Technetium mainly forms during artificial nuclear fission; it exists primarily as TcO4(-) in nuclear waste, and it is among the most hazardous radiation-derived contaminants because of its long ...half-life (t1/2 = 2.13 × 10(5) years) and environmental mobility. The high water solubility of TcO4(-) (11.3 mol L(-1) at 20 °C) and its ability to readily migrate within the upper layer of the Earth's crust make it particularly hazardous. Several types of materials, namely resins, molecular complexes, layered double hydroxides, and pure inorganic and metal-organic materials, have been shown to be capable of capturing TcO4(-) (or other oxoanions) from solution. In this review, we give a brief description about the types of materials that have been used to capture TcO4(-) and closely related oxyanions so far and discuss the possibility of using metal-organic frameworks (MOFs) as next-generation ion-exchange materials for the stated application. In particular, with the advent of ultra-stable MOF materials, in conjunction with their chemical tunability, MOFs can be applied to capture these oxyanions under real-life conditions.
Volatile loss of technetium (Tc) during vitrification of low-activity wastes is a technical challenge for treating and immobilizing the large volumes of radioactive and hazardous wastes stored at the ...U.S. Department of Energy's Hanford Site. Various research efforts are being pursued to develop technologies that can be implemented for cost effective management of Tc, including studies to understand the behavior of Tc during vitrification, with the goal of eventually increasing Tc retention in glass. One of these studies has focused on identifying the form or species of Tc and Re (surrogate for Tc) that evolve during the waste-to-glass conversion process. This information is important for understanding the mechanism of Tc volatilization. In this paper, available information collected from the literature is critically evaluated to clarify the volatile species of Tc and Re and, more specifically, whether they volatilize as alkali pertechnetate and perrhenate or as technetium and rhenium oxides after decomposition of alkali pertechnetate and perrhenate. The evaluated data ranged from mass spectrometric identification of species volatilized from pure and binary alkali pertechnetate and perrhenate salts to structural and chemical analyses of volatilized materials during crucible melting and scaled melter processing of simulated wastes.
•Literature data were critically evaluated to clarify volatile species of Tc and Re.•Tc and Re volatilize as M(Tc,Re)O4 without decomposition for M=Cs, K, Na, Li.•Volatility can be related to concentration and thermodynamic properties of volatile species.
Although the vitrification of nuclear waste has a decades‐long history, numerous opportunities still exist to improve its efficiency and to increase the waste loading in glass. This is especially ...true for the vitrification of low‐activity waste (LAW), which has been historically treated by other immobilization technologies and is less mature than high‐level waste (HLW) vitrification. In this work, we address one of the least understood phenomena during the conversion of nuclear waste feeds to glass—the formation of molten salt and transient glass‐forming melt. Using high‐temperature environmental scanning electron microscopy (HT‐ESEM) in combination with X‐ray diffraction, thermogravimetry, and evolved gas analysis, we have analyzed the complex chemical reactions and phase transitions as they occur during melting of representative HLW and LAW melter feeds. We evaluated the compositions of amorphous phases and the fractions of salt components, and estimated the fractions of molten salt phases present in the feeds as a function of temperature. We show that the maximum fraction of molten salts is ∼4 % and ∼28 % during HLW and LAW feed melting, respectively, and discuss the possibility of molten salt migration in LAW feeds. We also argue that the presence of significant fractions of molten salt phase can hinder the retention of rhenium (and, hence, radioactive technetium), and discuss how the properties of molten salt phase and transient glass‐forming melt are related to primary foam formation and behavior. Finally, we summarize key unanswered questions requiring further research to increase the understanding of the conversion process and enhance the nuclear waste vitrification efficiency.
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•Silver-functionalized silica aerogel (AgAero) is novel material for iodine capture.•AgAero completely and fast removed I− from different aqueous environments.•AgAero exhibited a ...preferred removal of I− over Br− and Cl−.•AgAero was able to remove IO3- in DIW through reduction to I−.
One of the key challenges for radioactive waste management is the efficient capture and immobilization of radioiodine, because of its radiotoxicity, high mobility in the environment, and long half-life (t1/2 = 1.57 × 107 years). Silver-functionalized silica aerogel (AgAero) represents a strong candidate for safe sequestration of radioiodine from various nuclear waste streams and subsurface environments. Batch sorption experiments up to 10 days long were carried out in oxic and anoxic conditions in both deionized water (DIW) and various Hanford Site Waste Treatment Plant (WTP) off-gas condensate simulants containing from 5 to 10 ppm of iodide (I−) or iodate (IO3−). Also tested was the selectivity of AgAero towards I− in the presence of other halide anions. AgAero exhibited fast and complete removal of I− from DIW, slower but complete removal of I− from WTP off-gas simulants, preferred removal of I− over Br− and Cl−, and it demonstrated ability to remove IO3− through reduction to I−.
Technetium-99 (99Tc) is a major radionuclide of concern in the Hanford low-activity waste (LAW), which will be vitrified into borosilicate glass at the Waste Treatment and Immobilization Plant (WTP). ...Sulfate in LAW has been known to be a critical factor affecting the volatile loss of 99Tc. We investigated rhenium (a nonradioactive surrogate for 99Tc) incorporation by crucible melting tests of two representative simulated LAW glass feeds, each prepared with varied sulfur concentrations in glass. The slurry feeds were dried and heated to 400–1100 °C. Soluble and insoluble phases of heated feeds were analyzed to profile the partitioning of various components during the melting process. The mechanism of how sulfate affects rhenium incorporation during feed-to-glass conversion reactions, and so final retention of rhenium in glass, is proposed based on the two hypotheses that segregated sulfate-rich phase forms during feed conversion reactions and Re preferentially partitions to this sulfate-rich phase.
The Hanford site in Washington State houses ~56 million gallons of radioactive wastes stored in 177 underground tanks. The waste must be immobilized and permanently stored, and the plan is to ...separate the tank wastes into low activity waste (LAW) and high-level waste (HLW) streams. The U.S. Department of Energy is building a Waste Treatment and Immobilization Plant at Hanford site to separately vitrify these two waste streams in borosilicate glass using Joule-heated ceramic melters (JHCM). Although the process of nuclear waste vitrification seems to be well established, in practicality, it is faced with complex problems starting from the design of glass compositions, to processing in melters and long-term performance of the final vitrified waste forms. The article presents an overview of our current understanding of critical challenges related to the development and performance of HLW glasses.
In this article, we argue that 1) the activation energy for viscous flow becomes independent of temperature when the viscosity of molten glass is sufficiently low at high enough temperatures, such as ...those that exist in a glass-melting furnace, and 2) the intercept of the linear function lnη versus T−1 (η is the viscosity and T is the temperature) is independent of glass composition. This hypothesis, which is hardly new and is well supported by experimental data, allows minimization of the number of fitting parameters. A new dataset of meticulously measured viscosities of a large composition region of simulated nuclear waste glasses that recently became available provided an excellent opportunity to test this hypothesis to verify it again. Also, we used this dataset to demonstrate that some popular functions designed for representing the high-viscosity segment (where the activation energy changes with temperature) are not recommendable for approximating the low-viscosity segment (where the activation energy is constant). Fitting such functions produces overparameterization and leads to physically meaningless (or at least esthetically unsatisfactory) outcomes, or, if the functions are constrained by the glass-transition viscosity and the high-temperature asymptote, the result is a significant lack of fit.
•When glass viscosity is low at high temperatures, activation energy becomes constant.•As temperature increases, viscosity approaches a composition-independent value.•High-temperatures activation energy is approximately linear function of composition.•These facts allow fitting parsimonious models for viscosity of many-component glasses.
Efficient and cost‐effective removal of radioactive pertechnetate anions from nuclear waste is a key challenge to mitigate long‐term nuclear waste storage issues. Traditional materials such as resins ...and layered double hydroxides (LDHs) were evaluated for their pertechnetate or perrhenate (the non‐radioactive surrogate) removal capacity, but there is room for improvement in terms of capacity, selectivity and kinetics. A series of functionalized hierarchical porous frameworks were evaluated for their perrhenate removal capacity in the presence of other competing anions.
Going nuclear: Functionalized hierarchical porous frameworks were used for the efficient removal of perrhenate from aqueous solution (see figure).
During the processing of low-activity radioactive waste to generate solid waste forms (e.g., glass), technetium-99 (Tc) is of concern because of its volatility. A variety of materials are under ...consideration to capture Tc from waste streams, including the iron oxyhydroxide, goethite (α-FeOOH), which was experimentally shown to sequester Tc(IV). This material could ultimately be incorporated into glass or alternative low-temperature waste form matrices. However, questions remain regarding the incorporation mechanism for Tc(IV) in goethite, which has implications for predicting the long-term stability of Tc in waste forms under changing conditions. Here, quantum-mechanical calculations were used to evaluate the energy of five different charge-compensated Tc(IV) incorporation scenarios in goethite. The two most stable incorporation mechanisms involve direct substitution of Tc(IV) onto Fe(III) lattice sites and charge balancing either by removing one nearby H(+) (i.e., within 5 Å) or by creating an Fe(III) vacancy when substituting 3 Tc(IV) for 4 Fe(III), with the former being preferred over the latter relative to gas-phase ions. When corrections for hydrated references phases are applied, the Fe(III)-vacancy mechanism becomes more energetically competitive. Calculated incorporation energies and optimized bond lengths are presented. Proton movement is observed to satisfy undercoordinated bonds surrounding Fe(III)-vacancies in the goethite structure.
Technetium ((99)Tc) is an abundant, long-lived radioactive fission product whose mobility in the subsurface is largely governed by its oxidation state. Tc immobilization is crucial for radioactive ...waste management and environmental remediation. Tc(IV) incorporation in spinels has been proposed as a novel method to increase Tc retention in glass waste forms during vitrification. However, experiments under high-temperature and oxic conditions show reoxidation of Tc(IV) to volatile pertechnetate, Tc(VII). Here we examine this problem with ab initio molecular dynamics simulations and propose that, at elevated temperatures, doping with first row transition metal can significantly enhance Tc retention in magnetite in the order Co>Zn>Ni. Experiments with doped spinels at 700 °C provide quantitative confirmation of the theoretical predictions in the same order. This work highlights the power of modern, state-of-the-art simulations to provide essential insights and generate theory-inspired design criteria of complex materials at elevated temperatures.
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