear waste glasses requires a thorough understanding of the vapor hydration mechanisms of these glasses. In this context, the effect of temperature and relative humidity on vapor hydration of the AVM6 glass, which is the most reactive among the studied French AVM glasses, was investigated in the present study. Polished glass monoliths were vapor hydrated for 1 year (i) at 97% relative humidity (RH), at temperatures 70°C and 90°C to study the effect of temperature and (ii) at 50°C and 50% RH, 76% RH, 88% RH and 97% RH to study the effect of RH. The altered glass samples were characterized by SEM, XRD and ToF-SIMS. The results were compared with published results of AVM6 glass vapor hydrated at 50°C and 95% RH. The nature of the secondary phases that precipitate varied with both temperature and relative humidity, leading to variation in the behavior of elements in the altered layer (gel) but the altered layer morphology is similar at all temperatures studied. The Arrhenius law for temperature dependence is respected between the temperature range 50–90°C and the activation energy suggests that network-hydrolysis is the predominant rate controlling mechanism at these temperatures and duration. The secondary phases that incorporate transition metals and rare-earth elements are identified by XRD only in experiments conducted at lower RH values.

Water/Ethanol and 13X Zeolite Pairs for Long-Term Thermal Energy Storage at Ambient Pressure

Thermal energy storage is a key technology to increase the global energy share of renewables - by matching energy availability and demand - and to improve the fuel economy of energy systems - by recovery and reutilization of waste heat. In particular, the negligible heat losses of sorption technologies during the storing period make them ideal for applications where long-term storage is required. Current technologies are typically based on the sorption of vapour sorbates on solid sorbents, requiring cumbersome reactors and components operating at below ambient pressure. In this work, we report the experimental characterization of working pairs made of various liquid sorbates (distilled water, ethanol and their mixture) and a 13X zeolite sorbent at ambient pressure. The sorbent hydration by liquid sorbates shows lower heat storage performance than vapour hydration; yet, it provides similar heat storage density to that obtainable by latent heat storage (40-50 kWh/m^3) at comparable costs, robustness and simplicity of the system, while gaining the long-term storage capabilities of sorption-based technologies. As a representative application example of long-term storage, we verify the feasibility of a sorption heat storage system with liquid sorbate, which could be used to improve the cold-start of stand-by generators driven by internal combustion engines. This example shows that liquid hydration may be adopted as a simple and low-cost alternative to more efficient - yet more expensive - techniques for long-term energy storage.

Influence of composition of nuclear waste glasses on vapor phase hydration

For the first time, the influence of glass composition on the vapor hydration kinetics of the French AVM nuclear waste glass simulants was investigated. Three complex borosilicate glasses (>20 oxides) along with three simplified Na/(Ca, Na)/(Mg, Na)-alumino-borosilicate glasses with four or five oxides were altered at 50 °C in water vapor (95% RH) for up to 557 days. The solid characterization of the altered samples (by SEM, TEM, XRD, ToF-SIMS, SAXS) revealed that the rate-controlling vapor hydration mechanism is composition dependent. The vapor hydration rate of the more durable glasses, whose molar ratio of Al2O3/(CaO or MgO) is ≥ 1, seems to be limited by network-hydrolysis, whereas the overall glass alteration rate of the less durable glasses, whose Al2O3/MgO ratio is < 1, seems to be driven by the precipitation of Mg-rich secondary phases. All the vapor hydrated glasses show the presence of a homogeneous gel layer of a few tens of nm thickness. The more durable glasses have a smaller quantity of secondary phases precipitated on their surface. The less durable glasses have a relatively significant quantity of secondary phases precipitated above the gel layer and irregularly shaped, scattered and highly porous altered zones formed beneath the gel layer. The overall alteration rate of the less durable glasses was 10–20 times faster than the more durable glasses. For all glasses, FTIR spectroscopy indicated an inflexion in the vapor hydration rate after 120–200 days of alteration, likely due to a passivating effect of the altered layer formed under unsaturated conditions. The average pore size of the altered layer in certain vapor hydrated glasses measured by SAXS is similar to the pore sizes of the gel layer formed in aqueous medium and varies very slightly with glass composition.

Iron phosphate glass waste forms for vitrifying Hanford AZ102 Low Activity Waste (LAW), part II: Property-composition model

Mathematical models for the chemical durability–composition relation for 5-component iron phosphate glasses, containing a nuclear waste similar to that of the high sulfate (~17 wt%), high soda (~80 wt%) Hanford AZ 102 LAW, have been developed using statistical analysis. The dissolution rate (DR) in deionized water at 90 °C was used as a representative property for the chemical durability. These waste forms contained P2O5, Fe2O3, Al2O3, Na2O and SO3. This 5-component system was used previously to develop mathematical models for predicting the dependence of glass formation on composition. The results from this study can potentially be used to develop other properties-compositions models for iron phosphate glasses containing nuclear wastes.Two separate statistical models, based on Backward Elimination and Stepwise procedures, were developed and both models produced similar results. The DR-values calculated from both models were in good agreement with the experimentally measured values. The DR-composition models developed for the present iron phosphate glass waste forms (containing Hanford AZ102 LAW), are shown to be valid for predicting DR-values for similar waste forms containing other types of nuclear wastes as well. The chemical durability for a few selected glasses whose compositions were generated by the models was also measured by the Vapor Hydration Test (VHT). The model predicted DR-values qualitatively confirmed the measured VHT-values, which provides further support for validating the model.