Product Consistency Test Research Articles

Effect of V2O5 on crystallization tendency and chemical durability of Mo-bearing aluminoborosilicate glass

AbstactThe effect of V2O5 addition on molybdates crystallization tendency, glass structure and chemical durability of aluminoborosilicate glass belonging to SiO2-B2O3-CaO-Na2O-Al2O3-MoO3 system has been studied. The results confirm that V2O5 addition can effectively suppress the crystallization tendency of powellite and enhance the molybdenum solubility in the glass. The MoO3 solubility limit is found to be 2.8 mol% in the V2O5-containing aluminoborosilicate glass. Raman results reveal that V2O5 addition seems to modify the local structural environment of isolated MoO4 units and increase their chemical disorder in the glass, which is favored for molybdenum incorporation in the glass. The molar volume and glass transition temperature of samples are found to depend on V2O5 content. Product consistency test (PCT) results show that the normalized leaching rates of the V2O5-containing aluminoborosilicate glass maintain at a fairly low level compared with standard borosilicate glassy waste form.

Synthesis and characterization of zirconolite-sodium borosilicate glass-ceramics for nuclear waste immobilization

Zirconolite-sodium borosilicate glass-ceramics were successfully prepared via slow-cooling methods, and the crystallization, microstructure and aqueous durability have been investigated with powder X-ray diffraction (XRD), backscattered scanning electron microscopy and energy dispersive spectroscopy (BSE-EDS), Raman spectroscopy and ASTM Product Consistency Test leaching method. The results show that the main crystalline phase in the prepared glass-ceramics is strip-shaped zirconolite phase and the quantitative fraction of the zirconolite phase is about 30 wt%, the chemical composition of zirconolite crystals grown from a glass matrix is determined by Rietveld refinement to be Ca0.93Zr0.76Ce0.31Ti1.95Al0.05O7 and 84.53% Ce are incorporated in zirconolite crystals. Moreover, the aqueous durability test shows the low normalized leaching rates of Si (LRSi), Ca (LRCa) and Ce (LRCe) of the glass-ceramics, and LRSi, LRCa and LRCe are about 4 × 10−4, 1 × 10−4 and 8 × 10−7 g m−2 d−1, respectively after 56 leaching days. The obtained conclusions provide useful information for the immobilization of high-level radioactive wastes by using borosilicate glass-ceramic as potential matrix through one-step method.

Non-volatile immobilization of iodine by the cold-sintering of iodosodalite

A cold-sintering process at a very low temperature (300 °C) achieved stable immobilization of simulated radioiodine waste incorporated in sodalite (iodosodalite). The reported sintering temperature was much lower than conventional ceramic waste form processing temperatures (600−1100 °C) and had no effect on the stability of the loaded iodine waste. Excellent iodine retention (>93%) with relative sintered density 91% were achieved by the cold-sintering at 300 °C, respectively. The sintered body exhibited a micro-hardness value of 3.9 ± 0.1 GPa and compressive strength of 198 ± 11 MPa. The seven-day product consistency test found iodine leaching rates on order of the magnitude 10-4 g/m2⋅d. These results are the first example of the low temperature consolidation of iodine-bearing sodalite without using any additional material (e.g. glass, cement, etc.). High retention of the loaded simulated radioiodine without volatilization warrants the cold-sintering process for the environmental-friendly disposal of radioiodine.

Chemical behavior of uranium contaminated soil solidified by microwave sintering

For investigating the chemical behavior of uranium-contaminated soil solidified by microwave sintering, the product consistency test method was adopted. The effective factors such as soil species, soil composition, temperature, pH values and their coupling effects have been researched. The maximum leaching rate of uranium was below 1.7 × 10−8 g m−2 day−1 and the cumulative fraction of leached uranium was below 0.0165 after 42 days. The acidic or alkaline leachate and higher temperature would enhance the uranium’s leaching rate, and it was found that the chemical stability of solidified soil is related to the molar ratio of Al2O3/SiO2.

Evaluation of thermal stability in deep geological repository and nuclear criticality safety of spent nuclear fuel vitrified in iron phosphate glass

Iron phosphate glasses with low melting temperatures of ∼1300 °C were developed to immobilize spent nuclear fuels (SNFs). The glasses have densities of ∼3.15 g/cm3 and glass transition temperatures of ∼540 °C that are high enough to endure the temperatures in geological repositories. The waste loading of CeO2 in the glass was ∼21.92 wt%, which corresponds to ∼30.38 wt% of UO2 at the same molar ratio. Normalized elemental releases from the product consistency test were well below the regulated limit of 2 g/m2. Effective neutron multiplication factor keff was 0.755, which is below nuclear criticality safety regulation (keff = 0.95). Vitrified SNF occupies one-eleventh of the repository space compared to direct disposal of the same amount of UO2, excluding left-over low-level wastes.

Preparation and characterization of powellite ceramics Ca1-xLix/2Cex/2MoO4 (0 ≤ x ≤ 1) for Mo-rich HLW condition

Powellite ceramic represent one candidate to immobilize minor actinides and Mo from reprocessed UMo nuclear fuel. In this work, the Ca1-xLix/2Cex/2MoO4 (0 ≤ x ≤ 1) series is prepared via a solid-state reaction using Ce3+ as trivalent minor actinide (Am3+) surrogate, with structure/microstructure characterized by XRD, XPS, HRTEM, and SEM as well. The Ca1-xLix/2Cex/2MoO4 (0 ≤ x ≤ 1) compositions crystallize in tetragonal and are isostructural with scheelite. Rietveld refinements show that Ce3+ and Li+ simultaneously enter into the eight-fold coordinated Ca site of powellite crystal. The chemical durability of powellite phases is evaluated by the ASTM C1285-14 product consistency test method B. The leaching behaviours of Ce and Mo are accordance with the interfacial dissolution-reprecipitation mechanism. For all the Ca1-xLix/2Cex/2MoO4 (0 ≤ x ≤ 1) ceramics, 7-days NLCe and NLMo are found to be in the order of 10−3-10−5 g·m−2 and 10−2-10−4 g·m−2 respectively, which exhibit the great retention of Ce and Mo. Interestingly, the values of 7-days NLCe and NLMo are predominantly controlled by the distortion of MoO4 tetrahedra and disordered arrangements of Ce3+ and Li+. Thus, our initial understanding on the structure and chemical durability relation will provide insight to design new waste forms for the Mo-rich HLW condition.

Sintering of metakaolin‐based Na/Ca‐geopolymers and their immobilization of Cs

AbstractMetakaolin‐based Na/Ca‐geopolymers with a designed composition close to feldspar were used as precursors of Cs immobilization form materials. The sintering behaviors of geopolymers and their sintered materials' immobilization of Cs ions were investigated; the results indicated that the major phases of sintered geopolymers are plagioclase feldspars and feldspathoid. With the addition of Cs to the geopolymers, Cs‐leucite phase and caesium silicates were formed at 1150°C and 1170°C, respectively. It was found that Cs addition can slightly decrease the sintering temperature of geopolymers and make the grains finer. In addition, in order to reduce the volatilization of Cs ions during sintering, sintering temperatures of Cs‐geopolymer were further decreased to 850°C by introducing B2O3. After the geopolymer and Cs‐geopolymer were sintered using a low‐temperature liquid‐phase process, they remained structurally plagioclase feldspars and feldspathoid. Leaching on Product Consistency Test indicates that the leaching concentrations of Cs‐geopolymer samples sintered at higher temperatures are lower than those of samples sintered at lower temperatures. Thus, the increased volatilization of Cs ions at the higher temperatures and the formation of Cs‐leucite phase and caesium silicates can lead to the decrease of leaching concentrations in leachates. Therefore, low sintering temperatures and the fabrication of Cs crystalline ceramics are the key factors toward improving the immobilization of Cs ions.

Structure features, crystallization kinetics and water resistance of borosilicate glasses doped with CeO2

Effect of Ce, as a surrogate for actinide elements, on structure, crystallization kinetics, glass transition kinetics and water resistance of the studied borosilicate glass has been investigated by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and Product Consistency Test (PCT). The results show that the addition of CeO2 induces an increase in the concentrations of non-bridging oxygens (NBOs) and a decrease of B-O-B bonds in [BO3] units, and promotes the conversion of [BO3] units into [BO4] units. Moreover, in the glass region, the glass transition activation energy (Eg) and crystallization activation energy (Ec) of the borosilicate glass decrease with the content of CeO2 and surface crystallization is the main crystallization process for the CeO2 doped borosilicate glass while annealing. The formed glass waste form containing 15 wt% CeO2 shows good stability and water resistance. The normalize leaching rates of the main elements B and Si (LRB and LRSi) and actinide surrogate Ce (LRCe) in the glass waste form keeps in the order of magnitude of 10−3 g·m−2·d−1 and 10−8 g·m−2·d−1 respectively in 28 leaching days. The obtained conclusions provide researchers with useful information for the disposal of high-level radioactive wastes which are rich in Ce or actinide elements using borosilicate glass as potential baseline glass.

Formulation of high-Al2O3 waste glasses from projected Hanford waste compositions

High alumina high-level waste (HLW) is stored at the Hanford Site. This waste will be vitrified into borosilicate glasses at the Hanford Tank Waste Treatment and Immobilization Plant (WTP). The efficiency of the WTP operation will be strongly influenced by the loading of high Al2O3 wastes in HLW glass and the melter processing rate. A study was performed to formulate glasses with high loading of Al2O3-rich HLW and demonstrate their processability. Glasses with Al2O3 concentrations ranging from 30 to 35 mass% were designed and tested. One example glass with Al2O3 loading of 34 mass% was processed in a small-scale continuous melter. These compositions showed promising properties and good processability, suggesting that loading Hanford HLW glasses with excess of 30 mass% Al2O3 can be successfully produced at the WTP. The ability of existing glass property-composition models to predict the properties of HLW glasses with high Al2O3 content was evaluated. Viscosity and electrical conductivity model predictions appeared reasonably close to measured values, while crystallization and product consistency test model predictions were found to be significantly different from measured values.

Reactive spark plasma sintering of Cs-exchanged chabazite: characterisation and durability assessment for Fukushima Daiichi NPP clean-up

ABSTRACT Ion-specific media (ISM) have played an integral role in the clean-up and remediation efforts at the Fukushima Dai-ichi disaster site, through the processing of contaminated wastewaters. The use of these materials generates a secondary nuclear waste stream, presenting its own series of engineering problems arising from stringent handling and long-term storage requirements. A reactive spark plasma sintering (SPS) method was investigated for conditioning of the spent cesium exchanged zeolite, chabazite. A natural form of the zeolite was used as an analogue to the engineered ISM used at the Fukushima NPP site. Simulant wasteforms were sintered using different temperature and pressure parameters followed by analysis of phase assemblage, density, and durability (using the product consistency test (PCT)). The results indicated that zeolite structure had collapsed completely, with the exchanged cesium partitioned primarily into a durable feldspar to assure stability of the sintered material for passively safe storage or geological disposal.

Kinetics of oxyapatite [Ca2Nd8(SiO4)6O2] and powellite [(Ca,Sr,Ba)MoO4] dissolution in glass-ceramic nuclear waste forms in acidic, neutral, and alkaline conditions

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.

Cold sintering and durability of iodate-substituted calcium hydroxyapatite (IO-HAp) for the immobilization of radioiodine

Cold sintering of iodine-hosted calcium hydroxyapatite was investigated for the development of a durable matrix for radioiodine immobilization. Single-phase, nano-crystalline, iodate-substituted calcium hydroxyapatite (IO-HAp) was synthesized by a wet precipitation method and sintering of the dried IO-HAp powder containing ∼7 wt% of substituted iodine was carried out at 200 °C under a uniaxial pressure of 500 MPa. It was demonstrated that a sintered relative density of 96.8% can be achieved without affecting the iodate nature of the substituted iodine. A product consistency test of the sintered samples was also carried out under a standard condition. The normalized leaching rates of Ca, P and I after seven days were 6.9 (±0.5) × 10−7, 2.6 (±0.2) × 10−7 and 2.4 (±0.4) × 10−5 g/m2/d, respectively, providing evidence of the durability of the cold sintered matrix and promise of using the cold sintering process for the conditioning of volatile element-bearing radioactive waste into solid waste forms.

Adsorption and immobilization of radioactive ionic-corrosion-products using magnetic hydroxyapatite and cold-sintering for nuclear waste management applications

A simple and efficient method for the adsorption and immobilization of the radioactive ionic-corrosion-products Co2+, Cr3+, Mn2+, Fe2+, Ni2+, Cu2+ and Zn2+ generated in nuclear reactor coolant has been developed using a magnetic hydroxyapatite nanocomposite (MA-HAP) and a cold-sintering technique. The structure, morphology, magnetic properties and zeta potential of synthesized MA-HAP adsorbent were studied to evaluate its suitability for cationic uptake. The incorporated magnetic nanoparticles significantly improved the Co2+ adsorption capacity of MA-HAP to 68.95 mg/g under optimized conditions such as a pH of 6, a contact time of 120 min and an adsorbent dosage of 4 g/L. More than 92% of Co2+ was removed from the simulated aqueous solution in the presence of other ionic-corrosion-products. Densification of the nanocomposite loaded with the corrosion product was carried out using a pressure-assisted cold-sintering technique at 200 °C for 10 min. The sintered waste form showed high relative density (>95%) and hardness (>2.5 GPa). The results of a product consistency test indicated a low normalized leaching rate in the range 10−6 to 10−7 g/m2/day for all ions adsorbed by the MA-HAP. Thus, the material and methods introduced here are highly capable of adsorbing and immobilizing radioactive waste.

Non-linear effects of alumina concentration on Product Consistency Test response of waste glasses

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 (xAl2O3 ≤ 0.035) the effect of Al2O3 additions is highly negative (dlnNLαdxAl2O3 = −72). At intermediate concentrations (0.035 <xAl2O3 ≤ 0.19) the effect of Al2O3 additions is moderately negative (dlnNLαdxAl2O3 = −11). At high concentrations (0.19 <xAl2O3) the effect of Al2O3 additions is highly positive (dlnNLαdxAl2O3 = +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.

Research on chemical durability of iron phosphate glass wasteforms vitrifying SrF2 and CeF3

A part of the radioactive waste produced in the Thorium Molten Salt Reactor (TMSR) will be present as fluorides. Because of some characters of fluoride, it is undesirable for immobilizing the waste as other usual oxide waste in borosilicate glass that often produces immiscibility. However, iron phosphate glasses have been made with up to 30 wt% SrF2 and 20 wt% CeF3, and about 20 wt% SrF2 and CeF3 combined. The iron phosphate glass matrix mixed with fluoride waste could be melted at temperatures between 1050 °C and 1200 °C in only 30–60 min, its melting temperature is lower than that of borosilicate glass. Powder X-ray diffraction (XRD) showed that the glassy wasteforms that contains more than 20% wt% SrF2 and CeF3 mixture, or more than 20 wt% CeF3 would start crystallizing. The product consistency test (PCT) was used to identify the chemical durability of the glass. Glass powder (−100 to +200 mesh) after washing was soaked in 10 ml of deionized water at 90 °C for 7 days. The normalized elemental mass release of elements Sr, Ce and F were calculated at the level of 10−3 g/m2, through the concentration of element measured after PCT, that was equivalent to or better than that of borosilicate glass wasteforms. Iron phosphate glasses are concluded to be a practical alternative for vitrifying the radioactive waste of TMSR.

Glass-bonded iodosodalite waste form for immobilization of 129I

Immobilization of radioiodine is an important requirement for current and future nuclear fuel cycles. Iodosodalite [Na8(AlSiO4)6I2] was synthesized hydrothermally from metakaolin, NaI, and NaOH. Dried unwashed sodalite powders were used to synthesize glass-bonded iodosodalite waste forms (glass composite materials) by heating pressed pellets at 650, 750, or 850 °C with two types of sodium borosilicate glass binders. These heat-treated specimens were characterized with X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, energy dispersive spectroscopy, thermal analysis, porosity and density measurements, neutron activation analysis, and inductively-coupled plasma mass spectrometry. For the best waste form produced (pellets mixed with 10 mass% of glass binder and heat-treated at 750 °C), the maximum possible elemental iodine loading was 19.8 mass%, but only ∼8–9 mass% waste loading of iodine was retained in the waste form after thermal processing. Other pellets with higher iodine retention either contained higher porosity or were incompletely sintered. ASTM C1308 and C1285 (product consistency test, PCT) experiments were performed to understand chemical durability under diffusive and static conditions. The C1308 test resulted in significantly higher normalized loss compared to the C1285 test, most likely because of the strong effect of neutral pH solution renewal and prevention of ion saturation in solution. Both experiments indicated that release rates of Na and Si were higher than for Al and I, probably due to a poorly durable Na-Si-O phase from the glass bonding matrix or from initial sodalite synthesis; however the C1308 test result indicated that congruent dissolution of iodosodalite occurred. The average release rates of iodine obtained from C1308 were 0.17 and 1.29 g m-2 d-1 for 80 or 8 m−1, respectively, and the C1285 analysis gave a value of 2 × 10−5 g m-2 d-1, which is comparable to or better than the durability of other iodine waste forms.

Phase structure evolution and chemical durability studies of Gd1−xYbxPO4 ceramics for immobilization of minor actinides

Rare earth phosphates (REPO4) are considered as promising materials to immobilize the high-level radioactive waste. In the paper, Gd1−xYbxPO4 (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8 and 1.0) series were prepared via the solid-state reaction using Gd as the surrogate for minor actinide Am and Cm, and the structure, phase transformation and morphology were characterized by XRD, Raman, TEM and SEM. With the calcination temperature ranging from 1200 to 1500 °C, the coexisted phases of monazite- and xenotime-type structure are observed in Gd0.9Yb0.1PO4. The monophasic xenotime-type Gd0.9Yb0.1PO4 is obtained at 1600 °C. The monazite → xenotime transformation of Gd1−xYbxPO4 depends on the substitution of Gd3+ by Yb3+ ions. With the decrease in Gd3+ content, the gradual changes of FWHM and the hypsochromic shift of P-O symmetrical stretching vibration (Vs) are observed in the Raman spectra, indicating that the distortion of PO4 tetrahedra occurs during the phase transformation process. The chemical durability test is measured by the ASTM product consistency test leaching method, and the normalized mass loss (NLGd) of Gd and (NLYb) of Yb is extremely low and shown in the order of 10−6–10−7 g/m2 for all the ceramics.

The effects of crystallization and residual glass on the chemical durability of iron phosphate waste forms containing 40 wt% of a high MoO3 Collins-CLT waste

The effects of cooling rate on the chemical durability of iron phosphate waste forms containing up to 40 wt% of a high MoO3 Collins-CLT waste simulant were determined at 90 °C using the product consistency test (PCT). The waste form, designated 40wt%-5, meets appropriate Department of Energy (DOE) standards when rapidly quenched from the melt (as-cast) and after slow cooling following the CCC (canister centerline cooling)-protocol, although the quenched glass is more durable. The analysis of samples from the vapor hydration test (VHT) and the aqueous corrosion test (differential recession test) reveals that rare earth orthophosphate (monazite) and Zr-pyrophosphate crystals that form on cooling are more durable than the residual glass in the 40wt%-5 waste form. The residual glass in the CCC-treated samples has a greater average phosphate chain length and a lower Fe/P ratio, and those contribute to its faster corrosion kinetics.

Synthesis and characterization of cerium containing iron phosphate based glass-ceramics

The structure and properties of xCeO2-(100-x)(40Fe2O3-60P2O5), where x = 0, 2, 4, 6 and 8 mol%, glass-ceramics prepared by melting and slow cooling method have been investigated by using X-ray diffraction (XRD), scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), differential thermal analysis (DTA) and the Product Consistency Test (PCT). The results show that the 40Fe2O3-60P2O5 sample is homogeneously amorphous and the sample containing 2 mol% CeO2 has a small amount of FePO4 phase embedded. For the sample containing up to 4 mol% CeO2, monazite CePO4 and a small amount of FePO4 appear. Spectra analysis show that the structure networks of the glass-ceramics mainly consist of orthophosphate, along with pyrophosphate and a small amount of metaphosphate units. Moreover, the leaching rates of Fe and Ce are about 3.5 × 10−5 g m−2 d−1 and 5.0 × 10−5 g m−2 d−1 respectively after immersion in deionized water at 90 °C for 56 days, indicating their good chemical durability. The conclusions imply that the prepared method may be a promising process to immobilize nuclear waste into glass-ceramic matrix.

Immobilization of Cs-trapping ceramic filters within glass-ceramic waste forms

A facile and simple process is described for the long-term management of Cs trapping fly-ash based filters. Glass-ceramic waste forms were synthesized from Cs-trapping filters simulated by the reaction of fly ash and Cs at a high temperature. Structural and morphological characterizations revealed that Cs was strongly immobilized in pollucite crystals, as well as in the binding glassy matrices that accommodate pollucite particles. The leaching results obtained by a product consistency test (PCT) suggested that borosilicate glass would be better than soda-lime glass to immobilize Cs-containing waste. These findings will be the basis of the final waste forms of Cs captured in fly ash filters.