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Technetium-99 immobilization in low-temperature nuclear waste forms often relies on additives that reduce environmentally mobile pertechnetate (TcO4-) to insoluble Tc(IV) species. However, this is ...

Investigations of the crystallization of aluminosilicate phases within Hanford nuclear waste glasses typically involve subjecting samples to the canister centerline cooling (CCC) schedule. This cooling schedule is representative of the slowest cooling thermal profile which these glasses will experience after the glass is poured into the high level waste (HLW) container. However, few investigations have observed how the crystallization behavior changes by varying the heat treatment schedule. In the present study, three Hanford HLW glasses are subjected to CCC and isothermal heat treatments (IHT) to better understand the evolution of phases and the chemical partitioning due to temperature schedule. Samples were characterized using electron probe microanalysis, X-ray diffraction, micro X-ray fluorescence, and micro X-ray absorption spectroscopy. From IHT, eucryptite and apatite phases were observed which were not observed during CCC. Spatially-resolved measurements demonstrated that the oxidation state of the iron was similar among glass and crystal, and we suggest a mechanism to describe the compositional fluctuations near the crystal-glass interface which influence crystallization.

A cadmium germanium arsenide compound, Cd3Ge2As4, was synthesized using a double-containment fused quartz ampoule method within a rocking furnace and a melt-quench technique. The crystal structure was determined from single-crystal X-ray diffraction (SC-XRD), scanning and transmission electron microscopies (i.e. SEM, STEM, and TEM), and selected area diffraction (SAD) and confirmed with electron backscatter diffraction (EBSD). The chemistry was verified with electron energy loss spectroscopy (EELS).

International audience; A glass‐ceramic waste form is being developed for immobilization of waste streams of alkali (A), alkaline‐earth (AE), rare earth (RE), and transition metals generated by transuranic extraction for reprocessing of used nuclear fuel. Benefits over an alkali borosilicate waste form are realized by the partitioning of the fission product fraction insoluble in glass into a suite of chemically durable crystalline phases through controlled cooling, including (AE,A,RE)MoO4 (powellite) and (RE,A,AE)10Si6O26 (oxyapatite). In this study, a simplified 8‐oxide system (SiO2‐Nd2O3‐CaO‐Na2O‐B2O3‐Al2O3‐MoO3‐ZrO2) was melted, then soaked at various temperatures from 1450 to 1150°C, and subsequently quenched, in order to obtain snapshots into the phase distribution at these temperatures. For these samples, small angle X‐ray and neutron scattering, quantitative X‐ray diffraction, electron microscopy, 23Na nuclear magnetic resonance, Nd3+ visible absorption, and temperature‐dependent viscosity were characterized. In this composition, soak temperatures of urn:x-wiley:00027820:media:jace16406:jace16406-math-00011250°C were necessary to nucleate calcium molybdate (~10‐20 nm in diameter). Further cooling produced oxyapatite and total crystallization increased with lower soak temperatures. Both Na and Nd entered the crystalline phases with lower‐temperature soak conditions. Slow cooling or long isothermal treatments at ~975°C produced significantly higher crystal fractions.

Immobilization of chemically complex aqueous waste streams from used nuclear fuel reprocessing is achievable at higher waste loadings with glass ceramics as compared to borosilicate glasses. Additionally, crystalline phases with similar chemistry are more durable than their amorphous counterparts. However, during glass ceramic fabrication, mechanical stresses at crystal-glass interfaces, which are caused by thermal expansion mismatching during cooling, create locales where water is capable of accessing and reacting with the various phases in the glass ceramic, thus releasing radionuclides into the aqueous phase. In the present work, we build on previous chemical durability investigations of a glass-ceramic containing crystalline oxyapatite [Ca2Nd8(SiO4)6O2] and powellite [(Ca,Sr,Ba)MoO4] secondary phases. The individual crystalline and bulk glass phases have been fabricated separately and the corrosion behavior has been investigated with single-pass flow-through (SPFT) testing at 90 °C in buffered pH(RT) 4, 7, 9 and pH 11 solutions and with the static product consistency test (PCT). The results demonstrate the varying dissolution kinetics of the individual phases in the range of pH studies. The consequence of the varying dissolution kinetics are described with a conceptual model of glass-ceramic dissolution mechanisms.

This paper details synthesis methods that were used to fabricate phase-pure Nd3BSi2O10 using a LiCl-flux method. In this study, a variety of conditions were evaluated to find the optimal processing parameters for maximizing the Nd3BSi2O10 yield including the mass ratio of LiCl to target mass of Nd3BSi2O10 (i.e., 1:3, 1:2, 1:1, 2:1, and 6:1) as well as the soak temperature (i.e., 700, 800, or 900 °C) at the 1:1 ratio. It was found that the optimal ratio of LiCl:Nd3BSi2O10 was 1:1 and the optimal heat-treatment temperature was 900 °C (of those temperatures evaluated). Product made from the 1:1 material synthesized at 900 °C was recovered and fired at temperatures of 925–1350 °C. After 4-h firings at temperatures of 925–1100 °C, the Nd3BSi2O10 phase purity was found to be 100% with powder X-ray diffraction, but started to decompose at the longer firing times at 1100 °C and higher temperatures (1350 °C) at the 4-h dwell time. Thus, the lowest firing temperature for achieving a phase-pure pellet was 925 °C under the conditions studied here. The phase-pure sample was found to contain 28 vol% open porosity determined by Archimedes’ method. The durability of the Nd3BSi2O10 product was examined in flow-through conditions at 90 °C with contacting solution between pH90oC 4.1 and 10.1. The release rate of Nd from the material had a maximum at pH90oC 4.1 calculated to be 4.93 g m−2 d−1 and a minimum release rate of Nd at pH90oC 7.7 of

Nuclear wastes generated from reprocessing of used nuclear fuel tend to contain a large fraction of rare earth (RE, e.g., Nd3+), transition (TM, e.g., Mo6+, Zr4+), alkali (A, e.g., Cs+), and alkaline earth cations (AE, e.g., Ba2+, Sr2+). Various strategies have been considered for immobilizing such waste streams, varying from nominally crystal-free glass to glass-ceramic to multi-phase ceramic waste forms. For glass and glass-ceramic waste forms, the added glass-forming system is generally alkali-alkaline earth-aluminoborosilicate (i.e., Na-Ca-Al-B-Si oxide). In a US-UK collaborative project, summarized here, we investigated the glass structure and crystallization dependence on compositional changes in simulated nuclear waste glasses and glass-ceramics. Compositions ranged in complexity from five – to – eight oxides. Specifically, the roles of Mo and rare earths are investigated, since a proposed glass-ceramic waste form contains crystalline phases such as powellite [(AE,A,RE)MoO4] and oxyapatite [(RE,AE,A)10Si6O26], and the precipitation of molybdenum phases is known to be affected by the rare earth concentration in the glass. Additionally, the effects of other chemical additions have been systematically investigated, including Zr, Ru, P, and Ti. A series of studies were also undertaken to ascertain the effect of the RE size on glass structure and on partitioning to crystal phases, investigating similarities and differences in glasses containing single RE oxides of Sc, Y, La, Ce, Nd, Sm, Er, Yb, or Lu. Finally, the effect of charge compensation was investigated by considering not only the commonly assessed peralkaline glass but also metaluminous and peraluminous compositions. Glass structure and crystallization studies were conducted by spectroscopic methods (i.e., Raman, X-ray absorption, nuclear magnetic resonance (NMR), optical absorption, photoluminescence, photoluminescence excitation, X-ray photoelectron spectroscopy), microscopy (i.e., scanning electron microscopy, transmission electron microscopy, electron probe microanalysis), scattering (i.e., X-ray and neutron diffraction, small angle measurements), and physical characterization (i.e., differential thermal analysis, liquidus, viscosity, density). This paper will give an overview of the research program and some example unpublished results on glass-ceramic crystallization kinetics, microstructure, and Raman spectra, as well as some examples of the effects of rare earths on the absorption, luminescence, and NMR spectra of starting glasses. The formal collaboration described here has resulted in the generation of a large number of results, some of which are still in the process of being published as separate studies.

A glass-ceramic borosilicate waste form is being considered for immobilization of waste streams of alkali, alkaline-earth, lanthanide, and transition metals generated by transuranic extraction for reprocessing used nuclear fuel. Waste forms are created by partial crystallization on cooling, primarily of oxyapatite and powellite phases. In-situ neutron diffraction experiments were performed to obtain detailed information about crystallization upon cooling from 1200°C. The combination of high temperatures and reactivity of borosilicate glass with typical containers used in neutron experiments, such as vanadium and niobium, prevented their use here. Therefore, methods using sealed thick-walled silica ampoules were developed for the in-situ studies. Unexpectedly, high neutron absorption, low crystal fraction, and high silica container background made quantification difficult for these high temperature measurements. As a follow-up, proof-of-concept measurements were performed on different potential high-temperature container materials, emphasizing crystalline materials so that residual glass in the waste form sample could be more easily analyzed. Room temperature measurements were conducted with a pre-crystallized sample in ‘ideal’ containers stable at low temperatures (i.e., vanadium and thin-wall silica capillaries) and compared to the same measurements in containers stable at high temperatures (i.e, platinum, single crystal sapphire, and thick-walled silica ampoules). Results suggested that Pt is probably the best choice if suitably sealed to prevent contamination from the sample after neutron activation.

This paper details the results from two laboratory-scale (∼1 L) melter experiments where the processability was evaluated for vitrifying a simulated high-MoO3 raffinate waste from reprocessing of used commercial nuclear fuel. The target glass (CSLNTM-C-2.5) contained roughly 18 mass% waste loading (2.5 mass% MoO3). A homogeneous glass product was produced after process optimizations. Following the experiments, condensates present in the experimental apparatus were analyzed and found to be primarily composed of SiO2, Al2O3, Zr-O, rare earth, molybdate, and nitrate phases. The composition of the glassy product resulting from the second melter experiment was very similar to that of the target composition with variations noted for Al, Mo, and Zr.

The effect of alumina mole fraction on the 7-day Product Consistency Test (PCT) responses, ln[NLα], of simulated nuclear waste glasses was examined. It was found that the effect is highly non-linear. At low mole fractions of Al2O3 ( x A l 2 O 3 ≤ 0.035) the effect of Al2O3 additions is highly negative ( d ln N L α d x A l 2 O 3 = −72). At intermediate concentrations (0.035 x A l 2 O 3 ≤ 0.19) the effect of Al2O3 additions is moderately negative ( d ln N L α d x A l 2 O 3 = −11). At high concentrations (0.19 x A l 2 O 3 ) the effect of Al2O3 additions is highly positive ( d ln N L α d x A l 2 O 3 = +21). This variable impact of Al2O3 on ln[NLα] is speculated to be caused by the rates at which the glass corrosion process changes through various reaction regimes in static conditions at 90 °C. A model for prediction of ln[NLα] as a function of glass composition is presented. This model represents the data from 2669 glass compositions spanning a broad nuclear waste glass composition region and it is useful up to much higher Al2O3 concentrations than previous models (from ∼10 to 25 mol% Al2O3). The model was validated using data subset validation methods and shown to predict validation data in the same composition region with roughly equal certainty as the model fit data. The potential causes for non-linear effects of Al2O3 on ln[NLα] were discussed in context of the progression of glass corrosion rates. Time resolved static corrosion data is needed to better understand these non-linear effects.

This paper discusses the synthesis, characterization, and chemical durability assessment of oxyapatite [Ca2Nd8(SiO4)6O2] and mixed-alkaline-earth powellite [(Ca,Sr,Ba)MoO4]. These are the major crystalline phases that precipitate from the melt during cooling of a glass-ceramic waste form currently being evaluated for immobilizing wastes produced during the aqueous reprocessing of used nuclear fuel. The oxyapatite was made at 99.7% purity using a solution-based process followed by heat treatments. The powellite, made by melting carbonates and slow-cooling the melt, formed two different phases, one rich in Ca (74.7%) and Sr (19.4%) (balance is Ba) and the other rich in Sr (25.6%) and Ba (65.6%) (balance is Ca). Following static dissolution tests after 5 days, the oxyapatite phase had a maximum normalized loss of 0.024 g m−2 for Ca and the powellite phases showed a higher normalized loss from the Ba-powellite (0.22 g m−2) compared with the Sr-powellite (0.06 g m−2) and Ca-powellite (0.02 g m−2). Additionally, crystal structure data were measured using X-ray diffraction and are compared in detail with literature data of powellites and oxyapatites of similar chemistries.

Transition metal and rare earth (RE) elements are important fission products present in used nuclear fuel, which in high concentrations tend to precipitate crystalline phases in vitreous nuclear waste forms. Two phases of particular interest are powellite (CaMoO4) and oxyapatite (Ca2RE8(SiO4)6O2). The glass compositional dependencies controlling crystallization of these phases on cooling from the melt are poorly understood. In the present study, the effect of rare earth identity and modifier cation field strength on powellite and apatite crystallization were studied in a model MoO3-containing alkali/alkaline-earth aluminoborosilicate glass with focus on (1) influence of rare earth cation size (for RE3+: Ce, La, Nd, Sm, Er, Yb) and (2) influence of non-framework cations (RE3+, Mo6+, Na+, Ca2+). Quenched glasses and glass-ceramics (obtained by slow cooling) were characterized by X-ray diffraction (XRD), Raman spectroscopy, X-ray absorption spectroscopy (XAS), and electron probe microanalysis (EPMA). All samples were X-ray amorphous upon quenching, except the Ce-containing composition which crystallized ceria (CeO2), and the sample devoid of any rare earth cations which crystallized powellite. On heat treatment, powellite and oxyapatite crystallized in the majority of the samples, with the former crystallizing in the volume and the latter on the surface. The EPMA results confirmed a small concentration of boron in the oxyapatite crystal structure. RE cations were incorporated in the glass, as well as in powellite, oxyapatite, and in the case of Yb3+, keiviite (Yb2Si2O7). Raman spectroscopy showed that the primary vibration band for molybdate MoO42− in the glasses was strongly affected by the ionic field strength of the modifying cations (alkali, alkaline earth, and RE), suggesting their proximity to the MoO42− ions in the glass, though the Mo O bond length and coordination according to XAS suggested little local change.

The work presented here is based off the concept that the rare earth chloride (RECl3) fission products within the used electrorefiner (ER) salt can be selectively removed as RECl3 (not yet demonstrated) or precipitated out as a mixture of REOCl and REOx through oxygen sparging (has been demonstrated). This paper presents data showing the feasibility of immobilizing a mixture of RECl3s at 10 mass% into a 78%TeO2-22%PbO glass while also showing that this same mixture of RECl3s can be oxidized to REOCl at 300 °C and then to REOx by 1200 °C, evolving Cl2(g). When the REOx mixture is heated at temperatures >1200 °C, the ratios of REOxs change. The mixture of REOx was then immobilized in a lanthanide borosilicate (LABS) glass at a high loading of 60 mass%. Both the 78%TeO2-22%PbO glass and LABS glass systems show good chemical durability. The advantages and disadvantages of tellurite and LABS glasses are compared.

Liquidus temperature (TL) was measured for 38 simulated high-level waste borosilicate glasses covering a Hanford composition region, using optical microscopy and crystal-fraction extrapolation methods to analyze isothermally heat-treated specimens. The glasses encompassed a one-component-at-a-time variation of 16 components from a representative Hanford HLW simulant baseline composition. The TL values ranged from 1006 °C to 1603 °C. First-order models were fit to data to obtain component effects on TL (per 1 mass% additions) and then the components were grouped into three categories: TL-increasing components [i.e., Cr2O3 (264 °C), “Others” (minor components, 163 °C), oxides of noble metals (137 °C), NiO (91 °C), as well as Al2O3 and Fe2O3 (~ 19–21 °C)]; TL-decreasing components [i.e., K2O (−26 °C), Na2O (−41 °C), and Li2O (−68 °C)]; and those of little effect [i.e., MnO, P2O5, ZrO2, F, Bi2O3, SiO2, B2O3, and CaO (9 to −12 °C)]. Also presented are temperatures at which 1 vol% of spinel is at equilibrium with the melt (T1%) as these values are considered relevant to the Hanford Tank Waste Treatment and Immobilization Plant. The measured and estimated values are compared and contrasted and the effect of TL and T1% on glass formulation is discussed. The different methods for measuring TL are compared and contrasted.

The crystal structure of a lanthanide borosilicate, Nd3BSi2O10, has been determined from laboratory X-ray powder diffraction data. It is composed of [SiO4]4− and [BSiO6]5− anionic layers linked by Nd3+ cations between them., A lanthanide borosilicate, trineodymium borosilicate or Nd3BSi2O10, was synthesized using a flux method with LiCl, and its structure was determined from X-ray powder diffraction (XRD) and electron probe microanalysis (EPMA). The structure is composed of layers with [SiO4]4− and [BSiO6]5− anions alternating along the c axis linked by Nd3+ cations between them.

Glass-ceramic waste forms present a potentially viable technology for the long-term immobilization of liquid nuclear wastes arising from used nuclear fuel reprocessing. Through control of chemistry during fabrication, such waste forms can have designed secondary crystalline phases within a borosilicate glass matrix. In this work, a glass-ceramic containing crystalline powellite and oxyapatite secondary phases was tested for its corrosion properties in dilute conditions using single-pass flow-through testing. Three glass-ceramic samples were prepared using different cooling rates to produce samples with varying microstructure sizes. In testing at 90°C in buffered pH(RT) 7 and pH 9 solutions, it was found that increasing solution pH and decreasing microstructure size (resulting from rapid cooling during fabrication) both led to a reduction in overall corrosion rate, indexed by boron release from the glass matrix. On the other hand, the corrosion rate of crystalline phase decreased with a decrease in pH. The...

This paper discusses work to develop Na2O-B2O3-SiO2 glass binders for immobilizing LiCl-KCl eutectic salt waste in a glass-bonded sodalite waste form following electrochemical reprocessing of used metallic nuclear fuel. Here, five new glasses with ∼20 mass% Na2O were designed to generate waste forms with high sodalite. The glasses were then used to produce ceramic waste forms with a surrogate salt waste. The waste forms made using these new glasses were formulated to generate more sodalite than those made with previous baseline glasses for this type of waste. The coefficients of thermal expansion for the glass phase in the glass-bonded sodalite waste forms made with the new binder glasses were closer to the sodalite phase in the critical temperature region near and below the glass transition temperature than previous binder glasses used. These improvements should result in lower probability of cracking in the full-scale monolithic ceramic waste form, leading to better long-term chemical durability.

During the vitrification of high-level radioactive waste (HLW) in HLW melters in the Waste Treatment and Immobilization Plant (WTP) located in Washington State, spinel crystals [Fe, Ni, Mn, Zn]2+ [Fe, Cr]23+ O4 may precipitate from glass and accumulate in the melter riser, preventing the discharge of molten glass into canisters. Therefore, an effort is being made to develop an electrical conductivity method to monitor crystal buildup in the melter riser. A vertically configured electrical conductivity (EC) probe with an alumina shaft and Pt-10%Rh-electrodes was designed and tested in standard conductivity solutions and glass melts both with and without spinel crystals. The EC probe measured conductivity in conductivity solutions within 10% of their certified values and showed a linear relationship with increased spinel layer thicknesses. Testing in silicate glass containing spinel crystals allowed for the determination of spinel conductivity as a function of temperature. The conductivities of spinel crystals of 20 to 24 S/m at 800°C were in excellent agreement with the conductivity of trevorite (NiFe2O4) crystals at 800°C reported in the literature. The conductivities of spinel crystals and measured changes in the conductivity across the accumulated layer allowed for a successful measurement of spinel crystal accumulation in simulated HLW glass. This article is protected by copyright. All rights reserved.

The Hanford Tank Waste Treatment and Immobilization Plant will vitrify the mixed hazardous wastes generated from 45 years of plutonium production at the Hanford Site in Washington State. The molten glasses will be poured into stainless steel containers or canisters and subsequently cooled for storage and disposal. For appropriate facility design and operations to handle such highly energy-consuming processes, knowledge of the material properties is required. The thermal properties (heat capacity, thermal diffusivity, and thermal conductivity) of representative high-level and low-activity waste glasses were studied as functions of temperature in the range of 200°C-800°C (relevant to the cooling process). Simultaneous differential scanning calorimetry-thermal gravimetry (DSC-TGA), Xe-flash diffusivity, pycnometry, and dilatometry were implemented. The study showed that simultaneous DSC-TGA would be a reliable method for obtaining the heat capacity of various glasses in the temperature range of interest. Accurate thermal properties from this study were shown to provide a more realistic guideline for capacity and time constraints of the heat removal process when compared to the original conservative design-basis engineering estimates. The estimates, though useful for design in the absence of measured physical properties, can now be supplanted and the measured thermal properties can be used in design verification activities.

Borosilicate glass-ceramics are being developed to immobilize high-level waste generated by aqueous reprocessing into a stable waste form. The corrosion behavior of this multiphase waste form is expected to be complicated by multiple phases and crystal-glass interfaces. A modified single-pass flow-through test was performed on polished monolithic coupons at a neutral pH (25 °C) and 90 °C for 33 d. The measured glass corrosion rates by micro analysis in the samples ranged from 0.019 to 0.29 g m−2 d−1 at a flow rate per surface area = 1.73 × 10−6 m s−1. The crystal phases (oxyapatite and Ca-rich powellite) corroded below quantifiable rates, by micro analysis. While, Ba-rich powellite corroded considerably in O10 sample. The corrosion rates of C1 and its replicate C20 were elevated an order of magnitude by mechanical stresses at crystal-glass interface caused by thermal expansion mismatch during cooling and unique morphology (oxyapatite clustering).

A cadmium germanium arsenide compound, Cd3Ge2As4, was synthesized using a double-containment fused quartz ampoule method within a rocking furnace and a melt-quench technique. The crystal structure was determined from single-crystal X-ray diffraction, scanning and transmission electron microscopies, and selected area diffraction and confirmed with electron backscatter diffraction. The chemistry was verified with electron energy loss spectroscopy., A cadmium germanium arsenide compound, Cd3Ge2As4, was synthesized using a double-containment fused quartz ampoule method within a rocking furnace and a melt-quench technique. The crystal structure was determined from single-crystal X-ray diffraction (SC-XRD), scanning and transmission electron microscopies (i.e. SEM, STEM, and TEM), and selected area diffraction (SAD) and confirmed with electron backscatter diffraction (EBSD). The chemistry was verified with electron energy loss spectroscopy (EELS).

To prevent the release of radioiodine during the reprocessing of used nuclear fuel or in the management of other wastes, many technologies have been developed for iodine capture. The capture is only part of the challenge as a durable waste form is required to ensure safe disposal of the radioiodine. This work presents the first durability studies in dilute conditions of two AgI-containing waste forms: hot-isostatically pressed silver mordenite (AgZ) and spark plasma sintered silver-functionalized silica aerogel (SFA) iodine waste forms (IWF). Using the single-pass flow-through (SPFT) test method, the dissolution rates respective to Si, Al, Ag and I were measured for variants of the IWFs. By combining solution and solid analysis information on the corrosion mechanism neutral-to-alkaline conditions was elucidated. The AgZ samples were observed to have corrosion preferentially occur at secondary phases with higher Al and alkali content. These phases contained a lower proportion of I compared with the matrix. The SFA samples experienced a higher extent of corrosion at Si-rich particles, but an increased addition of Si to the waste led to an improvement in corrosion resistance. The dissolution rates for the IWF types are of similar magnitude to other Si-based waste form materials measured using SPFT.

It has been very difficult to use popular elemental imaging techniques to image Li and B distribution in glass samples with nanoscale resolution. In this study, time-of-flight secondary ion mass spectrometry, nanoscale secondary ion mass spectrometry, and atom probe tomography (APT) were used to image the distribution of Li and B in two representative glass samples, and their performance was comprehensively compared. APT can provide three-dimensional Li and B imaging with very high spatial resolution (≤2 nm). In addition, absolute quantification of Li and B is possible, although there remains room for improving accuracy. However, the major drawbacks of APT include poor sample compatibility and limited field of view (normally ≤100 × 100 × 500 nm3). Comparatively, time-of-flight secondary ion mass spectrometry and nanoscale secondary ion mass spectrometry are sample-friendly with flexible field of view (up to 500 × 500 µm2 and image stitching is feasible); however, lateral resolution is limited to only about 100 nm. Therefore, secondary ion mass spectrometry and APT can be regarded as complementary techniques for nanoscale imaging of Li and B in glass and other novel materials. Copyright © 2016 John Wiley & Sons, Ltd.

High-alumina high-level waste (HLW) glasses are prone to nepheline precipitation during canister-centerline cooling (CCC). If sufficient nepheline forms, the chemical durability of the glass will be significantly impacted. Overly conservative constraints have been developed and used to avoid the deleterious effects of nepheline formation in U.S. HLW glasses. The constraints used have been shown to significantly limit the loading of waste in glass at Hanford and therefore the cost and schedule of cleanup. A 90-glass study was performed to develop an improved understanding of the impacts of glass composition on the formation of nepheline during CCC. The CCC crystallinity data from these glasses were combined with 657 glasses found in the literature. The trends showed significant effects of Na2O, Al2O3, SiO2, B2O3, CaO, Li2O, and potentially K2O on the propensity for nepheline formation. A pseudo-ternary submixture model was proposed to identify the glass composition region prone to nepheline precipitation. This pseudo-ternary with axes of SiO2 + 1.98B2O3, Na2O + 0.653Li2O + 0.158CaO, and Al2O3 was found to divide glasses that precipitate nepheline during CCC from those that do not. Application of this constraint is anticipated to increase the loading of Hanford high-alumina HLWs in glass by roughly one-third.

Nepheline crystallizes upon slow-cooling in some melts concentrated in Na2O and Al2O3, which can result in a residual glass phase of low chemical durability. Nepheline can incorporate many components often found in high-level waste radioactive borosilicate glass, including glass network ions (e.g., Si, Al, Fe), alkali metals (e.g., Cs, K, Na, and possibly Li), alkaline-earth metals (e.g., Ba, Sr, Ca, Mg), and transition metals (e.g., Mn, and possibly Cr, Zn, Ni). When crystallized from melts of different compositions, nepheline composition varies as a function of starting melt composition. Five simulated high-level nuclear waste borosilicate glasses shown to crystallize large fractions of nepheline on slow-cooling were selected for study. These starting melt compositions contained a range of Al2O3, B2O3, CaO, Na2O, K2O, Fe2O3, and SiO2 concentrations. Compositional analyses of nepheline crystals in glass by electron probe micro-analysis (EPMA) indicate that nepheline is generally rich in silica, whereas boron is unlikely to be present in any significant concentration, if at all, in nepheline. Also, several models are presented for calculating the fraction of vacancies in the nepheline structure.

Chlorosodalite has the general form of Na8(AlSiO4)6Cl2 and this paper describes experiments conducted to synthesize sodalite with a solution-based approach to immobilize a simulated spent electrorefiner salt solution containing a mixture of alkali, alkaline earth, and lanthanide chlorides. The reactants used were the salt solution, NaAlO2, and either Si(OC2H5)4 or Ge(OC2H5)4. Additionally, seven different glass sintering aids (at loadings of 5 mass%) were evaluated as sintering aids for consolidating the as-made powders using a cold-press-and-sinter technique. This process of using alkoxide additives for the Group IV component can be used to produce large quantities of sodalite at near-room temperature as compared to a method where colloidal silica was used as the silica source. However, the small particle sizes inhibited densification during heat treatments.

This article presents a case study of developing an experimental design for a constrained mixture experiment when the experimental region is defined by single-component constraints (SCCs), linear m...

Iodosodalite powders were synthesized using aqueous (70 °C) and hydrothermal (180 °C) methods and then hot isostatically pressed (HIP), without or with (10 or 20 mass%) sodium borosilicate glass binders, at 900 °C and 175 MPa for 3 h to convert into glass-bonded iodosodalite waste forms. After HIPing, comparisons were made of the structures, compositions, morphologies, porosities, and chemical durabilities of the iodosodalite samples produced through aqueous and hydrothermal methods. X-ray diffraction patterns of HIPed samples showed that iodosodalite remained as the dominant phase. However, the iodosodalite phase concentrations decreased after HIPing, indicating iodosodalite decomposition (or amorphization) during the HIP process. Scanning electron microscopy and elemental mapping analyses on cross sections of HIPed samples showed homogeneous elemental distributions for aqueous-grown iodosodalite, whereas hydrothermally grown iodosodalite samples had greater heterogeneity and porosity. The densities of HIPed samples with more glass binder were generally higher than samples HIPed without binder. The leach tests on samples containing 20 mass% glass binder showed that the iodine dissolution rate of HIPed hydrothermally grown iodosodalite was about 4 × higher than iodosodalite grown with the aqueous method. This study provides alternative techniques for immobilizing iodine streams in iodosodalite waste forms.

Epsilon metal (e-metal) is the metallic phase that forms as inclusions at the grain boundaries in the UO2 fuel during reactor operation. This metal is composed of Pd, Mo, Rh, Ru, and Tc. These metallic inclusions are insoluble in strong acid and remnants of these metallic inclusions have been found in the UO2 matrix that remains from the natural reactors in Gabon that were active 1.8 billion years ago, therefore e-metal should be an excellent waste form for the immobilization of the long-lived isotopes 107Pd (6.5 × 106 a) and 99Tc (2.13 × 105 a), with 99Tc being the isotope of interest for repository performance. Therefore, the chemical durability of this potential waste form is assessed in this study. Typically, corrosion rates for metallic materials are measured electrochemically because they are quick, inexpensive, and can reveal the mechanism by which a metal corrodes, at least initially. However, in a repository the waste form would be subjected to slowly flowing water without an applied electrical potential over long time periods. Therefore, the corrosion rates of e-metal specimens were measured with both electrochemical tests and the single-pass flow-through test (SPFT). Potentiodynamic and potentiostatic polarization results suggest that a thin passive film exists on the alloy surface, which seems to be responsible for its high corrosion resistance. Additionally, X-Ray photoelectron spectroscopic results suggest that Pd oxides are significantly enriched in the passive film Results from the SPFT show that the dissolution rates were weakly dependent on pH. Only Mo and Re were found in solution and were used for the calculation of the dissolution rates. In general, the electrochemically determined corrosion rates agree reasonably well with the initial dissolution rate measured with the SPFT test, but they are about one or two orders of magnitude higher than the steady state rates. The causes for this discrepancy are discussed.

As the glass-water reaction proceeds, certain glass compositions undergo a delayed acceleration in the alteration rate – a phenomenon that is commonly referred to as Stage III behavior – which generally coincides with zeolite formation. However, studying Stage III in the laboratory is difficult because the time scale required to observe Stage III behavior can vary from months to several years, and the solution conditions that initiate this acceleration remain poorly understood. Consequently, the development of a corrosion test method capable of quantifying Stage III rates in a timely manner would be of great benefit to both the scientific community and policy makers concerned with nuclear waste disposal. In this work, we induced and measured the acceleration in the corrosion rate of the AFCI glass by seeding static corrosion tests with zeolite Na–P2 at imposed pH90°C values ranging from 9.5 to 11.5. The log10 induced Stage III rates increased linearly with increasing pH and were measured to be between the forward and residual rates of AFCI. Tests designed to compare induced Stage III rates obtained for different seeding times showed that the induced Stage III rates were faster in the absence of an alteration layer. In summary, the use of the seeded test method allows for a quantification of induced Stage III rates in a relatively short, one-month time period.