Glass production using tailing of upgraded rare metal mineralization,Abu Rusheid area – Egypt for nuclear waste immobilization

Most research activities in the nuclear mining industries are directed to separation of heavy and radioactive minerals from rocks and sediments. This process is completed without utilizing tailing and mineral waste materials. Tailing materials containing radioactive wastes are considered as a major environmental problem; especially with increasing the exploration of metals and lower grade mineral deposits. In the present work, borosilicate glass could be prepared for immobilizing these nuclear wastes in the designing compositions by melting at 1200–1250 °C. NaI (TI) gamma-ray spectroscopy system was used to detect ²³⁸U, ²³²Th, ²²⁶Ra, and ⁴⁰K concentrations in tailing materials and prepared glass to emphasize their safe dose. The produced glass was characterized by XRD, SEM, FTIR, and UV spectrophotometer. Vickers hardness and leaching properties were also studied. The results showed that compact - structured glass with high durability towards groundwater was obtained. All studied environmental parameters indicated that the glass radiation dose was lower compared to international averages, thus the glass can be safely used in nuclear waste immobilization.     

The sintering kinetics of porcelain bodies made from waste glass and fly ash

Porcelain is a material produced from kaoline, quartz and potassium-feldspar. Recently, research of new materials, for example non-hazardous wastes, that are able to replace traditional fluxing agents without changing the process or quality of the final products has been realized. The aim of this work is to study the possibility of the use of glass powder waste and fly ash together for manufacturing porcelain. Instead of quartz, fly ash was used at the selected porcelain composition. The waste glass was added partially and fully in replacement of potassium-feldspar. Samples were fired in an electric furnace with a heating rate of 10 °C/min at 1100, 1150 and 1200 °C for a period of 1, 2, 3 and 5 h. The sintered samples were characterised by XRD (X-ray diffraction) and SEM (scanning electron microscopy). Sintering activation energies were determined based on the bulk density result. At 10, 15, 20 and 25 wt.% glass waste addition, the apparent activation energies were calculated to be 145, 113.5, 70.4 and 53.74 kJ/mol, respectively. It was found that the sintering activation energy decreased with increasing waste glass addition.     

Effect of phase evolution and acidity on the chemical stability of Zr1-xNdxSiO4-x/2 ceramics

Zircon ceramics (ZrSiO₄) are promising candidates for actinide immobilization. Here, a series of Zr_1-xNd_xSiO_4-x/2 (x?=?0, 0.02, 0.20, and 1.0) ceramics are prepared to study the effects of phase evolution and acidity on the chemical durability of ZrSiO₄-based nuclear waste forms. The results show that the chemical stability of the single-phase ZrSiO₄ sample is better than that of the biphasic Zr_0.8Nd_0.2SiO_3.9 sample due to the existence of a secondary highly soluble phase (Nd₂Si₂O₇), which increases the contact area with leachate. The normalized release rates of both zirconium (Zr) and neodymium (Nd) in the Zr_1-xNd_xSiO_4-x/2 ceramic waste forms increase with increasing Nd loading and acid concentration. LR_Zr in ZrSiO₄ ceramics and LR_Nd in Zr_0.98Nd_0.02SiO_3.99 ceramics are the lowest, while LR_Zr and LR_Nd reach the highest values in the Zr_0.8Nd_0.2SiO_3.9 and Nd₂Si₂O₇ ceramics, respectively. The normalized release rates of Zr are lower than those of Nd due to the difference in the energies of their bonds with oxygen atoms. Moreover, the changes in the surfaces of the leached ceramics are consistent with their leaching results.     

Immobilization of hospital waste incineration ashes in glass-ceramic composites

In the paper process of sintering of hospital incineration ash as a counterpart of low-level active waste with borosilicate glass frit is presented. It is shown that low porosity glass-ceramic waste-form can be obtained at a temperature range of 850–900 °C. In the sinter, the main crystal phases are wollastonite and aegirine-augite pyroxenes which have a large isomorphous capacity of binding hazardous elements. The crystal phases are fully encapsulated by the glass that provides additional protection against environmental influence. Thus, multibarrier material can be obtained at a temperature considerably lower than vitrification. This, in turn, can improve the economics of immobilization process.     

Rapid preparation of Gd2Zr2−xCexO7 waste forms by flash sintering and their chemical durability

A simple and low-energy-consuming approach for preparing ceramic nuclear waste forms is greatly preferred for disposal of ever-increasing amounts of radioactive nuclear wastes. Herein, simulated radionuclide Ce could be rapidly incorporated into Gd₂Zr₂O₇ ceramic via flash sintering technique. Under an electric field of 250 V/cm, Gd₂Zr_2−xCe_xO₇ (0.0 ≤x ≤ 1.2) waste forms with a single phase of defect-fluorite were flash sintered at relatively low temperatures of 889–997 °C in 60 s. The onset temperature of flash sintering was decreased with the enhancement of Ce content. Furthermore, the density and grain size of Gd₂Zr_2−xCe_xO₇ waste forms were increased with the increase of the current limit. The nearly full dense Gd₂Zr_2−xCe_xO₇ waste forms were flash sintered at a current limit of 200 mA. The normalized leaching rates of Gd, Ce, and Zr in Gd₂Zr_0.8Ce_1.2O₇ after 28 d were 6.5475 × 10⁻⁵, 1.1624 × 10⁻⁷, and 1.1613 × 10⁻⁷ g·m⁻²·d⁻¹, respectively, which exhibited a good chemical durability.     

Characterising highly active nuclear waste simulants

Reprocessing of spent nuclear fuel produces a highly active liquor (HAL) waste stream, which is typically stored over extended periods of many years in waste tanks equipped with extensive heat exchange capability. Over time, particulates are known to precipitate from the HAL within these tanks. Particle simulants provide a route for understanding the physical behaviour of these HAL solids under different agitation and transfer conditions. Particle and dispersion characterisation techniques are used here to understand the behaviour of two types of simulant HAL solids, viz. caesium phosphomolybdate (CPM) and zirconium molybdate (ZM), in dispersion. Distinct properties are established for CPM and ZM and compared to a common oxide particle material titanium dioxide (TiO₂). The results of this study highlight the influence of key aspects of the HAL particulates, such as size and shape, on relevant solid–liquid properties such as sedimentation and rheology. The influence of bulk liquid properties such as electrolyte concentration and pH is also investigated. The results indicate various possible behaviours within the tanks which may impact the storage, remobilisation and pipeline transport of this class of nuclear waste.     

Phase evolution and microstructural studies in CaZrTi2O7 (zirconolite)–Sm2Ti2O7 (pyrochlore) system

From the characterization of a series compositions with general stoichiometry as Ca_1−xZr_1−xSm_2xTi₂O₇ (0.00 ≤ x ≤ 1.00), the phase evolution between zirconolite (CaZrTi₂O₇) and pyrochlore type Sm₂Ti₂O₇ has been elucidated. All the compositions were prepared by high temperature solid state reaction and characterized by powder X-ray diffraction (XRD) and electron probe for microanalyses (EPMA). Three major phase fields, namely two layer (2-M) or four layer (4-M) monoclinic zirconolite and cubic pyrochlore structure types were observed in this system. In addition, a feeble amount of perovskite type phase is found to coexist with zirconolite phase. 4-M zirconolite phase is observed as single phase field at the composition with x = 0.30 and 0.35, while cubic pyrochlore phase is observed as single phase at the compositions with x ≥ 0.60. Further, the composition and microstructure of coexisting phases are verified by back scattered electron image and EPMA studies.     

Study on the leaching behavior of salt waste forms prepared by using zeolite and lime glass

When the reaction of salt and zeolite was used to minimize the free salt in waste forms (r = 0.1), Cs showed the lowest leaching rate, 1.015 × 10⁻¹ g/m² d. Because alkali chloride is chemically stable, the reaction that alkali elements become components of glass does not happen and thus the leach resistance of the waste form solidified with soda glass was not much different from that solidified with borosilicate glass. The crystalline phase containing Cl was sodalite, but the tendency that Cs exists prior to sodalite phase was not confirmed. From a result of a long-term leaching, the predicted leaching fraction of Cs in 900 days was as high as 5.13%, but that of Sr was as low as 0.24%. The leaching experiment with a varying pH showed the major nuclides such as Cs, Sr, and Li in salt waste had different leaching characteristics each other.     

Phase evolution and chemical durability of Zr1-xNd xO2-x/2 (0 ≤ x ≤ 1) ceramics

A series of Zr_1-xNd _xO_2-x/2 (0 ≤ x ≤ 1) ceramics was prepared by solid-state reaction method. The effects of Nd content on the phase evolution were investigated. The chemical durability of resulting waste forms was also examined. The results show that the ceramics with x < 0.1 show monoclinic and cubic zirconia phase, with 0.2 ≤ x < 0.4 exhibit a single cubic phase, with 0.4 ≤ x ≤ 0.6 exhibit a single pyrochlore phase, with 0.6 < x < 0.8 exhibit a single cubic phase and remain cubic phases and hexagonal Nd₂O₃ when 0.8 ≤ x ≤ 1. The unit cell parameters of the Nd-doped zirconia samples increase as the Nd content increases. Moreover, the normalized element release rates of Nd element in Nd-doped zirconia ceramics firstly decrease with leaching time and almost no change after 21 days (∼0⁻⁶ g m⁻² d⁻¹), demonstrating its good chemical durability.     

Phase evolution,microstructure and chemical stability of Ca1-xZr1-xGd2xTi2O7 (0.0 ≤ x ≤ 1.0) system for immobilizing nuclear waste

In order to ascertain the structural relationship of zirconolite and pyrochlore for their potential application in HLW immobilization, the Gd-doped zirconolite-pyrochlore composite ceramics (Ca_1-xZr_1-xGd_2xTi₂O₇) were systematically synthesized with x?=?0.0–1.0 by traditional solid-phase reaction method. The phase evolution and microstructure of the as-prepared samples have been elucidated by XRD and Rietveld refinement, Raman spectroscopy, BSE-EDS and HRTEM analysis. The results showed that zirconolite-2M, zirconolite-4M, perovskite and pyrochlore, four phases were identified in Ca_1-xZr_1-xGd_2xTi₂O₇ system and could be coexisted at x?=?0.4 composition. With the increase of Gd³⁺ substitution, the phase evolution was followed by zirconolite-2M→zirconolite-4M→pyrochlore. It is illustrated that the phase transformation from zirconolite-2M to zirconolite-4M was promoted by the preferential substitution of Gd³⁺ for Ca²⁺. And the solubility of Gd³⁺ in zirconolite-2M, zirconolite-4M and pyrochlore increased in sequence. The chemical stability test was also measured by the PCT leaching method. The normalized elemental release rates of Ca, Zr, Ti and Gd in Ca_1-xZr_1-xGd_2xTi₂O₇ system were fairly low and in the range of 10^?6?10^?8 g?m^?2 d^?1, which indicated a potential ceramics composite ensemble of CaZrTi₂O₇-Gd₂Ti₂O₇ system for nuclear HLW immobilization.     

Chemical durability and surface alteration of lanthanide zirconates (A2Zr2O7: A = La-Yb)

Chemical durability of lanthanide zirconates (A₂Zr₂O₇) (A = La-Yb) under near-field environments is important for evaluating their application as potential nuclear waste forms. In this work, A₂Zr₂O₇ (A = La-Yb) are synthesized by spark plasma sintering with controlled microstructure and their chemical durability are evaluated in a nitric acid solution (pH = 1). Scanning transmission electron microscopy analysis reveals an amorphous passivation film either enriched with Zr or lanthanide. The complex chemistry of the passivation films can be correlated with a transition in corrosion mechanisms from a preferential release of lanthanide in La₂Zr₂O₇ to a preferential release of Zr in Er₂Zr₂O₇ and Yb₂Zr₂O₇. These results suggest a dominant mechanism of incongruent dissolution and surface reorganization for the formation of passivation films. Strong correlations are identified between the leaching rates and cation ionic size, ionic potential, electronegativity differences between A-site cation and Zr, and bonding valence sum of oxygen, suggesting important impacts of structural and bonding characteristics in controlling chemical durability of lanthanide zirconates.     

Effect of waste aluminosilicate material on cement hydration and properties of cement mortars

The effect of waste material (catalyst used previously in catalytic cracking of petroleum in fluidized bed—fluidized bed cracking catalyst denoted as FBCC) on cement hydration kinetics was investigated in terms of fineness of this admixture. The compressive strength and microstructure of cement mortars were also examined. Variable percentage of this aluminosilicate admixture, originating from batches of quite different grain size composition, was introduced to cement pastes. Further on, cement mortars were produced using the material of higher activity, as it has been found in admixtured cement investigations. The waste was added as cement replacement or, partially, as sand replacement. The activity of waste catalyst was strongly related to the fineness—finer grains indicate better activity. In the presence of a FBCC admixture, the Ca(OH)₂ content decrease in cement pastes due to the pozzolanic reaction is observed. The surface area of hydrated paste becomes higher and, simultaneously, the mean pore diameter decreases, as compared to reference sample, without admixture. The strength improvement is observed particularly when the aluminosilicate material is introduced as partial sand replacement.